Tanta University Faculty of Engineering Electrical Power and Machines Engineering Department

PLC-Based Smart Elevator Control System

PLC-Based Smart Elevator Control System

Abd-Elmoghney Talaat Shahin Ahmed Magdy Awad Ebrahim Mostafa Shokr Hala Said Mohammed Islam Mohammed Sarhan Mohammed Ahmed Almozayen Samar Mohammed Dabour Walaa Ahmed Ebrahim

Supervised by Dr. Said M.Allam

Chapter 1 Introduction With the overall rapid development taking place in all spheres, the living standard of human being has tremendously increased as such the high rise buildings are constructed for malls, and housing purposes. Thus the installation of elevators in these high rise buildings becomes an integral part of the infrastructure for the movement of goods and people. So, the control system is essential in the smooth and safe operation of the elevator. It guides the elevator in what order to stop at floors, when to open or close the door etc.

1.1 Historical Review Over years, man has developed in somehow an elevator shape to use in his everyday life to raise water, food and other objects to higher levels, the first reference to an elevator is in the works of the Roman architect Vitruvius, who reported that “Archimedes” built his first elevator probably in 236 BC. In some literary sources of later historical periods, elevators were mentioned as cabs on a hemp rope and powered by hand or by animals. It is supposed that elevators of this type were installed in Sinai, Egypt.

Chapter 1

Introduction

1.2 Types of Elevators 1.2.1 Traction Elevators These elevators have steel ropes that raise and lower cars from above. In a machine room above the elevator shaft, a control system operates a motor that turns a sheave. Cables roll over this deeply grooved pulley to pull a car up or lower it down. The cables are also attached to a counterweight that weighs about as much as the car on the other side of the sheave when it is at 40 percent of capacity purpose of the counterweight is to create a balance to conserve energy. With a counterweight, the elevator operates much like a see-saw. The motor can move the car by just overcoming friction between the ropes and sheave and the difference in weight between the elevator car and the counterweight.

1.2.2 Hydraulic Elevators Use a fluid to lift and lower the car. The car has a piston in a cylinder beneath it; the elevator lifts when an electric motor powers a hydraulic pump to push a fluid (typically oil) into the cylinder, which pushes the piston up. To lower the car, the control system opens a valve and the fluid flows back into the tank as the weight of the car pushes down on the piston. Hydraulic machines can effectively multiply the relatively weak force of the pump to generate the stronger force needed to lift the car. 2

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Introduction

Hydraulic elevators are simple and inexpensive, but they are comparatively inefficient because they lack the counterweight that traction elevators have.

1.2.3 Traction-Hydraulic Elevators The traction-hydraulic elevator has overhead traction cables and counterweight, but is driven by hydraulic power instead of an overhead traction motor. The weight of the car and its passengers, plus an advantageous roping ratio, reduces the demand from the pump to raise the counterweight, thereby reducing the size of the required machinery. The great development in variable speed drives nowadays made traction elevators the most widely used type, also its advantage of energy saving managed it to become favorable.

1.2.4 Smart Elevator Choice System Due to the wide need for more than one elevator car in the big buildings, a smart choice algorithm has been developed recently to choose the nearest elevator car to the passenger current location, for saving time and energy. In this system, there is one destination panel at every floor for passengers to specify their desired destination and then the control system chooses the nearest elevator car and moves it to the passenger current place. 3

PLC-Based Smart Elevator Control System

Chapter 1

Introduction

1.3 Modern Elevators Drive System Good and reliable drive system is the major component of a high quality elevator. The most commonly used motor type in modern elevators is three phase induction motor as they are simple, has low maintenance requirements, has relatively high starting torque especially with the aid of variable speed drives which give a great aid in the enhancement of three phase induction motor performance. Microprocessors are the most commonly used controllers for operation and speed control of elevators, position sensors (commonly, magnetic sensors) give a feedback signal of the elevator’s cabin position to the controller, and then the controller receives the destination order from user panel to execute the control algorithm and move the elevator to its destination. The most commonly used speed control algorithm moves the elevator under two different speeds, low speed at starting and before mechanical braking but the most advanced algorithm named variable voltage variable frequency (VVVF) which moves the elevator under increasing speed steps at starting and under decreasing speed steps before mechanical braking which gives very smooth movement.

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Chapter 1

Introduction

1.4 Project Objectives The main objectives of this project are to:  Save time and energy in big buildings when more than one elevator is needed by including smart control algorithm to choose the nearest elevator to the passenger leaving other elevators unmoved.  Use

programmable logic

controller (PLC) as

controller rather than using microprocessor, as it is more reliable and easier to maintain.  Use closed loop speed control method of three phase induction motor. This enhances dynamic response and transient operation of the motor.

1.5 Project Outlines Chapter 1: Introduction Chapter 2: System description 

Presents an overall description of the

proposed smart elevator system

Chapter 3: PLC-based destination control system 

Presents

a

detailed

description

of

implemented PLC programming and choice algorithm of the smart elevator system

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PLC-Based Smart Elevator Control System

Chapter 1

Introduction

Chapter 4: System modeling and control 

Presents a theoretical study of induction

motor modeling including the application of both scalar and vector speed control methods

Chapter 5: Experimental system 

Presents

a

full

description

implemented elevator system

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of

Chapter 2 System Description The proposed system presents a full smart elevator control system including driving motors, inverters and control unit which derives these motors to lead elevators to the desired destination specified by passengers precisely, also to control motor's speed during journey up or down to guarantee maximum movement softness, soft starting and soft stopping which gives the elevator users the highest service quality.

