IEEE TRANSACTIONS ON EDUCATION, VOL. 32. NO. 2, MAY 1989 153

Graphics Robot Simulator for Teaching Introductory Rob0 tics

TZVI RAZ

Abstract-The design and features of microcomputer software de- veloped for teaching robot motion concepts are presented. The expe- rience acquired in the first semester of use in an introductory robotics course is also reported.

INTRODUCTION OURSES in robotics and other modem manufactur- C ing technologies are being offered by increasing num-

bers of universities, colleges, and even high schools. These courses are perceived as valuable and important by both students and educators since they prepare the next generation of engineers and technologists to enter the highly competitive area of manufacturing. Robotics cur- ricula are offered by community colleges, while program options or course sequences with emphasis on robotics are found in most industrial, electrical, and mechanical en- gineering or technology departments, and even computer science departments. Among the main subjects that stu- dents are expected to master in their introductory robotics course are robot motion and robot programming.

Most robotics courses utilize laboratory assignments to assist in conveying motion control and robot program- ming concepts. In the light of the growing demand, the use of laboratories poses a number of problems. Labora- tory equipment is expensive to acquire and maintain, and requires support personnel (maintenance technicians, fac- ulty supervisors, teaching assistants). Equipment failures and breakdown, which are even more frequent when the operators are undergraduate students with minimal prior experience, can disrupt the sequence of the course and affect its effectivity. Educational robots used in teaching laboratories are quite limited in the range of functions they support and in the type of programming environment they offer. Industrial grade robots are much more expensive and require special installation to accommodate structural weight, power consumption, and operator safety. In ad- dition, the amount of training needed is quite extensive, requiring more time than what most college level courses can afford to devote.

An alternative to the use of actual robots consists of using software for simulating robot motion. Typically, such software provides a graphical model of the robot on

Manuscript received March I S , 1988. The author is with the Department of Industrial and Management En-

IEEE Log Number 8927490. gineering, University of Iowa, Iowa City, IA 52240.

the VDU, helping the student-users visualize the response of the robot to various motion commands. Graphical sim- ulation software is also useful to evaluate different robot configurations for suitability to certain types of tasks, and to support the development and verification of robot con- trol programs for specific applications.

Several software systems for graphical robot simulation are available at various levels of development and sophis- tication. Some of the major ones, are listed next, followed by recent references: PLACE (positioner layout and cell evaluation) from McDonnell-Douglas Automation Com- pany [ 3 ] , [ 6 ] ; ROBOT-SIM, from Calma Inc. [8], Pre- View, from Wavefront Technologies [7] and GRASP (graphical robot applications simulation package) from the University of Nottingham [l] . Other software is men- tioned in [2], [4], [ 5 ] . The great majority of these pro- grams offer numerous features but require at least the power of a minicomputer and several weeks of training to reach proficiency in their use. Clearly, such systems are not suitable for educational use by large numbers of oc- casional users, and a different approach is needed for ed- ucational environments. In this paper, we describe a graphic robot simulation program designed specifically for instructional use in college level courses in intoductory robotics, and report on the experience acquired during the first semester of use.

BACKGROUND

The Department of Industrial and Management Engi- neering of the University of Iowa consists of 7 1 / 2 FTE faculty, about 1 10 undergraduate students with an annual graduating class of 25-30, and about 60 graduate students in masters and doctoral programs. The introduction of modern manufacturing technologies and principles in the curriculum received a major boost with the establishment of the Computer Integrated Systems laboratory, docu- mented elsewhere [9], and the addition of a series of new courses. The course “Introduction to Industrial Robot- ics” was first offered in the Summer of 1985, and soon become a very popular course, with the result that enroll- ment had to be restricted due to constraints in the amounts of laboratory resources (equipment and supervision time) available.

The need for an effective program to complement hands- on laboratory experience become apparent by the second time the course was offered, when half of the students

0018-9359/89/0500-0153$01 .OO O 1989 IEEE

154 IEEE TRANSACTIONS ON EDUCATION, VOL. 32. NO. 2 , MAY 1989

wishing to enroll had to be turned away. The development of the software described here began in December 1986 with a grant from the Council on Computer-Aided Edu- cation of the University of Iowa. The development team included three programmer/analysts and their manager from the computing center, and the faculty responsible for the course, with an undergraduate assistant added later, all involved at various FTE levels. The version described here was completed by August 1987 and used during the Fall 1987 offering of the course.

