1. ADVISER:CHENG-HSIEN LIU 劉承賢 REPORTER: 劉宗和、葉致成 ID:9733530 、 9733593 HOMOPOLAR MICROMOTOR WITH LIQUID METAL ROTOR Teimour Maleki and Babak Ziaie School of Electrical and Computer Engineering, Purdue University, West Lafayette, USA (Tel : +1-765-494-0725; E-mail: bziaie@purdue.edu) TRANDUCERS & EUROSENSORS’07 The 14 th 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 GA homopolar motor concept model MEMS structure

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

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. AdvantageDisadvantage simplehigh current requirement which is typically mitigated by using superconducting wires compact no force ripple do not require current or magnetic field controllers

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. The homopolar micromotor consists of a mercury droplet as the liquid rotor. homopolar electric motor

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

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

14. Algorithm i B F

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

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 Fig. Simulation result for magnetic field and current distribution in the micromotor The magnetic field in the location of the motor is mostly in z direction.

21. 3-D computer aided simulation 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. Fig. the current density distribution in the rotor and top silicon part Fig. The magnetic flux density z- component magnitude on top of the magnet.

22. Result Fig. Electromagnetic force vs. electric current. Fig. Measured output RPM vs. current. 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. 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))

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 Key Properties High strength over a wide temperature range High fracture toughness High hardnessOutstanding wear resistance, both impingement and frictional modes Good thermal shock resistanceGood chemical resistance Typical Uses Rotating bearing balls and rollersCutting tools Engine moving parts — valves, turbocharger rotors Engine wear parts — cam followers, tappet shims Turbine blades, vanes, bucketsMetal tube forming rolls and dies Precision shafts and axles in high wear environments Weld positioners