course planning on motor control laboratory

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COURSE PLANNING ON MOTOR CONTROL LABORATORYYih-Ping Luh*, Associate Professor

Department of Mechanical EngineeringNational Taiwan University, Taipei, Taiwan 106, R.O.C.

[email protected] Yang, National Taiwan University

ABSTRACT

In this paper, a complete set of experiments on Motor Control Laboratory are designed. The main purpose ofthis course is to teach the basic techniques of designing electro-mechanical systems and to gain theknowledge on tuning and controlling dynamic systems. The design of this course should attract specialinterests of many universities because it actually requires students to build the most expensive part of theinstruments so that the instrumentation budget would fit in a reasonable amount of money. Theseinstruments include a simple PC based spectrum analyzer, a motor driver and motor speed/positioncontrollers. Experiments use these instruments include design of analog circuits, design of power electroniccircuits and motor drives, identification of dynamic systems, and speed/position control of servo motors. Allexperiments are specially designed for mechanical engineering students to gain basic electrical engineering

techniques, control experiences, and dynamic system instincts. The responses from the students who tookthis course are overwhelming. Also many industrial companies found very useful to employ students whohas taken this course.

INTRODUCTION

Mechanical engineers these days are different from the past. They need more knowledge in the field ofElectrical Engineering and Computer Science. The work they need to do also covers electronics,

programming, electric machinery, and signal processing. A course designed specifically on integrating thesefields becomes very important to mechanical engineering students. We strongly feel that a laboratory that

covers these fields should be designed.

The course is designed in the following procedures:

1. Lecture material to cover a very broad area of electrical and computer engineering fields (1).2. Design experiments to build the course required instruments. Students in the process of building

these instruments can learn the basics of analog/digital circuits, power electronic circuits, and signalprocessing.

3. Use instruments build in the previous experiments to identify and control a system.Consider a standard block diagram of motor control system infigure 1. In order to perform this experiment,

we need to teach students how to design an analog circuit, how to design a motor driver, how to identify themathematical model of a dynamic system, and how to control a dynamic system. Instruments required tocomplete this experiment are controller, driver, motor, sensor, sensor conditioner, power supply, functiongenerator, spectrum analyzer and other small instruments. Students in the process of building controllers,drivers, sensor conditioners, spectrum analyzers and using these instruments to construct a control systemcan gain the hands on experience on designing, tuning and understanding the behavior of both componentlevel and system level of real dynamic systems.

Figure 1 Block Diagram of Motor Control Systemsup
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COURSE PLANNING AND EXPERIMENTS

The course is divided into six laboratories and a final project. They are listed as follows:

Labl - Design and Analysis of Analog Circuits - 1 Week

Lab2 - Design A Linear Driver For D.C. Motors - 2 Weeks

Lab3 - System Identification Of A D.C. Motor - 2 Weeks

Lab4 - D.C. Motor Speed Control - 1 Week

Lab5 - Decoder Design For Encoders And Linear Scales - 1 Week

Lab6 - D.C. Motor Position Control - 2 Weeks

Final Project: Design and Control Selected Systems - 6 Weeks

The contents of Lab1covers the basic OP circuits [2] including adder, multiplier, differentiator, integrator,filter and buffer. Students are required to build each components to test the bandwidth and the functionalityof each circuit and the performance of different operational amplifiers. Students should be very careful aboutavoiding the saturation condition and unbalanced circuits. After the test of each component, students arerequired to build an analog circuit to simulate the behavior of a mass-spring-damper system. In the process

of constructing the simulation circuits, they are required to avoid saturating op-amps during the normaloperation. After this lab, students should learn the basic circuit design and tuning techniques to build ananalog servo controllers.

Lab2 covers the basic theory of power electronics [3]. Students are required to understand transistor,MOSFET, and SCR circuits. A linear drive for d.c. servo motors is constructed as shown in figure 2.Students should build and test the driver in three portions - the differentiation portion, the common emitter

plus common base amplification portion, and the Darlinton amplification portion. The bandwidth of eachportion should be very carefully designed and tested so that the entire circuit would be stable at allfrequencies [1]. A pre-made circuit board along with the complete circuit design are handed out to thestudents in the lab. Aggressive students can further modify the circuits to either improve the frequencyresponse or to increase the power capacity of the driver. However, most of the students will follow thecircuit diagram in the handout to construct the driver.Figure 2shows the block diagram and the circuitdesign of this linear drive. The cost of parts and wires for this experiment is only $20. The driver they makenot only give students useful hands on experiences on power electronics, but also provide the necessaryactuating device for d.c. servo motor which will be used in later labs.

Figure 2. ( 1 ) Block Diagram Of Linear Driver For D.C. Servo Motorsup
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Lab3 is divided into three parts. The first part is to identify the motor mathematical model using the timedomain system identification technology. Given step input from the function generator, the students arerequired to measure the motor velocity response, record it on the oscilloscope and compute the dynamicmodel in the Laplace transformation format. Driver designed in lab2 is used to complete this experiment.Figure 3shows the experiment setup.