DESIGN OBJECTIVES The purpose of the software was to help students grasp

and visualize robot motion concepts and to allow experi- mentation with different types of robots. The key concepts covered are: robot definition as a chain of links, joint and Cartesian spaces, homogeneous transformation matrices, forward and reverse kinematic tranformations, and pro- gramming robots by defining motion sequences.

The software was designed to support the following main functions: robot definition by specification of kine- matic parameters; robot motion in either joint or Cartesian space; graphical display of robot position accompanied by numerical displays of the position, orientation and dis- placement of each joint; and creation and editing of robot programs and execution in batch mode, including the cal- culation of motion duration.

The program had to be very easy to learn to use. The College of Engineering is committed to Macintosh com- puters, with over 160 machines available to the 2000 stu- dents in the college. Consequently, the program, named GRS for graphic robot simulator, was written in Pascal for the MacIntosh. The run time compiled version is con- tained in a 99 K file.

OVERVIEW OF THE SOFTWARE The software was designed to support the following

functions: 1) Robot definition, accomplished with a series of

statements, each corresponding to a single joint and con- taining information about the joint type (rotational or translational) and its various kinematic parameters: dis- placement angle (theta); twist angle (alpha); joint offset ( d ); link length ( a ) ; speed (in degrees or inches per sec- ond); and lowest and highest allowed displacements from the joint’s initial (home) position. The definition of the various quantities is compatible with the major textbooks on robotics. The robot definition statements may be saved on a file for future use.

2) Display of a graphical model of the robot in a win- dow that occupies most of the screen area.

3) Presentation of position, orientation, and displace- ment information in numerical form for up to six joints.

4) Simulation of robot motion by changing the graph- ical model and the numerical values as the robot executes motion commands. These commands specify the amount of motion in absolute terms (relative to the zero displace- ment position) or relative to the current position. Two

modes of motion specification are supported: motion in joint space, whereas joint displacements (either absolute or relative) are provided, or motion in Cartesian space, whereas the desired position of the wrist is provided as a triplet [x, y, z ] . The second mode, which does not specify the orientation of the wrist, is available only for those robots for which the reverse kinematic transformation is contained in the program. Two modes of interpolation (linear or joint) at three levels of detail (2, 4, and 8 inter- mediate points) are available to control the path followed by the wrist and the amount of detail shown for the mo- tion.

5 ) Creation and execution robot motion programs. These programs may include any valid motion command, interpolation commands (which change the mode and de- tail of the interpolation), or delay commands (which in- crement the timer by a constant provided as argument. Robot motion programs may be created from within the software or by any word processor. In addition, it is pos- sible to append to the active program file the last com- mand executed from the menu. This feature facilitates program development by allowing students to try different commands until the desired results are achieved, and only then add the command to the program.

6) On-line help, available from the main menu. Help files are organized by function and by use, providing in effect a built-in tutorial. In addition, the dialog boxes con- tain a description of the command they support, to facil- itate input and editing.

7) Other features supported include suspending and stopping the execution of the robot motion program, re- seting the timers (which are displayed below the position readout window), display of robot characteristics, editing of robot definition and motion control programs, and gen- eration of hard copy of the various windows.

EXTERNAL DESIGN The program was designed to take full advantage of the

user oriented MacIntosh interface. All the commands and their parameters may be entered through pull-down dialog boxes or by direct typing into a command window located below the main menu bar, or a combination of both modes. Robot definition files and robot motion programs can be created from within GRS or with any word pro- cessor and saved as “text only” files.

The screen is divded into four main areas, as shown in Fig. 1. As customary with MacIntosh programs, the top of the screen contains the main menu bar, which includes the following entries: files, edit, robot commands, simu- lation commands, windows, and help. The selections available from each one are showed in Fig. 2 and are de- scribed in some detail later.

