2007:010 CIV

MASTER'S THESIS

A Comparison between Fieldbuses and Remote I/O for Instruments in the Process Industry

Lars Persson

Luleå University of Technology MSc Programmes in Engineering Electrical Engineering Department of Computer Science and Electrical Engineering Division of EISLAB 2007:010 CIV - ISSN: 1402-1617 - ISRN: LTU-EX--07/010--SE

Abstract New technology arises in all areas. Some just last for a few years before someone else develops a better technology. This makes it hard to decide whether and when to use new technology. Being a contractor for the process industry, like Outokumpu Technology, does not make it easier to decide since these systems usually runs for at least 15 years. The aim for this Thesis is to provide information about fieldbus technology for the process industry. Although it has existed for several years far from everyone uses it. The Thesis focus on fieldbuses at instrument level. The knowledge, both of the contractor and the buyer, is of great importance. Both advantages and disadvantages have been identified with fieldbuses that can be used to decide whether to use fieldbuses or not. The examiner of this Thesis was Per Lindgren at Luleå University of Technology

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Preface This Master’s Thesis is the final part required for my Master of Science degree in Electrical Engineering at Luleå University of Technology. The work has been carried out during the winter 2005/2006 at Outokumpu Technology AB in Skellefteå. The aim with this Thesis is to identify advantages and disadvantages with fieldbus technology in Industrial applications. I would like to thank the following persons who have helped me with this Thesis: my supervisor Pär Norman at Outokumpu Technology for his time, support and ideas; Leif Nyberg, Manager of the Electrical and automation department at Outokumpu Technology and Per Lindgren at LTU for allowing me to do this Thesis; Leif Karlsson from ABB who has answered a lot of my questions; Jan Östensson, Mats Näätsaari and Jan Malmström at Husum and Magnus Normell at Eurocon for the interviews; my sweet girlfriend Freja for proofreading and correcting my English; and finally the rest of the staff at Outokumpu Technology for help and support throughout the work on this Thesis. Skellefteå, 17th December 2006 Lars Persson

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Table of content 1 Introduction ........................................................................................................................... 1 1.1 Background ...................................................................................................................... 1 1.2 Purpose ............................................................................................................................. 1 1.3 Delimitations .................................................................................................................... 1 1.4 Outokumpu Technology AB, Skellefteå .......................................................................... 1 2 Method.................................................................................................................................... 3 2.1 Literature review .............................................................................................................. 3 2.2 Interview........................................................................................................................... 3 2.3 Test system....................................................................................................................... 3 3 Literature review................................................................................................................... 5 3.1 Communication levels in a plant...................................................................................... 5 3.2 History.............................................................................................................................. 5 3.3 Fieldbus in general ........................................................................................................... 7 3.4 4-20 mA vs. fieldbus instrument technology ................................................................. 10 3.5 Wireless fieldbus ............................................................................................................ 11 3.6 Description of Profibus, Foundation Fieldbus, DeviceNet and Interbus ....................... 13 3.7 Practical experience from fieldbus installations............................................................. 22 4 Interview .............................................................................................................................. 27 4.1 Background .................................................................................................................... 27 4.2 Result.............................................................................................................................. 27 5 Test system ........................................................................................................................... 31 5.1 Background and problem ............................................................................................... 31 5.2 Material .......................................................................................................................... 32 5.3 Method ........................................................................................................................... 34 5.4 Result.............................................................................................................................. 34 5.5 Problems......................................................................................................................... 41 6 Result .................................................................................................................................... 43 6.1 Comparing fieldbuses with remote I/O .......................................................................... 43 6.2 Requirement of knowledge ............................................................................................ 43 6.3 Differences between Foundation Fieldbus H1 and Profibus PA.................................... 43 6.4 Wireless fieldbuses......................................................................................................... 44 7 Discussion and Conclusion ................................................................................................. 45 8 Future work ......................................................................................................................... 47 References ............................................................................................................................... 49 Appendix A: AS-Interface (Actuator-Sensor Interface) Appendix B: Test system specification Appendix C: GSD file Appendix D: Interview questions Appendix E: Abbreviations

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1 Introduction 1.1 Background The society is constantly developing and changing and therefore we have to adapt to the changes. The automation of metal process industry is no exception. The 4-20 mA analogue standard for instrumentation is still used at most plants in Sweden, but the standard has been around since the 1960’s. However, the time of change seems to have reached the process industry as well. In the early 1990’s, digital communication between instruments and the control system began to develop. Still, new developments, improvements and changes are made every year, making it problematic to decide when to invest time and money into the new systems. It has to be taken into consideration that a control system for the process industry runs for at least 15 years. The research question of this thesis is: is it time for new technology to take over?

1.2 Purpose Outokumpu Technology AB in Skellefteå (OTSk) is currently trying to decide whether the company should learn how to use fieldbuses and recommend it to their costumers. A fieldbus is like a data bus where several devices are connected to the same cable and communicate through digital messages. If the answer to the previous question is yes, which fieldbus should be used? Should OTSk recommend different fieldbuses depending on for example where in the world the installation is taking place? To facilitate the decision, OTSk wants to know which advantages fieldbuses can give regarding installation, operation and maintenance. Additionally to this, this thesis includes a limited search of wireless fieldbus solutions. The thesis will 1. Identify advantages and disadvantages with fieldbuses compared to remote I/O 2. Describe the differences between the fieldbuses 3. Identify advantages and disadvantages with wireless fieldbuses and make a study of wireless products and their manufacturers. The purpose is not to give a recommendation, but to provide data for decision-making.

1.3 Delimitations Due to time limits, the thesis will only look at four different fieldbuses for field devices. These are DeviceNet, Foundation Fieldbus H1, Interbus and Profibus PA. This thesis will then focus on two of them, Foundation Fieldbus H1 and Profibus PA, since these are operating at the instrument level. To know more about how a fieldbus functions in a plant, visits will be arranged to Husum and ETEK. Interviews will be performed with installation, maintenance and operation personnel at both plants. They are both located near Örnsköldsvik in Sweden. A system was provided by OTSk to evaluate Profibus PA. It included an ABB AC 800F control system and selected instruments from different manufacturers.

1.4 Outokumpu Technology AB, Skellefteå Outokumpu Oy is an international company specialized in stainless steel and technology. The company has about 13 000 employees and operates in over 40 countries. Outokumpu Tech-

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nology is a part of the Outokumpu group, which mainly sells technology for the metal and mineral process industry. This includes designing, developing and supplying tailored plants, processes and equipment. Outokumpu Technology in Skellefteå, Sweden was formerly owned by Boliden and was known as Boliden Contech AB. The company provide engineering and project services in Sweden and the rest of the world. The contact between OTSk and Boliden is still strong and Boliden is the major customer. With time new customers have been acquired and Outokumpu now have customers worldwide. China has fast become an important market due to the rapidly growing metal process industry. Outokumpu has an area of knowledge stretching from mining to melting and chemical activities. Around 100 employees are located in Skellefteå and about 25 of these works at the Electrical and Automation department.

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2 Method 2.1 Literature review The Thesis began with a literature review of fieldbuses and their advantages compared to remote I/O, which is currently used. The information was mainly taken from technical descriptions and from the standard-makers official homepages. Articles discussing advantages and disadvantages were also searched for on the Internet. The information about wireless fieldbus instruments has been found on the Internet. Mainly www.google.com and homepages of instrument manufacturers’, like www.abb.com, have been used. In addition to the two above Internet sources, www.scholar.google.com and ieeexplore.ieee.org, have been used to find general information about fieldbuses and information about fieldbuses in practice.

2.2 Interview A number of questions were prepared in advance and used as base for the interviews. The question sheet was constructed in cooperation with my supervisor and sent to the interviewees in advance.

2.3 Test system The AC 800F control system was new to me and the manual was used to learn how to use it. Parts of the personnel at Outokumpu Technology in Skellefteå had used the AC 800F control system before and helped when the manual was not enough. However, information about how to use Profibus in the AC 800F control system was not known at Outokumpu Technology in Skellefteå and personnel at ABB was used as an information source.

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3 Literature review 3.1 Communication levels in a plant The amount of data sent in a plant is increasing. Not only are the plants growing larger and the number of instruments increasing, the size of data retrieved from each device is also increasing. This puts higher demands on the communication. Industrial communication can be divided into three levels: Cell, Field and Sensor/Actuator level, see Figure 1. The communication can be both vertical and sometimes pass a level as well as horizontal. Binary signals are sent on the Sensor/Actuator level and the devices are usually powered and communicating on the same cable. At the Field level lie the distributed devices such as transmitters, drive units and I/O modules. Some devices at the Field level like a transmitter send a limited amount of data and can be powered by the bus while others like a driver have external power but send a lot of data. At Cell level we have the process stations which usually send lots of data. However, the data is not as time critical as the data sent at Field and Sensor/Actuator level. All fieldbuses studied in this Thesis communicate at the field level. Cell Level

Field Level

Sensor/Actuator Level Figure 1. The different communication levels in a plant.

3.2 History In a conventional DCS (Distributed Control System) each device is connected with two wires, see Figure 2. A device can be an instrument for measuring temperature, pressure etc. or an actuator that act on the system like a valve. In conventional communication, an instrument sends its measured values with an analog signal. In a similar way, an actuator is controlled by varying the current sent to it. If the device does not have large power consumption, it can be fed through the cable. The instrument sends its measured value by varying the current it uses between 4 and 20 mA. It is a standard that will be referred to as 4-20 mA in this Thesis. If 420 mA instruments are used in a large plant, lots of cables have to be installed. Even if several devices are located at the same place each one has its own cable, containing two wires.

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DCS

Figure 2. The conventional way to connect devices. Every device has its own cable to the DCS.

One step towards fieldbuses and digital communication came with remote I/O. It was mainly used to reduce the cables that have to be installed. In remote I/O, the I/O cards are moved from the DCS and placed closer to the instrument, see Figure 3. These I/O cabinets are then connected to the control system with a single cable. To be able to send several measured values across this cable the communication is made digital.

DCS

10110110…

Remote I/O cabinet

Figure 3. Remote I/O. Devices located close to each other are connected to the same remote I/O cabinet. The cabinet is then connected to the DCS with a bus cable.

