a virtual electromagnetic laboratory for the classroom and the www

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IEEE TRANSACTIONS ON MAGNETICS, VOL. 33, NO. 2, MARCH 1997 1990 A Virtual Electromagnetic Laboratory for the Classroom and the W K.Preis, I. Bkrdi, 0. Bir6, R. Hoschek and M. Mayr IGTE, Technical University Graz, Kopemikusgasse 24, A-8010 Graz, Austria I. Ticar University of Maribor, Smetanova 17, SLO-2000 Maribor, Slovenia Abstract-A virtual laboratory for the classroom as well as for distance education on World Wide Web is presented making use of the various FEM software packages for the numerical calculation of electromagneticfield problems developed at IGTE. For creating interactive educational examples, the HTML- format specification was chosen. The interaction between the HTML document and the FEM software is performed by the widespread Tool Command Language (TCL). I. GENERAL DESCRIPTION The educational software can be used interactively. This means for instance, that certain parameters of interest like impressed currents in coils or material properties may be changed within a given range followed by an automatic recalculation of the problem and presentation of the desired results [ 11. It is also possible to change some essential geometrical data interactively. As an example, the width of the air gap, the position of the windings of an iron coil or the position of a rectangular dielectric obstacle within a planar capacitor (see Fig. 1) may be changed by a simple mouse movement or per dialog box without the necessity of remeshing. The finite element mesh, usually invisible for the user is drawn in these figures to show the distortion of the mesh due to the movement. For this purpose, the finite element preprocessor has been extented to allow such changes easily without being confronted with any finite element environment. The parameters to be changed together with the appropriate variation of these parameters can be set by the teacher when creating the educational example in oder to avoid an inappropriate action by the users. Animated electromagnetic fields can be shown by displaying precalculated bit maps [2]. For this purpose, an animation player has been written which has similar control functions as a video player. Furthermore, the generation of such animation sequences should be possible in an easy way. This can be performed with a special preprocessor, where for instance the start and end position of an air gap together with the number of different positions in between can be defined by mouse or in a dialog box and the whole sequence is then generated automatically without having anything to do with a FEM package. Manuscript received March 18, 1996 This work was supported in part by Fonds zur Forderung der Wissenschaftlichen Forschung, Vienna, Austna, under Grant No P9271-TEC . . . . . . . . . . . . . . . . . . . . . . . . . . . . ........ I . , ..., ................. I , , , . .. i .... :; I . ............................................... ............ I ................... ~.~ :---,.. ., ............................. . . . . . f;;;;.' : I- .. I . ; L. . :.. .. L ,;.---.+ .................. ! S : 0 ' ...... .................. Fig. 1. Movement of a rectangular &electric obstacle inside a capacitor by mouse or in a dialog box During the session, the students should not be confronted with any finite element procedure as far as only education in electromagnetic theory is intented. In the course of education in numerical field analysis, a direct access to the pre- and postprocessing facilities is possible at different levels from a beginner level up to full access for the experienced stude Again, the level of access can be set by the teacher. For creating interactive educational examples, the HTML- 0018-9464197$10,00 0 1997 IEEE

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Page 1: A virtual electromagnetic laboratory for the classroom and the WWW

IEEE TRANSACTIONS ON MAGNETICS, VOL. 33, NO. 2, MARCH 1997 1990

A Virtual Electromagnetic Laboratory for the Classroom and the W

K.Preis, I. Bkrdi, 0. Bir6, R. Hoschek and M. Mayr IGTE, Technical University Graz, Kopemikusgasse 24, A-8010 Graz, Austria

I. Ticar University of Maribor, Smetanova 17, SLO-2000 Maribor, Slovenia

Abstract-A virtual laboratory for the classroom as well as for distance education on World Wide Web is presented making use of the various FEM software packages for the numerical calculation of electromagnetic field problems developed at IGTE. For creating interactive educational examples, the HTML- format specification was chosen. The interaction between the HTML document and the FEM software is performed by the widespread Tool Command Language (TCL).

I. GENERAL DESCRIPTION

The educational software can be used interactively. This means for instance, that certain parameters of interest like impressed currents in coils or material properties may be changed within a given range followed by an automatic recalculation of the problem and presentation of the desired results [ 11.

It is also possible to change some essential geometrical data interactively. As an example, the width of the air gap, the position of the windings of an iron coil or the position of a rectangular dielectric obstacle within a planar capacitor (see Fig. 1) may be changed by a simple mouse movement or per dialog box without the necessity of remeshing. The finite element mesh, usually invisible for the user is drawn in these figures to show the distortion of the mesh due to the movement.

For this purpose, the finite element preprocessor has been extented to allow such changes easily without being confronted with any finite element environment. The parameters to be changed together with the appropriate variation of these parameters can be set by the teacher when creating the educational example in oder to avoid an inappropriate action by the users.

