Magnetic Bearing : An Integrated Platform for Teaching and Learning

S.C.Mukhopadhyay, C. Gooneratne and G. Sen Gupta

Institute of Information Sciences and Technology Massey University (Turitea), Palmerston North

New Zealand email : S.C.Mukhopadhyay@massey.ac.nz

Abstract The paper has described the different components of a hybrid magnetic bearing model which is used as an integrated platform for teaching and learning. The main purpose is to encourage the students to utilise their knowledge learnt through the lecture to solve a complex engineering problem. The magnetic bearing as an integrated platform provides an environment for innovation, verification of their acquired knowledge, application of their design skills and a common link among many subjects in their curriculum. Keywords: magnetic bearing, integrated platform, engineering curriculum, teaching, learning.

1 Introduction Sometimes it becomes very simple and handy to teach a complex and abstract subject such as Electromagnetics with the help of a practical model used as an integrated platform. In this project the magnetic bearing model has been used as an integrated platform to teach a few subjects, the authors are involved with. The selection of a proper platform can be a topic of debate and that is the reason the area of research is coming into picture. For the selection of a platform, the first question is to be asked, is, “What am I trying to teach?” The next question is, “Can I assist the learning process with a platform for learning?’ With the long teaching experience the authors believe that the magnetic bearing can be properly used as an integrated platform for the subjects such as Electromagnetics, Sensing technology, electronics and power electronics, control systems, motor dynamics etc. to enhance the learning process of the students. Many authors have augmented the conventional textbook presentations by introducing laboratory works in the form of problem based learning to teach the students [ 1 – 4]. In problem based learning a specific problem situation is used to focus the learning activities to achieve the target. Applications have been designed to assist students to teach modern embedded computing subject with the help of computer vision [5]. Computer based simulations and implementations have been developed to illustrate the important practical applications of PID control [6] and measurement procedure for viscous and coulomb friction [7] to show how the theory can be used to solve practical problems. In addition

some authors [8] have used fuzzy logic algorithm to teach speed control of DC motor with some success. 2 Operating principle of magnetic bearing The magnetic bearing works just like normal mechanical bearing except the bearing functionality is achieved utilizing magnetic field. In magnetic bearing system the rotor is levitated in the magnetic field. The magnetic bearing system can be either of two types: (i) Active magnetic bearing in which all are electromagnets and (ii) Passive magnetic bearing in which a combination of electromagnets and permanent magnets (PM) are used. In repulsive type magnetic bearing system usually the rotor is levitated by the repulsive forces between stator and rotor permanent magnets. The system is unstable in nature. The controlled electromagnet is used to keep the rotor in the desired position. The repulsive magnetic bearing system has the advantages of using a smaller number of electromagnets and simplified control scheme compared to active magnetic bearing system. The advantages of using magnetic bearing system compared to mechanical bearing in high speed motors are long life, frictionless and lubrication free operation, feasible operation at high speed etc. Many research papers have been published on magnetic bearings using permanent magnets [9, 10]. But the satisfactory performance of this type magnetic bearing is strongly dependent on the characteristics of the permanent magnet and its configuration in the bearing system. The configuration of permanent magnets depends on many factors such as type of machine in which it is used, the amount of weight to be levitated, the material characteristics of the magnet, and its availability etc [11, 12, 13].

2nd International Conference on Autonomous Robots and AgentsDecember 13-15, 2004 Palmerston North, New Zealand

283

3 Magnetic bearing as an integrated

platform for teaching and learning Fig. 1 shows the picture of the magnetic bearing setup used as an integrated platform for teaching and learning. The system was designed and fabricated from the scratch and is still under development. This is a part of the students project and they develop a few things and use this as a model to experimentally verify their knowledge. In the electromagnetics course they learn that like poles of permanent magnets repel each other and in this model they can experience that the repulsive force acting between two sets of permanent magnets are used to levitate the rotor of the motor.

Fig. 1 The magnetic bearing set-up used for teaching and learning

The repulsive type magnetic bearing system shown in Fig. 1 is for vertical shaft machine and has been fabricated using many small circular permanent magnets which are arranged along the periphery of a circular disc. Two such discs as shown in Fig. 2, are used one of which is fixed to the stator and the other one is fixed to the rotor shaft. They form a magnetic bearing. Two such bearings are used to levitate the rotor. The thickness and number of magnets and their arrangements is a matter of interest from stability consideration. So the selection of magnet thickness and the gap between the disks to be selected based on some criterion. The outer diameter of the bearing disk is kept same to that of the outer diameter of the motor. Initially the distance of the center of the magnets from the disk center is 50 mm is chosen for the stator disk and that of rotor disk is 47.5 mm as shown in Fig. 2. In this scheme 24 magnets are used in both the rotor and stator disk. The specification of the magnets are shown in Table 1.