2.1 System Components

Figure (2.1) system description

Chapter 2

System Description

2.1.1 Driving Motor The driving motor of the proposed system is a three phase induction motor, due to its simple construction, low maintenance requirements and the wide spread and development of using power electronics in speed control of these motors to give a high efficient and accurate speed control methods.

2.1.2 Inverter The most commonly used speed control method of three phase induction motor in modern systems is varying both the stator supply voltage and frequency. The variation of supply voltage and frequency was a very complicated matter until the wide development in industrial power electronic devices. Inverter is a power electronic device that can be used to obtain a variable voltage and variable frequency. It consists of power electronic switches which can be controlled by driving pulses generated from the control unit.

2.1.3 Control Unit A. Destination Control Unit A programmable logic controller (PLC) is used to control elevator movement until reaching the desired destination. The PLC-code which represents the destination control algorithm must have input data from sensors or buttons to be able

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Chapter 2

System Description

to control elevator movement precisely. These data includes elevator car current location determined by magnetic sensors or limit switches, passenger current location and passenger desired destination which determined by push buttons panel at every floor.

B. Speed Control Unit In modern elevators drive systems, speed control algorithms are included with the inverter in a compact device called “digital inverter” or “variable speed drives (VSD)” which have ports to be connected directly to PLC.

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Chapter 3 PLC-Based Destination Control System 3.1 Introduction Peak-hour traffic in a busy high-rise often means long waits, a scramble for each arriving elevator, crowded elevator-cars and too many stops. A Programmable Logic Controller (PLC) is used in a destination control system for elevator selection to minimize these inconveniences by instantly assigning each passenger to the nearest elevator.

3.2 Control System Layout In conventional-elevators systems, passengers press randomly an up or down call button and wait. Then they crowd abroad the first arriving car and after entering this car, the other cars reach without interest. This is depreciation to these elevators and their motors and loss of power and time. On the other hand, in smart-elevators systems, instead of pressing traditional up or down buttons, passengers enter their desired destination floors before entering an elevator. The control system instantly directs each passenger to the nearest car assigned to the requested floor. The system continuously reviews passenger destination data and adapts to changing traffic patterns, ensuring optimal traffic flow throughout the day.

Chapter 3

Using

PLC-Based Destination Control System

smart-elevator

system

leads

to

the

following

advantages:  Saving a large amount of energy  Less-crowded cars  Reducing passenger waiting time  Reducing number of stops per trip to decrease elevator travel time  Eliminating crowding during heavy traffic  Reducing the maintenance of the elevators and the driving motors  Increasing the life span of elevators

3.3 Control System Description In large buildings, more than one elevator can be used. Firstly, the passenger enters the desired destination from a control panel beside the elevators. Limit switches are used to specify elevator place. Secondly, the control system using a PLC unit compares elevator place and passenger place to select the nearest elevatorcar to the passenger and display the name of this elevator on screen. Finally, the nearest elevator-car takes the passenger from his place to his destination. This can be easily described as shown in table 3-1.

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Table 3-1

Passenger location Floor (1) Floor (4) Floor (2)

Location of Location of Elevator A Elevator B Floor (2) Floor (1) Floor (3)

Floor (3) Floor (3) Floor (4)

Control system selection Elevator (A) Elevator (B) Elevator (A)

3.3.1 Programmable Logic Controller Programmable Logic Controllers (PLCs), also referred to as programmable controllers, are in the computer family. They are used in commercial and industrial applications. A PLC monitors inputs, makes decisions based on its program, and controls outputs to automate a process or machine. PLCs consist of input modules or points, a Central Processing Unit (CPU) and output modules or points. An input accepts a variety of digital or analog signals from various field devices (sensors) and converts them into a logic signal that can be used by the CPU. The CPU makes decisions and executes control instructions based on program instructions in memory. Output modules convert control instructions from the CPU into a digital or analog signal that can be used to control various field devices (actuators). A programming device is used to input the desired instructions. These instructions determine what the PLC will do for a specific input. An operator interface device allows process information to be displayed and new control parameters to be entered. 12

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Microprocessors are commonly used as a control unit in elevators control systems. PLC is used instead of microprocessors because of the great advantages of PLC against microprocessors. These advantages can be summarized as follows:  Visual observation and control: When running a PLC program a visual operation can be seen on a screen.  Control board: PLC board is easy to design rather than microprocessors board.  Interfacing: PLC is easy to interface with other system.  Programming: Using ladder diagrams for programming are very simple and easy to understand.  Program: PLC program can be tested, validated and corrected easily that leads to save a large time  Troubleshooting and fault detection: PLC troubleshooting and fault detection are really quick, easy and simple.  Noise

effect:

PLC

is

not

affected

by noise as

microprocessors.