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However, the remote I/O did not change the communication between the instrument and DCS. The devices still gave an analogue value that had to be interpreted. And no data could be sent to the device, which is needed to configure it out in the field. Another step towards digital communication came with the HART protocol. Highway Addressable Remote Transducer (HART) was a first step towards intelligent instruments. It uses the conventional 4-20 mA to send the process value but it also sends a digital message in the same cable. Information can be sent in both directions and be used for diagnostics or to calibrate the device. It sends a low frequency, FM (Frequency Modulated) sinusoidal signal superimposed on the analog signal, see Figure 4. It has to use a point-topoint connection, and has a speed of 1200 bps. It is a widely used technology and can be used with new remote I/O systems. Before it was supported by remote I/O, a handheld device was used to communicate with the instrument. [1]

Figure 4. HART communication superimposed on an analog signal. (Source: http://www.romilly.co.uk/hartwave.gif Accessed 2005-10-07)

By sending the process value as an analog signal, the device is still compatible to older systems and whether to use the advantages or not is up to the user. A problem is that special instruments have to be bought to communicate with HART devices, and the operator had to go out on the field and connect it to the cable for the device. [1], [2], [3]

3.3 Fieldbus in general In the 1980s, a new way to communicate between the devices and the control system began to develop, called fieldbus. The fieldbus is an all-digital way of communicating, which gives new possibilities for intelligent devices and new solutions. A fieldbus is constructed as a bus, which means that several devices share the same cable, see Figure 5. Because of digital communication, more than one variable can be measured in one 7

device. These reductions in cable and number of instruments can reduce installation time and cost. Fieldbus devices can also be made more advanced, like a thermal camera, which measures several points. The devices can also signal when they are about to break down or if there is something else wrong, for example a valve that can not close. Since all devices share the same cable, it enables direct communication between the devices. This can be used to move the control from the central system and put it out in the field. By doing this, important segments can work properly, even if process station stops or a cable breaks between the device and process station. The data can also be shared to operators by connecting the host to an ordinary intranet (Ethernet) or even Internet. In this way the electrician does not have to be at the site to do for example diagnostics.

Fieldbus Controller

10110110…

T

Figure 5. On a fieldbus, all devices are connected to the same cable.

The cable can be of different types depending on desired speed, electromagnetic interference, cable length etc. Copper, fibre optics or even radio links can be used in some cases. Some fieldbuses are bound to use one type while other supports several. Fieldbus gives the opportunity to use multivariable devices, which is devices that can measure more than one process variable. One example is the multivariable transmitter 2100T from ABB that measures mass flow with absolute pressure and temperature compensation. The additional information can be used both to get a more accurate reading, by compensating for temperature, and also send more information to the operator thus reducing the number of devices. [2], [4]

3.3.1 Geographical differences When the fieldbuses began to develop, each manufacturer made their own bus with their own communication protocol. The problem was that only devices from that manufacturer could be connected to that bus. To avoid this problem, the busses where made independent from the manufacturer by creating organizations which was open for everyone. These organizations then tried to make their communication a standard, so that it would be used everywhere. It usually began at a national level where it was rather easy to be accepted as standard. It happened simultaneously in different countries and the problems began to arise when these where trying to become international standards. BSi British Standards describe a standard as “…a published document that contains a technical specification or other precise criteria designed to be used consistently as a rule, guideline, or definition.” (British Standards (2006); What is a standard? accessed 2006-02-09, available online at http://www.bsi-global.com/British_Standards/Standardization/what.xalter). The idea is that if everyone follows the standard, all instruments would be compatible with all systems. The problem was that there were two promising fieldbus solutions in Europe, German Profibus and French FIP. They had different approaches and both had spent too much time 8

and money developing their fieldbus to give it up. Several articles, like [5] and [6], have studied what could be called “the fieldbus war”. It all resulted in a compromise standard (IEC 61158), which included 8 fieldbuses, all of them incompatible with each other. The fieldbuses are Foundation Fieldbus H1, Controlnet, Profibus, P-net, Foundation Fieldbus HSE, Swiftnet, World-FIP and Interbus-S. [7] The different fieldbuses where supported in different parts of the world and therefore, there are geographical differences between them. Below are the geographical locations of the four fieldbuses studied in this Thesis. [5], [6] Profibus originates from Germany (Siemens) and it is therefore natural that it has a strong hold in Germany and the whole of Europe. It is growing in the rest of the world as well. It has about 20% of the world’s fieldbus market and is used in a wide range of industries, such as automotive, production machinery and metal processing. PNO (PROFIBUS Nutzer Organization) is the organization that controls Profibus. DeviceNet comes from the US (Allen-Bradley), which is also its main market. It is especially developed for the factory automation and is competing with fieldbuses like Profibus DP and Interbus. It is used in general production machinery, automotives etc. It is also found in factories in Asia. Interbus originates from Germany (Phoenix Contact) with Europe as the main market. Interbus was developed in 1990 and was the first fieldbus that was manufacturer independent. It is controlled by Interbus Club. The fieldbus is mainly used by automotive manufacturers. Interoperable System Project (ISP) and WorldFIP North America created Fieldbus Foundation, as a result of that the Europeans could not agree to one standard. It is a rather new fieldbus and therefore not as large as the others, but they are growing rapidly. The main market is North America. For the process industry, for which Foundation Fieldbus was specified, they have equal shares with Profibus PA in Asia. [8], [9], [10]

3.3.2 Diagnostics and predictive maintenance According to [11], one of the main costs for industries is maintenance. Therefore, managers often try to reduce costs in this area. The maintenance today, is mainly based on time limits or a consumption trigger, like the number of full strokes for a valve. This is exactly what we do with our cars, when specific parts are changed after a number of kilometers. It is called preventive maintenance. If the intervals are short, the risk for failure gets lower, but the costs get higher. Even when set at a reasonable level, one problem still exist. Preventive maintenance does not avoid failure only reduce the probability. Another maintenance strategy is “maintenance on fault”, which basically means that the instrument is used until it breaks down and is then replaced. This strategy is not acceptable for devices that the process depends on and a fault would cause a standstill, since they cost a lot of money. There is a third maintenance strategy, called predictive maintenance. As the name suggests, the aim is to predict when a fault will occur, and hopefully be able to predict it just in time to save money and avoid faults. [11] However, predictive maintenance is not as easy as it sounds. Very little is known about which data that is useful or how to use the data to predict faults. It also depends on what kind of device and fault it is. Measure instrument faults are hard to predict because it mostly consist of electronic components, while valve positioners are easier since they have a relevant mechani-

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cal part. Motors are other devices where faults can be and has been predicted. Motors, however, sends lots of data and is usually not connected to Profibus PA or Foundation Fieldbus H1. Since these are the fieldbuses focused on in this thesis no more effort will be put into predictive maintenance for motors. [11], [12] In theory, the fieldbus is not a requirement for diagnostics of intelligent devices. HART devices are fully capable of sending diagnostics, but the problem is to get the information into the control system. The usual way to get diagnostics from a HART device it to connect a handheld device directly to the analog line connecting the I/O and device. It can however be sent to the operator or engineer from the I/O cabinet by using fieldbuses like Profibus DP [13]. With fieldbus connected instruments, the diagnostic data is available to the operator or engineer with little or none work, which is not the case for HART devices. [14] Two articles have been found, each describing a plant that practice predictive maintenance [15], [14]. The first one, Cargill Vitamin E Plant in Eddyville, Iowa has been using predictive maintenance since 2001. They have documented savings through improved maintenance which resulted in better reliability. The software they are using is the AMS Suit from Emerson, which is capable of instrument commissioning, configuration and trouble shooting. All maintenance activities are also automatically documented, like which changes are made in an instrument configuration. This reduces documentation time and makes it easier to track changes. [15] An example, where diagnostics were used, was a travel deviation alert from a control valve sent to the engineer. From the control room it seemed to work properly. When the technicians checked out the valve, they found that the air supply line of plastic was melted, because it was too close to a steam line. It resulted in a collapse of the line, which limited the air supply to the valve, which caused it to respond very slowly. This increased the variability of the process, without the operators knowing why. [15] In [15] the importance of knowledge is told. They recommend that one person should be asset manager, with good knowledge about the process, instruments and the software used. Another recommendation is not to expect immediate result, because it takes time to learn the system, software and how to work with predictive maintenance. The Total Solvants' Oudalle plant in the Normandy region of France is another example of a plant that uses predictive maintenance. Like the Vitamin E Plant, this plant uses AMS from Emerson for diagnostics. They have also found that the documentation is easier since the software documents all changes of the device settings. Before they used predictive maintenance, all of their 100 control valves were checked annually. Today, only 10 valves have to be checked each year, saving 90% of the maintenance work on control valves. [14]

3.4 4-20 mA vs. fieldbus instrument technology There are plenty of advantages with fieldbus. One feature is the ability to use more advanced instruments like a temperature-measuring camera that measures more than one point. The fieldbus can also increase the accuracy and reduce noise. The value sent from a fieldbus device is not altered on its way to the process station, while the analog signal is subjected to noise and interference. The fieldbus also has only one analog to digital conversion while the 4-20 mA has two and one additional D/A conversion, see Figure 6. These are all subjected to quantization errors. The deviations that show up in a fieldbus system are deviations in the

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process itself and not some noise picked up on the way. This gives the opportunity to analyse the process itself, with greater extend.

4-20 mA

Fieldbus

Control system

Control system

A/D

A/D

Unidirectional

Bidirectional

D/A

D/A

Micro processor

Micro processor

Micro processor

Micro processor

A/D

A/D

A/D

A/D

Sensor

Sensor

Sensor

Sensor

Figure 6. Comparison of communication link between the sensor and control system with 4-20mA (left) and fieldbus (right).

The diagnostic functions of a fieldbus device can warn the operator before an accident or a breakdown occurs that could cause an unplanned standstill. The factories are growing bigger and so is the cost for a standstill. In case of a breakdown, either a cable or a device, a fieldbus can in some cases make the error detection easier. In hazardous areas, the enhanced diagnostic abilities mean fewer visits to devices in hazardous areas thus minimizing personnel risks. Some fieldbuses have the ability to distribute the control out to the devices instead of using a centralized control. By doing that, the central system only needs to monitor the process. The system becomes less dependent on a single computer or PLC and by creating independent subsections, each of these can be shut down and maintenance can be preformed individually, if the process allows it. However, there are drawbacks as well. 4-20 mA is an industrial standard that is used everywhere. It is simple to understand and easy to use. Even if the support groups of different fieldbuses states that they are an international standard, there are several of them. Currently, they do not seem to come together to form one united standard, but instead make their bus more unique to push it forward. [3]

3.5 Wireless fieldbus Below is a short literature review of advantages and disadvantages with wireless fieldbuses and after that, an overview of wireless products, mainly for fieldbus usage.