Fig. 2 Two disks with 24 magnets in both form

a magnetic bearing Table 1 Specification of magnet Make The Magnet Source, USA Part number Neodymium 27H Size Dia = 0.375” and

Thickness = 0.1” Residual flux density 1.08 T Coercive force 0.779 X 106 A/m The repulsive force between two disks is used to levitate the rotor. The system is unstable along the vertical axis. The electromagnet is used for positioning the rotor. The control system is required to maintain the rotor position. The design of control system is based both on analog electronics design and microcontroller design has been studied here. The sensor is used to measure the position of the rotor. The mechanical design and fabrication is very important in the magnetic bearing. So the magnetic bearing can be effectively used as an integrated platform for teaching as well as problem based learning. 4 Magnetic levitation The repulsive force between the two disks containing permanent magnets is used for levitating the rotor. A novel arrangement of permanent magnets has been used in this configuration. The repulsive force has been measured by using the JJ Instruments, England and is shown in Figs. 3. The instrument can measure either the tensile force or the repulsive force. The fixing of the disks are shown in Fig. 4. As long as the system measures the repulsive force the data is recorded and the operation is stopped as soon it goes to the tensile mode.

Electromagnet Permanent Magnet

Upper bearing

Lower bearing

Aluninum disk Permanent magnet

Gap- sensor

2nd International Conference on Autonomous Robots and AgentsDecember 13-15, 2004 Palmerston North, New Zealand

284

Fig. 3 Experimental setup for force measurement

Fig. 4 The fixing of the disks The measured repulsive force between the disks as a function of separation between them is shown in Fig. 5 for a magnet length of 0.1” (2.54 mm). It is seen that the repulsive force changes its nature and becomes attractive when the separation between them reduces to 1.8 mm. In order to utilise the repulsive force to levitate the rotor and considering the stability consideration the separation should not be less than 2.5 mm. Fig. 5 Measured repulsive force as a function of disk

separation The result shown in Fig. 5 is quite interesting to investigate and is verified analytically. The schematic arrangement of the permanent magnets in two-dimension and the forces are shown in Fig. 6. The analytical expression of force between two magnet of pole intensity Qm1 and Qm2 is given by (1).

0 1 2

122124

m mQ QF ur

µπ

= (1)

Fig. 6 Schematic representation of bearing to show the different forces

There are four forces acting between two magnets. FN1S2 and FN2S1 are the attractive forces and FN1N2 and FS1S2 are the repulsive forces. The resultant force is given by the vector sum of all the forces. In the disk arrangement, each magnet of stator disk interact with all the 24 magnets of rotor disk and vice-versa. The force has been calculated for different length of magnets. The variation of the calculated force as a function of disk separation is shown in Fig. 7. It is seen that the theoretical calculation matches very close to the measured one. The repulsive force is used to levitate the rotor whereas the radial force is important from the stability consideration. The arrangement is so chosen that the rotor automatically comes back to original central position in case of any displacement from the central position due to any disturbance. Fig. 7 Calculated repulsive force as a function of disk

separation

6 Field modeling The use of high electrical conductivity aluminum as base material has been thoroughly checked as it may produce appreciable amount of eddy current loss during high speed running condition. The flux distribution due to the worst situation of the

Iron plate

Aluminium plate

Magnet

FRotor disk

Stator disk

N1

S1

S2N2

F

F

axial

radial

N1

S1

S2

N2

Upper disk

Lower disk

Force sensor

Upper disk

Lower disk

0.002 0.004 0.006 0.008 0.0100

2

4

6

8

10

12

Separation of disks (m)

Rep

ulsi

ve F

orce

(N)

0 0.002 0.004 0.006 0.008 0.01

-4

-2

0

2

4

6

8

10

12

Separation of disks (m)

Rep

ulsive

For

ce (N

)

Length of Magnet (mm)

8 6 4 2

2nd International Conference on Autonomous Robots and AgentsDecember 13-15, 2004 Palmerston North, New Zealand

285

permanent magnet has been analyzed using finite element method. Figs. 8 and 9 show the distribution of magnetic vector potential and the flux density of the closest orientation of the permanent magnets. It is seen that the magnitude of the flux density is too less to produce any loss in the aluminum disk used for holding the permanent magnets. The students get the opportunity to work on finite element software package to calculate the field parameters. This comes as a part of their course of electromagnetics.

Fig. 8 The magnetic vector potential

Fig. 9 The flux density distribution

7 Control of rotor position The system is unstable along the vertical direction. A controlled electromagnet has been used for controlling the rotor position along this axis. This type of magnetic bearing system is stable along the radial axis but is unstable along the vertical direction. In order to maintain the desired position a gap sensor is used as shown in Fig. 1 to measure the actual gap. The gap signal is used in the control circuit to send the necessary control current to the electromagnet. 7.1 Analog PID controller After a fair amount of trial and error an analog PID controller has been implemented for the stable positioning of the rotor as shown in Fig. 10. Agilent make 6554A (60 V, 9A) DC power supply with the analog programming of output voltage and current

facility has been used to supply the current to the electromagnet. In the absence of the current in the electromagnet the rotor rests on the stopper. While the current is passed through the electromagnet the rotor is lifted and maintained at the desired position as shown in Fig. 11. The peak to peak disturbance is less than 20 micrometer as shown in Fig. 12. The bottom waveform shows the signal from the gap sensor while the top waveform corresponds to the signal to the input of the power supply.