3.3.2 PLC-Unit Specifications The SIMATIC S7-300 universal controller, shown in figure (3.1), saves on installation space and features a modular design. A wide range of modules can be used to expand the system centrally or to create decentralized structures according to the task at hand, and facilitates a cost-effective stock of spare parts. 13

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PLC-Based Destination Control System

Figure (3.1) SIMATIC S7-300 (S7 313c) PLC

 Features of S7 313C PLC:  16 digital inputs  16 digital outputs  Memory: o Ram  Integrated: 32KB for program and data  Expandable: no o Load memory  Upgradable FEPROM : with micro memory card (MMC)UP TO 4 MB o Backup  Without battery: program and data  Profibus-DP/Device Net  Execution times o Pit operation: 0.1 ㎲ to 0.2 ㎲ o Word operation: 0.5 ㎲ 14

PLC-Based Smart Elevator Control System

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PLC-Based Destination Control System

3.4 Control System Flow Chart

Figure (3. 2) Flow chart of the PLC program (part 1)

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Figure (3. 3) Flow chart of the PLC program (part 2) 16

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PLC-Based Destination Control System

3.5 Simulation Results 3.5.1 PLC Inputs and Outputs Table 3-2: PLC Inputs and Outputs

PLC Inputs I0.0 Elevator A –limit switch I1.0 at floor 1 I0.1 Elevator A -limit switch I1.1 at floor 2 I0.2 Elevator A -limit switch I1.2 at floor 3 I0.3 Elevator A -limit switch I1.3 at floor 4 I0.4 Elevator B -limit switch I1.4 at floor 1 I0.5 Elevator B -limit switch I1.5 at floor 2 I0.6 Elevator B -limit switch I1.6 at floor 3 I0.7 Elevator B -limit switch I1.7 at floor 4 PLC outputs Q0.0 Elevator (A) up Q0.2 Q0.1 Elevator (A) down Q0.3

Ok switch at floor 1 Ok switch at floor 2 Ok switch at floor 3 Ok switch at floor4 Call button 1 Call button 2 Call button 3 Call button 4

Elevator (B) up Elevator (B) down

3.5.2 Case-Study Simulation Results Firstly, the existing conditions are entered to the simulation program of S7 313c software. The conditions of the presented case-study are:  The location of elevator (A): floor (3)  The location of elevator (B): floor (4) 17

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PLC-Based Destination Control System

 Passenger location: floor (1)  Passenger desired destination: floor (4) It can be noted that the elevator A is the nearest one, therefore, the control system should select elevator A. This smart choice algorithm can be illustrated as follows: 1. Figure (3.4) shows PLC inputs and outputs.

Figure (3.4)

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2. Figure (3.5) shows that the location of elevator (A) is floor (3) and the location of elevator (B) is floor (4).

Figure (3.5)

3. Figure (3.6) shows that passenger location is floor (1) and the desired destination is floor (4).

Figure (3.6)

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4. Figure (3.7) shows that the control system selection is elevator (A).

Figure (3.7)

5. Figure (3.8) shows that elevator (A) arrived floor (2).

Figure (3.8) 20

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6. Figure (3.9) shows that elevator (A) arrived floor (1), and timer (T3) operates for 5 seconds till the passenger entering the elevator.

Figure (3.9)

7. Figure (3.10) shows that elevator A is moved up after the passenger entered the elevator.

Figure (3.10) 21

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8. Figure (3.11) shows that elevator (A) arrived floor (2).

Figure (3.11)

9. Figure (3.12) shows that elevator (A) arrived floor (3).

Figure (3.12) 22

PLC-Based Smart Elevator Control System

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PLC-Based Destination Control System

10.Figure (3.13) shows that elevator (A) arrived floor (4).

Figure (3.13)

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PLC-Based Smart Elevator Control System

Chapter 4 System Modeling and Control 4.1 Dynamic Model of a Three-Phase Induction Motor Induction motor, which is the most widely used motor type in industry, has been favored because of its good self-starting capability, simple construction, low cost and reliability. Along with variable frequency ac inverters, induction motors are used in many adjustable speed applications. Induction motors can be controlled to achieve dynamic performance as good as that of DC motors [1]. The dynamic model of the machine subjected to control must be known in order to understand and design modern speed control drives. Due to the fact that every good control has to face any possible change of the plant, it could be said that the dynamic model of the machine could be just a good approximation of the real plant. Nevertheless, the model should incorporate all the important dynamic effects occurring during both steady-state and transient operations [1].

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Figure (4.1) shows schematic diagram of a three-phase induction motor (axes relationship)

Figure (4.1) Schematic diagram of a three-phase induction motor (axis relationship)

4.1.1 Dynamic d-q axis Model The dynamic performance of three phase induction motor is somewhat complex because the three-phase rotor windings move with respect to the three-phase stator windings. The machine model can be described by differential equations in natural (abcaxes) seventh order model with time-varying mutual inductances. But such a model tends to be very complex[2]. Dynamic d-q model solves the problem of high system order by solving machine model with respect to two quadrant d-q axes.

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Using reference frames, the problem of time varying mutual inductances can be solved.

