MEMS Sensors and Actuators Laboratory (MSAL)

Electrostatic Micromotors Supported on

Electrostatic Micromotors Supported on

Microball Bearings

Microball Bearings

Nima Ghalichechian, Alireza Modafe, Mustafa Beyaz, Nick Ganing, and Reza Ghodssi

Dept. of Electrical and Computer Engineering, Institute for Systems Research, University of Maryland – College Park, MD 20742, USA

INTRODUCTION

SUMMARY AND FUTURE WORK

•

The

dynamic characterization

of a 6-phase, bottom-drive, linear, variable-capacitance

micromotor (B-LVCM) supported on microball bearing was presented.

•

The motion of the micromotor was captured using a high-speed video camera.

• Instantaneous velocity (11 mm/s), acceleration (1.9 mm/s

2

), and net force (0.19 mN) (all

in amplitude) were measured.

•

The motor was modeled with a mass-dashpot-spring system and

dashpot coefficient

was

measured to be b=6.25×10

-4

Kg/s.

• Future work:

Design, fabrication, and characterization of a

rotary

variable-

capacitance micromotor supported on microball bearings.

CHARACTERIZATION

DESIGN AND FABRICATION

CHARACTERIZATION

OBJECTIVE:

To characterize the dynamic behavior of a bottom-drive, linear,

variable-capacitance (B-LVCM) micromotor supported on microball bearings.

•

Microball bearing technology in silicon provides a reliable support mechanism for the
rotor of micromotors and microgenerators.

•

The primary application of the B-LVCM is long-range, high-speed, linear micro-
positioning.

•

Mechanical properties of roller bearing can be studied by characterization of the B-
LVCM.

•

B-LVCM is a platform for developing a rotary micromotor.

Time (s)

0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

Di

s

p

la

c

e

m

e

n

t

(m

m

)

-0.05

0.00

0.05

0.10

0.15

0.20

0.25

0.30

Time (s)

0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

D

is

p

la

c

e

m

e

n

t

(m

m

)

8.75

8.80

8.85

8.90

8.95

9.00

9.05

9.10

Time (s)

0.04

0.06

0.08

0.10

0.12

0.14

D

is

p

la

c

e

m

e

n

t

(m

m

)

8.76

8.78

8.80

8.82

8.84

8.86

8.88

Time (s)

0.04

0.06

0.08

0.10

0.12

0.14

V

e

lo

c

it

y

(

m

m

/s

)

-8

-6

-4

-2

0

2

4

6

8

10

12

Time (s)

0.04

0.06

0.08

0.10

0.12

0.14

A

c

c

e

le

ra

ti

o

n

(

m

m

/S

2

)

-2000

-1000

0

1000

2000

N

e

t

F

o

rc

e

(

m

N

)

Schematic 3D view of the B-LVCM.

Optical micrograph (top-view)

of the 6-phase stator.

Optical micrograph

(top-view) of the slider.

440C stainless

steel

Microball material

290

Trench width, µm

284.5

Microball diameter, µm

26-34

Air gap, µm

84/36

Electrode/pole number

120/180

Electrode/pole pitch, µm

90/90

Electrode/pole width, µm

Value

Parameter

•

The micromotor comprises three major components:

stator, slider, and microballs

.

•

Bottom-drive

design results in increased active area

and increased motor force.

•

Six-phase design results in small force ripples.

•

Microball bearing

design provides a mechanical

support for maintaining a uniform air gap.

•

A thick film of

Benzocyclobutene (BCB)

low-k polymer

(k=2.65) was used as an insulating layer to

1.

Reduce the parasitic capacitances and increase the
motor efficiency.

2.

Reduce wafer curvature (compared to SiO

2

) due to

low residual stress.

•

The stator fabrication process comprises of three BCB
deposition, two metal deposition steps, and a deep
reactive ion etching (DRIE) step.

Micromotor geometry specifications.

(C) Transient Response Measurement at 635-1000 fps
•

Transient response of the micromotor to 120-V square pulses was measured using
high-speed camera system.

•

Micromotor has a second-order step response modeled as a damped sinusoidal
function.

Startup of the micromotor from rest
position captured at 1000 fps.

Position of the slider, X(t), in 130-ms
time window captured at 635 fps.

Local slider displacement, X(t)-V

avg

×t,

showing damped sinusoidal transient
response captured at 635 fps.

