Robotics and Computer-Integrated Manufacturing 31 (2015) 1–10

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Robotics and Computer-Integrated Manufacturing journal homepage: www.elsevier.com/locate/rcim

A complete CAD/CAM/CNC solution for STEP-compliant manufacturing Wenlei Xiao n, Lianyu Zheng, Ji Huan, Pei Lei School of Mechanical Engineering and Automation, Beihang University, 100191 Beijing, China

art ic l e i nf o

a b s t r a c t

Article history: Received 30 October 2013 Received in revised form 23 June 2014 Accepted 23 June 2014 Available online 23 July 2014

STEP-NC or ISO 14649 is the next generation of data models between CAD/CAM and CNC systems. After a decade of investigation, the STEP-NC technology is still under developed. The lack of a complete CAD/ CAM/CNC prototype system with full bidirectional data flow hinders the improvement of STEP-NC. This paper proposes a complete CAD/CAM/CNC solution for STEP-compliant manufacturing, so as to explore the functionalities and emphases of STEP-NC technologies. Frameworks of individual CAD/CAM and CNC systems are illustrated in detail. Architectures of STEP-compliant CAD/CAM and CNC systems are studied and several criteria are summarized. Finally, this paper proposes a complete prototype STEP-compliant solution, which consists of a secondary developed STEP-compliant CAD/CAM system on the CATIA platform and an open structured STEP-compliant CNC system. & 2014 Elsevier Ltd. All rights reserved.

Keywords: STEP-NC CAD/CAM CNC

1. Introduction Since the 1950s, CNC systems have become the kernel devices of machine tools. With the advancement of information technology, CNC hardware and software systems have been rapidly promoted [1]. However, the programming interface between CAD/CAM and CNC systems has remained almost unaffected for a long time until the emergence of STEP-NC [2]. STEP-NC, namely ISO 14649 [3], provides a new data exchange interface between CAD/CAM and CNC systems and is considered as the next generation of CNC programming languages. The STEP-NC standard defines not only the tool path data model but also other related process and resource data models for the use of object-oriented manufacturing tasks. In order to realize the STEP-NC concept, both of the CAD/CAM and CNC systems have to be updated. Essentially, the seamless connection of STEP-NC needs a powerful STEP-NC post-processor in CAD/CAM and a stable STEP-NC interpreter in CNC. A CAD/CAM system with STEP-NC post-processor is usually called STEPcompliant CAD/CAM system. Similarly, a STEP-compliant CNC (STEP-CNC) means a CNC controller which takes STEP-NC as input and controls the machine tool motion [1]. Although it is one of the main motives to replace the conventional G/M codes, the vision of STEP-NC is not just for providing a modern programming language. Inspection data definitions (ISO 14649-16) enable a reverse data flow from a STEP-CNC system back to a STEP-compliant CAD/CAM system [4], so as to construct a close-loop machining system [5]. Since 2001, STEP-NC has been enhanced and attracted large amounts of research interests. However, until recent years there has

n

Corresponding author. E-mail addresses: [email protected], [email protected] (W. Xiao).

http://dx.doi.org/10.1016/j.rcim.2014.06.003 0736-5845/& 2014 Elsevier Ltd. All rights reserved.

been very little practical use of STEP-compliant CAD/CAM and STEPCNC systems. The inevitable challenge and complexity of STEP-NC implementation make it hard to be accepted by both CAD/CAM and CNC vendors [6]. Hence, the process of replacing G/M codes with STEP-NC is delayed, and the feasibility of STEP-compliant systems is doubted by many researchers and engineers. Although there have been many researches on STEP-compliant CAD/CAM [7,8] and STEPCNC systems [9,10], they are essentially individual rather than collaborative work. We consider that the recent bottleneck of STEP-NC is the lack of a complete CAD/CAM/CNC solution, so that many of the advanced functionalities of STEP-NC cannot be revealed, such as intelligent control, tool path regeneration, and close-loop manufacturing. In this paper, we summarize a variety of frameworks and criteria for establishing STEP-compliant systems. Afterwards, we introduce a secondary developed STEP-compliant CAD/CAM system, and a STEP-CNC system with complete NCK (Numeric Control Kernel) and PLC (Programmable Logical Control) functions. Combining these two STEP-compliant systems together, a complete CAD/CAM/CNC solution for STEP-compliant manufacturing can be obtained. In this solution, the STEP-NC data flow can be fully realized without a reluctantly developed STEP-NC to G codes convertor [2]. Many STEP-NC functionalities have been implemented, including feature-based tool path regeneration and close-loop machining. Though the proposed STEP-compliant CAD/CAM and CNC systems are still on the prototype stage, they have expectable capabilities to control a future intelligent machine tool. 2. Frameworks for implementing STEP-compliant CAD/CAM/CNC systems STEP-NC requires complete improvements inside CAD, CAM and CNC systems. Fig. 1 depicts a bidirectional STEP-NC data flow