Figure (4.2) Schematic diagram of a three-phase induction motor in both abcaxes and d-q models

Assume that the ds - qs are oriented at angle  .The voltages

vdss and vqss can be resolved into as-bs-cs components and can be represented in the matrix form as: sin( ) vas  cos( ) v   cos(  120) sin(  120)  bs   vcs  cos(  120) sin(  120)

s 1  vqs   s 1 vds   s  1 vos  

(4.1)

The corresponding inverse relation is: vqss  cos( )  s  2 vds   sin( )  s  3 0.5  vos 

48

cos(  120) sin(  120) 0.5

cos(  120)  vas  sin(  120)  vbs   vcs  0.5

PLC-Based Smart Elevator Control System

(4.2)

Chapter 4

System Modeling and Control

4.1.2 Dynamic model of three-phase induction motor in arbitrary reference frame The stator voltage equations are given by: vqs  R si qs 

d Ψqs  ωΨds dt

v ds  R s i ds 

d Ψ ds  ωΨ qs dt

(4.3) (4.4)

The rotor voltage equations are given by:

vqr  Rr iqr 

d qr  (  r )dr  0 dt

(4.5)

vdr  Rr idr 

d dr  (  r )qr  0 dt

(4.6)

For synchronous reference frame ω  ωe For stationary reference frame ω  0 For rotating reference frame ω  ωr The flux linkage expressions:

qs  Lqsiqs  Lmiqr

(4.7)

ds  Ldsids  Lmidr

(4.8)

qr  Lqriqr  Lmiqs

(4.9)

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dr  Ldridr  Lmids

(4.10)

The electromechanical developed torque expression in terms of d-q axis Variables:

Te  Tl  J

dm  Bm dt

(4.11)

4.2 Dc to ac Converter (Inverter) Inverter is a power electronic device that converts dc to ac power by switching the dc input voltage (or current) in a predetermined sequence so as to generate ac voltage (or current) output as shown in figure (4.3).

IDC + VDC



Iac

+ Vac 

Figure (4.3) General block diagram of the dc-ac converter

The inverter is so named because it performs the opposite function of a rectifier. A variable output voltage can be obtained by varying the input dc voltage and maintaining the gain of the inverter constant. On the other hand if the dc input voltage is fixed and it is not controllable, a variable output voltage can be obtained by varying the gain of the inverter, which is normally

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accomplished by Pulse Width Modulation (PWM) control within the inverter. The inverter gain may be defined as the ratio of the ac output voltage to dc input voltage[2].

4.2.1 Three-Phase Inverter Three-phase inverters are used for variable-frequency drive applications with three phase ac motors and for high power applications such as HVDC power transmission. A basic threephase inverter consists of three single phase inverter switches each connected to one of the three load terminals as shown in figure (4.4).

Figure (4.4) Three phase voltage source inverter using power transistor

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Figure (4.5) explains the fabrication of the output voltage waves in square wave or six step mode of operation.

Figure (4.5) Equivalent circuit indicating voltage vno between the neutral points

If the three phase load neutral (n) is connected to the center tap of the dc voltage Vd, then the load phase voltages are Vao, Vbo and Vco. From figure (4.5) the following relations can be derived:

van 

2 1 1 vao  vbo  vco 3 3 3

(4.12)

vbn 

2 1 1 vbo  vao  vco 3 3 3

(4.13)

vcn 

2 1 1 vco  vao  vbo 3 3 3

(4.14)

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4.2.2 Pulse Width Modulation (PWM) Inverter The energy that a switching power converter delivers to a motor is controlled by Pulse Width Modulated (PWM) signals applied to the gates of the power electronic switches. PWM signals are pulse trains with fixed frequency and magnitude and variable pulse width. There is one pulse of fixed magnitude in every PWM period. However, the width of the pulses changes from period to period according to a modulating signal. When a PWM signal is applied to the gate of a power electronic switch, it causes the turn on and turns off intervals of the power electronic switch to change from one PWM period to another PWM period according to the same modulating signal. The frequency of a PWM signal must be much higher than that of the modulating signal, the fundamental frequency, such that the energy delivered to the motor and its load depends mostly on the modulating signal. In sinusoidal PWM, output signals are constructed by comparing two signals, a carrier signal and a modulation signal as shown in figure (4.6). The carrier signal is a high frequency (switching frequency) triangular waveform. The modulating signal is a controlled sine wave.[2]

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Figure (4.6) PWM operation

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4.3 Speed Control of a Three-phase Induction Motor Induction motor drives have been and are the workhorses in the industry for variable speed applications in a wide power range that covers from fractional horsepower to multi-megawatts. These applications include pumps and fans, paper and textile mills, subway and locomotive propulsions, electric and hybrid vehicles, machine tools and robotics, wind power generation systems, etc. the control of induction motor drives for variable speed applications is included in this chapter. The control schemes available for the induction motor drives are the scalar control, vector or field oriented control, direct torque and flux control and adaptive control. Because of advances in solid state power devices and microprocessors, variable speed ac Induction motors powered by power converters are becoming more and more popular. Switching power converters offer an easy way to regulate both the frequency and magnitude of the voltage and current applied to the motor. As a result much higher efficiency and performance can be achieved by these motor drives with less generated noises. The most common principle of this kind is the constant V/Hz principle which requires that the magnitude and frequency of the voltage applied to the stator of the motor maintain a constant ratio. By

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doing this, the magnitude of the magnetic field in the stator is kept at an approximately constant level throughout the operating range below base speed.

