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Tatsuya Kikuchi, Member, IEEE, Takashi Kenjo, Member, IEEE, and Shuichi Fukuda, Senior Member, IEEE
Abstract - The objective of this study is to investigate remote-learning methods in the context of mechatronics education, and in particular, for the study of brushless DC motors, which are extensively employed in robots, information devices, home appliances and other areas. While hypermedia-based courseware and computer-assisted instruction are widely used in conventional desk-type learning, very few examples exist of remote learning that involve experiments. The authors therefore developed a prototype client-server system for remotely conducting experiments on brushless DC motors, including Web-based courseware and other software. The server computer is connected to the motor laboratory, and the visual image and sounds of the experiment are transmitted to the client computer in real time. The remotely located user can operate the motors and conduct experiments through the client computer. Through demonstrations to a class, the authors conclude that the remote lab combined with a simulation of the motorís dynamic behavior can be a quite effective teaching aid for the study of precision motors.
The authors are also engaged in instructing (Japanese) university-level students on the subject of small motors and their driving methods using microprocessors or PCs (i.e., mechatronics), where they have found workbench demonstrations to be highly effective aids. They have so far developed several instructional materials and experiment benches on mechatronics, which are currently being used in Japanese universities and technical schools as well as overseas countries receiving ODA (official development assistance). The first model of the mechatronics workbench, called MECHATRO LAB, can be used to demonstrate various kinds of electric motors and their electronic operations .
Of these small precision motors, the use of the brushless DC motor (or DC brushless motor) is growing at a spectacular pace in many areas including PC peripherals, robots, medical equipment, home appliances (air conditioners, refrigerators, washing machines), and industrial applications (pumps and ventilators). Brushless DC motors are supplanting conventional motors like induction motors, brush DC motors and stepping motors to become the major actuator in mechatronics because of its simple construction, reliability and energy-saving characteristics.
Furthermore, they are not subject to certain weaknesses of these conventional motor types: the brush-type DC motor generates arcs and is subject to mechanical wear, the small induction motor has a very poor energy conversion performance, while the stepping motor requires a very small air gap between the rotor and stator core.
The brushless DC motor can be constructed in various arrangements, such as the normal inner-rotor type seen in CPU coolers, outer-rotor type, axial-gap type (pancake type), or with a air/magnet bearing configuration (Fig. 1), and so will likely see many more applications in such areas as automobiles. Because of its versatile construction, the brushless DC motor can be incorporated into various devices.
Brushless DC motors can basically be classified into single-phase and three-phase types. The former is widely used for small fans, while the latter finds many applications requiring high performance. In this study, we employ the latter type.
It is known that the rotor's position must be detected by some means in a brushless DC motor to generate switching signals for the six transistors or MOSFETs in the drive circuit, known as an inverter (Fig.2). Hall-effect sensors are widely used for this purpose, though the so-called sensor-less method also sees such applications as the spindle drives of hard disks. In the sensor-less scheme, the current or voltage waveform is used to determine the rotor's angular position. The use of Hall-effect sensors is considered to be the basic method. Fig. 1 shows a typical construction of a brushless DC motor, consisting of the following basic components:
1) Permanent magnet as the rotor,
2) Stator coils: At least three coils in this sample, but often 6, 9, 12 coils wound on teeth in the steel core as illustrated in Fig.3,
3) Three Hall-element sensors, which are basically placed at 120-degree intervals (Fig.1), but also often at 60-degree intervals (Fig.4).
4) Three-phase inverter consisting of six transistors (or MOSFETs) and its control circuit .
For these components to work properly as a motor, which in the brushless DC motor can take various configurations, they must be arranged and operated according to certain rules or principles. The aim of the present remote lab lies in imparting to students an understanding of this rule. Although brushless DC motor technology is an important subject, it is not taught in most technical schools or universities. The authors therefore see the need for post-university education, where effective experimental tools can be of great value. Yet, Mechatro Lab is the only lab apparatus that the authors are aware of that can demonstrate the basic operations of the brushless DC motor. This general unavailability of lab material can be remedied to some extent by the use of remote learning, which can also contribute to increasing the pool of qualified technical instructors.
The use and study of remote educational systems via the Internet or through satellite broadcasting is becoming a global trend. The authors too see great potential applications for such remote systems for education/training conducted across national boundaries, particularly in the training of technical instructors. The authors made an initial study on a related subject aimed at Japanese students with particular reference to a claw-pole stepping motor . Remote international study/training can supplement and improve training programs, such as the one described above. For instance, the overseas training period can be shortened, or more material covered over the same period, if trainees can prepare for the course beforehand by Web-based courseware, review the material covered afterwards in their home country over the Internet, or follow up with advanced topics and remotely conducted labs. When such remote learning tools are made widely available and the necessary communication networks have been installed, this should greatly expand the sphere for international cooperation.