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Fig. 1. The bidirectional STEP-NC data flow between STEP-compliant CAD/CAM and CNC systems.

between a CAD/CAM system and a CNC system. In the bidirectional data flow, the exchange data should be defined using the objectoriented modeling method, and no data leakages are allowed on the CAD-CAM-CNC data chain. Consequently, the STEP-compliant CAD/CAM system should have a STEP-NC post-processor and a feature-oriented STEP-NC kernel, while the STEP-CNC system should have a STEP-NC code parser and some intelligent functions to support STEP-NC operations. Both of them are able to import and export related data in STEP-NC format. 2.1. Feature-oriented CAD/CAM system The feature-oriented STEP-NC data in the STEP-compliant CAD/ CAM system can be managed in three ways, which are subject to three CAD/CAM frameworks respectively [11]:


A CAD/CAM system which imports and exports STEP-NC data.
A CAD/CAM system with STEP-NC data support structures.
A CAD/CAM system with kernel STEP-NC data structures. The first framework only provides a CAD/CAM system with the capability to map the ISO 14649 output and input to its native geometric and manufacturing features. The second framework is integrated with an external or internal package to manage the STEP-NC data. The third framework supports a STEP-NC kernel for both geometric and manufacturing data models, so that it can be considered as the highest level of frameworks with STEP-NC compliance. For actual implementations, it requires considerable time and efforts to develop a brand new CAD/CAM system with the highest level of compliance, while it has great functional limitations to develop a STEP-NC post-processor. Besides, many commercial software systems, like CATIA, Pro-E, SolidWorks, can provide most of the necessary CAD/CAM functions. Therefore, the second framework becomes popular, which means to embed a secondary developed STEP-NC package into a mature CAD/CAM system.

In this paper, we propose the system structure of a STEPcompliant CAD/CAM system developed in the CATIA environment. Unlike many other stand-alone STEP-compliant CAD/CAM systems [7,8], the developed system provides a more flexible and interactive environment for STEP-NC manipulations. In addition, the ISO 14649 data is remedied with AP203 representations, so that the STEP-CNC system will have abundant geometric data to represent the manufacturing project in a 3D viewer. Fig. 2 demonstrates the structure diagram of the proposed CAD/CAM system. The CAD/CAM system consists of five main function-blocks, each of which deals with some certain issues in a STEP-compliant CAD/ CAM system. (1) Feature definition block: The use of features is a magnificent contribution of the STEP-NC technology. It can combine geometric, process planning and manufacturing data into one unitive entity. Therefore, it becomes more apt to represent a manufacturing project with some manufacturing terms (hole, boss, pocket, etc.) instead of the CSG (Constructive Solid Geometry) entities that are widely used in contemporary CAD systems for solid modeling. Thus, it is a major function in the feature definition block to remedy this lack of consensus. In previous CAD/CAM researches, two feature generation approaches, designed by features and feature recognition, have been mostly used [12–14]. The first approach can be only implemented in a newly developed CAD/CAM system. So only the second approach is worth considered in a secondary developed system. Nonetheless, recognition of interactive STEP-NC features is much more complex than prior cases, since not only geometric but also manufacturing information has to be added into the newly generated features. The mixture of multiple aspects in a feature entity causes ambiguous interpretations during the feature recognition process. For example, a hole can be recognized as either a drilling hole or a boring hole contingent upon the hole size and related cutting technologies. Sometimes, this kind of feature determination can be only mastered by a skilled engineer. In view of the above analysis, the STEP-NC feature definition block uses an elementary yet reliable method for feature generations. That is to manually define the feature with its geometric and manufacturing parameters. By this way, the required STEP-NC features are clearly instantiated and further manipulated in the process planning block. (2) Process planning block: The process planning block generates process related STEP-NC entities. As the core function block of a STEP-compliant CAD/CAM system, it provides a STEP-NC mechanism to plan the manufacturing process. The process planning tasks are categorized into high-level and low-level. The high-level process planning handles with the major manufacturing entities, such as project and workingstep and workplan. In addition, it provides some functions to regulate the interrelationships among those defined entities, for example optimizing the execution order of massive workingsteps. The low-level process planning defines the rest necessary data models involved in a machining operation, including manufacturing operation and resource definitions. Operation definitions (strategy, technology, tool path, etc.) indicate how a workpiece is made, while resource definitions (cutting tool, probing tool, etc.) provide necessary information to generate a machining operation. All the process planning functions have to be developed from scratch, as no similar definitions have been implemented in the CATIA environment. (3) Data exchange block: A STEP-compliant CAD/CAM system should have the capabilities to export and import STEP-NC codes. It is important to exchange the NC data in an ISO 14649 manner, so that the lack of consensus can be avoided. The exported STEP-NC codes contain feature, process and resource data that are interrelated in a manufacturing project, while the imported STEP-NC