4.3.1 Scalar Control Scalar control as the name indicates is due to magnitude variation of the control variables only, and disregards any coupling effect in the machine. For example the voltage of the machine can be controlled to control the flux, and the frequency or slip can be controlled to control torque. Using scalar control, also called "volts per hertz" control or v/f control, a drive essentially acts as a power supply of a selected frequency and proportional voltage. At a given speed, the motor performs much as it would when supplied by utility power. For each frequency setting, motor operation is governed by a torque vs. speed curve. However, flux and torque are also functions of frequency and voltage respectively. Scalar controlled drives give somewhat inferior performance than the other control schemes but they are the easiest to implement. A. Open Loop Volts/Hz Control The open loop volts/Hz control of an induction motor is very popular because of its simplicity.

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In Figure (4.7) the primary control variable is the frequency er. The commanded phase voltage is generated by a gain stage based on the speed e to maintain a constant air gap flux.

Figure (4.7) open loop V/F speed control with voltage-fed inverters

Figure (4.8) shows the drive's steady state performance on a torque speed plane with a fan or pump load (TL=kωr2). As the frequency is gradually increased, the speed also increases proportionally as indicated at points 1, 2, 3, 4, etc. The operation can be continued smoothly in the field weakening region (after base speed) where the supply voltage Vs saturates.

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Suppose the load torque is changed from TL to TL' for the same frequency command, the speed will drop slightly from r to r'. This type of speed variation can easily be tolerated by a fan or pump where precision speed control is not necessary[2].

Figure (4.8) Torque speed curves showing effect of frequency variation, load torque and supply voltage changes

It can be noted that, in the open-loop control mode, the speed of the motor cannot be controlled precisely. B. Closed Loop Volts/Hz Control with Slip Regulation An improvement over open loop Volts/Hz control is closed loop Volts/Hz control with slip regulation as shown in Figure (4.8).However, additional feedback control loops increases system complexity and potential stability problems but lead to more smooth variation.

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Here, the speed loop error generates a slip command sl* via a proportional-integral controller (P-I controller) and limiter. This slip command is added to the feedback speed signal r to get the frequency command e* which, in turn, generates the voltage command through a volts/Hz function generator.[2]

Figure (4.9) close loop speed control with V/F control and slip regulation

Since slip is proportional to torque at constant flux, this approach may be considered as open loop torque control within a speed control loop. If a step-up speed command is provided, the motor accelerates freely until a slip limit (corresponding to the motor’s torque limit) is achieved and then settles down to the steady state load-limited torque[2]. If r* is stepped down, the drive behaves as a generator and decelerates with constant negative slip sl*. However, the value of

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sl* must be limited to a safe margin below the slip speed corresponding to the pull-out torque point. Since the slip speed is relatively small compared to the rotor speed, this mode of operation requires precise measurement of the rotor speed. Also, in negative slip mode of operation, the regenerated power must either be dissipated in a braking resistor or fed back to the ac mains.

4.3.2 Vector Control Scalar control is simple to implement, but the inherent coupling effect, both the flux and the torque are functions of voltage or current and frequency, gives sluggish response and the system is prone to instability because of a high order system effect. If the torque is increased by incrementing the slip or frequency the flux tends to decrease and this flux variation is very slow. The flux decrease is then compensated by the flux control loop, which has a large time constant. This temporary dipping of flux reduces the torque sensitivity with slip and lengthens the response time. The variations in the flux linkages have to be controlled by the magnitude and frequency of the stator and rotor phase currents and their instantaneous phases. Normal scalar control of induction machine aims at controlling the magnitude and frequency of the currents or voltages but not their phase angles[2].

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There is a close parallel between torque control of a dc motor and vector control of an ac motor. In a dc machine, the field flux f (f) produced by field current If is orthogonal to the armature flux a (a) produced by the armature current Ia as shown in figure (4.10).

Figure (4.10) separately excited dc motor

The developed torque Te can be written as: Te  k 't I a I f

(4.15)

Dc motor-like performance can be achieved with an induction motor if the motor control is considered in the synchronously rotating reference frame (de-qe) where the sinusoidal variables appear as dc quantities in steady state. Two control inputs ids and iqs can be used for a vector controlled inverter as shown in Figure (4.11).

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Figure (4.11) vector controlled induction motor

Thus torque is given by: Te  k t Ψ r i qs  k 't i ds i qs

(4.16)

This dc motor-like performance is only possible if i*qs only controls iqs and does not affect the flux, i.e. iqs and ids are orthogonal under all operating conditions of the vectorcontrolled drive. Thus, vector control should ensure the correct orientation and equality of the command and actual currents. There are essentially two general methods for vector control. One, called the direct or feed-back method, and the other, known as the indirect or feed forward method. A. Direct Vector Control In direct vector control the field angle is calculated directly by using terminal voltages and current or Hall sensors or flux sense windings.