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3.5.1 Advantages and disadvantages with wireless Some of the advantages with wireless connections are reduction in cost and time for installation and maintenance of the signal cable. It is also useful for connecting instruments from mobile units or other places, where chemicals or vibrating parts, are likely to wear of the cables. It will also make the system more dynamic since instruments are easier to move and the maintenance personnel can use a handheld wireless device to configure or get diagnostics. [16], [17] Fieldbuses are dependent on deterministic behavior to live up to hard real-time demands for cyclic polling of data. If a part of, or the entire system, is built on wireless links, these have to live up to the same demands. Wireless connections are more affected by noise and interference then a copper wire, messages can be lost and the message has to be re-sent. This makes it hard for wireless links to meet real-time demands. Security is another issue. It is easy to eavesdrop, send malicious packets or disturb the transmission. Energy supply is an important issue, mainly since field devices are usually fed with power by the signal cable. And if a cable is needed it is not truly wireless. [16], [17] These issues have to be solved before wireless can become the industries first choice. Even if it might not be ready to install in every part of any plant in any industry, it is already used in more specialized areas where cables are likely to break or cables are difficult to install. [18] shows that an aluminum plant, with large mobile units, could reduce the number of production stops due to wire wear.

3.5.2 A product overview Although wireless technology has been known for a long time, it is not widely used for device communication in the process industry. One reason can be that they still need a power supply. A separate power cable can be used, but then most advantages are lost. Batteries are another option, but usually instruments are placed in tight and not central areas and to change batteries would be a hard job. Instead manufacturers are looking for new alternative power sources. One that is already in use is induction. ABB has a wireless proximity switch, which is powered by an electric field. The field is generated by an alternating current in a power loop and a coil inside the instrument that draws power from the field. [19] There are products available to send fieldbus data across the network. One example for Profibus DP is an optical link from HIRSCHMANN. It provides a wireless link, 0.5 to 15 m, between two or more segments. Since it is an optical link, line-of-sight is needed. One limitation is that masters are only allowed on one of the segments. Supported speeds are 9.6 kbit/s to 1.5 Mbit/s. Each or these segments still have to be supplied with power. [20] Omricon has a similar product for DeviceNet. It uses radio transmission and has a range of up to 60 m in indoor environments. The system has master and slave modems, and several masters can operate in the same area. The speed is however limited to 100 kbit/s. One advantage is that the usual six meters limits for spur cables can be overcome. The power supply is unfortunately still needed for each subnet. [21] ELPRO Technologies has a wireless modem for longer ranges. It uses radio frequency for communication and if it has line-of-sight it can work for up to 20 km. The usual working range however is about 1-2 km. The range can be extended with up to four repeaters. To be able to transmit this range, it uses more power than the other wireless links described in this Thesis. It has support for and can connect to several fieldbuses / protocols for example 12

Profibus, DeviceNet and Ethernet. Each modem also has eight discrete I/O that can be configured as either input or output. The modems support the same transfer rates as the fieldbus. [22]

3.6 Description of Profibus, Foundation Fieldbus, DeviceNet and Interbus A fieldbus is usually designed for one kind of industry. Figure 7 shows which are best suited for production or process industry. The fieldbuses designed for process industry are Foundation Fieldbus, Profibus PA, HART and AS-Interface. HART is not a fieldbus but merely a digital extension of the analog devices. However, it plays an important role in development of intelligent field instruments. AS-Interface, described in Appendix A, is a fieldbus, but is very limited. It is constructed for binary devices with a maximum bus length of 300 m. It is usually used in combination with an ordinary fieldbus. To extend the knowledge about fieldbuses, and not only for the process industry, a short description of DeviceNet and Interbus is also included in this section.

Figure 7. Area of usage for the different fieldbuses. Source: [23], page 15, figure 6

3.6.1 Profibus Profibus consists of three different busses, Profibus PA, Profibus DP and PROFInet. Profibus PA and Profibus DP are used at the field level. Profibus PA has a lower speed but instead it can supply power in the cable. Profibus DP is more suited to send lots of data from for example drivers. PROFInet is used to connect the different networks and can be used to give the operators information about the system on their PC through the local network (Ethernet). This also gives the opportunity for the electrician to configure the devices from any PC connected to the intranet. [24], [25] PROFInet PROFInet is a wide automation concept that has been developed due to the increased use of modular, decentralized control. PROFInet is both a specification and an open, system independent software that handles the run time communication.

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A Profibus DP or Profibus PA section can be connected to PROFInet by using a proxy. With these, all devices can be accessed directly from the PROFInet network. PROFInet uses Ethernet to communicate between devices. [26] Profibus DP (Decentralized Periphery) The Profibus DP bus is mainly used to communicate between the PLC (Programmable Logic Controller) and the decentralized periphery. Profibus DP can connect segments of Profibus PA and remote I/O cabinets with its high-speed bus. It supports DP-V2, which is described below. Profibus DP communicates through RS 485 and has a maximum speed that depends on the length of the bus, see table 1. Profibus DP can be used in hazardous areas if RS 485-IS and Profibus DP-Ex barriers are used. With RS 485, one segment can have up to nine repeaters. If fibre optic cables are used, any number of repeaters can be used. The maximum number of devices that can be connected to a Profibus DP bus is 126, due to limitations in addresses. That limit is however theoretical. In practise about 40 – 60 devices are maximum for Profibus DP since the response time would become too high otherwise. [24] Table 1. Maximum segment length at different transmission speeds for Profibus DP with copper cable.

Transmission speed (kbit/s) 3 000 – 12 000 1 500 500 187.5 93,75 – 9,6

Maximum segment length (m) 100 200 400 1000 1200

Profibus PA (Process Automation) The Profibus PA bus was developed for the process industry. The bus communicates through a two-wire cable and corresponds to the MBP (Manchester coded, Bus Powered) standard. This standard is used in process automation because it can power the devices connected to it. As Profibus DP, Profibus PA also has an intrinsically safe version, where MBP-IS is used and can therefore be used in hazardous areas like chemical plants. The specification states that the bus cable should be a twisted, preferably shielded two-wire cable and must be terminated at both ends to avoid the signals from reflecting back. The bus speed is 31.25 kbit/s and up to 32 devices can be connected on one bus segment. This number can be reduced because the devices demand too much current or the cycle time becomes to long. To send 1 or 0 on the bus, a device increases or decreases its power consumption by 9 mA. It will increase or decrease the voltage of the bus with 0.5 V which can be detected by the other devices connected to the bus. A special power source has to be used so that it does not try to compensate for this loss. The signals are Manchester coded which means that the signals are read at the middle of a bit cell, see. A rising edge is a 1 while a trailing edge is a 0. This coding is self-clocking, which means that the receiver can determine the clock rate of the transmitter from the signal. In each bit cell the signal is high for half of it and low for the other half, which means that the mean bus voltage is independent of the number of ones and zeros that are sent. MBP supports all bus topologies and have a maximum length of 1900 m. Repeaters can be used on a Profibus PA segment to extend the number of devices or the maximum cable length. Up to four repeaters are allowed and give a total of 126 devices and a maximum length of 9500 m. Profibus PA allows cyclical communication, DP-V0, to send

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process values and acyclical communication, DP-V1, for diagnostic and configuration values. [24], [27]

Figure 8. Manchester coded data.

Communication protocol DP Profibus uses the communication protocol DP to communicate between the devices and the process station. There are three different versions, DP-V0, DP-V1 and DP-V2. They are compatible with each other and can be used on the same bus. Profibus DP can use either of them while Profibus PA uses DP-V0 and DP-V1. A higher version supports all functions from the versions below. The functions are:

DP-V0 • • •

Cyclic data transfer Diagnostic Configuration via GSD files

DP-V1 • • • • •

Acyclic data transfer1 Alarm handling FDT/DTM and EDD device management Function blocks acc. IEC 61131-3 PROFIsafe

DP-V2 • • • • • • [26]

Broadcast communication (one to one/many) Time and time stamp Isochronous mode2 Up- and download functions Hart on DP Redundancy

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Acyclic data transfer enables data to be transferred in between the cyclic data. By using it, setting parameters and calibration of instruments are possible in runtime. 2 With isochronous mode, highly precise positioning process with less than one microsecond in clock deviation is possible. This works independent of the busload.

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As there are several devices sharing the same bus, rules are needed to control which one is allowed to send. Otherwise, conflicts can occur if two devices send at the same time. It is solved by using a MAC (Medium Access Control) protocol. Profibus usually works on a master/slave system where the master asks a slave for data and the slave immediately respond. The slave does not initiate any communication without being allowed by a master. Profibus allows several masters on the same bus and therefore need to control which master that is allowed to send. It is controlled by a timed token that is passed between the masters. Only one token exists on the bus and the master that has it is allowed to pull data from its slaves. When all slaves have been pulled for data or the maximum allowed time has elapsed the token is passed to the next master. [26] Process value diagnostics Process values in a Profibus system are sent as a 32-bit floating-point number (IEEE 754). The value is calculated as follows Process value = (-1)sign * 2(E-127) * (1 + F) Where sign, E and F are in the bits shown in Figure 9. Bit

31 30 29 28 Sign Exponent (E) 27

Bit

27

26

25

24

23

26

25

24

23

22

21

20

15 14 13 Fraction (F)

12

11

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8

2-11

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2-8

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22 21 20 Fraction (F)

19

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16

2-1

2-2

2-3

2-4

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2

1

0

2-16

2-17

2-18

2-19

2-20

2-21

2-22

2-23

Figure 9. The value of each bit in an IEEE 754 floating-point number.