In particular, we feel that such remote learning systems can be highly useful in training technical instructors in developing countries on the subject of motion control (or mechatronics) and other technical subjects for which practical workshop experience is invaluable.
The following section briefly surveys related works on electrical motors
and distance learning, sections III and IV present the system configuration
and outline of the remote lab, section V discusses reactions by overseas
instructors and issues for further improvement. Finally, our conclusions
are given in section VI.
Fig. 1. Brushless DC motor assembled in a dental piece, with a typical
construction using only three simple coils. The rotor has three permanent
magnets: the largest is for generating torque and the two thin ones are
magnetic bearings. The white ceramic pipe, together with the rotorís ceramic
sleeve, constitutes an air-film bearing.
Fig. 2. Brushless DC motor drive system with a three-phase bridge inverter
and Hall sensors.
Fig. 3. Typical stator of three-phase brushless DC motor (cross section):
(a) four-pole, six-coil configuration, (b) eight-pole, nine-coil configuration,
and (c) eight-pole, 12-coil configuration. Each tooth carries one coil.
Fig. 4. Printed circuit board with four Hall-element sensors. The three units placed at 60-degree intervals are used as the position sensors. The peripheral rack is for rotating the board, combined with a stepping motor (see Fig. 8).
Yet, all of these WBC are "closed" systems in the sense that they are self-contained within the userís personal computer. This has led R. Jain to propose "wiring the laboratory" . Among such studies involving remote laboratories, B. Aktan et al. stress the importance of combining theory and practice in the study of control engineering. They developed a paradigm for remote laboratory use and demonstrated its feasibility through its implementation in robot control . They give three points that must be considered when developing remote labs: 1) active learning, 2) data collection facilities, and 3) safety.
Meanwhile, H. Shen et al. developed a remotely operated lab apparatus for semiconductor experiments . Such a system "gives students the opportunity to work with sophisticated equipment, of the kind they are only likely to find in an industrial setting, and which may be too expensive for most schools to purchase." They state that remote laboratories offer advantages that "cannot be replaced by simulation software packages."
The authors too have developed an effective study aid based on networking technology that would enable the student to connect via his/her terminal to the laboratory (in particular, MECHATRO LAB), that is, we aimed to provide a remote laboratory environment tied into a WBC.
A. System description
As shown in Fig. 5 and Fig. 6, the remote laboratory system consists of a client-server computer architecture and software.
Client-Server: The client is the remote computer at the user end. The client specification consists of a Windows multimedia type computer with a LAN card. Also, we use Microsoft Internet Explorer 5.0 (IE5) and NetMeeting 2.1 (NM2). The former is for WBC browsing, while we use NM2ís streaming video function for transmitting the experimentís images. The loudspeaker conveys the atmosphere of the experimental via sounds.
The remote server consists of a multimedia server and a motor control server. The reason for using two PCs is that the closed loop operation of the brushless DC motor requires a high execution speed, where sensor signals must be received and switching signals generated at close to 50 microsecond intervals, and also that continuous measurement of the motor current is needed.
The role of the computer which we call a multimedia server is to provide the clients with multimedia tools, such as WBC, simulation, and video. The experiments are captured by a CCD camera and digitized by a video capture card and sound card. The motor control server (MCS) is used for operating the motor and measuring the voltages, and incorporates a 12-bit A/D converter (AD574) and digital I/O (8255) card. The signal lines are connected to the control bench, which is discussed later.
Motor laboratory: For the lab apparatus, we used Mechatro Lab 2 . A brushless DC motor can be constructed with a four-pole, six-coil stator, a magnet disk, and a printed circuit board having three Hall-element sensors, as shown in Fig.7. Mechatro Lab2 has a MOSFET power circuit that serves multiple purposes; here it is used as a three-phase inverter to drive the motor. By giving the proper switching signals to the inverter based on the Hall sensor signals, the rotor will start up and keep running.
To use Mechatro Lab 2 in the remote lab, we built a control bench and a microcontroller with an RS-232C transmission function. With this, the motor's line-to-line voltage, neutral point voltage, and input current are transmitted via an insulated transformer and filter circuit to the A/D converter in the MSC. A relay is used to change the motor connection between star and delta. The Hall-element board is driven using a stepping motor (Fig.8).
B. Client window
The client window is shown in Fig.9. It consists of three subwindows: WBC, video window, and the I/O remote-control board. As Window 1 shows, the WBC is an electronic textbook that can be read using a WWW browser, and has a hypertext structure employing multimedia including textual explanations, photos, cross-sectional drawings, and animated schematic drawings. Image size and color of the live video, Window 2, are set at 320pixels x 240pixels (width x height) and 16-bit color. Video quality of the client may be below server video settings because of increased network traffic.