Animated electromagnetic fields can be shown by displaying precalculated bit maps [2]. For this purpose, an animation player has been written which has similar control functions as a video player. Furthermore, the generation of such animation sequences should be possible in an easy way. This can be performed with a special preprocessor, where for instance the start and end position of an air gap together with the number of different positions in between can be defined by mouse or in a dialog box and the whole sequence is then generated automatically without having anything to do with a FEM package.

Manuscript received March 18, 1996 This work was supported in part by Fonds zur Forderung der Wissenschaftlichen Forschung, Vienna, Austna, under Grant No P9271-TEC

............................ ........ I . , ..., ................. I , , , . . . i....:;

I . ............................................... . . . . . . . . . . . . I ................... ~.~ :---,.. ., .............................

. . . . . f;;;;.' : I- . . I ..... ; L. .... :.. . . L ,;.---.+ .................. ! S : 0 ' ...... ..................

Fig. 1. Movement of a rectangular &electric obstacle inside a capacitor by mouse or in a dialog box

During the session, the students should not be confronted with any finite element procedure as far as only education in electromagnetic theory is intented. In the course of education in numerical field analysis, a direct access to the pre- and postprocessing facilities is possible at different levels from a beginner level up to full access for the experienced stude Again, the level of access can be set by the teacher.

For creating interactive educational examples, the HTML-

0018-9464197$10,00 0 1997 IEEE

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be used for viewing the educational examples. Furthermore, this standard is widespread and well known.

The mechanism of interacting between the HTML document and the FEM software is performed by the widespread TCL (Tool Command Language) [3] which has been enlarged by several special functions like DDE (Dynamic Data Exchange) under MS-Windows [4].

11. EXAMPLES

Several examples havc already been worked out for distant education in electromagnetics on the web and interactive training in the classroom. Some of them are described here shortly. For all discussed examples, special data like geometrical dimensions, number of turns, material properties etc. are not given here, because they can be easily accessed from the web.

The first example, an electrostatic lecture is concerned with the prediction of the potential of a metallic obstacle within a planar capacitor. Assuming a certain potential of the obstacle, its total charge will be non-zero, in general. Another potential gives again a non-zero charge. By applying the superposition principle, the correct potential resulting in zero charge of the body has to be calculated using the results of the first two experiments. In a final calculation, the correct potential has to be verified. In another step, the metallic body can be replaced by a dielectric one with a very high permittivity resulting in a potential almost identical to that of the metallic one.

In the last part of this lecture, a metallic cylinder can be moved by the mouse within a planar capacitor (Fig. 2). The contours of all electrodes are appropriately colored in order to display the surface charge density. Furthermore, the variation of potentials and total charges on the electrodes is shown for the respective cylinder positions. This virtual experiment can be performed with the capacitor either connected to or disconnected from the battery.

In a second example, a nonlinear magnetostatic problem, a choke coil with air gap shown in Fig. 3 is investigated (Fig. 3). Several parameters like the width of the air gap and the thickness of the yoke as well as the excitation and the number of turns can be chosen within a certain range. Since the BM- curve is exactly linear up to 1.1 T (Fig. 4), the inductances obtained from the magnetic energy and the flux linkage are the same. For higher flux densities this is no longer true, as can be shown easily by the virtual experiments which have to be performed in this lecture. As a special computer animation attached to this lecture, the width of the air-gap for a certain arrangement can be continously changed by mouse, whereby the flux distribution in the whole region together with the B field in the symmetrie plane along the r-axis is shown.

In addition, the transient nonlinear behaviour of the coil can be studied for different external circuit elements R and C and voltages like voltage steps, sinusoidal voltages, etc.

Fig. 2. Two different positions during the movement of a metallic cylinder within a capacitor by mouse .

Fig 3. Nonlinear choke coil with air gap

In Fig. 5, the typical transient behaviour of an R-L circuit connected to a voltage step can be seen. After chosing all neccessary parameters, the differential equation for the current is solved including nonlinear finite element calculations during each time step. As can be seen in Fig. 5, the phenomenon is linear at the beginning and is identical with the analytical solution. With increasing saturation the inductance decreases resulting in a lower time constant.

In Fig. 6, the same nonlinear R-L circuit was excited sinusoidally, whereby the effect of hysteresis is neglected

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Fig. 4. B/H-curve with linear behavior up to 1.1 T. Fig 7 Transient behavior of a RLC circuit with nonlinear L switched by a voltage step

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Fig. 5. Comparison of the transient behavior of an R-L circuit switched to a voltage step for a linear and a nonlinear L, respectively.