Together with the process value, a status code is sent which is one byte. This diagnostic byte gives an easy access to the status of the device, especially for transmitters with limited diagnostics. Valves have more diagnostics where this byte is not enough. Examples of useful information sent with the byte are; the process value has exceeded a limit, the thermo element is not connected or the device is out of service. The first two bits, 0 and 1, represent the quality, bit 2 to bit 5 is sub-quality and bit 6 and 7 shows limit status. The errors that are reported depend on the device. Some status codes are the same for all devices like Good value and Out of service while other are device specific like lead breakage of the sensor for the temperature transmitter. Table 2 shows some common status codes. Table 2. Common status codes for Profibus

Status 0x00 0x1F 0x80 0x89 0x8A 0x8D 0x8E

Description Bad value Device Out of Service Good value Good, LOW_LIM alarm active Good, HI_LIM alarm active Good, LOW_LOW_LIM alarm active Good, HI_HI_LIM alarm active

PROFIsafe Tasks with high demands on security like emergency stop buttons usually have to use a special bus or conventional technology. To be able to connect these devices to Profibus DP, 16

PROFISafe was developed. It defines how security devices should communicate with each other on the Profibus so that it can be used for secure automation. PROFISafe checks for several errors that can occur on a serial link like delays, lost or repeat of data, data in the wrong order, wrongly addressed data or corrupt data. PROFISafe is a software that put itself on top of the Profibus DP protocol, in the ISO/OSI model. It can work with devices that do not use PROFISafe without disturbing the rest of the devices. It uses acyclic transmission to communicate and works on MBP, RS485 and fibre optic cable. [24] Interaction with HART Because of the large number of HART devices installed in the industry today, it has been made possible to connect it to Profibus. The HART software is implemented into the slave and master and can be send on the Profibus. The HART devices can be connected to the Profibus via a HMD (HART Master Device). Several HART devices can be connected to one HMD. [26] Device configuration To be able to communicate with different devices, a GSD (General Station Data) file is used to specify the communication properties for a device. For the application functions of a device, like configuration of parameters and variable ranges, EDDL (Electronic Device Description Language) can be used. For more complex applications, FDT/DTM is used. GSD is a plain text file with information about the communication with a specific device. There are both required data, like Vendor_Name, and optional data depending on if it is a master or a slave. Other information that can be stored in a GSD file is parameters, with data type and range, and product identification number. Diagnostic messages can also be set in this file. The GSD files belong to a product and should come with the device. There are also general GSD files, which makes it possible to connect for example a pressure transmitter from any manufacturer to the bus. GSD is not suitable for application related parameters and functions for field devices. Therefore EDDL is used for a more precise description of the device. The main reason for developing EDDL is its ability to describe functions for a fieldbus. EDDL and GDS are limited in use and are not useful for describing complex devices and not standardized, specialized properties in intelligent devices. With FDT (Field Device Tool), a DTM (Device Type Manager) is written by the manufacturer and can contain specific device information. DTM can be compared with a PC driver, like a driver for a printer. Once installed, the PC can communicate directly to the printer and also use its special features. In the same way, the electrician can access all parameters and diagnostic data from the device, if the correct DTM is used. The DTM also specifies the Human Machine Interface (HMI) for the device. The FDT program is needed to use the DTM files and connects the device with the electrician. [26], [28]

3.6.2 Foundation Fieldbus Foundation Fieldbus has two different busses. H1 is a slow and, if needed, intrinsically safe bus that can supply power to the devices connected to it. High Speed Ethernet (HSE) is the second one used for communication between process stations. On a Foundation Fieldbus the control can be placed inside the actuator instead of in the process station. It puts higher demands on the actuators and more hardware in the devices, but at 17

the same time reduces the work for the process station. One advantage of putting the control in each actuator is that the entire system does not have to go down if the process station loose contact with the segment. H1 The H1 bus, like Profibus PA is based on MBP, see chapter 3.5.1 for details. The following applies to the Foundation Fieldbus H1 bus. Each device should have at least nine Volts to work properly. Without repeaters, the maximum length of a one segment is 1900m. With up to four repeaters, five segments can be connected can reach 9500m. Maximum numbers of devices per segment is 32. By using repeaters, 240 devices can be connected to the network. But there can be no more than four repeaters between two devices. The bus speed is 31.25 kbit/s. [29], [30] Foundation Fieldbus supports both scheduled and unscheduled transmission. Real-time applications that are time critical, like the control of valves, uses scheduled transmission to assure they are executed in time. Tasks like parameterization and diagnostics, which are less time critical uses unscheduled communication services and uses the time between the scheduled once. As stated earlier the control can be done directly by the actuators. It can contain a PD (Proportional/Derivative) or PID (Proportional/Integral/Derivative) function block that can be connected to an instrument. If they are on the same bus, actuators can read the instruments whenever they send their data. However, there is still need for controlling access to the bus. A LAS (Link Active Scheduler) is used to control the transmission. It is a device that has a schedule of all scheduled transmission and controls the communication by polling information from the instruments. There are two types of devices on a H1 bus, basic devices and Link Masters. Link Master devices are devices that have the potential to become a LAS. Therefore, each Link Master is configured with the same schedule. If the active LAS goes down another Link Master takes over. This makes the bus less dependent on one device. To synchronize the time of all devices the LAS sends out a Time Distribution on the bus. When the engineer configures the system, a schedule is made for each device. It states when scheduled tasks should be preformed. For an instrument it could be 1: reading the Analog In (AI) and 2: sending the information. The second task should be executed a fixed time after the first to ensure that the value has been received. From the configuration of all devices a schedule is created, specifying when a device is allowed to send. The LAS also polls for unassigned device addresses in between the cyclical communication, which makes it possible to connect devices during operation and integrate them into the system. [30], [31] Example of a scheduled data transmission To give an example of how a scheduled data transmission is handled by the H1 bus, one cycle of the system in Figure 10 will be described. Assume we have two instruments and one actuator that depend on the instruments. The tasks should be executed at time offset in Table 3.

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Figure 10. Example of a Foundation Fieldbus system. Table 3. A time scheme for devices.

Device S1 S2 A1

Type Action Instrument Read AI Transmit read data Instrument Read AI Transmit read data Actuator Execute PID Set AO

Offset 0 30 0 40 50 79

The following is happening: At time 0, both instruments read their value from AI (Analog In). At time 30, the LAS send a message to S1 to broadcast its data. A1 is dependent on S1 and when S1 broadcasts its value A1 stores it. At time 40, S2 sends its data after permission from LAS. No devices can write to the bus unless the LAS permit it. It is done to avoid conflicts on the bus. At time 50, A1 has received both values and can therefore execute the PID controller. At time 79, the PID has calculated the desired value of AO and it can be set. After a fixed time, the loop begins from the top again. HSE (High Speed Ethernet) HSE can create a control backbone for all devices in the plant. It provides a wide network where the electrician can manage functions like calibration or diagnostics throughout the plant. Users can connect different segments for basic control, emergency shutdown etc. By using a LD (Linking Device), data from one H1 segment can be send directly on the HSE network. HSE uses standard Ethernet technology and provides peer-to-peer communication, removing the need for a central computer. Ethernet is available at low cost and widely used. It has high speed (100Mbit/s), which makes it possible to send lots of data. The drawback is that Ethernet uses random bus access and can therefore not be used for time critical tasks. The same function blocks are used in H1 and HSE and therefore the same programming language can be used for the entire system. [30]

3.6.3 DeviceNet DeviceNet is an open standard that is controlled by Open DeviceNet Vendor Association, ODVA. It uses the CAN (Controller Area Network) bus to send information across the network and CIP (Common Industrial Protocol) for interpretation of the data. As with Profibus and Foundation Fieldbus, several manufacturers supply the same product and these are inter19

changeable. Interchangeable means that a device from one manufacturer can be replaced with one from another. DeviceNet supports multidrop topology and can provide power through the cable. But it has four wires, two for power and two for sending signals. The communication over the bus is controlled by CAN, which uses a priority system to avoid several devices to send messages on the bus at the same time. The system can use either master-slave or a peer-to-peer if the control is distributed throughout the system. Multimaster and change-of-state, which is an event driven message, are also allowed. Several devices, independent on which buss communication is used can receive one message. Devices can be added or removed from the network and new data paths can be added while the system is on-line. Power taps can be added at any point on the network and can therefore give redundant power supply. The maximum speed depends on the length of the bus and the drop lines, which is shown in Table 4. Table 4. The end-to-end network distance varies with data rate and cable thickness. (Source: http://www.odva.org/10_2/05_tech/PUB00026R1.pdf, page 2, table 2)

Data Rates Thick trunk length (m) Thin trunk length (m) Flat trunk cable (m) Maximum drop length (m) Cumulative drop length (m)

125 KBPS 500 100 380 6 156

250 KBPS 250 100 200 6 78

500 KBPS 100 100 75 6 39

According to the CAN specification the bus can be in two states: dominant (logic 0) or recessive (logic 1). Any device can put the bus in dominant state, while recessive is only possible when no device is in dominant state. All devices listens on the bus, even when they are sending, and if the state they receive is not the same as it sends, another device is sending at the same time. This is used to make a priority system that makes sure there are no collisions on the bus. An 11-bit identifier is being sent at the beginning of each transmission and that is where the priority is put. Each packet contains 0-8 bytes of data, which is enough for most devices. But if longer messages are needed, DeviceNet has a fragmentation protocol that handles larger data amount. At the end of the frame, a CRC field is used to check for transmission errors. There is also an ACK bit that is used by the receiving devices to acknowledge that the message has been received. The data is sent with the CIP which is object oriented. An object has attributes, services and behaviour. In common devices there are a standardized set of objects. These devices can then be exchanged with devices from different manufactures without changing the programming. CIP is not dependent on a specific transport network, like a CAN bus or Ethernet, and can therefore travel between these. Even if DeviceNet fundamentally uses peer-to-peer communication, it also has a communication schedule for Master/Slave connections. This can be used when the data communication links are known at power-up. The data can be either polled, where one or several can receive the message, cyclic, where the data is produced at a predefined rate or change-of-state, where the data is sent when it changes. The last option also has settings for minimum and maximum time between transmission, to give an “alive” signal and avoid flooding the bus. [32]

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3.6.4 Interbus Interbus is a fieldbus that has well developed diagnostic abilities. It is always build with a ring topology. This is however not always shown out in the field, because the bus passes the devices twice, on its way out and on the way back. Because of this, the communication is virtually bidirectional. Smaller loops, often containing simple devices, can use a “true” ring topology. Figure 11 shown what an Interbus connection can look like.

Figure 11. An example of an Interbus fieldbus segment. (Source: http://www.interbusclub.com/en/doku/pdf/interbus_basics_en.pdf page 8, figure 4)

Because of the ring topology, the devices have point-to-point connections with their neighbors, which means it can cover long distances. Interbus is a mono master system and one frame send through the entire system contains information to and from all devices. The addressing of the devices is automatically assigned, by using their position in the ring. The engineer is able to assign aliases for the device addresses, which makes it easier to add or remove devices without re-addressing existing devices. The system is divided into three different structure types, Remote Bus, Local Bus and Loop. The controller board is the master in the system and controls the data traffic. It is the connection between Interbus and a higher-level network like Ethernet. It also handles diagnostic messages and if it has a display it can display these messages there. The controller board is connected to the Remote Bus. The data can be transmitted using several medias, copper (RS485), fiber optics, infrared etc. It can also supply current to devices connected to it. The transmission speed is 500 kbps and the maximum distance between Remote Bus devices is 400m. This can be achieved since each device works as a repeater and allows a total maxi21

mum bus length of 13 km. If fiber optic cable is used instead, the maximum length is 80 km. Bus terminals are connected on the Remote Bus and can branch out Local Buses. These also divide the system into subsystems, which can be individually closed. Decentralized instruments and actuators are connected by a loop. These have to be connected as a ring and the bus does not pass it twice. They are connected by a two wire cable which both supply power and transports data. 63 devices can be connected in the loop at a maximum distance of 20 m between two of them and a total distance of 200 m. To integrate planning, configuration, diagnostics etc. into one tool Interbus uses CMD (Configuration, Monitoring and Diagnostic). After the system is finally connected, it starts up automatically and can be configured with the CMD tool. Each subsystem can be tested separately using the monitor function. The controller board can be programmed to do limited processing by using IEC 61131 programming language, Function Block Diagram. It can be used to do time critical control tasks and reduce the load of the control system. To connect/configure a device most manufacturers provide electronic device description. There are also standard profiles for these that make it possible to change a device from one manufacture to another without changing the programming. When a bus error occur, like a broken wire or device, the ring structure makes it possible to both localize the error and continue to run the part not effected by the error. This is done by letting the device closes to the error send the information back instead of sending it to the faulty link. A CRC is used on each point-to-point transmission between devices to make sure the received information is correct. By looking at the error statistics from these, parts with large disturbances caused by for example a weary slip ring can be detected and replaced in time. [33], [34]

3.7 Practical experience from fieldbus installations After searching the Internet for articles describing fieldbus installation in practice, I found that that I am not the only one trying to find out how the fieldbuses work in practise. Three documents [35], [36] and [37] were found useful and reliable enough to use. The first article [35] comes from värmeforsk, an organisation where heat and power producers meet to share experience and form research groups. The author visited four installations in Sweden. The first one was an energy supply central with a heat pump and two refrigerating machinery at Bo01, which is a residence trade fair in Malmö. The second one was Barsebäcksverket waste disposal, which has a system for handling of radioactive waste. The third one was Nimrod in Stockholm, a district refrigeration plant with four refrigeration units. And the last visited plant was Scanraff, a Propane plant. The second article [36] tries to estimate the economical differences in installing a fieldbus instead of remote I/O. A new plant was also constructed based on the report. The third document [37] was found on the Foundation Fieldbus web server. It describes the experience from implementing a fieldbus in Western Australia at a sodium cyanide plant. It is the least reliable of these articles but it shows that knowledge and experience is important factors when it comes to fieldbuses.

3.7.1 Reduction in time and money The largest saving was found at Nimrod where the refrigeration plant saved as much as 50% of the project time and an estimate of 30% cut in costs. The major cuts where made in com22

missioning. One reason for the large cuts can be that the plant has a sister plant that was commissioned one year before. The experience from that was used in constructing the new one. However, it reveals that there are huge potential savings if fieldbuses are used with experience. The electronic installation gave a cut of 25%. The cut was not larger because the fieldbus has to be protected from disturbance, grounded correctly and a more expensive cable has to be used. The documentation is another area where a lot of time was saved, because the number of circuit diagrams was reduced. At Bo01 the commission was a part that saved a lot of time. Mainly because when one device was configured, that information could be reused for the other devices of the same sort. The fieldbus is therefore more profitable the larger the plant is. There where some problems, but it was not worse than if conventional I/O would have been used. The fieldbus devices are a little more expensive but the reduction in commissioning time makes the fieldbus installation cheaper than remote I/O. At Barsebäcksverket the modernisation would be taking place during operation. Another condition was that a lot of the existing equipment should be used, for example cables and instruments. Because of it, the savings in commissioning was very small, since the fieldbus had to be adapted to the existing installation. Barsebäcksverket estimate that the cost was slightly higher compared to a conventional installation. For Scanraff, the instrumentation went faster than conventional technology, while the time for meetings and work with construction, configuration and layout took more time. The theoretical calculations conducted in FuRIOS estimate the cost reduction to 4.2%. They state that a reduction of up to 20% suggested in former studies is due to the fact that they compare to conventional wiring and not remote I/O. The cost reduction when changing from conventional wiring to remote I/O is therefore roughly 15%. The greatest cut in cost where the I/O system, with a reduction of 18.1%. This was mainly because the I/O cabinets and their power supply were replaced with fieldbus barriers. The fieldbus barriers are light, small and easy to install compared to remote I/O cabinets that have to be carefully assembled. The fieldbus is more dynamic since it does not have to be decided in advance where each device has to be connected. The barriers are more decentralized which can make the spurs to the devices shorter. The cost for fieldbus devices where slightly increased (+0.6%). However, the devices where replaced by an equal one, and if multivariable devices where used instead, it could have lowered the cost. The use of standards, reduction of error sources and easier fault diagnostics due to transparency have the potential to make commissioning much faster. It is not necessary to make a loop check. The device just has to be connected and checked whether it shows a “sign of life”. For a 12-18 month project, the FuRIOS report estimate the reduction in time to 10 days. This report uses prudent calculations and it is fair to assume that some areas can save more time and money.

3.7.2 Practical experience – installation and commissioning At Bo01 they have established that the usage of fieldbus advantages in devices differs a lot between the manufacturers. While some uses fieldbus to its full extend, others just support the necessary basic functions. While installing, the data cables where continuously checked with respect to impedances to assure it was as resistant toward interference as possible. A wrongfully grounded cable can give random errors that are hard to find the source of. To make the Profibus DP bus more resistant towards interference, the communication speed was set to half the maximum speed. When configuring and connecting the devices to the control system 23

some problems occurred. One GSD file did not work in DeltaV, which is a Digital Automation System from Emerson Process Management. It was also problematic to find correct information about the memory mapping, which defines where in the memory different parameters are stored. Barsebäcksverket experienced minor problems during commissioning but no worse then if conventional technology would have been used. It is important to have the right version of the GSD file and suppliers were unfortunately not good at either giving a GSD file nor give the right one. They also found that a segment can work properly, even without termination, but intermittent errors can occur that are hard to trace. Since the signal cable is more sensitive to interference, Nimrod decided to use a separate cable ladder for fieldbus cables. They also made it easy to identify and follow a buss segment by using different colors for the different segments. At Scanraff there were problems with finding the driver for the devices. There have also been problems with the first generation DeltaV H1 cards that were not stable. It stopped to respond and had to be restarted. Upgrading the software solved the problem. The sodium cyanide plant in Australia experienced a lot of problems, mainly because they could not find a contractor with fieldbus experience. During installation, they made sure the screening was properly connected since they thought it to be a possible issue during commissioning. When commissioning the host vendors’ pressure and differential pressure transmitters, it was fairly straightforward. However, the third party devices, for example valves from three different vendors, took longer time. This was mainly a result from lack of experience with third party Foundation Fieldbus devices. They where also more advanced devices where data was sent in both directions. While the commissioning of host transmitters took minutes, it took on average one day to commission each valve. In FuRIOS 2, the plant build on the FuRIOS report confirmed that there are saving potentials in commissioning. They also found that the installation gets more flexible since it does not have to be decided in advance, exactly which and where the devices should be placed.

3.7.3 Practical experience – maintenance and running a fieldbus system At Bo01, the personnel are generally positive to the usage of fieldbuses. A lot of information about the devices is presented which makes the fault finding easier. Diagnostic tools have for example been used to find fault on Foundation Fieldbus transmitters. Barsebäcksverket has not experienced any major problems with the bus. The only problem with the bus itself was that the communication heads on a few valves stopped to work because they had old versions. Nimrod uses the extended diagnostics that the fieldbus provide. One example is alarm from malfunctioning devices and communication errors. The configuration software that is used to get diagnostics from the devices is an integrated part of the software packet PCS7 from Siemens. A new type of error that can occur on a fieldbus is consequence alarms. This error can be triggered by a short circuit or a device that interrupt the communication on the segment, which result in alarm from practically every device on the segment. All alarms can make it very hard to identify the error source.

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Another communication disturbance occurred at Scanraff when they got moisture in an outlet, which stopped the communication on that segment. This “short circuit” was not enough to trigger the short circuit protection but it disturbed the communication. All devices can be configured to send alarms, but to avoid a lot of alarms showing up at the operator station, only critical alarms and sum alarms are presented at the operator station. More specific alarms and diagnostics can then be reached from the engineering station.

3.7.4 Demands on knowledge Nimrod has found that finding errors and using the fieldbus diagnostics needs both education and new work procedures. At a typical plant there are usually one electronic, one instrument and one control department with different assigned tasks. With the introduction of “intelligent devices” these tasks can move between the departments, where the knowledge is not present. This problem has to be taken into consideration and either educate the personnel or change the organisation. Education is an important issue that has to be looked into, otherwise it might cause problems. Another problem might be oppositions from the personnel, they might be negative to changes or unwilling to learn a new system. At Scanraff, the personnel made major parts of the fieldbus installation, programming of operator stations and control system, with support from Emerson. Three workers have been educated for three weeks and the remaining operators got 2 + 2 days of education. Besides the education, the involved personnel had learned them selves. New changes in the system comes so quickly that not even the supplier can keep up with how to solve certain problems. At the constructed plant in the FuRIOS 2 article, the managers of the report admit that they underestimated the demands on training. However, they agree with the FuRIOS report that it is less training compared to remote I/O because once trained with Profibus, little more training is needed, while new devices in remote I/O systems has different operation philosophies which requires new training. They also state the importance of teaching everybody, installation personnel, engineers and managers about the fieldbus, in order to take advantages of the new technology and also know its limits.

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4 Interview 4.1 Background The literature review, of sites with fieldbuses installed, provided information about the installation of a fieldbus. However, the information about operation of a fieldbus system was insufficient. To complement this with more information, two sites in Örnsköldsvik, Sweden, were visited. The sites were ETEK, a pilot plant for ethanol, which has been running for about a year, and Husum, a paper mill that has been using fieldbuses for several years. The interviews where conducted in November 2005.