Immediately below the main menu area is the command window, which begins with the keyword “command.” Here the student enters the commands in immediate exe- cution mode. Commands may also be selected from the menus, with the arguments provided through dialog boxes. In such a case, the result of the interaction with

RAZ: TEACHING INTRODUCTORY ROBOTICS

6 Files Edit Robot Cmds Simulation Cmds Windows Help 3 : 2 1 3 3 ’

+Z

155

Posi t ion Units Oeqrees & Inches

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Quit

A 0 0 0 0 0 0 6 0 0 0

X 1 2 9 6 1 2 9 6 1 2 9 6 Y 0 0 0 0 0 0 0 0 0

2 1 0 3 8 1 0 3 8 1 0 3 8

R 0 0 0 0 0 0 0 0 0

Stopprogram [ ~ I $ ~ ~ o t c m d s 51>1)1i11 I ipl Ionri j RppendLastCmd X HelpSirnulationcrnds

P 1 0 0 0 1000 7000

L + Y\ I 1045 I 5 4 5

Fig. 1. Screen display of GRS.

J

the menu and dialog box will appear in the command win- dow, allowing further editing prior to execution.

The larger portion of the screen (about 75 percent) is devoted to the display of the graphical model. The model represents the robot as a chain of straight links of decreas- ing width. The robot is shown in an isometric projection, with the joints shown as circles and the links represented as rigid beams with width decreasing as they are further away from the base. The projection is located at the center of a labeled set of Cartesian axes and is scaled to accom- modate the largest possible extension of the links.

The fourth area, on the right side of the screen, consists of position and time readouts. The top line displays the units: degrees for angular displacement, inches for linear displacement. Below, the absolute displacement from the “home” position, the Cartesian coordinates and the ori- entation in terms of roll, pitch, and yaw for up to six joints are displayed. The bottom part displays time elapsed in seconds: the simulated time of the last motion command executed on the right and the cumulative simulated time since the clock was last reset on the left. The simulated time clock is reset automatically when a motion program starts execution, and may be reset explicitly from one of the selections in the simulation commands menu.

The program is designed to be self explanatory to the

extent possible. When the application is launched, an ini- tial credit screen with the version number and the authors names will appear. Clicking on the conrinue button allows the user to proceed.

COMMANDS The following sections describe the functions available

from the seven pull-down menus in the main menu bar. The “apple” menu provides access to the installed desk

accessories. The “about robot * * * ” selection calls the initial credit screen.

The jiles menu differs somewhat from the MacIntosh file menu, even though it performs the same types of ac- tivities-opening, closing, and saving files. The choices available from this menu are as follows:

OpenProgram loads a robot program for execution or editing. The standard MacIntosh dialog appears to facil- itate the selection of the file from different drives and folders. It is also possible to enter the name of the file to load directly after the OpenProgram command on the command line.

SuveProgrum becomes available for selection when- ever changes are made to the currently active robot pro- gram file.

CloseProgrum puts away the currently active robot pro-

156 IEEE TRANSACTIONS ON EDUCATION. VOL. 32, NO. 2. MAY 1989

command Mom10

* * * * * * + + + + + + tyntaw:

MOM I< jo ln t l , l I . k J o l n t 2 ~ I IS ja ln t3 l ... ll p n t n ::- number of degrees. Inches. e tc .

14.00. 0.00. 8. ld

MweReI I ~ j a i n l l ~ l [ . l ~ j o i n t 2 ~ l l , l joint3l ... 11 Jolntn ::- number 01 degrees. Inches. etc.

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command: Interpolate

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Fig. 3 . Dialog boxes for robot motion commands

gram; if changes have been made, a reminder to save will be displayed.

LoudRobot loads a robot definition file; once its exe- cution is completed, the readout area on the right hand side of the screen is filled with initial values and the graphical model of the robot appears in the “home” po- sition.

SuveRobot becomes available for selection if the cur- rently active robot definition file has been edited.

CZoseRobot puts away the currently active robot defi- nition file. If any changes have been made to it, you will be asked if you wish to save it.