W. Xiao et al. / Robotics and Computer-Integrated Manufacturing 31 (2015) 1–10

Manufacturing feature definition

3D model design

3

New project creation ISO 14649-10

hole

plane

slot

pocket

surface

...

Workpiece definition Attributes definition AP 203 / AP 224

1 2

Resources definition

probing_tool ...

STEP-NC data exchange

P21 File

Reverse optimization

ARM AIM

P28 File

EB

STEP-CNC

toolpath ...

strategy

Sequence optimization

technology

Workingstep creation

its_secplane

Operation definition

its_feature

Workplan creation

Low-level process planning

cutting_tool

High-level process planning

LB

Procedure optimization

Operation optimization

Inner connection

Resource optimization

4

3

ARM : Application Reference Model AIM : Application Integrated Model EB : Early Binding LB : Late Binding

1 Feature definition block

2 Process planning block

3 Data exchange block

4 Off-line closed-loop manufacturing block

Fig. 2. Framework of the proposed STEP-compliant CAD/CAM system.

codes contain measured and probed data which are also constructed in STEP-NC format. Generally, nearly all the proposed STEP-compliant CAD/CAM systems have proposed ISO 6983 exporters in parallel, so as to connect conventional CNC controllers that accept G/M codes. However, G/M codes exportation disobeys the intent of STEP-NC, which causes information loss during the post-processing process and hinders the realization of intelligent manufacturing. This reluctant solution is mainly due to a lack of real STEP-CNC systems that are able to accept ISO 14649 codes as their machining instructions. With the development of STEP-CNC technologies, this transitional solution will become unnecessary. Using a reverse post-processor, the STEP-compliant CAD/CAM system is able to translate the imported data into its intrinsic geometric and manufacturing data. The imported data contain essential information for closed-loop manufacturing. Recently, the most attended reverse manufacturing data are the probing based inspection [15], as described in the current draft of ISO 14649-16 (Data for touch probing based inspection) [16]. However, there are not merely inspected data that should be fed back to the CAD/CAM system. The closed-loop manufacturing needs to consider its feedback as more generalized process data, such as inspection, chatter, cutting force, servo drive signal, and machine tool capabilities [15,17].

(4) Off-line closed-loop manufacturing block: One of the advantages in using STEP-NC is the support of closed-loop manufacturing. Reverse processing block deals with the off-line closed-loop optimization using feedback process data that are imported from a STEP-CNC system. The optimization process consists of three phases, which are procedure optimization, operation optimization and resource optimization. Those three levels of optimization conform to the object-oriented and top-down design concepts of STEP-NC [18]. Procedure optimization calculates global optimizing results for workingstep executing sequences. Operation optimization adjusts corresponding manufacturing strategies and technologies, which will afterward affect the tool path regeneration in the CAD/CAM system. Resource optimization adjusts the resource parameters that are used in former manufacturing operations and not well suited. 2.2. Intelligent STEP-CNC system Since the emergence of STEP-NC in 2001, there has existed a long-standing negative view of the STEP-NC technology. The conventional CNC system is considered as powerful enough to handle with most of the machining tasks, so that STEP-NC is unnecessary. In fact, this viewpoint has misread the vision of

