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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: [email protected])
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
Outline
 Abstract
 Introduction
 Algorithm
 fabrication
 3-D computer aided simulation
 Conclusion
 Reference
Abstract
GA homopolar motor concept model
MEMS 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
Homopolar motor
 Current, magnetic field and magnetic force directions.
Here the exerted torque causes the disc to rotate in
an anti-clockwise direction.
introduction
general DC motor
introduction
Homopolar DC motor compared with other DC motors
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
The liquid rotor simplifies electrical connection to the
rotating part and reduces friction and power loss.
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.
Algorithm
Faraday’s setup: magnet, disk
and closing wire.
Rotational torques acting on the magnet and on
the closing wire.
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
motor
(Ampere’s low)
current
Magnetic field
generator
Changing the magnetic field
Algorithm
B
i
F
Algorithm
Fabrication
Mercury
2mm
Highly doped
silicon wafer
(0.001 Ω-cm)
Neodymium
super magnet
A micromachined
circular hole with the
diameter of 2mm.
200μm
A small hole was
created in Silicon
nitride layer using
RIE.
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
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
3-D computer aided simulation
Fig. The magnetic flux density zFig. the current density distribution in the
component magnitude on top of the
rotor and top silicon part
magnet.
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.
Result
Fig. Electromagnetic force vs. electric
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 .
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))
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