Quit exits GRS. The Edit menu provides standard editing functions (cut,

copy, past, clear, and undo). The ShowClipboard selec- tion is not implemented in the current version.

The RobotCmds menu contains the commands for ma- nipulating the robot and becomes available once a robot has been loaded. The commands may be typed on the command line or selected from the pull-down menus, in which case they are followed by dialogue boxes. Fig. 3 shows the various dialogue boxes reached from the robot commands menu. The commands available are as fol- lows:

Move allows direct movement of the robot by specify- ing the desired position in terms of absolute (i.e., from the “home” position determined from the robot definition file) displacements for each joint. The current absolute displacement for each joint is available in the Position Readout. The syntax for the Move command is

RAZ: TEACHING INTRODUCTORY ROBOTICS

~

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lfit 2 f i f 3 f i t 4 f i t 5 f i f 6mt

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0 0 0 0 000 000 0 0 0 000 0 0 0

I RobotProgram %2(

INTERPOLATE JOINT LOW

MOVE 2 29,-68 21.53 8 8 MOVE -26 57,2 24.1 5 01 MOVE 45,54 74,37 47

Fig. 4. Information available from the windows menu

where each ( j x ) is a real number in degrees for rotary joints or inches for linear joints. The default value for unspecified parameters is 0.

MoveRel specifies the desired position in terms of off- sets from the current position. The syntax and the default values for unspecified parameters are the same as in Move.

MoveTo specifies the desired position of the robot wrist in Cartesian coordinates relative to the origin of the ref- erence frame displayed in the screen. This command and the next one (MoveToRel) can be executed only for ro- bots for which the reverse kinematic transformation is available in GRS. The syntax of MoveTo is

MoveTo ( X ) , ( U ) , ( Z ) .

MoveToRel specifies the desired position of the robot wrist in Cartesian coordinates relative to the current po- sition of the robot. Its syntax and default values are the same as in MoveTo.

Delay simulates pauses that occur during robot motion sequences by incrementing the simulated time displays below the position readout window. The actual delay time is always 2 s. The syntax is Delay (number of simulated seconds ) .

Interpolate specifies the interpolation method (joint or linear) and the resolution for displaying the motion (two, four, or eight intermediate points corresponding to low, medium, and high resolution). Linear interpolation is available only for robots for which the reverse kinematic transformation is available in GRS. The syntax is as fol- lows:

Interpolate ( joint I linear ), ( low 1 medium I high ) .

The SimulationCmds menu contains the commands that control the creation and execution of robot motion pro- grams.

RunProgram executes the current robot program. It

triggers the execution of a ‘ ‘ ResetProgram” command which moves the robot to its home position and resets the readouts. RunProgram is for immediate execution only and is not allowed inside a robot program.

ResetProgram moves the robot to its home position and resets the time and position readouts. If issued from inside a robot program, it will also cause the program to stop.

CleurProgrum deletes all the commands in the active program window.

StopProgram stops the execution of the robot motion program; it has no effect when typed in the command win- dow.

AppendLmtCmd appends the last command entered on the command line to the end of the current active robot program. This command facilitates the creation of robot motion programs.

ChangeRobot displays the file that contains the defini- tion of the currently loaded robot for editing. Changes to the current robot definition will not be reflected until it is saved and then loaded with the “LoadRobot” command.

The Windows menu contains information about the ro- bot and robot program in use. The two possible windows, which are shown in Fig. 4, are reached from the following commands:

RoborChars opens a window containing information about the robot currently loaded. Information displayed includes the name of the file containing the robot defini- tion, the units used with the current robot (e .g . , degrees and inches), the original values of the parameters loaded from the robot definition file.

RobotProgram opens a window showing the commands contained in the active robot program for editing.

The Help menu provides access to text files containing instructions for each of the main menus and their selec- tions. Additional function specific help displays are also available on the following topics: robot definition, pro-

158 IEEE TRANSACTIONS ON EDUCATION. VOL. 32. NO. 2, MAY 1989

TABLE I DISTRIBUTION OF STUDENT RESPONSES TO T H E QUMTION

T O WHAT EXTENT DID THE P R O G R A M HELP YOU U N D F R S T A N D T H E F O L L O W ~ G CONCEPTS?