4.2 Result 4.2.1 ETEK ETEK is a pilot plant for ethanol production with forest residues as raw material. They use DeltaV from Emerson as control system. Foundation Fieldbus H1 is used to connect instruments and valves, AS-I for discrete valves and motor on/off and Profibus DP for AC drivers and interlocking controllers. ETEKs reasons for choosing to use a fieldbus were easy changes and less cable, compared to conventional I/O. It reduced the number of cable ladders by half and also made the cross-connection rooms unnecessary. Since ETEK is a pilot plant they constantly change the process and therefore have to add, move or remove instruments and valves. The system is build to be dynamic and all segments have spare outlets to be able to add devices while online. At ETEK they are satisfied with the fieldbus and think that the fieldbus have lived up to its expectations. There are two areas where a fieldbus could not be used. The first one is security applications, which has high demands on reliability. The other is the small Ex area where it would be too expensive to use Foundation Fieldbus H1 Ex-barriers on only a few devices (about 10 devices). On the Foundation Fieldbus H1 bus ETEK connects up to 16 devices, but no more than 4 can be valves. This is because the load of the bus would otherwise be too great, resulting in long cycle times. The devices are connected to the bus through a multibarrier box, which has overload protection. This is used to avoid a shortcut or electrical fault to make the bus unusable. ETEK has experienced some problems with commissioning, where some devices were lost from the system after the bus had been without power. They found that these devices had lost their address. This was due to different initiation procedures for different manufacturers, which resulted in that devices did not store the given address. Another drawback was that the cost for programming the system got somewhat higher than planned. Finding faults on the Foundation Fieldbus H1 bus is done in two different ways. The first is with diagnostics that is sent from the device, through the bus and into the engineering station. The second is with a hand held device “Field communicator” connected to a specific device. It is mainly used when a device is in the workshop for inspection. Although some diagnostics where available before, with HARTS, it is being more used now with the fieldbus.

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4.2.2 HUSUM Husum is a paper mill in the north of Sweden that began to be built in 1915. The mill produces both market pulp and paper, coated and uncoated. 1450 people are employed at the mill. The personnel at Husum have always been curious about new control technology and have been using fieldbuses for several years. They have both upgraded from 4-20 mA to fieldbus as well as built a new fieldbus installation. The control systems used for the fieldbuses are Freelance 2000 from ABB and Siemens PCS7. As fieldbus advantages they see additional diagnostics, faster commissioning and easier configuration. An I/O card has to be set up individually and mistakes can be done because the signal has to be converted from analog to digital, and the range has to be set both on the instrument and the I/O card. With a Profibus PA device, a 32 bit value is sent to the operator station and does not have to be interpreted. As soon as a connection with the device is established, there is usually no problem with commissioning. The personnel have found that it is not always possible to exchange an instrument while online. With some systems it is possible to replace it online if the devices are of the same type and manufacturer. However, if the configuration (DTM) file has to be changed, the system must be reprogrammed. The maintenance personnel experiences fewer small errors (like sensor drifting), but in total it is not that much of a difference. One good thing with fieldbus instruments is that the devices can show the actual value, and not only values inside their calibration range. A pressure transmitter can have a physical range 0 – 200 kPa but a working range of 20-40 kPa. While the 4-20 mA transmitter would only show values within this range and the accuracy depends on the width of the calibration range, a fieldbus can show the entire physical range at very high accuracy. Husum uses Fieldcare to diagnose and configure their devices. One reason is that DTMs, from other manufacturers than ABB, has problems working properly in the Freelance control system. Therefore the system uses GSD files. A new segment does however use DTM in Freelance as well. A diagnose device is connected to the Profibus DP bus and can, if the system is transparent, be used to diagnose several Profibus PA segments. When an instrument has been exchanged, a configuration file can be downloaded to commission it as fast as possible. To make it easier to find errors and configure devices, they always order the devices with displays. A maximum of twelve devices on each segment is used to keep the cycle time low, about 500 ms. Even if the cyclic data exchange takes about half of it, the rest is used for acyclic exchange like alarms. When an alarm occurs it is not unusual with consequence alarms, which then cause a high load on the bus. To minimize the risk of disturbing the Profibus PA bus when adding, moving, removing and even when commissioned, multibarrier boxes are used. All Profibus PA segments have the same cycle time although some of them could have shorter. Husums reason for using the same cycle time is that they want to be able to connect new devices to any segment, without having to check which cycle time it has. Up to 4 Profibus PA segment are connected to one Profibus DP/PA segment coupler. As for education, the operators got none while the maintenance personnel got 3 to 4 days. The errors occur seldom resulting in the personnel forgetting how to find the errors.

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The devices themselves are robust and have very few errors. The reason for changing a device is usually wear, vibrations or other unplanned physical accidents with the device. These errors occur both on fieldbus devices and 4-20 mA in about the same amount. However, the bus communication can cause problems. When a 4-20 mA system was replaced with fieldbus, the grounding was not done correct, which caused devices to temporary loose connection and send an alarm. There were also sections where the bus cable was placed close to the voltage cable for motors, when for example passing through a hole in the wall. To avoid this, the installer should have the knowledge about how to install a fieldbus and inspect the bus prior to commission. Husum recommend that a good cable is used, with well-made connections, and not reuse old 4-20 mA, unless shielded.

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5 Test system 5.1 Background and problem The aim for this system test is to test a number of possible scenarios that might occur when installing and maintaining a Profibus PA system. The result can be used to compare with similar situations when conventional I/O is used instead. The test should also try to find and prepare system engineers for problems that might occur. The test consists of several tasks divided into two groups, installation and maintenance. The result will describe how to accomplish these tasks in the specified control system with the specified software. Parts of the result should therefore not be used to draw conclusions about fieldbuses in general. However, it can show problems that might occur in other control systems as well. These tasks where defined after a meeting with Per Norman, my supervisor. The first two tasks are related to installation and commissioning of fieldbus devices and the remaining are related to maintenance and function of fieldbus devices. These are the tasks

5.1.1 Connecting and installing devices on the fieldbus: Installation of fieldbus connected devices can be done in different ways. Therefore perform the following: • Describe in detail, how configuration and installation of an instrument with a DTM file is done. • Describe in detail, how configuration and installation of an instrument with a GSD file is done. • Summarize the difference between DTM and GSD files.

5.1.2 Configuration and commissioning of a control loop: The task is to program a traditional control loop consisting of one positioner, one pressure transmitter and a tank. The operator station view should also be programmed. The system should be controlled by a PI-controller. There is no physical system and therefore a simulated process should be programmed. The intension with the control loop is to use it for the maintenance tasks, of this test. The operator view should display process values, enable the operator to change set points and display some alarms and diagnostics. The status and settings of the devices should be displayed on the process values faceplate. The regulator should support two modes, automatic and manual. • Describe how configuration, installation and commissioning of the devices are made and how the control loop is programmed (including the simulated process).

5.1.3 Exchange of pressure transmitter (ABB) of the same type: The test system has lost contact with the pressure transmitter. The operator station shows the error “Slave not existent”. A quick check out in the field shows that the transmitter is not working at all. There is a pressure transmitter of the same type in storage, which should be used to replace it. The settings from the old device should be used in the new one. The only documented information, except information stored in CBF, is the node number on the bus. All information from the engineering station can of course be used. • Describe how the faulty device is replaced with a new one.

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5.1.4 Exchange of pressure transmitter of different type: Same task as above but with one exception, the stored pressure transmitter is equivalent but not of the same type. • Describe how the faulty device is replaced with a new one.

5.1.5 Disturbance of a positioner: The positioner of the control valve has problems with the compressed air. In the high winds there is a ladder that squeezes the air tube, which finally blocks the air supply totally. The positioner is no longer responding to control, which can be observed from the operator station. • Describe how the error finding is conducted. Assume that the problem with the ladder is not known. • Describe if there are any diagnostics or alarms that occur before the air supply is blocked, which could alert the operator before the final block.

5.1.6 Safety position of the positioner: When problems occur in a process, which causes it not to work properly, positioners can move to a safety position. A power failure would for example make controlling impossible. For these cases the valves usually have a default position that is mechanically constructed. A spring valve would become either fully opened or closed. One case that can occur when using fieldbuses are that the bus and positioner is powered but the communication with the process station is lost. • Describe in detail, how to configure the positioner to freeze in the current position when communication with the process station is lost.

5.1.7 Alarm and diagnostics: The fieldbus devices have extended diagnostics compared to analog devices (without HART). A temperature transmitter (ABB) is showing an error. Simulate the error by removing the sensor from the transmitter. • Describe in detail, how the error is presented to the operator and engineer. • Describe how to configure which value should be shown/sent when the error occurs.

5.2 Material To conduct the test, a system was borrowed from ABB. It was an AC 800F control system, which consists of one process station and a DP/PA segment coupler. A positioner, a temperature transmitter and a pressure transmitter were also borrowed from ABB. To test instruments from other manufacturers, a temperature transmitter was borrowed from ALNAB and a pressure transmitter from Endress+Hauser. The hardware was set up as shown in Figure 12

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Figure 12. The hardware set-up of the test system.

A PC was used both as engineering and operator station, and the software used consisted of CBF (Control Builder F 8.1) and DigiVis, an operator station software. CBF runs on Windows XP and therefore it was installed on the PC. The PC was connected to the process station with Ethernet. The process station had one Profibus DP card, which was connected to the segment coupler. This test will not evaluate the performance of the bus or how it can be connected. Therefore the connections can be made as simple as possible. All devices were connected in parallel to the same point, where a terminal block with screw connections where used, to make it easy to add or remove devices on the bus. No terminators were included in the test system and therefore none were used on the Profibus PA bus. The limited length of the bus and number of devices connected on it made it possible to use without termination. No lost of devices or communication has been noticed during the tests. To conduct the test, a GSD and DTM file is needed for each device. With most of the devices came a GSD file, but the DTM had to be downloaded from the Internet. For the ABB devices, the DTMs could not be downloaded separately and the file was large, about 350 MB. It also included ABBs configuration and diagnostic program SMART VISION. A similar file had to be downloaded from Endress+Hauser, but it was not available for everyone. The device from ALNAB did not actually have a DTM, but after talking to them over the phone, they sent a beta version of the DTM for their transmitter.

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5.3 Method To be able to conduct these tests, knowledge of both software and hardware is needed. Most of the information was learned from manuals, data sheets and similar documentation. CBF has been used by two employers at Outokumpu (but not with Profibus PA), which were asked when specific problems occur. Leif Karlsson at ABB had experience from both DTMs and GSD files in CBF.