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STEP-NC on the next generation of intelligent control. In spite of that, without the support of STEP-CNC instances, this debate would still persist for a longer time. In this regard, the lack of STEP-CNC controllers has been considered as a major hindrance to the popularization of STEP-NC. Currently, very few STEP-CNC prototypes have been developed and commercial CNC controller vendors seem to have limited interests in promoting STEP-CNC technologies, though many researchers have pointed out that it will be a future drawback to CNC vendors who do not follow the new standard. Fortunately, the development of open-CNC technologies provides another option to develop STEP-CNC prototypes [19,20]. It becomes technically feasible to develop an intelligent STEP-CNC system based on the open-CNC architectures. According to the way how STEP-NC is implemented, future STEP-CNC systems are categorized into three classes [1]:


STEP-CNC with conventional control, which simply incorporates ISO 14649 in a conventional controller via post processing.


STEP-CNC with new control, which has a STEP-NC interpreter


and can execute motions based on the machining strategies and sequences defined in a ISO 14649 program. STEP-CNC with new intelligent control, which is able to perform NC tasks intelligently and autonomously based on comprehensive information of ISO 14649.

The first type of STEP-CNC system is a transitional scheme before conventional CNC systems get eliminated. It only uses the STEP-NC format to define tool path data, thus most advantages of STEP-NC have not been revealed. Basically, it can only be considered as a CNC controller that can parse “G-code” in STEP-NC format. Therefore, a real STEP-CNC system has to be established, which can fully parse feature-based STEP-NC codes and execute some tool path generating and intelligent manufacturing functions [10,21]. However, due to the great developing workload and some technical difficulties, very few STEP-CNC prototypes fall into the second category, and until recently no third type of STEP-CNC has been developed. The lag of STEP-CNC implies a fact that, without the support of commercial CNC vendors, it is difficult to develop a practically useful intelligent STEP-CNC system. In order to facilitate the development of STEP-CNC, criteria have to be addressed concretely, considering both requirements from CAD/CAM systems and CNC controllers. Therefore, we summarize some criteria for developing an intelligent STEP-CNC system, where criteria (1)–(5) are for the need of CAD/CAM and (6)–(10) are for the need of CNC. (1) STEP-NC interpreter: The STEP-NC interpreter is a standard component in an intelligent STEP-CNC system to accept ISO 14649 programs. Correspondingly, feature-oriented objects are implemented inside. It is important that the interpreter is strictly compliant with the ISO 14649 standard, so that the “Made once and run anywhere” (MORA) concept of STEP-NC can be realized. (2) STEP-NC executor: STEP-NC requires the motions of a machine tool to be faithfully executed according to machining strategies and workingstep sequences that are specified in a ISO 14649 program. The STEP-NC executor tries to accomplish this task and avoid misunderstanding the manufacturing intent designed previously in a STEP-compliant CAD/CAM system. (3) Tool path generation: According to the definition of ISO 14649, toolpath is an optional attribute of a workingstep. Thus, a STEP-CNC system should be able to generate the tool path data in an ISO 14649 manner. With an integrated tool path generator, machine tool users can locally and instantly regenerate the tool path when they want to change some related machining parameters. In this way, the STEPCNC system possesses a flexible manufacturing capability. (4) 3D STEP-NC viewer: Normally, a feature-based and objectoriented STEP-NC program contains much more information than