The I/O remote-control board supports the remote laboratory, as illustrated
in Window 3. On this window, one can select either 120- or 180-degree operation,
in either CW or CCW. Moreover, one can rotate the remote Hall sensor board
by pressing down on the mouse at the CW or CCW button for 'Hall Sensor
Position' and observe this on the live video. By releasing the mouse button,
the sensor board will stop and hold its position. An oscilloscope window
is provided to observe the line-to-line voltage and neutral position voltage.
>From this, the user can learn how the driving mode, winding connection
and sensor position affect the motor's characteristics. This program and
the simulation software discussed below were written in Microsoft Visual
Basic (VB). Here, we adopted Microsoft Active X for making these programs
usable as Internet applications.
Fig. 5. System diagram connecting server and client.
Fig. 6. Function diagram for client-server system.
Fig. 7. Brushless DC motor construction used for experiments.
Fig. 8. Brushless DC motor assembly; the position of the Hall sensor
board is adjusted by rotating with a stepping motor.
Fig. 9. Client windows for brushless DC motor lab. 1) Web-based courseware; 2) video window of motor lab; and 3) I/O remote-control board.
Chapters 1 and 2 describe the basic construction of a modern brushless DC motor. Chapter 1 focuses on the mechanical construction, and Chapter 2 deals with the connection for the three sets of coils, for which there are two basic schemes: star and delta. Chapter 3 discusses the two fundamental switching modes in relation to the delta and star connections, which constitute a logic circuit problem with regard to the position-sensor signals. In this study, a Z-80 microprocessor and software is used for this purpose instead of a hardware logic circuit. Chapter 4 gives explanations on the rotor's permanent magnet and the back-emf waveform in relation to the two connection types and switching modes.
As stated above, one can start the motor after selecting the following items: 120- or 180-degree operation, delta or star connection, and CW or CCW. In the 120-degree mode with the correct sensor positions, the input current will be minimized, the neutral potential will be triangularly-shaped, and the line-to-line voltage trapezoidal-shaped, as in Fig.10(a). Neutral point here means the common terminal of the three coils in the star connection. If the sensor is positioned correctly, the terminal voltage's waveform and frequency and the input current do not change even when the rotating direction is reversed. If the Hall sensor position is shifted to either direction, the line-to-line voltage will be as shown in Fig.10(b), and there will be a noticeable speed difference when the revolving direction is reversed.
In the 180-degree mode, however, the waveforms of the line-to-line voltage and neutral point voltage are both rectangular regardless of the sensor position, and so the correct position is found by first finding a range where current is minimized and then honing into a position where the waveform frequency is unaffected when the motor is reversed.
We stated above that the brushless DC motor is built into various devices.
While the required motor construction is simple, certain rules must be
observed in order for these arrangements to work as a proper motor, which
are deeply related to electromagnetics, dynamics and electronic circuitry.
If these equations are properly described and computed by a computer, this
can provide a basis for simulation. We created such software using VB.
11 shows the Window of this simulation program. As will be stated in
Section V, the usefulness of simulation software was pointed out by a foreign
trainee who had studied simulation techniques for analyzing stepping motor
behavior from the authors.
|1||Fundamental construction: stator core, stator windings, rotor, and position sensor.|
|2||Three-phase: delta and star connection, neutral point.|
|3||Switching: 6-step operation: 120-degree and 180-degree.|
|4||Magnet and back-emf, waveform.|
Fig. 10. Remote oscilloscope window showing line-to-line voltages in
120-degree switching. (a) Good condition between Hall sensor positions
and drive timing, (b) Irregular condition.
Fig. 11. Simulating the brushless DC motor's behavior. One can set parameters, select switching mode (120- or 180-degree) and vary Hall sensor positions. Shown (from top to bottom) are the neutral point voltage, line-to-line voltage and input current. The Hall sensor position can be changed even after the RUN button has been clicked.
A. Positive response of foreign trainees
Positive responses from the trainees are now discussed. First, several saw that the remote lab could improve the training program conducted in Japan. The training program currently lasts for about six months, but a remote lab could shorten the period of stay in Japan since it would become possible to study some lab subjects from their home countries. The oversea training period can be shortened if remote learning can be employed for preparatory study or follow-up. This could reduce the costs of such a training program.
Or it could be used to follow up on their training program on a continuing basis to study the subject in more depth (upgrading, updating of skills).
Monographs on specialized topics and lab equipment are often expensive or not readily available in many developing countries, and students and researchers would benefit greatly if they could access WBC or remote labs with relative ease at a relatively low cost. While the costs of purchasing computers and installing networks can be high initially, once they are in place, as in many technical training facilities abroad, the use of instructional aids through networking can cut further costs if they can substitute for conventional books and equipment. Some trainees mentioned the benefits of being able to receive instruction directly from experts abroad.