5.4 Result 5.4.1 Connecting and installing devices on the fieldbus: How to install and configure a device with a GSD file To be able to communicate with a Profibus device the master has to have a GSD file, which is like a driver for the device. These are the same for both Profibus DP and Profibus PA. A GSD file is a plain text file that describes how to communicate with the device. Each row has one variable and a value for that variable, see Figure 13. Examples of variables are Vendor_Name and Model_Name. A variable of importance is the Ident_Number. These numbers are given by PNO when a company wants to have a device validated for Profibus. If the Ident number of the GSD file and device are not the same, the wrong GSD file is used and no communication will be set up. The GSD file also specifies the speed supported by the device. Since Profibus PA only supports the lowest speed, 31.25 kbit/s, therefore these variables are only useful for Profibus DP devices. The GSD file specifies what cyclic data that can be received from it and what the process station should send to retrieve it. The meaning of the different diagnostic bits are also described in the file.

;GSD File for Profibus DP (EN 50170) #Profibus_DP Vendor_Name = "ABB Automation"; Model_Name = "TF12 Temperature Transmitter"; OrderNumber = "ABB, TF12" Ident_Number = 0x04c4 93.75_supp =1 31.25_supp =1 Bitmap_Device = "TF12___N" Unit_Diag_Bit( 0) = "Hardware failure electronics" Unit_Diag_Bit( 4) = "Memory error" Unit_Diag_Bit( 5) = "Measurement failure" ; Module Definition Module = "Temperature 1" 0x00, 0x42, 0x84, 0x08, 0x05, 0x00 2 Info_Text = "Secondary Variable 1 (Channel 1)" EndModule Module = "Temperature 1 & Temperature 2" 0x00, 0x42, 0x84, 0x08, 0x05, 0x42, 0x84, 0x08, 0x05 5 Info_Text = "Secondary Variable 1 (Channel 1) + Secondary Variable 2 (Channel 2)" EndModule

Figure 13. Selected parts of a GSD file. Complete file in appendix C.

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The GSD file only specifies how the communication is done, which cyclic data that can be retrieved and description of diagnostic bits. Therefore, when using GSD files, there are other ways to configure the device with acyclic data through the bus. DPV1 is the acyclic service of Profibus and here variables can be read or written to in different blocks of the device. CBF has its own way of reading and writing the acyclical data. The user can specify a slot and index for a variable, which is a memory position, and the data type. This variable can then be read and, if allowed, written to. This is however not as easy as it sounds. The table that specifies slot and index is long and it is not easy to find which variable to change. It depends mostly on the vendor and how well documented their device is. The pressure device from Endress+Hauser was well documented and the memory table was send with the manual. As for the ABB devices, none came with the devices and it could not be found on their homepage. However, after a call to Lars Forslund, ABB technical support Sweden I got memory maps for the ABB devices as well. Although it seem to work well with acyclic communication with CBF it is time consuming and hard to find the interesting variables. A good thing is that once the variables has been identified and set up for one device, the configuration can be exported and used on other devices. It can also be exported between different PCs. These files could not be downloaded from the ABB website but Leif Karlsson sent me a hardware structure file for the TZID-110. It is probably worth the time to call around and try to find an existing file instead of writing a new one. How to install and configure a device with a DTM file The DTM, unlike GSD, is not only a file. It is a program that has to be installed on the computer it should be used on. When a DTM is used in the ABB 800F system it does not only handle the acyclic communication but also set up the cyclical. This is not specified in the FDT/DTM standard and the DTM files has to be adapted to do this, which can cause DTMs not to work in the system even if they follow the standard. When starting the DTM for the first time it asks for which cyclical data that should be retrieved. If the device has not been configured before, the address is usually 126 and does not correspond to the address set in the hardware structure. The address 126 is not allowed except för new devices that will be given a new address. While in commissioning mode, the device address can be easily changed by right-clicking in the hardware structure in the right window and choose Set Address.... Here it asks for the current device address, in this case 126. It can also be changed from any other address as long as that address does not have a device configured on it in the hardware structure. If the devices address is set using this function, the devices have to be connected one by one since only one device can have address 126. These addresses can also be set in advance either by ordering them with a specific address, if possible, or setting up the devices with a separate configuration tool like SMARTVISION. For some devices the address can be set either by DIP switches or software. When the system is online and the address has been configured, the online configuration can be brought up by starting the DTM. This will open a separate window, see Figure 14. For the TF12, the data is not automatically downloaded from the device and therefore click on Load from device in the Device menu. The device can now be configured and when all changes have been made, it can be uploaded to the device.

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Figure 14. An example of what a DTM can look like.

DTMs data can be saved and loaded from the project. The configurations can be saved as templates and used several times in the same project. That means that if the settings should be the same, the configuration can be reused. This function was not available at this test since only a demo version was used. Whether or not it is sufficient to keep the data saved in the project cannot be said, since it could not be done in this test. If the device is replaced with another which is not identical, the DTM has to be replaced and it is unlikely that the configuration can be reused in the new one. It would therefore be recommended to have it stored in some other way as well. Most DTMs tested here even has a documentation option, where all settings of the device can be printed out. Comparison between GSD and DTM While a GSD file only specifies how to communicate with a device and which cyclic communication is available, a DTM can also configure a device. One advantage with GSD when using CBF is that it is a part of the project. That means that if the project is moved to another computer, it would still work there. If DTM is used, the same DTM has to be installed on that computer. This has not been tested but Leif Karlsson, ABB Skellefteå, informed me about it. There have been some problems saving the DTM configuration in the project. This can be avoided by configuring the devices with a FDT program specially written for configuration and diagnostics, like SMARTVISION or Fieldcare. These programs need a special card in the PC that can be connected directly to the Profibus DP bus and where unfortunately not available for this test.

5.4.2 Configuration and commissioning of a control loop: The system was programmed using FBD (Function Block Diagram), a part of the standard 36

IEC 61131-3 which describes five programming languages. FBD is a graphical language where blocks are placed and connected to each other. The execution order of the blocks has to be set as well. The operator station is also programmed in a graphical programming language, where objects and their properties are connected to the variables in the system. The task specifies that a faceplate should be present for the PID controller. CBF already have a faceplate for it that will be used. The system was simulated by a first order system, see Figure 15, and works as follows. The valve position runs through a delay block which works like a low pass filter, which has a delay time of 5s. It is then multiplied with fifteen to make the valve able to give a pressure of 1500 mbar when fully opened. To make the process more real, a sinusoidal disturbance is added. The disturbance has two parameters that can be changed, the amplitude and the period. To make it easy to access these variables they are displayed and can be changed from the operator station. The resulting value is added with the value from the pressure transmitter before it is send to the controller. The delay and counter blocks has properties not showed here but can be accessed by double-clicking on it. The counter adds up the value on pin IN at a specific time interval, in this case every second. When the system was configured, the hardware was connected and installed as described earlier. When setting the set point for the valve, a status byte should be send as well. Unless a good value is send, like 16#80, the positioner will not accept the set point. In DigiVis 16#80 means the number 80 with base 16, which is 128 with base 10. Values like 16#00 and 16#1F (out of service) results in a set point not valid error in the valve, which then enters FAIL SAFE mode. This can be used to avoid the positioner to act on an invalid value. In this system, the status from the pressure transmitter could be sent to the valve. If the pressure sensor would stop working, which means not sending a good process value, the valve would not act on this value. Status codes like warning and alarm for high values are considered valid data.

Figure 15. FBD blocks of the simulated pressure.

When configuring the TZID-110 with the DTM program, one problem occurred. When trying to write changes to the device an error message was displayed "Write access denied! Please check the Operation Mode of the Device". After a call to Lars Forslund, ABB technical support in Sweden it was found out that the positioner has to be in "Out of Service" mode to be configured. It can be found in the menu Operate and then choose Positioner. After all changes have been made, the mode should be changed back to Automatic.

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Figure 16. The screen of the operator station. In the graph the set point is red (dots) and process value is blue (squares).

Figure 16 shows the operator station view for the test system. The pressure measured is absolute pressure and is added to the simulated value. The PID controller has its own faceplate seen to the right in the figure. To be able to send different status messages to the positioner, a status field was created. The PID controls the positioner, and the read back value form that, which is its actual position, is used to calculate the simulated value. The valve has some additional diagnostics besides the process values status, called CHECK_BACK. It consists of 3 bytes where 11 bits are used to indicate different statuses, like simulation or local operation. To make the data easy to interpret, a user function block was created which had 3 byte inputs and 11 Boolean outputs, each indicating a status. A faceplate was also build to show the value of each status bit, see Figure 17.

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Figure 17. Faceplate of the positioners extended status. A red dot indicates an active status.

One status is Travel time limit exceeded. A travel time limit can be specified using the DTM and if the positioner has not reached the desired position within this time, the bit is set. It is not indicated on the process value status that one of the CHECK_BACK bits are set and therefore the user function block also has a sum output indicating that at least one CHECK_BACK bit is set. This has then been connected to a red or green light on the operator display in the positioner square. A click on this light will bring up the faceplate.

5.4.3 Exchange of pressure transmitter (ABB) of the same type: A potential gain when changing transmitters of the same type is that the settings can be downloaded into the new device from the project. Since this option was not available this test will give the same result as exchanging pressure transmitters of different types, except that the DTM does not have to be changed.

5.4.4 Exchange of pressure transmitter of different type: Before connecting the new device to the bus, the address of it should be checked. Even if the factory standard address is 126, it is possible to order them with a preset address. The address can be set locally with a display or buttons or on a separate configuration bus, where all setting can be configured as well. When programming with the display and buttons, 9-32 V is all that is needed to supply the device. Some devices might also have DIP switches to set the address with hardware. The reason for checking the address is that if it has the same as another device on the bus, both devices will probably fail and thus risking one more device to fail. The address should be set to either 126, and later configured from the engineering station, or the address it should have in the system. Whether or not to configure the high and low warnings in the devices itself or in the process station is an issue that arises with the use of fieldbuses. If they are configured in the device they must be set up in the new device as well, but if they are configured in the process station this is not necessary. For devices of the same type, this is not a problem, but when they are not of the same type, the settings have to be made all over again. It must be assured that the DTM for the new device is installed on the computer or at least are

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on a CD beside the device. Since the DTM has to be installed on every computer using it, it is probably a good idea to have them beside the device even if the DTM is installed on the engineering station. After the new DTM has been installed in the system, the device should be configured as the device it is replacing.