conventional G-code. Except for tool path data, it contains feature data for workpiece, process data for manufacturing strategies and technologies, resource data for related auxiliary devices (cutting tool for example), etc. Because of the extremely abundant information it carries, a STEP-NC program is friendly for computerized processing instead of human reading. Therefore, some auxiliary tools are needed to help people understand the STEP-NC program. Since many entities have their geometric aspects, it becomes necessary to develop a 3D viewer to present the STEP-NC information. (5) E-manufacturing ability: The conventional manufacturing process is limited in a sole CAD/CAM/CNC system, while the STEPNC technology enables a distributed and networked manufacturing process. The support of XML (STEP-XML) gives a STEP-CNC system the capabilities to access STEP-NC data through network. (6) Interaction of STEP-NC info: The object-oriented structure of STEP-NC data makes it possible to edit the machining parameters on a STEP-CNC controller. Practically, an intelligent STEP-CNC controller must provide users the possibility to interact with the controller. STEP-NC data are then read, edited and updated directly on the Man–Machine Interface (MMI) of a STEP-CNC system. Flexible manufacturing is thus realized. (7) On-line closed-loop manufacture: On-line closed-loop manufacture means that the feedback information is collected and processed on a CNC controller rather than uploaded to a CAD/CAM system. In contrast to off-line closed-loop manufacture, on-line closed-loop manufacture reduces the periods number of the closed-loop optimization, so as to obtain a faster response. For example, a CNC controller can determine the machining errors by machining features and inspection data, and immediately construct a fine machining workingstep for the following machine tool motion. (8) Sensor-based manufacture: An intelligent STEP-CNC system must support sensor feedbacks. In order to make efficient use of sensors in machine tools, future CNC controllers need to change their machining parameters or trajectories in real time according to the sensor signals and events [22,23]. In this case, tool paths become volatile and need to be on-line computed. This advanced technology requires a reconfigurable sensor network in advance. Hence, both the software and hardware architectures of an intelligent STEP-CNC system have to be improved. (9) Open-CNC architecture: The open-CNC architecture is noteworthy during the implementation of an intelligent STEP-CNC system. Many intelligent machining functions are so complicated that they need to be developed on a universal PC platform, which means the conventional CNC system with a hardware-based customized structure is unfitted for establishing an intelligent STEP-CNC system. A CNC system mainly consists of three key components: man–machine interface (MMI), programmable logical controller (PLC) and numerical control kernel (NCK). Until recently, the technology of a fully PC-based industrial controller with a soft MMIþPLC þNCK structure has been tending to mature, and commercial products have also emerged. In this regard, it is encouraged to develop an intelligent STEP-CNC based the openCNC architecture. (10) Reconfigurable control network: As indicated in criterion (8), a reconfigurable control network is important for constructing the hardware architecture of an intelligent STEP-CNC system. The current best solution is an industrial Ethernet fieldbus. Using the industrial Ethernet, it is convenient to reorganize the network structure.

3. Developing a STEP-compliant CAD/CAM system As a part of the integrated STEP-compliant systems proposed in the previous section, a STEP-compliant CAD/CAM system which

W. Xiao et al. / Robotics and Computer-Integrated Manufacturing 31 (2015) 1–10

ENTITY manufacturing_feature (* m1 *) ABSTRACT SUPERTYPE OF (ONE OF(region, two5D_manufacturing_feature, transition_feature)); its_id: identifier; its_workpiece: workpiece; its_operations: SET [0:?] OF machining_operation; END_ENTITY; ENTITY two5D_manufacturing_feature (* m1 *) ABSTRACT SUPERTYPE OF (ONE OF(machining_feature, replicate_feature, compound_feature)) SUBTYPE OF (manufacturing_feature); feature_placement: axis2_placement_3d; END_ENTITY; ENTITY machining_feature (* m1 *) ABSTRACT SUPERTYPE OF (ONE OF(planar_face, pocket, slot, Step,Round_hole, toolpath_feature, Profile_feature, boss, spherical_cap, rounded_end, thread)) SUBTYPE OF (two5D_manufacturing_feature); depth: OPTIONAL elementary_surface; END_ENTITY; ENTITY pocket (* m1 *) ABSTRACT SUPERTYPE OF (ONEOF(closed_pocket, open_pocket)) SUBTYPE OF (machining_feature); its_boss: SET [0:?] OF boss; slope: OPTIONAL plane_angle_measure; bottom_condition: pocket_bottom_condition; planar_radius: OPTIONAL toleranced_length_measure; orthogonal_radius: OPTIONAL toleranced_length_measure; END_ENTITY; ENTITY closed_pocket (* m1 *) SUBTYPE OF (pocket); feature_boundary: closed_profile; END_ENTITY;

Workplan Top milling Security plane Plane milling technology function strategy Top inspecting Drilling Boring Hole inspecting Pocket rough milling Pocket pre-inspecting Pocket fine milling Pocket post-inspecting

5

project -top milling |-security plane |-plane milling |-technology |-function |-strategy -top inspecting -drilling -boring -hole inspecting -pocket rough milling -pocket pre-inspecting -pocket fine milling -pocket post-inspecting

Fig. 3. Definition of the entity pocket using EXPRESS language.

Defining working_step

Defining cutting_tool

Fig. 5. Process planning block. (a) High-level process planning. (b) Low-level process planning. Fig. 4. Create a pocket feature in the STEP-compliant CAD/CAM system.

could process STEP-NC manufacturing information and export STEP-NC programs is presented in this section. The system was developed as a plug-in component into the CATIA environment. Until recently, CATIA has been widely used as an excellent secondary development platform for CAD/CAM applications. It provides a number of powerful functions and four approaches to access secondary developments, including Automation API (Application Programming Interface), Knowledge Ware, Interactive User Defined Feature and CAA (Component Application Architecture). This paper adopts CAA (V5, C þ þ) as the main programming method for it has the best performance of integration, functionality and efficiency. CAA functions in an object-oriented programming manner, especially relies on COM (Component Object Model) and OLE (Object Linking and Embedding) technologies [24]. 3.1. Feature definition In the current STEP-compliant CAD/CAM system, the STEP-NC feature definition block is the foundation of all other function

blocks. According to the framework described in Section 2, all the manufacturing features are defined manually. Here we propose an example for defining a pocket feature. As referenced from ISO 14649-10, the pocket entity is defined as Fig. 3. At first, the feature is defined as a closed pocket with its attributes (its_id, its_workpiece and its_operations) automatically assigned by default. Then, the feature_placement attribute is determined by a user-given coordinate system. Afterwards, by selecting a set of related geometric elements, the attributes depth and feature_boundary are determined. The boss attribute needs to be manually given by the user if any boss feature exists. Since the boss feature is an optional attribute, it is set to null as default. Detail parameters (slope, planar_radius and orthogonal_radius), which are correlative to a pocket, are specified by selecting the corresponding dimension labels on a CAD model (as shown in Fig. 4). At last, the attribute bottom_condition is manually given as planar_pocket_bottom_condition. Fig. 4 shows the result after defining the pocket feature. Here, we would like to point out that the manual mechanism of feature definition is not the most efficient way on all conditions.

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Sometimes, it is more suitable to combine both feature definition and recognition for some complex features. For example, it is believed as possible to automatically recognize attributes after the feature is preliminarily defined as a closed pocket and all correlative geometric elements or facets have been selected. This mechanism will be studied in future researches.

Paths Inspecting path.1 View.1 Inspecting path.2 View.2

3.2. Process planning

Inspecting path.3 View.3 Inspecting path.4 View.4

Fig. 6. Off-line close-loop manufacturing. (a) Theoretical inspecting point generated by CAD/CAM. (b) Measured inspecting point probed by CNC.

The process planning block is a core component in the proposed system, containing both the high-level and low-level process planning functions. The high-level process planning is to create a workplan and its sequenced workingsteps. A workingstep can be either a milling, drilling, boring or inspecting task. All the workingsteps will be executed one by one in sequence. Fig. 5 (a) demonstrates the tree structure of a workplan and its workingsteps. The low-level process planning is to define the detail parameters of each workingstep. The major parameters of a workingstep entity are its_secplane, its_feature and its_operation. The attribute its_secplane is determined by selecting an existing plane, and the attribute its_feature is set by choosing an existing feature defined by the feature definition block. The attribute its_operation is defined in a dialog as shown in Fig. 5(b) (defining working_step). Involved operation attributes include approach&retract planes, cutting tool, strategy, technology, function, toolpath, etc., wherein some of them can be automatically generated (for example the toolpath). Therefore, most efforts of the low-level process planning are made to define the attribute its_operation. In this paper, we further divide the defining tasks into operation definition and resource definition. The operation definition provides the process information to a machining operation, while the resource definition provides the resource entities used in the operation. As shown in Fig. 5(b), the user can create a workingstep and a cutting tool by using corresponding dialogs.

ISO-10303-21; HEADER; FILE_DESCRIPTION((''),'1'); FILE_NAME('D:\casestudy','2013/9/10','Administrator','MICROSOF-1DE82C of BUAA720-513',$,'ISO 14649',$); FILE_SCHEMA(('INSPECTION_SCHEMA')); ENDSEC; DATA; #1=PROJECT('EXECUTE EXAMPLE1',#4,(#2),$,$,$); #2=WORKPIECE('SIMPLE WORKPIECE',$,$,$,$,#3,($)); #3=BLOCK('Rectangular Block',$,100.2,120.2,50.2); #4=WORKPLAN('Main Workplan',(#5,#15,#32,#42,#52,#73,#83,#105,#115),$,$,$); #5=MACHINING_WORKINGSTEP('Top milling',#6,#,#7,$); #6=PLANE('secplane.1', $); #7=PLANE_FINISH_MILLING($,$,'FINISH PLANAR FACE1,10,$,#8,#12,#13,0,$,$,#14,2.5,0); #8=MILLING_CUTTING_TOOL('',#9,(#11),,$,$); ENDSEC; END-ISO-10303-21;