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A block diagram of a direct vector control method using a PWM voltage-fed inverter is shown in figure (4.12).

Figure (4.12) direct vector control block diagram with rotor flux orientation

The principal vector control parameters, i *ds and i*qs, which are dc values in the synchronously rotating reference frame, are converted to the stationary reference frame (using the vector rotation (VR) block) by using the unit vector cose and sine. These stationary reference frame control parameters idss* and iqss* are then changed to the phase current command signals, i a*, ib*, and ic* which are fed to the PWM inverter[2]. A flux control loop is used to precisely control the flux. Torque control is achieved through the current i*qs which is

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generated from the speed control loop (which includes a bipolar limiter that is not shown). The torque can be negative which will result in a negative phase orientation for iqs in the phasor diagram in figure (4.13).

Figure (4.13) stationary and synchronous phasors showing correct rotor flux orientation

Here the de-qe frame is rotating at synchronous speed e with respect to the stationary reference frame ds-qs, and at any point in time, the angular position of the de axis with respect to the ds axis is (e =et). From this phasor diagram we can write: s Ψ dr  Ψ r cosθ e

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(4.17) PLC-Based Smart Elevator Control System

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s Ψ qr  Ψ r sinθ e

where; cosθ e 

Ψr 

(4.18) s Ψ dr Ψr

, sinθ e 

s Ψ qr Ψr

(Ψsdr ) 2  (Ψsqr ) 2

(4.19)

These unit vector signals, when used in the vector rotation block, cause ids to maintain orientation along the de-axis and the iqs orientation along the qe-axis. In low speed region the rotor flux component can be synthesized more easily with the help of speed and current signals. Representing the flux equations using the currents and speed results in: s dΨ dr



Lm s 1 s s i ds  w r Ψqr  Ψ dr τr τr

(4.20)



Lm s 1 s s i qs  w r Ψ dr  Ψqr τr τr

(4.21)

dt

s dΨ qr dt

Where

65

r =

Lr / Rr is the rotor circuit time constant.

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These two equations are defined as the current model for flux estimation as shown in figure (4.14).

Figure (4.14) Current model flux estimation The main advantage of using current model is that the drive operation can be extended down to zero speed so the current model estimation can be made at any speed however; note that the estimation accuracy is affected by the variation of the machine parameters such as rotor resistance variation due to skin effect and temperature.

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B. Indirect Vector Control The indirect vector control method is essentially the same as direct

vector

control,

except

the

unit

vector

signals

are generated in feed forward manner. Indirect vector control is very popular in the industrial applications. Figure (4.15) explains the fundamental principle of indirect vector control with the help of a phasor diagram. The fixed on the stator and the

axes are fixed on the rotor.

Synchronously rotating axes axis by positive slip angle frequency and

axes are

are rotating ahead of corresponding to slip

. Since the rotor flux is directed on the

axes

[2].

θ e   ωe .dt   (ωr  ωsl )dt  θ r  θsl

(4.22)

Figure (4.15) phasor diagram explaining indirect vector control

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The rotor circuit equations can be written as: dΨ dr  R r i dr  (ωe  ω r )Ψ qr  0 dt dΨ qr dt

 R r i qr  (ωe  ωr )Ψ dr  0

(4.23) (4.24)

Ψdr  L r i dr  L mi ds

(4.25)

Ψ qr  L r i qr  L mi qs

(4.26)

From equations (4.25) and (4.26), the dq-axis rotor current can be written as:

i dr 

1 L Ψ dr  m i ds Lr Lr

(4.27)

iqr 

1 L Ψqr  m i qs Lr Lr

(4.28)

The rotor current in equation (4.25) and (4.26) which are inaccessible, can be eliminated with the help of equation (4.27) and (4.28) as:

dΨ qr dt dΨ qr dt



Rr L Ψ qr  m R r i qs  ωsl Ψ dr  0 Lr Lr

(4.29)



Rr L Ψ dr  m R r i ds  ωsl Ψ qr  0 Lr Lr

(4.30)

Where ω

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ω

ω has been substituted.

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For decoupling control, it is desirable that: Ψ qr  0 That is,

dΨ qr dt

0

So that the total rotor flux ̂ is directed on the

axis

Substituting the above condition in equations (4.29) and (4.30), we get L r dΨ r  Ψ r  L mi ds R r dt

ωsl 

(4.31)

Lm R r i qs Ψ r Lr

̂ Where Ψ

Ψ

(4.32)

has been substituted

̂ If rotor fluxes Ψ

, which is usually the case, then

from equation (4.31):

Ψ r  L mi ds

(4.33)

In other words, the rotor flux is directly proportional to current

in steady state.

C. Implementation of indirect vector control The indirect vector controller takes only the speed from the machine while all other parameters are estimated. The implementation of the indirect vector control is as shown in figure

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(4.16). The torque command is generated as a function of the speed error signal, generally processed through a PI controller. The flux component of current r

for the desired rotor flux

is determined from this equation:

r

m ids

and

maintained constant here in the open loop manner for simplicity. The slip frequency

is generated from

in feed forward

manner, Slip gain KS is given by:

ks

ωsl Lm R r    i qs Lr Ψ r Slip speed Signal

generate frequency signal generated from

(4.34)

is added with speed signals

. The unit vector signals are then

by integration and look up table as indicated in

the figure (4.16).