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Fig. 6. Sinusoidal excitation of a nonlinear R-L circuit

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Fig. 8. Single mode propagation with PML on the right hand side

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Fig 9. Dual mode propagabon with PML on the right hand side

The transient behaviour of a nonlinear R-L-C circuit excited by a voltage step is shown in Fig. 7. Here it can be noticed that the eigen-frequency decreases with time since the saturation decreases too. Again, hysteresis of the iron core has been neglected. Time harmonic eddy currents in bus bar systems with circular and rectangular cross sections can be investigated in another example. Different parameters like distance, cross sebtion and frequency can be chosen fi-eely.

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Results are displayed and computer animations of 3 phase bus bar systems are shown.

In a microwave example, the behaviour of guided waves can be studied in the form of computer animations in several rectangular waveguide configurations. In Fig. 8 the propagation of the Hlo - mode in a rectangular waveguide is shown for one time instant. The waveguide is matched on the right hand side by a perfectly matched layer (PML). The PML results in a strong decay of the fields with almost zero reflections. In another example, an additional propagating Hzo - mode is excited at the H-plane step. In this case, both modes are matched and absorbed by PMLs at the right hand side (Fig. 9).

111. WWW- HARDWARE

Putting the IGTE services on the WWW was not possible with a standard WWW-server. While presenting standard information via internet can be done by every low-end workstation or PC, the goal was to provide interactive capabilities which allow users from the net access to the IGTE-software.

So, a configuration of two PC's and a workstation was chosen to provide the WWW-services. The address www- igte.tu-graz.ac.at is the standard WWW-server which provides information about the institute and the research fields.

Another PC acts as an application server which is controlled using the HTTP-protocol. It is not dedicated as a WWW-server, so there are no pages available.

The workstation is an application server for time consuming applications (e.g. 3-dimensional examples). Since communication between the WWW-server and the workstation is cumbersome, the workstation is not used to reduce the process-time of simple (often 2-dimensional) examples.

a time. So in March 1996, we switched to a personal computer running OS/2 Warp Connect. The WWW-server program is GoServe 2.47, written by Mike Cowlinshaw from IBM. This program package was enhanced by the OS/2 Forum Austria to simplify installation and maintainance. The performance of the server is better now, up to 20 clients may connect at the same time and we can still increase the number.

As we use a systeim of three computers with different operating systems and different tasks some words about the interface between these computers is necessary.

Normally a WWW-client (a WWW-browser) requests some static informatioin from the server. Using forms which are defined in the HTML-standard allows the client to interact with the server. Within a form the client (the user) can put some data in entry fields or click on a button. This information is transmitted to the server. A special subroutine takes the data and starts a specific task depending on the user's input.

In our case, a special program is started on the application server. This program performs some necessary administration tasks before starting the TCL-interpreter. The interpreter uses a script file defined for the chosen example.

The programs for numerical field-calculations and visualisation of results are started and controled via DDE. Finally, the interpreted script creates an output file in the HTML-format containing the calculated results and images.

The interpreter finishes by sending back the output file to the calling client.

Starting programs on the workstation is done by a simple REXEC command submitting some additional parameters. Only the numerical field calculation is done on the station, visualisation has to be done using the application server. This way is necessary since the visualisation software is not available on the workstation

CONCLUSIONS IV. WWW- SOFTWARE

The choice of software had to take into account the existence of IGTE-software which should be part of the services provided via WWW.

With respect to the application software the application server has to run under an operating system supported by the IGTE-software. Since the visualisation software has been developed under Windows the use of Windows NT seems to be a good choice. The numerical field computation is done by programs written in FORTRAN which have been transfered to Windows NT without much effort. For time consuming computations, a workstation running under DEC OSFA is provided.

The WWW-server ran under Windows 3.1 for almost a year using WinHTTPD 1.4. The system crashed down regularly and was not able to support more than one client at

An environment has been developed enabling access to the IGTE-software via W W . Several educational examples on electromagnetics are already available attracting considerable attention on the web.

REFERENCES

[1] M. Mayr, I. Bardi, R. Hoschek and K. Preis, "Neue grafische Konzepte fur FEM Pre- und Postprocessing," Proc. 40. Internationales wissenschaftliches Kolloquium Ilmenau, Bd. 2, pp. 204-209 K. Preis, I. Bardi, 0. Biro, R. Hoschek, M. Mayr, U. Peterhi, I. Ticar and K.R. Richter, "Computer Animation of Electromagnetic Phenomena," IEEE Truns. on Mugn., vol. 31, No. 3, pp. 1714-1717, 1995.

[3] J. K. Ousterhout, Tcl and the Tk Toolkit, Addison-Wesley, 1994 [4] K. Preis, "Application of the IGTE-FEM-Package for Educational

Purposes in Electrodynamics," Panel Session 2, COMPUMAG Berlin, Germany, July 10-13, 1995.

[2]