P21 Code

'EXECUTE EXAMPLE1' 'SIMPLE WORKPIECE' 'Rectangular Block' '100.2' '120.2' '50.2' 'Plane milling' 'Security plane 1'

P28 Code



Fig. 7. Data exchange.

W. Xiao et al. / Robotics and Computer-Integrated Manufacturing 31 (2015) 1–10

STEP-NC program

Sync manager Real time arrangement

STEP-NC interpreter

STEP-NC memory structure

Spindle Coolant Air pressure Door Light Emergency Axis limits Lubrication

In

Out

0 1 2 3 4 5 6 7

0 1 2 3 4 5 6 7

PLC mapping

+24V

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Machine functions

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Manufacturing feature Tool path generator

Y axis acc/dec Ring buffer

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D/A

Fine interpolation Position feedback

Look ahead buffer

MMI

Servo Motor

NCK

PC-based CNC MMI+PLC+NCK

PLC-IO, Sensors and Servo motors Software based

Hardware based

Fig. 8. System architecture of the STEP-CNC prototype.

3.3. Off-line closed-loop manufacturing The off-line closed-loop manufacturing function is established based on the bidirectional data flow. The probing inspection task is a typical application [25]. Fig. 6 presents a touch probing inspection example. Fig. 6(a) shows the probing points generated by the STEP-compliant CAD/CAM system, and (b) shows the comparison between theoretical and measured points. This kind of information can be further used as a reference for the optimization process. 3.4. Data exchange STEP-NC programs could be generated and exported after process planning. The exported STEP-NC files can be formatted either in P21 (clear text) or P28 (XML), as shown in Fig. 7. The P28 format is specified in ISO 10303-28. It uses the Extensible Markup Language (XML) to represent EXPRESS schema and STEP data. Using the XML format, it is convenient to represent and exchange STEP-NC data. Since many mature XML parsers have been developed, people have no need to spend too much time and effort on the SDAI (Standard Data Access Interface) technology for a P21

parser. Moreover, the use of XML also helps to exchange data via the Internet, so that an e-Manufacturing network can be constructed.

4. Developing a STEP-compliant CNC system This section proposes a prototype STEP-CNC system with the above mentioned framework. A fully PC-based CNC system is realized by means of soft-PLC and soft-NCK technologies. There are a number of resources available on a PC platform for implementing the PC based CNC system. However, the poor real-time performance of the PC platform is a major impediment to this purpose. This problem has hindered the development of open-CNC for a long time until industrial control was greatly promoted by the fast-growing IT technologies in recent years. Many innovative techniques, such as industrial PC (IPC), industrial Ethernet, compilers, have been gradually introduced into the industrial control field. Some commercial products have also come out, among which the TwinCAT platform is considered as one of the most prominent. A soft-PLC that conforms to the IEC 61131-3 standard

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MMI is a major difference between conventional and STEPcompliant CNC systems. In view of this, we will discuss the MMI of STEP-CNC in detail, whereas PLC and NCK of STEP-CNC are not in the scope of this article.

Coordinates panels STEP-NC program structure - project - workingstep - security_plane - feature - operation

STEP-NC 3D Viewer

cutting_ tool

security_plane distance

approach_plane reference_plane

toolpath

profile_length

workpiece

STEP-NC Data Viewer Control Panel

STEP-CNC Interface

Machine functions

Operation Fig. 9. STEP-CNC MMI.