5.4.5 Disturbance of a positioner: Thanks to the extended diagnostics, the red light in the positioner square turns red. It can also be connected to send an alarm but this has not been done in this system. The operator opens the faceplate to retrieve the additional information, in this case Travel time limit exceeded. However, it does not indicate what is wrong, just that the positioner has not reached its set point in time. The first step for the engineer would be to start the DTM at the engineering station. From there, messages and malfunctions can be checked. Low air pressure is however not one of them and no other message/malfunction would give a hint of what is wrong. The next step would be to put it into manual mode and change the set point from there. After established that the positioner is not responding to a change in position, the air supply would be the next thing to check. To do this, one would have to walk out into the plant, and hope that a pressure meter is present at the positioner, which is not the case for the one borrowed for this test, but can be ordered. If there are no pressure meters and a valve is present at the end of the air supply pipe, one could close it, disconnect it, and slowly open it to find out if there is any pressure in the pipe. One would now have to follow the air supply line to find why the air supply has to low pressure. The positioner itself cannot find this problem before it occurs. By setting the travel time as small as possible, it might give some indication, but that could also trigger false alarms.

5.4.6 Safety position of the positioner: The DTM is started on the engineering station. First the Target Mode is changed to Out of Service. Now enter menu Configure and choose Operation. Below the tab Failure Behavior the positioner can be set to remain in the last position of it enters failsafe mode. Failsafe mode occurs for example if the status sent to it is not a good one, or the communication with the process station is lost. If the error occurs on the Profibus PA bus, and causes the voltage to drop, failsafe is not entered and the valve goes to the valves mechanical safe mode. A single acting spring valve for example goes to either fully opened or closed depending on construction.

5.4.7 Alarm and diagnostics: A temperature transmitter was not a part of the control loop configured earlier in this test. It was connected in the same system but got its own operator display which only displays the process value and the status byte send with it. The transmitter has two independent sensors connected to it which can be read from the process station. It also has a third cyclic value, which can be calculated from the two sensors. The third value can be chosen from a list. Examples of what can be sent are: Sensor 1's value, sensor 2's value, mean value and redundant which means the mean value if both work, or the correct one if one is broken. For this test, the third cyclic value was set to redundant. All three values where shown at the operator station. After configuring the system, an error was simulated by removing one of the sensors. At the operator station the following is displayed. The value from the removed sensor is freezed at its last good value. The status for the value becomes 16#44 instead of 16#80. This can be used 40

when programming the operator station to tell the operator what has happened. In this case it makes the background of the status field red. Besides the changed status, an alarm message is send as well, telling that the unit has a diagnostic fault. The following is shown at the engineering station. In the Hardware structure the error can be traced down because all parent object to the faulty object becomes red. Beside the faulty object, an icon is visible telling that diagnostic data is available, see Figure 18. To find out more the DTM for the device is run. In the diagnostic tab, there is a status for each sensor that tells the engineer that the sensor has a lead breakage, see Figure 19.

Figure 18. Hardware structure when diagnostic data is present.

Figure 19. The DTM shows a lead breakage of the Temperature sensor.

I have not been able to find a setting for which value to send when an error occurs, it automatically sends the last valid value. However, it is not always good to do so. One time when the sensor was detached, the last valid value became 80 ºC instead of the 23 ºC that the room was. It was probably caused by a reading when the wire was removed which increased the resistance. The value to send when an error occur can still be solved in the process station, by looking at the status of the process value.

5.5 Problems Upon the arrival of the test system the included software was CBF version 7.2. The software requirements for CBF 7.2 to run DTM files where Windows 2000 SP 4. It was installed on one computer to be able to run it. Everything except the DTMs was working. Several calls where made to Leif Karlsson, who had it working on his PC, to try to identify the problem. After several reinstallations of both the program and the operating system, it still did not

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work. One possibility was to use a later version, CBF 8.1, which had been ordered by Leif Karlsson. This delayed the tests but the time was used to learn how to use and program in CBF. GSD files were used to read data from the devices. When CBF 8.1 arrived and installed, the DTMs worked right away. CBF 8.1 runs in Windows XP. When trying to save the settings for the positioner for the first time, a popup appeared informing that the positioner is in the wrong operation mode. After a call to ABB it was found that the positioner has to be in Out of Service as Target Mode to be configured. This was not found in the documentation, and generally, the documentation for DTMs is very limited. There have been problems using DTMs from other manufacturers than ABB in CBF 8.1. One from Endress+Hauser and one from PRelectronics (beta) has been installed and tried in CBF. These are having problems setting up the cyclical communication for the devices and no solution was found. GSD files were used instead. The answer was found several weeks later, when contacting Endress+Hauser. I was told that the cyclic communication set up that CBF needs, is not a part of the DTM standard. Endress+Hauser offered to investigate whether to, and how long time it would take to make their DTM work in CBF. Due to the little time left of the thesis their offer was declined. At first it was not clear why the settings of the DTM could not bee saved. It was not until the end of the Thesis that it was found that it was because it was only a demo version. Unfortunately no solution to the problem could be found and one of the tests could not be evaluated.

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6 Result 6.1 Comparing fieldbuses with remote I/O One advantage of fieldbuses is faster and easier installation and commission. Fieldbuses can give huge cuts in both installation and commissioning time and cost, if the contractor has experience from fieldbus installations. Nimrod refrigeration plant is one example. They reduced their time with up to 50% and cost with 30%. I believe the main reason for them having such a good result, compared to the other is because a sister plant using the same technology was commissioned one year before. The experience gathered with that project was used in Nimrod. This shows that with the right knowledge and experience the savings are huge. The sodium cyanide plant in Australia however, shows that without experience the fieldbus commissioning can be difficult. Most places have found it easier or at least not harder to install and commission a fieldbus system. It also becomes more flexible during the planning, since the final placement of devices and which I/O cabinet to connect it to can be changed. Another advantage of fieldbuses is the diagnostic ability. Several articles show that there are savings both by increasing the availability of the plant and increasing the quality of the final product. The diagnostics from positioners can be used to replace valves before they break down by counting the number of full strokes. All positioners also have a read back value, corresponding to the actual position of the valve. This can be compared with 4-20 mA positioners, where most of them do not send back a value. Instead a flow transmitter somewhere else on the pipe is used to validate that it is open or closed. The diagnostic can help the maintenance personnel, by giving a hint of what is wrong and even be used for predictive maintenance. However, since so much data is available, it is also important to decide which data to present and how to present it to the operator or maintenance person.

6.2 Requirement of knowledge Third part case studies as well as the interviews showed that training is very important when changing from remote I/O to fieldbus. Both for the one planning and building the system as well as those who will operate and maintain it. New aspects have to be considered when evaluating which instruments to use. The way of programming can be changed due to the build in intelligence of the devices. When maintaining the system the multimeter can not be used since the signal is digital and not analog. Changes and adoptions at all levels are required when changing to fieldbuses. If the upgrade is done without using the benefits and just replacing the old system with newer technology, it is hardly worth it. As with the plant in Western Australia, where there were no prior experience of fieldbuses, a fieldbus can cause a lot of problems and can even be more expensive to install, than conventional technology.

6.3 Differences between Foundation Fieldbus H1 and Profibus PA Profibus PA and Foundation Fieldbus H1 are very similar and they have no major differences. They use the same physical interface, which means they can both power instruments through the bus and have the same limits in wire length. Most instruments found, can be ordered to fit either bus, so the range of instruments available is about the same. When searching for articles regarding fieldbus installations and predictive maintenance, it looked like H1 was more focused on the chemical industry and predictive maintenance. The main functional difference is that H1 already from the beginning was build for device to device communication. A PID controller can be placed inside a positioner and the value from a flow transmitter can be send directly to it. There is also a difference in the MAC protocol of the fieldbuses. Profibus uses a

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timed token between the masters and the masters pull data from the slaves. Foundation Fieldbus H1 has a schedule which includes not only when data will be sent on the bus but also when function blocks are executed inside the devices. The access to the bus is however still controlled by a master (LAS), which allows slaves to send their data at the scheduled time. In between the scheduled transmissions, the LAS send a token to each device in turn, allowing them to send for example diagnostic messages. The geographical differences are noticeable, Profibus are large in Europe, Foundation Fieldbus in North America and they have equal shares in Asia. It is therefore likely that a costumer in Europe will ask for Profibus. However, like ETEK, some choose Foundation Fieldbus. Which fieldbus to learn to use therefore depends on where in the world the installation will be done. For Asia, where metal process industry is growing rapidly, it is more difficult to speculate which fieldbus a company would prefer.

6.4 Wireless fieldbuses Wireless instruments are a dream for most installation personnel. No cables have to be installed, and instruments can be added or removed without much effort. However, there are problems that have to be solved before wireless instruments can become a reality. The power supply is probably the most difficult problem to solve. If a device have a power supply wire installed, it is not truly wireless. Wireless energy transmission can be used for small areas with many instruments but it is unpractical when the distances between instruments are large. Another way to power the instruments is with batteries. The drawback with batteries is that they have to be replaced at regular intervals. Energy scavenging instruments are a third choice, but these becomes dependent on its’ environment. The wireless links are subjected to more disturbances than wire links, which can cause problems in specific environments. The wireless communication of today, however, works in environments with disturbances. One truly wireless device was found, a proximity switch from ABB. It receives energy trough energy transmission. Other wireless devices for fieldbuses are fieldbus communication links, where parts of fiber or copper cable can be replaced with a wireless link. It can be used at difficult places where signal wires are likely to break. The instruments, however, still have to be connected to the communication link with a copper cable and be supplied with power.

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7 Discussion and Conclusion This thesis has provided data to make it easier for Outokumpu Technology AB to decide whether to learn and recommend fieldbuses at instrument level. The data collected from the system test is however closely connected to the ABB AC 800F system. A test carried out on a different system would probably give different answers. Parts of the tests were also dependent on functions of different instruments. By changing instruments the result could change as well. The literature review of wireless fieldbuses shows that the signal cable can be replaced with a wireless link. No satisfying power source has been found that would work in the metal process industry, where the instruments are long distances from each other. However, new technology is constantly developing and long lasting batteries or energy scavenging can solve the problem and make wireless instruments an alternative.

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8 Future work The question whether to use fieldbuses or not is not an easy one to answer, and which one to use might be even harder. For the instrument level there are mainly two buses that are interesting, Profibus PA and Foundation Fieldbus H1. This thesis has a simple practical test on one of them, Profibus PA, and a future work could be to test the Foundation Fieldbus H1 as well. The DTMs are not working perfectly in CBF 8.1 but they should work in a separate FDT/DTM program like SMARTVISION or Fieldcare. These should therefore be tested to make use of DTM files. Although a test system can give hints of some of the problems that can occur, it is a totally different thing to install it into a real plant. To learn how it can be integrated with other systems it should be installed in a section of a plant.

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