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Development, Implementation, and Assessment of a Web-Based Power Electronics Laboratory William Gerard Hurley, Senior Member, IEEE, and Chi Kwan Lee
Abstract—A Web-based laboratory exercise with remote access is presented, through which a student of Electrical/Electronic Engineering is introduced in both a theoretical and practical way, to many fundamental aspects of power electronics. The system is flexible and can expand the range of laboratory exercises where fullscale laboratories are not feasible. In the electrical environment, limits can be placed on voltages and currents for safety reasons. Prelaboratory investigations allow students to take an active involvement in the learning process by addressing some challenging and critical aspects of the design before approaching the physical system. Further understanding is gained by studying the circuit in a Web-based, interactive power electronics seminar (iPES) by simulating the circuit using PSpice and then analyzing the control and feedback issues with MATLAB. In the final stage, a real power converter is tested remotely over the Web, and the cycle of design, simulation, and test is completed using Web-based tools. Index Terms—Control engineering, dc–dc converters, distance learning, power electronics, Web-based laboratory.
NOTATION The instantaneous variable is lower case; the quiescent or average value is upper case; and the incremental component is . The Laplace translower case with a tilde, e.g., form of the incremental variable is upper case with the variable , e.g., . I. INTRODUCTION
A
WEB-BASED real-time laboratory is described where all of the instrumentation used in the experiment is remotely accessed over the Web, and the student can carry out the measurements in his or her own time while continuously refining the design as the measurements are made. The student sees all the instruments on the screen and controls the inputs as required to see the results of actions taken. The experiential learning setting afforded by the Web stimulates the student in a highly interactive environment. Further advantages over the traditional laboratory setting are that scheduled time slots are eliminated, safety with live electrical circuits is not an issue, and the number of users is not limited. Web-based laboratories have been developed in the area of control [1], [2]. The emphasis in the paper is on the broader concepts and assessment of Web-based learning and comparison with traditional learning methods. In a traditional laboratory exercise, the students carry out a prelaboratory assignment consisting of design and simulation. Manuscript received July 20, 2004; revised July 6, 2005. This work has been funded by Enterprise Ireland. The authors are with the Department of Electronic Engineering, National University of Ireland, Galway, Ireland (e-mail:
[email protected]). Digital Object Identifier 10.1109/TE.2005.856147
The student then goes to the laboratory with up to 20 others, who work in groups of two and measure the results to confirm the calculations, normally in a two-hour slot. The major drawback of this process is that the student does not have an opportunity to repeat the design component if the theory and measurements do not match, because of time constraints; in other words, the essential feedback link between theory and practice is missing. The feedback link is provided by Web-based laboratory measurements. The student sees all the instruments on the screen, controls the inputs as required, and monitors the results of actions taken. Unlike simulation tools, the instruments are operating in real time on real hardware. A power supply must be designed to have good line regulation, good load regulation, and good transient response to system disturbances and be basically stable under all operating conditions. All these requirements are satisfied by using a closed-loop controlled converter, which compares a reference voltage to the actual output voltage, thereby varying the duty ratio of the power transistor switch, which restores the output voltage to the desired value. Pulsewidth modulation (PWM) feedback control achieves this goal [3]–[6]. In the course of the exercise from prelaboratory to postlaboratory assignments, the student is introduced to the basic principles of power electronics, the dynamics of switching systems, the averaged and linearized circuit model techniques, and the application of compensation techniques in a typical control system. An excellent Web-based interactive power electronics seminar (iPES) with animated applets is available at [7]. The use of modern Web-based tools for circuit design (PSpice1) and control systems (MATLAB2) has removed the traditional paper-based approach, which normally requires sweeping approximations, while motivating the student in an innovative learning setting to gain insights into the underlying principles. Combining the simulation tools with iPES gives the student widely transferable skills beyond the specifics of the experiment under investigation. II. THE WEB-BASED POWER ELECTRONICS LABORATORY SETUP A. Equipment and dc–dc Power Converter The development of the remote-access laboratory is based on general-purpose interface bus (GPIB) instruments and LabVIEW 7.3 All the instruments, including a four-channel digital 1http://www.orcad.com/ 2http://www.mathworks.com/ 3http://www.labview.com/
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Fig. 1. Hardware setup in the laboratory.