The second and third part of lab3 are the frequency domain system identification experiment. In the secondpart, students are required to build a simple spectrum analyzer from the PC 586 computer. The followingprograms are handed to the students:

1. A preconditioned "random signal" generator program.2. A fast Fourier transform program which takes the time domain signal and returns the frequency

domain signal3. A program that divides two frequency domain signals at each frequency and plot the figure in db-

decade scale.4. A ADC/DAC IO control and data acquisition program designed for an ADC/DAC/DIO interface

board that also handed out to the students.
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(2) Circuit Design Of Linear Driver For D.C. Servo Motors

Figure 2 Construction of Linear Drivers For D.C. Servo Motors

Figure 3 Experiment Setup For Time Domain System Identificationup
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These programs are then loaded into PC. After the carefully selected ADC/DAC/DIO board is plugged intothe computer, students can then connect the system shown inFigure 4to identify the mathematical model ofd.c. servo motor. It is important to teach students how to choose a suitable ADC/DAC/DIO board. For amotor voltage to speed transfer function calculation, the system time constant varies from 0.05 sec to 0.5sec. We taught students how the precision and bandwidth of system ID results and the speed and number ofdigits of the IO boards are related. Also we taught students how to select the frequency bands of the randomsignal to fit into the nature of this experiment.

Figure 4 Construction of the Spectrum Analyzer for Lab3up

The last part of lab3 is to identify the system using frequency domain techniques. The random signalgenerated by the random signal generator is first sent to the driver input via the DAC port. The driveramplifies the signal and drives the motor. The angular velocity signal is taken from the tachometer andcollected by the computer via the ADC port. The random signal and the velocity signal are then transformedinto frequency domain using the FFT program. A transfer function plot is then shown in db-decade scale. Bymeasuring the break frequency, students can easily find the transfer function from the motor voltage input tospeed output. At the end of lab3, students are also required to compare the time domain and frequencydomain identification results. They can also discuss how identification errors are related to the design of theexperiments and the instrumentation.

Lab4 requires student to design a motor speed controller. They should use the result obtained from lab3 to

design the controller transfer function. Students are required to do the lab in two steps- simulation andimplementation. They use MATLAB or MATRIXx to do the simulation and use op-amps to construct thecontroller. Students are required to compare the performances between proportional and IP controllers[4]which are shown in Figure 5.
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(2) IP Motor Speed Control System

Figure 5 Block Diagram Of Motor Speed Control System

Students are required to try the proportional and IP controllers in this lab. They are also required to measuresteady state error, time constant, and to watch how the driver saturation may affect the control system. Labltechniques are also pointed out again to the students to watch on the circuit balancing technology when theydesign the controller circuits.

The main function of Lab5 is to prepare the sensor conditioner for motor position control. It also teachesstudents how to design a simple logic circuit. Instead the traditional logic gate that may create a bulky circuit

board, we use PLD to construct our decoder circuit. Students are required to use ABEL program to write thedecoding logic, simulate the circuit on the computer, and then burn the circuit into Lattice PLD. This

procedure is very useful to those students who are not major in electrical engineering to understand how todesign logic circuit using computer-aided software. Students also learn how to take the encoder outputs, puta buffer circuit after them, connect to the PLD and then send the decoded signal to a DAC to generate theanalog position signal. Figure 6 shows the block diagram of the decoding circuit.


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(2) The Circuit Diagram of the Decoding Circuit

Figure 6 The Block and Circuit Diagram of the Decoding Circuit


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(3) Multi-Loop Motor Position Control System

Figure 7 Block diagrams of Motor Position Control System down

The last lab is to design a motor position controller. Students are required to test the PD, IPD and multi-loopcontrollers in this experiment.Figure 7shows the block diagrams of these four controllers.

It is important to let students first build their own analog controllers, try to avoid op-amp output saturationduring the control operation, and to add their own anti-windup circuits[5]. During the experiment, they canalso compare the performances of all four types of controllers, learn how to tune the controller gains. We

found this is extremely useful to students in the future job hunting process. Industrial companies are veryhappy about having an employee that has controller tuning experiences. Many students also integrate theirvelocity controller, decoding circuit, and different position controller into one box, and build a nice andrigorous motor controller.
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At the end of this course, students are required to design controllers for various dynamic systems. They areallowed to pick any system they like and design a controller for it. Systems that have been used in the pastinclude a.c. induction motors, d.c. brushless motors, XY tables, inverted pendulum systems, etc. The main

purpose of this portion of the course is to let students have experiences to attack a problem from scratch.Students who were successful in this project are also found more confident in their future works.

CONCLUSION

In this paper, we gave the course design on motor control laboratory. Students who have taken this courseshould build most of the instruments themselves so that they not only save a huge budget but also learn howto design and construct basic components of a control system. The course is also mainly designed forMechanical Engineering students to gain experiences on handling electro-mechanical systems. Many

physical hard to understand behavior of dynamic systems can be easily found and understood in theseexperiments. Also students are required to prove all facts they have learned from the control and dynamicsystem text books[4][5]. Very effective learning processes can be generated. The result of this course is verygood. Many students graduated are working in the related field as a project leader, and some teachingassistants of this course are currently working as technical directors of large companies.

REFERENCES

[1] Yih-Ping Luh, "Lecture Notes On Electro-Mechanical Systems", Vol. 1- 2, Department of MechanicalEngineering, National Taiwan University, 1992.[2] Paul Horowitz, Winfield Hill, " The Art Of Electronics", 2nd Edition, Cambridge University Press, 1989.[3] Ned Mohan, Tore M. Undeland, and William P. Robbins, "Power Electronics Converters, Applications,and Design", 2nd Edition, John Wiley & Sons, Inc, 1995.[4] Richard M. Phelan, "Feedback Control System", Cornell University, 1991.[5] Gene F. Franklin, J. David Powell, and Abbas Emami-Naeini, "Feedback Control of Dynamic Systems",Addison-Wesley Publishing Company, l991.

course planning on motor control laboratory

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