At Least At Least Very At Least Quite Somewhat Not

Concept Helpful* Helpful* Helpful* Helpful* Helpful

Robot configuration 4 48 88 100 0 Tranformation matrix 12 40 88 96 4 Kinematic transforms 24 64 76 92 8 Robot motion 8 32 68 96 4 Point definition 8 48 80 96 4 Robot programming 40 64 92 96 4

*Entries are percentages out of 25 respondents. The columns with the asterisk are cumulative percentages.

gram creation and editing, report generation (which, by the way, is done using the standard MacIntosh procedures for screen and window dumps), and miscellaneous tips and hints.

CLASSROOM EXPERIENCE The first released version of the software was used dur-

ing the Fall 1987 semester. Its availability allowed the previously existing enrollment limit (12 students) to be removed, with the result that 29 students completed the course, including several from computer science and me- chanical and electrical engineering. Three assignments that required then use of the simulator were given: for- ward kinematic transformation, reverse kinematic trans- formation, and optimization of robot design. In the first two assignments, the students had to compare the results of robot motion obtained with three approaches: using an educational robot in the laboratory, performing the cal- culations using the kinematic transformation formula they developed with pencil and paper, and creating and mov- ing a robot model with GRS. The third assignment re- quired the students to experiment with different robot con- figurations created in GRS in order to minimize the execution time of a given task. The assignments were fol- lowed by a short evaluation questionnaire, the main re- sults of which are summarized in Table I .

Another question required the students to rate how easy it was for them to learn to use the GRS. The breakdown of responses was as follows: very easy-8 percent; quite easy-44 percent; moderately easy-40 percent; quite dif- ficult-8 percent; and very difficult-0 percent. The re- sponses to the questionnaire indicate that GRS was per- ceived to be useful and was well .received by the students. The instructor experience indicated that, compared to stu- dents who took the course in previous semesters, those who used GRS had a better understanding of robot kine- matics and robot programming. In addition, they ap- peared to be more effective in their laboratory work, com-

identified for implementation in future development ef- forts:

1) Provide time versus displacement graphs for the var- ious joints under different interpolation modes.

2) Provide multiple views of the robot (at least three main orthogonal views in addition to the isometric pro- jection, which is the only one currently available).

3) Capability to handle the reverse kinematic transfor- mation for different robot configurations (currently only the basic articulated configuration is supported).

4) Improve the command language by adding the UndoMotion command, and the ability to define and refer to points in either Cartesian or joint space.

5 ) Display a trace of the path followed by the end ef- fector.

CONCLUDING REMARKS Graphical simulation of motion has its limitations,

mainly due to software and hardware constraints on the extent of features that can be represented, on the accuracy of numerical computations and on the resolution of the display unit, and also to the fact that only a two-dimen- sional projection can be shown. However, as the experi- ence reported in the paper suggests, it can be a useful and effective tool for helping students understand the main concepts. Used as a complement to actual laboratory ex- perience, graphical simulation can augment the students exposure to robotics while actually reducing the demand for equipment and other laboratory resources.

ACKNOWLEDGMENT The author wishes to thank S. W. Wessels, S. Bowers,

L. Finken, and the other members of the Software De- velopment Group of the University of Iowa, and Kerry Kellog of the Department of Industrial and Management Engineering of the University of Iowa for their contribu- tions to the development of the software presented in the paper.

REFERENCES pleting their assignments on the small scale educational robots in roughly 25 percent less time. [ l ] M . C. Bonney. J . Moser, and Y. F. Yong, “Evaluation and use of a

graphical robot simulator,” Internutionul Conference on Sirrmlutiorl FUTURE DEVELOPMENT in Munufucfurinn. 1985. DD. 57-61. * .

Several additional features that have the potential to en- hance the instructional capabilities Of the Software were

P I R . R . Boren, “Graphics simulation and programming for robotic

131 S . Derby. “Computer graphics simulation of robot arms.” f r o c . M I T workcell design,“ Robotic Age, vol. 7. pp. 30-33. 1985.