TABLE I TECHNICAL SPECIFICATIONS OF THE dc–dc BUCK CONVERTER
Fig. 2.
dc–dc buck converter circuit.
oscilloscope, a dc power supply, and a digital voltmeter, communicate with the host computer using LabVIEW 7 and the GPIB interface to perform data exchange. Fig. 1 shows the hardware setup in the laboratory. In Fig. 1, the actual dc–dc power converter and associated power supply, resistive load, and oscilloscope are shown on the left; the host computer running LabVIEW 7 is shown on the right. The gate voltage, input current, output current, and output voltage ripple waveforms of the converter are measured. The input voltage of the dc–dc power converter can be varied by the dc power supply from 8 to 15 V. The power conversion part of the system is implemented by a dc–dc buck converter, using a power metal–oxide–semiconductor field-effect transistor (MOSFET) as the switching device, as shown in Fig. 2. The detailed technical specifications of the dc–dc buck converter are listed on Table I. B. LabVIEW and LabVIEW Internet Toolkit LabVIEW by National Instruments (NI), Austin, TX, is a graphical development environment for data acquisition, instrument control, measurement analysis, and data presentation. Fig. 3 shows two examples of virtual instrument (VI) setups, which are used to send a command and receive a data string from the instruments through the GPIB interface. Fig. 3(a) is a VI setup that sends the value of voltage and current limits to the dc power supply. The output voltage and current of the dc power supply can be measured and received using a simple receive command, which is shown in Fig. 3(b). All the functionality,
configuration, and appearance of the system are simply assembled by connecting different blocks. The LabVIEW Internet Toolkit incorporates the VI setups on the Internet Web browser. Fig. 4 shows the final instruments and control front panel as they appear on the Web browser interface. III. CIRCUIT ANALYSIS AND CONTROL A. Circuit Analysis The dc–dc buck regulator circuit operates in two different modes. An analysis of these modes, found in any undergraduate text [3], [4], reveals the following equation, where is the duty cycle of the switch: and
(1)
The iPES [7] is an ideal vehicle for the student to become familiar with the circuit; a sample screen is shown in Fig. 5. The dynamic behavior of the circuit is based on perturbations about the steady state or average values of voltage, current, and duty cycle. The process of linearization is described in [6], resulting in the linearized equivalent circuit shown in Fig. 6.
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Fig. 3. Example of LabVIEW VI. (a) Setup 1: Sending a command to an instrument. (b) Setup 2: Receiving a data string from an instrument.
Fig. 4. Final instruments and control front panel on the Web browser.
The transfer function of control to output will be required for stability analysis and may be found by setting , yielding (2) where
and
.
B. Block Diagram Fig. 7 shows a closed-loop system of a regulated dc–dc buck converter, consisting of a compensation error amplifier, a PWM generator/comparator, and the converter transfer function. The full circuit and the derivations of the transfer functions are described in the Appendix.
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Fig. 5.
Interactive power electronics seminar (iPES) [7].
Fig. 6.
Linearized equivalent circuit.
Fig. 8. PSpice waveforms of buck converter (upper trace output voltage and lower trace output current). Fig. 7. Block diagram of the converter.