Instantaneous velocity of the slider.

Instantaneous acceleration and the net
force of the slider.

Microballs

Electrodes

Motion

Stator

Slider

Microball Housing

Interconnect

3945

µ

µµ

µm

BCB Island

Test pad

Trench

Interconnect

Electrodes

Trenches

Poles

1600

µ

µµ

µm

Phase A:
V(t=0

−

)= 0 V

V(t=0

+

)= 120 V

Region 2: V

avg

=1.9 mm/s

Region 1: Acceleration
from rest position

Average Velocity













+

+

=

′

−

ϕ

β

π

α

t

e

X

X

t

m

b

2

sin

2

0

•

Velocity

and

acceleration

of the slider were calculated from the first and second

derivatives of the displacement data, respectively.

•

Instantaneous

net force

on the 0.1-gram slider was calculated to be 0.19 mN in

amplitude.

•

The micromotor was modeled as a

mass-dashpot-spring

system where m is the mass of

the slider, b is the dashpot coefficient representing the friction of microballs and silicon
housing, and k is the spring constant of the electrode-pole variable capacitance.

•

The differential equation describing such a system is given by

•

Dashpot coefficient was found to be b=6.25×10

-4

Kg/s (at V=120 V and f

e

=10 Hz).

Schematic 3D view of the rotary micromotor

X

k

b

m

Mass-dashpot-spring model for the
B-LVCM.

R

2

=0.9999

)

1

(

0

=

+

+

kx

x

b

x

m

&

&

&

AKNOWLEDEMENTS

•

This research was funded by Army Research Office through MURI Program under Grant No. ARMY-
W911NF0410176, with Dr. Tom Doligalski as the technical monitor, the Army Research Lab under Grant
No. CA#W911NF-05-2-0026, and the National Science Foundation under Grant No. ECS-0224361. The
authors would like to thank Nitta Hass Company, Japan for providing the CMP supplies.

•

Advantages of the proposed design

to other rotary micromotors are:

1.

microball bearings support: reliable

2.

BCB electrical isolation: highly efficient

3.

Bottom-drive design: large active area and torque

4.

Wide variety of applications

Dynamic characterization of the 6-phase B-LVCM is performed to obtain
instantaneous velocity, acceleration, net force, and friction coefficient from the
displacement measurement.

(a) Test Setup
•

The micromotor test station comprises the
following units

•

Six-channel, high-voltage actuation

•

Low noise, monochromic,

high-speed video

camera

: 1000 frames-per-second (fps)

•

Image processing and tracking

PC and Proanalyst

TM

Tracking Software

Record /

Playback

High-speed

Camera

LabVEIW

Interface

PC

Micromotor

NI-6722 DAQ

High-Voltage

Amplifier

T

ri

g

g

er

Time (s)

0

1

2

3

4

5

6

7

D

is

p

la

c

e

m

e

n

t

(m

m

)

-0.8

-0.6

-0.4

-0.2

0.0

0.2

0.4

0.6

0.8

Time (s)

0

1

2

3

4

5

6

7

D

is

p

la

c

e

m

e

n

t

(m

m

)

-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

7.21-11.10

14.40

80

7.37±0.12

7.20

40

3.98±0.02

3.60

20

1.94±0.02

1.80

10

Measured

average
velocity

(mm/s)

Predicted

average
velocity

(mm/s)

Excitation
frequency

(Hz)

Block diagram of the micromotor test

station.

(b) Displacement Measurement at 30 fps

•

Position of the slider is measured versus time.

•

The average velocity is measured from the slope of the graphs.

•

The predicted average velocity of the slider is given by

where W is the width of the electrode and f

e

is the frequency of

the excitation voltage.

Displacement of the slider when excited by 120V, six-phase, square pulses

Comparison of predicted and

measured average velocity

•

The results show a good

agreement

between the

predicted

and

measured

values for the

average velocity for f

e

≤ 60Hz.

•

The slider fabrication process comprises a
DRIE step for fabricating poles and
trenches.

e

avg

Wf

V

2

=

f

e

=10 Hz

f

e

=20 Hz

0

:

=

+

+

kx

x

b

x

m

DE

&

&

&

Rotor

Stator trench

Microballs

Electrode

Test pad

BCB Island

( )

(

)

φ

ω +

=

−

t

Sin

C

e

t

x

Solution

d

t

m

b

2

: