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 ZiaieSchool of Electrical and Computer Engineering,

Purdue University, West Lafayette, USA(Tel : +1-765-494-0725; E-mail:

bziaie@purdue.edu)

TRANDUCERS & EUROSENSORS’07The 14th international Conference on Solid-State Sensor. Actuator and Microsystems, Lyon. France. June 10-14,2007

Outline

AbstractIntroductionAlgorithmfabrication3-D computer aided simulationConclusion Reference

Abstract

GA homopolar motor concept modelMEMS structure

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

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.

introduction

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

Homopolar motor

introduction

general DC motor

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.

Advantage Disadvantage

simple high current requirement which is typically mitigated by using superconducting wires

compact

no force ripple

do not require current or magnetic field controllers

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

Rotational torques acting on the magnet and on the closing wire.

Faraday’s setup: magnet, disk and closing wire.

Algorithm

Algorithm

Electric field(E) electric charge (q)

Magnetic field(B)velocity of the particle (v)

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.

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

Algorithm

i

B

F

Algorithm

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

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.

3-D COMPUTER AIDED SIMULATION

Find the generated electromagnetic force.

Ampere’s law

Taking divergence of (1)

COMSOL 3.3

3-D computer aided simulation

Fig. Simulation result for magnetic field andcurrent distribution in the micromotor

The magnetic field in the location of the motor is mostly in z direction.

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.

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))

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.

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

Thanks of your attention

About Silicon Nitride

Key Properties

High strength over a wide temperature range

High fracture toughness

High hardness Outstanding wear resistance, both impingement and frictional modes

Good thermal shock resistance Good chemical resistance

Typical Uses

Rotating bearing balls and rollers Cutting tools

Engine moving parts — valves, turbocharger rotors

Engine wear parts — cam followers, tappet shims

Turbine blades, vanes, buckets Metal tube forming rolls and dies

Precision shafts and axles in high wear environments

Weld positioners