IV. PRELABORATORY ASSIGNMENT AND STUDENT EXERCISE Before carrying out the Web-based laboratory, students are required to complete a set of prelaboratory assignments. The set of prelaboratory assignments provides a good fundamental and theoretical background to the design of a dc–dc switching mode converter, which is later remotely tested on the Web. The assignments include theoretical circuit analysis, computer simulation, averaged and linearized circuit modeling, control-loop design and compensation, and inductor design.
and specifications. A PSpice model for an idealized dc–dc buck regulator includes a voltage-controlled switch, an ideal diode, an ideal inductor, and an ideal output capacitor. The switch is controlled by a pulse voltage source. Fig. 8 shows the PSpice output for the output capacitor voltage and the inductor current. The student is expected to compare the voltage ripple and current ripple with the well-known calculations based on the output capacitor and inductor values. The calculations may be compared with the actual measurements later. B. Stability Analysis With MATLAB
A. Computer Simulation With PSpice Students are asked to perform a circuit simulation using PSpice. Students can observe the operation principle of the converter from the simulation and verify the design equation
In this exercise, students are required to select the values of , , , and of the compensating error amplifier (Fig. 10) to ensure a phase margin of at least 45 and a gain margin of at least two to ensure stability. Stability is achieved
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Fig. 9.
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Bode plot of converter control system (MATLAB).
by the proper selection of the pole and zero of the compensation error amplifier. The student must generate a Bode plot of the open-loop converter control system (Fig. 9) using MATLAB. V. REMOTE LABORATORY SESSION In the laboratory exercise, the student observes and records waveforms from probe points on the power conversion board. Fig. 4 shows the input current, output current, output voltage ripple, and gate voltage of the converter on the Web browser 10 V regulated to give an output screen for an input of of 5 V (read from the dc voltmeter) across the 3.3- resistive load. The students can repeat the measurements for various input voltages from 8 to 15 V, noting the little or no change in the regulated 5-V output voltage. The objective is to observe the change in duty cycle of the converter according to the change of input voltage. Students are asked to calculate the inductor value (using the slope of the input current/inductor current), average input/output current, average diode current, input/output power, and efficiency of the converter. Other experiments may be considered, such as step response to input voltage changes and changes in load using an electronic load.
VI. ASSESSMENT AND EVALUATION Independent assessment of the Web-based laboratory exercise was based on the principles enunciated in [8], with emphasis on usability. The students were required to write a detailed project report for assessment purposes. Student evaluation was carried out by the National University of Ireland, Galway’s Centre for Excellence in Teaching and Learning (CELT). Feedback from the students dealt with the effectiveness of the approach as a teaching tool and the relative strengths and weaknesses of the curriculum content. The students were also asked to identify the principal advantages of Web-based learning over traditional approaches. The students rated each component highly (iPES, PSpice, MATLAB, and measurements) and identified flexibility in terms of access and time as the main advantages. The students were asked to 1) express their confidence level using the new system, 2) rate the quality of the materials, and 3) indicate their overall satisfaction. The results are summarized in Table II, indicating that over 80% of the students were satisfied with the exercise. The students suggested that the live experiment be made available over an extended period to gain full benefit from the experiment. The
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Fig. 10.
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Circuit for dc–dc converter.
feedback has been very useful to the instructors for the future running of the experiment.
TABLE II ASSESSMENT DATA
VII. CONCLUSION A Web-based power electronics laboratory with remote access has been described. A board-mounted power conversion system has been developed, offering an interactive teaching and demonstration facility to the students. The complete exercise includes interactive simulation (iPES and PSpice) and control software (MATLAB) for electronic systems. The paper verifies that the design and control realization of the physical system performs in a correct and robust manner and can thus be used as an educational tool to highlight the various concepts of switch mode power supplies and their control. The paper demonstrates
that Web-based laboratory exercises remove the traditional limitations on space, time, and staff costs, while offering the individual student more flexibility.
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APPENDIX The block diagram of Fig. 7 represents the full circuit diagram shown in Fig. 10. The transfer function of the main DC-DC converter has already been established in (2). The compensation error amplifier is a straightforward inverting amplifier, and the transfer function is readily established, yielding (A1) where is and is . The operation of the Pulse Width Modulated (PWM) controller is fully explained in [6]. In summary the control voltage is compared to a repetitive ramp waveform and the output of the comparator controls the duty cycle of the switch. The transfer function of the PWM circuit is [4], [6] (A2) where
is the peak value of the ramp waveform. ACKNOWLEDGMENT
The authors would like to thank W. H. Wölfle, M. Hynes, and S. C. Tang for their contributions. They also would like to thank Dr. I. MacLabhrain and M. Keating of the Centre of Excellence in Teaching and Learning (CELT) at the National University of Ireland, Galway for their assistance.