6:

to

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Figure (4.16) indirect vector control block diagram with open loop flux control

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4.6 Simulation Results In this chapter, simulation results are presented by solving the aforementioned models using the MATLAB_R2009a7.8 software, including three-phase induction motor, three phase voltage source inverter, scalar control and vector control of three phase induction motor. The presented simulation results are taken using a 0.32hp, 220v, 4pole, 50Hz three phase squirrel cage induction motor. The induction motor parameters are listed in table (4-1). Table (4-1): Induction motor parameters

Rr

28 ohms

Rotor winding resistance

Rs

38 ohms

Stator winding resistance

Lm

0.9944 H

Mutual inductance

Ls

1.1541 H

Stator winding inductance

Lr

1.1541 H

Rotor winding inductance

J

7e-4 Kg.m2

Inertia constant

B

0.0

Friction coefficient

In this section a sample of the obtained simulation results of both scalar control and vector control are presented considering the following cases: 1. Constant speed reference and constant load torque

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2. Step change of speed reference with a constant load torque 3. Step change of load torque with a constant speed reference

4.6.1 Scalar Control Simulation Results: Case 1: Constant speed and constant load torque Figure (4.17) shows the run up response, developed torque and stator phase a current of three phase induction motor for a speed reference of 1500 rpm under a constant load torque of 0.5 N.m.

Figure (4.17) run-up response of a three-phase induction motor with a load of 0.5 N.m

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Case 2: Step change in the speed reference with a constant load torque Figure (4.18) shows the run up response, developed torque and stator phase a current of three phase induction motor under step change in speed reference from 1500 rpm to 1000 rpm and a constant load torque of 0.5 N.m.

Figure (4.18) run-up response of a three-phase induction motor with a step change in speed reference at a constant load of 0.5 N.m

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Case 3: Step change in load torque with a constant speed reference Figure (4.19) shows the run up response, developed torque and stator phase a current of three phase induction motor under step change in load torque from 1 N.m to 0.5 N.m and a constant speed reference of 1500 rpm.

Figure (4.19) developed and load torque with a step change in load at a constant speed reference of 1500 rpm

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4.6.2 Vector Control Simulation Results: In this section, simulation results of both direct and indirect vector control methods are introduced.

A. Direct vector control Case 1: Constant speed and constant load torque Figure (4.20) shows the run up response, developed torque and stator phase a current of three phase induction motor under constant speed reference 1500 rpm and a constant load torque of 0.5 N.m.

Figure (4.20) run-up response of a three-phase induction motor with a load of 0.5 N.m

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Case 2: Step change of the speed reference with a constant load Figure (4.21) shows the run up response, developed torque and stator phase a current of three phase induction motor under step change in speed reference from 1500 rpm to 1000 rpm and a constant load torque of 0.5 N.m.

Figure (4.21) run-up response of a three-phase induction motor with a step change in speed reference at a constant load of 0.5 N.m

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Case 3: Step change of load torque with a constant speed reference Figure (4.22) shows the run up response, developed torque and stator phase a current of three phase induction motor under step change in load torque from 0.5 N.m to 1 N.m and a constant speed reference of 1500 rpm.

Figure (4.22) developed and load torque with a step change in load at a constant speed reference of 1500 rpm

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B.

System Modeling and Control

Indirect vector control

Case 1: Constant speed reference and constant load torque Figure (4.23) shows the run up response, developed torque and stator phase a current of three phase induction motor under a constant speed reference of 1500 rpm and a constant load torque of 0.5 N.m.

Figure (4.23) run-up response of a three-phase induction motor with a load of 0.5 N.m

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Case 2: Step change in speed reference with a constant load torque Figure (4.24) shows the run up response, developed torque and stator phase a current of three phase induction motor under step change in speed reference from 1500 rpm to 1000 rpm and a constant load torque of 0.5 N.m.

Figure (4.24) run-up response of a three-phase induction motor with a step change in speed reference at a constant load of 0.5 N.m

7:

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Case 3: Step change in load torque with a constant speed reference Figure (4.25) shows the run up response, developed torque and stator phase a current of three phase induction motor under step change in load torque from 0.5 N.m to 1 N.m and a constant speed reference of 1500 rpm.

Figure (4.25) developed load torque with a step change in load torque and a constant speed reference of 1500 rpm

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It can be observed from the obtained simulation results that:  For the same required load torque, motor under vector control draws lower current compared with current drawn under scalar control.  Scalar control suffer from coupling effect which leads to sluggish response, this problem is solved in vector control by controlling the value and the direction of motor flux and torque ,which leads to faster response.  Due to free of coupling effect, compared to scalar control, Vector control gives faster and smoother transition from a speed to another. Generally, it can be concluded that the overall system performance using vector control is much better than using scalar control. Therefore, vector control is suggested to be a more suitable control method for traction elevator.