RAZ: TEACHING INTRODUCTORY ROBOTICS 159

Conf CAD/CAM Technology for Manufacruring Eng., Cambridge, Tzvi Raz received the B Sc degree in 1977 from MA, 1982, pp. 215-221 the Technion-Israel Institute of Technology. the

[4] S J . Derby, “Computer graphics robot simulation programs’ A corn- M A Sc degree in 1979 from the University of parison,” Proc. Winter Annual Meeting oJASME, 1982, pp 203- Toronto. Toronto, Ont , and the Ph D degree i n

21 1 1982 from the University of Missouri, Cohrnbid. [5] R. N Stauffer, “Robot system simulation,” Robotics Today, vol. 6, all in industrial engineering.

pp 81-90, 1984 He is currently as Associate Profe\sor of In- [6] P Howie, “Graphic simulation for off-line robot programming,” Ro- dustrial and Management Engineering at the Uni-

botics Today, vol. 6, pp 63-66, 1984 versity of Iowa, Iowa City. where he teaches [7] W Kovacs, “Previewing robotic motion with computer graphics,” courses in information systems design, micro-

Robotics Age, vol 7, pp. 16-19, 1985 computer applications, robotics, and other IE sub- [SI B Novak, “Robotic simulation facilitates assembly line design,” jects He has published papera in the Journal ofQualit\ Technolog\. Coin-

Simulation, vol. 43, pp 298-299, 1984. pulers and Industrial Engineering. the Jourtlal of Meditol SI ?teinr, the [9] T. Raz, “Integration of New Computer Technologies in an Industrial International Journal of Production Rerearch. IIE Transatrionr, dnd other

Engineenng Academic Department,’’ Computers and Industrial En- technical journals gineertng, vol 12, no 3, pp. 159-165. Dr Raz is a Senior Member of IIE and an Associate Editor of IIE Trcins-

[ IO] R. N. Stauffer, “Robot system simulation,” Robotics Today. vol 6, actions pp 81-90

160 IEEE TRANSACTIONS ON EDUCATION. VOL. 32. NO. 2. MAY 1989

Short Papers

The Induction Machine Operation Demonstrated by Arago’s Disc

JOHANNES G. NIESTEN A N D OWEN T. TAN

Absfract-D. F. J. Arago’s biography revealed him as the patriarch of the induction machine. An Arago disc has been developed in the spirit of this invention. The disc can be used to demonstrate the oper- ation principle of the induction machine as well as to illustrate readily the characteristic shape of the torque-slip curve of this machine.

INTRODUCTION A survey concerning motor usage by the U.S. Federal Energy

Administration showed that at least 88 percent of the power used for electrical drives in industry is consumed by induction motors [ I ] . The application range of these machines has even been con- siderably broadened by the advancements in power electronics in the last decade. At this time, therefore, it seems appropriate to pay tribute to the ultimate inventor of the induction machine as well as to introduce, related to this invention, a demonstration model which was developed at Eindhoven University of Technology.

DOMINIQUE FRANCOIS JEAN ARAGO (1786-1853) Arago’s fascinating biography by R. Hahn [2] shows that from

the very start young Arago set his sights on a military career in the artillery. Therefore, after finishing a classical preparatory educa- tion, he prepared for admission to the renowned Ecole Polytech- nique by mastering the works of Euler, Lagrange, and Laplace. In 1803 he passed the entrance examination with grea,’ distinction.

It is true that eventually Arago’s study at the Ecole Polytech- nique did not lead him to his original goal; however, an extremely important invention later came his way. After two years as the front- runner of his classes, Arago was named secretary of the Bureau des Longitudes in 1806 and was sent to Spain with Jean-Baptiste Biot on a geodetic expedition. He returned to France in 1809, and was elected astronomer to the Institut de France. In that year. he was also appointed at the Ecole Polytechnique as successor of the world- famous Gaspard Monge, professor of descriptive geometry. Until his resignation in 1830, the universal scholar taught a variety of subjects besides descriptive geometry. Arago also taught astron- omy to the general public at the Paris Observatory from 1813 to 1846. His main contributions in astronomy were the exact deter- mination of planet diameters and the discovery of the sun chro- mosphere [3].