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[4] N. Mohan, T. M. Undeland, and W. P. Robbins, Power Electronics, Converters, Applications and Design. New York: Wiley, 1995. [5] R. Erickson and D. Maksimovic, Fundamentals of Power Electronics, 2nd ed. New York: Chapman & Hall, 1997. [6] W. G. Hurley, M. Hynes, and W. H. Wölfle, “PWM control of a magnetic suspension system,” IEEE Trans. Educ., vol. 47, no. 2, pp. 165–173, May 2004. [7] U. Drofenik and J. W. Kolar, “Interactive power electronics seminar (iPES)—A Web-based introductory power electronics course employing Java-applets,” in Proc. 2002 IEEE Power Electronics Specialists Conf. (PESC’02), vol. 2, Jun. 2002, pp. 443–448. [8] Y. Amigud, G. Archer, J. Smith, M. Szymanski, and B. Servatius, “Assessing the quality of Web-enabled laboratories in undergraduate education,” in Proc. 32nd Annu. ASEE/IEEE Frontiers in Education (FIE’02), vol. 2, Nov. 2002, pp. F3E-12–F3E-16.
William Gerard Hurley (M’77–SM’90) was born in Cork, Ireland. He received the B.E. degree (first-class honors) in electrical engineering from the National University of Ireland, Cork, in 1974; the M.S. degree in electrical engineering from the Massachusetts Institute of Technology, Cambridge, in 1976; and the Ph.D. degree from the National University of Ireland, Galway, in 1988. From 1977 to 1979, he was a Product Engineer for Honeywell Controls, Toronto, ON, Canada. From 1979 to 1983, he was a Development Engineer in transmission lines at Ontario Hydro, Toronto, ON, Canada. He lectured in Electronic Engineering at the University of Limerick, Ireland, from 1983 to 1991 and is currently Vice-President and Professor of Electrical Engineering at the National University of Ireland, Galway, and the Director of the Power Electronics Research Center. His research interests include high-frequency magnetics, power quality, and automotive electronics. Prof. Hurley is a Fellow of the Institution of Engineers of Ireland and a Member of Sigma Xi. He has served as a Member of the Administrative Committee of the IEEE Power Electronics Society and was General Chair of the Power Electronics Specialists Conference in 2000. He received a Best Paper Prize for the IEEE TRANSACTIONS ON POWER ELECTRONICS in 2000.
REFERENCES [1] C. C. Ko, B. M. Chen, J. Chen, Y. Zhuang, and K. C. Tan, “Development of a Web-based laboratory for control experiments on a coupled tank apparatus,” IEEE Trans. Educ., vol. 44, no. 1, pp. 76–86, Feb. 2001. [2] K. W. E. Cheng, C. L. Chan, N. C. Cheung, and D. Sutanto, “Virtual laboratory development for teaching power electronics,” in Proc. 2002 IEEE Power Electronics Specialists Conf. (PESC’02), vol. 2, Jun. 2002, pp. 461–465. [3] D. W. Hart, Introduction to Power Electronics. Englewood Cliffs, NJ: Prentice-Hall, 1997.
Chi Kwan Lee received the B.Eng. degree (with honors) and the Ph.D. degree, both in electronic engineering, from the City University of Hong Kong, Kowloon, Hong Kong, in 1999 and 2004, respectively. He then joined the National University of Ireland, Galway, where he is currently a Postdoctoral Research Fellow in the Department of Electronic Engineering. His research interests include random-switching techniques, analysis of multilevel inverter, flexible ac transmission systems (FACTs), and active power filter design.