4.2. MMI of STEP-CNC The MMI of the proposed STEP-CNC system is presented in Fig. 9. It contains a STEP-NC data viewer and a STEP-NC 3D viewer for processing and displaying the imported STEP-NC data. The STEP-NC data viewer provides a concise way for the STEPNC interaction between users and machines. It interprets a STEPNC program and manages information in an object-oriented manner (as shown in Fig. 9). A STEP-NC program has at least one workplan, which contains many workingsteps, and each workingstep consists of three primary attributes: security_plane, feature and operation. The detail parameters can be accessed by clicking on each item, and corresponding dialogs are pulled up, where parameters can be inquired and edited. The STEP-NC 3D viewer renders the STEP-NC information in a 3D environment. By this way, various geometric entities can be visually represented. The displayable data include workpiece, toolpath, cutting_tool, etc. A tool path generator is implemented in the prototype STEP-CNC system. Although the generator is still on the prototype stage, the flexibility of intelligent STEP-CNC has already revealed by adjusting machining parameters locally on the CNC system. Fig. 10 presents such an example, in which the tool path generating function is tested on different conditions. Machining parameters involved in milling a planar face (security plane, cutting depth, size of raw piece, approach and retract strategies) are individually changed. Tool paths are accordingly generated on line, and simultaneously displayed in the 3D viewer. 4.3. Virtual manufacturing

(CoDeSys) is integrated in the TwinCAT system. In the CoDeSys environment, a soft-NCK kernel can be then developed [26]. 4.1. Architecture Fig. 8 shows the architecture of the prototype STEP-CNC system, consisting of both software and hardware aspects. In order to establish an open-structured and reconfigurable hardware platform, the EtherCAT bus is adopted for connecting servo motors and PLC-IOs. EtherCAT is a fast industrial Ethernet technology, which can support a synchronized cycle time up to 100 μs. Moreover, sensor and actuator devices from different vendors can be seamlessly integrated together, as long as they have the EtherCAT interfaces. The fast, open and reconfigurable characteristics of EtherCAT can perfectly fulfil the demands of STEP-CNC, as aforementioned in Section 2. Using the IPC technology, it is possible to develop a fully software based CNC system (soft-CNC), including MMI, PLC and NCK. The soft-CNC architecture provides open and portable CNC packages for machine tool integrators, so that a minimal effort need to be undertaken when adjusting the CNC to different machine tools. Besides, there is no need to replace any devices on the master side. Usually, MMI processes the interaction between humans and machines, PLC deals with multiple inputs and output arrangements (including both digital and analog IOs), and NCK provides the necessary functions for motion controls. For a STEP-CNC system, most of the STEP-NC related functions are implemented in MMI. Hence, comparing to conventional CNC systems, the MMI of STEP-CNC is much more powerful and complex. Besides, MMI has become not only an interaction interface, but also an STEP-NC data processing center.

In order to verify the STEP-NC technologies, a real machine tool equipped with STEP-CNC is needed. However, it is expensive to construct a real machine tool. Besides, the difficulty and cost are usually beyond the abilities of most CAD/CAM and CNC researchers. In order to solve this problem, the STEP-CNC can be firstly verified in a virtual manufacturing environment before connected to a real machine tool. For this purpose, we established a simulated machine tool and make it virtually controlled by the STEP-CNC controller through an Ethernet interface. Since the STEP-CNC system was developed on a IPC platform, it is very easy to share the same LAN (Local Area Network) port in real and simulated controls. The system structure is shown in Fig. 11. By switching the Ethernet cable to real and simulated controls, the STEP-CNC system can be verified.

5. Discussion and conclusion In order to further popularize the STEP-NC technology, a complete CAD/CAM/CNC solution is proposed. For a long time, STEP-compliant CAD/CAM and STEP-CNC systems have been separately studied. The major contributions in this paper are to comprehensively study the STEP-compliant systems and summarize their frameworks and criteria. A real STEP-NC data flow between a STEP-compliant CAD/CAM system and a STEP-CNC system is developed. Although the systems proposed in this paper are still on the prototype stage, they are very useful attempts, as many hardware technologies and software architectures have been tested. The authors also find that the present most feasible approach for implementing STEP-NC is to develop a secondary developed CAD/CAM system and an open structured CNC system. In our future researches, more detailed STEP-NC technologies will

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Fig. 10. STEP-CNC prototype. (a) Change the security plane. (b) Change the cutting depth. (c) Change the size of raw piece. (d) Change approach & retract strategies.

EtherCAT

LAN Port

Real control Simulated control

STEP-CNC TCP/IP

Virtual Machine Tool

Fig. 11. Virtual manufacturing.

be studied, such as feature recognition, complex tool path generation, intelligent sensor-based control, and closed-loop optimization.

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