ADVISER:CHENG-HSIEN LIU 劉承賢 REPORTER: 劉宗和、葉致成 ID:9733530 、 9733593

1. HOMOPOLAR MICROMOTOR WITH LIQUID METAL ROTORTeimour Maleki and Babak ZiaieSchool of Electrical and Computer Engineering, Purdue University, West Lafayette, USA(Tel : +1-765-494-0725; E-mail: bziaie@purdue.edu) ADVISER:CHENG-HSIEN LIU 劉承賢 REPORTER:劉宗和、葉致成 ID:9733530、9733593 TRANDUCERS & EUROSENSORS’07 The 14th international Conference on Solid-State Sensor. Actuator and Microsystems, Lyon. France. June 10-14,2007

2. Outline • Abstract • Introduction • Algorithm • fabrication • 3-D computer aided simulation • Conclusion • Reference

3. Abstract MEMS structure GA homopolar motor concept model

4. introduction • First invented in 1821 by the famous ninetieth century English scientist Michael Faraday (1791-1867), he built a type of electric motor which nowadays is referred to as a homopolar motor. Michael Faraday

5. introduction What is “Homopolar” ? -> Requiring only the same electric polarity for its operation, substituting the word “same” with its Greek equivalent homos one arrives at the name homopolar.

6. introduction Homopolar motor • Current, magnetic field and magnetic force directions. Here the exerted torque causes the disc to rotate in an anti-clockwise direction.

7. introduction general DC motor

8. introduction Homopolar DC motor compared with other DC motors The liquid rotor simplifies electrical connection to the rotating part and reduces friction and power loss.

9. Introduction • Homopolar Motor，made with drywall screw, alkaline cell, wire, and neodymium disk magnet. The screw and magnet contact the bottom of the battery cell and are held up by magnetic attraction. homopolar electric motor • The homopolar micromotor consists of a mercury droplet as the liquid rotor.

10. Algorithm Faraday’s setup: magnet, disk and closing wire. Rotational torques acting on the magnet and on the closing wire.

11. Algorithm • Electric field(E) electric charge (q) • Magnetic field(B)velocity of the particle (v)

12. Algorithm the force on a point charge due to electromagnetic fields ： Lorentz Force Equation Faraday’s law of induction ： is the magnetic flux through the loop. is the electromotive force(EMF) experienced.

13. Algorithm motor Moving charge Changing the direction of moving charge or wire (Ampere’s low) Magnetic field current generator Creating the current (faraday’s low) Changing the magnetic field

14. Algorithm B F i

15. Algorithm

16. Fabrication Mercury 2mm A micromachined circular hole with the diameter of 2mm. Highly doped silicon wafer (0.001 Ω-cm) 200μm A small hole was created in Silicon nitride layer using RIE. Neodymium super magnet

17. Fabrication Fig. The optical image of the fabricated device showing the magnet , two layers of high doped silicon wafer ,a SU-8 cap and a Teflon rotor.

18. 3-D COMPUTER AIDED SIMULATION

19. Find the generated electromagnetic force. Ampere’s law Taking divergence of (1) COMSOL 3.3

20. 3-D computer aided simulation The magnetic field in the location of the motor is mostly in z direction. Fig. Simulation result for magnetic field and current distribution in the micromotor

21. 3-D computer aided simulation Fig. The magnetic flux density z-component magnitude on top of the magnet. Fig. the current density distribution in the rotor and top silicon part • The magnet diameter should be as big as possible. • The distance between the magnet surface and bottom of the mercury droplet should be kept at a minimum. • Increasing the mass of the thickness of the top silicon does not change the electromagnetic force.

22. Result Fig. Electromagnetic force vs. electric current. Fig. Measured output RPM vs. current. Because the measurement setup limitations.The author mention that the micromotor can rotate much faster than what is indicate in the figure(300 round per minute (rpm)) The output of electrostatic MEMS micromotor which is in the order of pN-m. The high-power MEMS electric induction motors needs power more than 100V .

23. Conclusions • Successfully simulated and fabricated a homopolar micromotor with a liquid rotor. • The simulation result show that important parameters in designing the micromotor are the magnet diameter and the thickness of the bottom silicon which controls the distance between the surface and the bottom the magnet metal liquid. • The other important parameter to increase both torque and rpm is the size of the hole in the top silicon which control the path length in the force equation.

24. reference • A SIMPLE ROLLING HOMOPOLAR MOTOR(Seán M. Stewart) • D.K. Cheng, Field and Wave Electromagnetics, Addison Wesley, 1992. • The homopolar motor: A true relativistic engine • http://zh.wikipedia.org/wiki/Wiki

25. Thanks of your attention

26. About Silicon Nitride