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4.6.4 Elevator-Movement Mechanism Control

system

should

provide

suitable

acceleration/

deceleration time and rate depending on the length of journey taken by the elevator to reach the desired destination. Figure (4.26) summarizes the operation mechanism of the elevator. When a passenger presses the call button, the direction of the motor is determined by comparing the elevator car current location and the passenger current location. Then the elevator moves under increasing speed steps at starting and under decreasing speed steps before mechanical braking which gives a very smooth movement.

Figure (4.26) the mechanism of elevator movement

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Figure (4.27) shows the elevator-movement mechanism for a short distance of one floor (i.e. low free speed). It can be observed that, the control system forces the elevator to track acceleration pattern of just two different speeds before reaching the relatively low free speed.

Figure (4.27) elevator-movement mechanism for a distance of one floor

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Figure (4.28) shows the elevator-movement mechanism for a medium distance of two floors (i.e. medium free speed). It can be observed that, the control system forces the elevator to track acceleration pattern of three different speeds before reaching the relatively medium free speed.

Figure (4.28) elevator-movement mechanism for a distance of two floors

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Figure (4.29) shows the elevator-movement mechanism for a long distance of three floors (i.e. high free speed). It can be observed that, the control system forces the elevator to track acceleration pattern of four different speeds before reaching the relatively high free speed.

Figure (4.29) elevator-movement mechanism for a distance of three floors

It can be concluded that, the passenger desired destination specifies the overall speed pattern that elevator will track.

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PLC-Based Smart Elevator Control System

Chapter 5 Experimental Set-Up In order to validate the proposed smart choice algorithm, an experimental system has been built as shown in figure (5.1). The proposed system consists of two dc motors driving two elevatorcars. Dc motors are used due to limited laboratory resources.

Figure (5.1) Elevator Model

Chapter 5

Experimental Set-Up

5.1 Experimental System Components The overall experimental system components can be described as follows:

1. PLC-Unit In this project, the used PLC-unit is s7313c as shown in figure (5.2) and has the following features:  16 digital inputs  16 digital outputs  Memory: o Ram  Integrated: 32KB for program and data  Expandable: no o Load memory  Upgradable FEPROM : with micro memory card (MMC)UP TO 4 MB o Backup  Without battery: program and data  Profibus-DP/Device Net  Execution times o Pit operation: 0.1 ㎲ to 0.2 ㎲ o Word operation: 0.5 ㎲

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Figure (5.2) Plc-Unit

2. Limit Switches In order to indicate the location of the elevator-car, eight limit switches are used. The employed limit switch and its operating conditions are shown in figure (5.3).

Figure (5.3) Limit Switch

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3. Push Buttons Generally, Push buttons, shown in figure (5.4) are used to choose the desired destination. This can be done by pressing the floor button (green) followed by the confirmation button (red). These push buttons are normally open and its operation includes two functions as follows:  Send signal to the PLC to determine the passenger location.  Send signal to the PLC to determine the passenger desired destination.

Figure (5.4) Push Buttons

4. Driving Motors Two geared dc-motors are used to drive the two elevators-cars as shown in figure (5.5).

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Figure (5.5) Dc-Motors

5. Relays Four relays, shown in figure (5.6), are used to control the rotating direction of the driving motors.

Figure (5.6) Relays

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Experimental Set-Up

6. Dc Power Supply The dc motors are supplied from a dc power supply as shown in figure (5.7) consisting of the following components:  1-phase transformer (220v/12v)  Bridge rectifier  Capacitor (400µf) as a filter

Figure (5.7) Dc-Supply

5.2 Case-Study Experimental Results Firstly, the existing conditions are entered to the proposed system. The conditions of the presented case-study are:  Elevator (A) location: floor (3)  Elevator (B) location: floor (4)  Passenger location: floor (1)

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Experimental Set-Up

 Passenger desired destination: floor (4) It can be noted that the elevator A is the nearest one, therefore, the control system should select elevator A. This smart choice algorithm can be illustrated as follows: 1. Figure (5.8) shows that the location of elevator (A) is floor (3) and the location of elevator (B) is floor (4).

Figure (5.8)

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2. Figure (5.9) shows that the desired destination is floor (4).

Figure (5.9)

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Chapter 5

3.

Experimental Set-Up

Figure (5.10) shows that passenger location is floor (1).

Figure (5.10)

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4.

Experimental Set-Up

Figure (5.11) shows that the control system selection is the

elevator (A) and elevator (A) moved to arrive to floor (2).

Figure (5.11)

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5.

Experimental Set-Up

Figure (5.12) shows the Elevator (A) arrived to floor (1).

Figure (5.12)

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6.

Experimental Set-Up

Figure (5.13) shows that elevator A is moved up after the

Passenger entered the elevator and then arrived to floor 2.

Figure (5.13)

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7.

Experimental Set-Up

Figure (5.14) shows that elevator (A) arrived floor (3).

Figure (5.14)

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8.

Experimental Set-Up

Figure (5.15) shows that elevator (A) arrived floor (4) and

stopped.

Figure (5.15)

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Experimental Set-Up

References [1] Dal Y. Ohm, “Dynamic Model of Induction Motor”, Drivetech, Inc., Blacksburg, Virginia. [2] Bimal K. Bose, “Modern Power Electronic and Ac Drives”, Prentice Hall PTR NJ07458, 2001.

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