Arago’s scientific life was dominated by a persistent interest in physical phenomena related to optics, electricity, and magnetism. In 1820, Arago interrupted his optical work to play a significant role in the advancement of electrodynamic theory of electricity and magnetism as proposed by AndrC-Marie Ampi-re.

In 1822 when Arago. together with one of his closest friends, Alexander von Humboldt. investigated the earth magnetic field on a hill at Greenwich. he noticed incidentally that the presence of

Manuscript received July 22, 1988: revised November 1 . 1988. J . G. Niesten is with the Department of Electrical Engineering. Eind-

0. T. Tan is with the Department of Electrical and Computer Engi-

IEEE Log Number 8927099.

hoven University o f Technology. The Netherlands.

neering. Louisiana State University. Baton Rouge. LA 70803.

metallic substances had a dampening effect on the oscillations of a compass needle. Initially, Arago did not perceive the importance of this original observation. After the Greenwich expedition was terminated, he first worked on other problems related to the speed of sound, the crystalline nature of ice, and the chemical and ther- mal effects of light. Only after two years had gone by, Arago pub- licly announced in 1824, that rotating nonmagnetic metallic sub- stances, especially copper, have a “magnetic effect’’ on a magnetized needle. This magnetic effect was demonstrated by a magnetic needle suspended freely over a copper disc. Rotation of the disc around its axis made the magnetic needle to follow the rotation. This invention became known as the Arago’s “disc” or “wheel.” Explanations concerning Arago’s disc were widely at variance.

Biot [4] considered the magnetic effect as a verification of Charles Augustin Coulomb’s ideas on the magnetic fluids and sug- gested that the magnetic fluids were separated by centrifugal force when the disc was rotated. Ampere restricted his discussion to the fact that the magnetic needle could be substituted by a current- carrying helix, thus only reinforcing his electrodynamic theory.

Based on Ampere’s physical model, John Herschel and Charles Babbage (21, [4], 1.51 went further by suggesting that the phenom- enon was based on magnetic attraction between the magnetic poles of disc and needle. The magnetic property displayed by the copper disc was induced on their molecules by the action of the magnetic needle causing the molecules to align themselves in a direction de- termined by the magnetic needle. The rotation effect was explained by the time lag introduced by the time needed for the electrody- namic molecules to align themselves in the proper direction.

It was left unexplained, however, why the induced magnetism occurred only when the disc was in motion relative to the magnet whereas, according to Ampere’s model, motion should be irrele- vant. The electric currents surrounding the molecules of the copper disc should be affected by the magnet without the medium of rel- ative motion.

Not satisfied with Herschel’s and Babbage’s explanation, Mi- chael Faraday [4] conducted his own experiments until later in 1831, after his invention of the dynamo, he found the explanation of Arago’s disc. When the metallic disc was revolved beneath the freely suspended magnetic needle, the magnetic field lines of the needle were cut by the disc, thereby inducing in the disc “eddy currents” which in turn created a magnetic field whose interaction with the needle magnetic field caused the needle to rotate with the disc. All the observed phenomena could now be explained by his electromagnetic induction law which Faraday made public univer- sally in 183 I .

Especially noteworthy is the fact that Arago himself did not have an airtight explanation of his discovery. Because of his loyalty to Ampere, Arago was never fully able to appreciate or accept the rival theory of Faraday.

In 1825 Arago was distinguished with the Royal Society‘s Copley Medal for the discovery of the magnetic effect, and in 1830 the Academie des Sciences elected him permanent Secretary to suc- ceed Fourier. So. in 1839 it was Arago who. also because of his personal interest. prepared a report on the photography invention of Daguerre and Niepce (the daguerreotype) for the Academie.

Arago also repeatedly played an important role in the political arena, especially during 1830-1848 [2], 131. In his last years. impeded by gradual loss of eyesight resulting in blindness. he was unable to conduct new experiments himself. However, he was sur- rounded by dedicated younger scientists who worked for him to

001 8-9359/89/0500-0 160$0 1 .OO 0 1989 IEEE