Fig. 10 Analog electronics based PID controller

Fig. 11 The starting and positioning of the rotor

7.2 Microcontroller based control The controller has also been implemented using a mixed signal microcontroller (µC) Cygnal C8051F020, the complete experimental set-up is shown in Fig. 13. The potentiometer on the expansion board is used for setting the reference gap. The experimental result of starting transient for a PD controller is shown in Fig. 14. The corresponding waveform for a PID control is shown in Fig. 15. The steady state waveform has improved in this case. The response of the bearing system to a disturbance is shown in Fig. 16. It is seen that the controller acts on the disturbance and damps out the oscillations and bring back to the normal steady state condition.

2nd International Conference on Autonomous Robots and AgentsDecember 13-15, 2004 Palmerston North, New Zealand

286

Fig. 12 Peak to peak vibration of the rotor

Fig. 13 Microcontroller (µC) based controller

Fig. 14 Transient result for µC based PD control

Fig. 15 Transient result for µC based PID control

Fig. 16 Response to a disturbance

8 Conclusion This paper has described different aspects of a repulsive type magnetic bearing system which is used as an integrated platform for teaching and learning. A few subjects in our curriculum such as electromagnetics, sensing technology, electronics, control system, microcontroller etc. can be effectively taught with this model. The students are allowed to do experiment of their own to get a feeling and verify their learning in the theoretical part of the subjects.

9 References 1. M. Hedley and S. Barrie, “An Undergraduate

Microcontroller Systems Laboratory”, IEEE Transactions on Education, Vol. 41, No. 4, Nov. 1998, pp 345.

2. 2. D.L.Maskell and P.J.Grabau, “A Multidisciplinary Cooperative Problem-Based Learning Approach to Embedded Systems Design”, IEEE Transactions on Education, Vol. 41, No. 2, May. 1998, pp 101 -103.

3. J.W.Bruuce, J.C.Harden and R.B.Reese, “Cooperative and Progressive Design Experience for Embeded Systems”, IEEE Transactions on Education, Vol. 47, No. 1, Feb. 2004, pp 83 - 91.

4. M.Mazo, J.Urena, et. al., “Teaching Equipment for Training in the Control of DC, Brushless, and Stepper Servomotors”, IEEE Transactions on Education, Vol. 41, No. 2, May. 1998, pp 146 - 158.

5. A. Hoover, “Computer Vision in Undergraduate Education: Modern Embedded Computing”, IEEE Transactions on Education, Vol. 46, No. 2, May. 2003, pp 235 - 240.

6. R.Kelly and J.Moreno, “Learning PID Structures in an Introductory Course of Automatic Control”, IEEE Transactions on Education, Vol. 44, No. 4, Nov. 2001, pp 373 - 376.

2nd International Conference on Autonomous Robots and AgentsDecember 13-15, 2004 Palmerston North, New Zealand

287

7. R.Kelly, J.Llamas and R.Campa, “A Measurement Procedure for Viscous and Coulomb Friction”, IEEE Transactions on Instrumentation and Measurement, Vol. 49, No. 4, Aug. 2000, pp. 857 – 861.

8. S.Saneifard, N.R.Prasad, H.A.Smolleck and J.J.Wakileh, “Fuzzy-Logic-Based Speed Control of a shunt DC Motor”, IEEE Transactions on Education, Vol. 41, No. 2, May 1998, pp 159 - 164.

9. J.P.Yonnet, “Passive magnetic bearings with permanent magnets”, IEEE Trans. on Magn., vol. 14, no. 5, pp. 803-805, Sep. 1978.

10. J.Delamare, E.Rulliere and J.P.onnet, “Classification and synthesis of permanent magnet bearing configuration”, IEEE Trans. on Magn., vol. 31, no. 6, pp. 4190-4192, Nov. 1995.

11. S.C.Mukhopadhyay, T.Ohji, M.Iwahara and S.Yamada, "Design, analysis and control of a

new repulsive type magnetic bearing", IEE proc. on Elect. Pwr. Appl. , vol. 146, no. 1, pp. 33-40, Jan. 1999.

12. S.C.Mukhopadhyay, T.Ohji, M.Iwahara and S.Yamada, "Modeling and control of a new horizontal shaft hybrid type magnetic bearing", IEEE Trans. on Ind.. Electronics Vol. 47, No. 1, pp. 100-108, Feb. 2000.

13. S.C.Mukhopadhyay, T.Ohji, T.Kuwahara, M.Iwahara, S.Yamada, F.Matsumura, "Comparative studies of levitation and control performances of two types single axis controlled repulsive type magnetic bearing", NASA periodicals, Vol. NASA/CP-1998-207654, pp 393-405, May 1998.

2nd International Conference on Autonomous Robots and AgentsDecember 13-15, 2004 Palmerston North, New Zealand

288