The foreign trainees also displayed a keen interest in the remote laboratory's subject itself, the brushless DC motor. Many hoped that the remote lab could be made available so that they could study the subject back in their home countries. A major reason for this interest could be the unavailability of this kind of lab equipment on small motors in most developing countries. The high interest was also due perhaps to the widespread use of the brushless DC motor in computers and other information devices. This suggests that the subject of remote labs such as ours should be tailored to meet the needs of expected users. For instance, there is likely to be a low need for a remote lab using materials and equipment that are readily available anywhere, or on very simple subjects that can be demonstrated locally. It seems that remote labs can be of more service when they require expensive or relatively unavailable equipment, or deal with advanced or specialized topics.
B. Limitations of the remote lab
What the trainees viewed as the remote lab's disadvantages centered on its operational aspects. Below are some of their comments.
1) Since there is only a single lab setup, it is not possible for several people to access the lab independently at the same time.
2) The lab topic is limited to only one: the brushless DC motor that we prepared. The user cannot change the topic to another one (such as the stepping motor, which was developed in a previous study )
3) How would the client lab participant deal with any problems that occurred during the experiment? "If the experimenter had mechanical or electrical trouble with the brushless DC motor lab, he cannot fix it on-line." This is a very real shortcoming of the remote lab, the solution for which is to have an on-site lab attendant, who can take care of any problems should they arise.
With regard to 1) above, there are many possible modes of remote learning, as B. Collis points out , they can be broadly divided into self-study (asynchronous) and group study (synchronous) situations. If several users are sharing a single lab setup, they must either take turns or collaborate as a group.
Thus, remote learning can take place with or without an instructor. With an instructor present, he or she will determine or coordinate the general flow of how to proceed with the remote lab. For instance, the instructor may directly explain the lab procedures to the users, or he/she may have them read an electronic textbook or instruction manual on the lab subject. When users access the remote lab for self-study, however, they must be able to read and follow a lab manual on their monitors.
In the present study, the authors prepared a WBC that included the remote lab exercises and a simulation program. Used before the lab, the simulation provides the user with analytical practice on some theoretical aspects, and after the lab, it could be used to gain insights on the lab results obtained. Or the WBC and simulation can be used apart from the lab, on a PC unit not part of the network environment. In this study, the combination of WBC, simulation, and the remote lab was received favorably by the class of foreign trainees.
The lab exercises focus on the relationship between the rotor magnet's sensor positions and the switching signals supplied to the inverter to drive the motor. The aim here is to show that there exists a proper position in relation to the three-phase windings and the inverter's operating modes, so that proper motion control and energy-saving design can be realized. The users can operate the motor by remote operation and view the generated voltage waveforms on screen.
The prototype system was demonstrated to a class of foreign trainees. They showed a keen interest in this remote lab, demonstrating that remote learning systems can induce a more active learning process than when printed or screen explanations are read alone. We also found that a suitable simulation of the motor's dynamic behavior complements the remote lab well. From this experience, we believe that such remote lab systems can be a helpful component of international technical cooperation.
Department of Electrical Engineering and Power Electronics
Polytechnic University of Japan
4-1-1, Hashimotodai, Sagamihara, Kanagawa 229-1196 JAPAN
Department of Production, Information and Systems Engineering
Tokyo Metropolitan Institute of Technology
6-6, Asahigaoka, Hino, Tokyo 191-0065 JAPAN
Takashi Kenjo (M'97) was born in Japan on February 2, 1940. He received the Masters Degree in 1964 and the Doctor-of-Engineering Degree in 1971 from Tohoku University, Sendai, Japan. His area of interest is in small precision motors and their controls, and he has written several monographs published by Oxford University Press. He has been with the Polytechnic University of Japan since 1965 and is currently a Professor in the Department of Electrical Engineering and Power Electronics.
Shuichi Fukuda (M'88-SM'99) received his B.S., M.S. and Ph.D.
in Mechanical Engineering from the University of Tokyo in 1967, 1969 and
1972, respectively. He is currently Professor of Production, Information
and Systems Engineering at Tokyo Metropolitan Institute of Technology.
He worked as associate professor at the Welding Research Institute, Osaka
University from 1976-1991; and concurrently as associate professor at the
Institute of Industrial Science,University of Tokyo from 1989-1991.
He was chairman of the design and systems division, JSME from 1992-1993. He was visiting professor at Stanford University and Osaka University concurrently in 1998. He was chair ASME Japan chapter 1996-1998 and is chair of the IEEE Reliability Society 1999-2000.
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