5"; -35"-

N94- 35908 ELECTROSTATICALLY

SUSPENDED

MICRO-MECHANICAL

R. Torti,

M. Gerver,

RATE

V. Gondhalekar,

SatCon

Technology

5"; -35"-

SENSED

S. Bart,

B. Maxwell

¢

Corporation

12 Emily Cambridge,

AND

GYROSCOPE

Street MA

02139

SUMMARY

angular

The goal of this work is development of fully electrostatically rate sensing micro-gyroscope fabricated according to standard

Fabrication driving

of test

circuits

including

structures

has

the FET

been

is proceeding.

tested.

input

The

stages

Off chip

prototype

electronics

device

suspended and rebalancing VLSI techniques.

for the electrostatic

will be assembled

sensing

in a hybrid

=

and

construction

.

of the sensors.

INTRODUCTION

SatCon

Technology

is currently

fully electrostatically suspended and structure will be fabricated according targets

include

angular

rate sensitivity

The program is supported Phase II SBIR. To date,

in the design rebalancing to current

under

of 0.01°/s

contract

and

fabrication

designed

Research

in-house

has

of a proof

of principle,

micro-gyroscope. The methods. Performance

at 100 Hz for 8.3 kHz

by NASA-Langley

a set of test microstructures

phase

angular rate sensing VLSI micromachining

(revolutions/s) Center

been

spin rate.

(# NAS1-19590)

fabricated

supplier, prototype capacitive sensing electronics and rotor drive electronics have the design and analysis of structures suitable for a prototype gyro is underway.

as a

by a VLSI been

tested,

and

Micro-electronic fabrication technologies have recently been applied to produce novel micro-mechanical devices such as motors, pressure sensors, and linear actuators. Their small

size,

APPROACH

integration

compatibility

micro-mechanical application

to aerospace,

Figure

1 and

battery

for a long-life, The

with

gyroscope, would

prototype

electronic

commercial,

consist

and potential

for this research and military

of entirely

wireless goal

circuits,

proposed

integrated

accelerometer

for this work,

invite

is a device

systems.

The

final

electronics

and

RF signal

and rate

however,

low cost

effort,

research.

that

would

instrument

The have

goal

transmitter

is shown and

in

lithium

sensor.

is more

modest.

As shown

in Figure

2, the 513

m,A_.,i _...AJ'4K NOT

FILMPD

P,,_ ......

NTE_,_AL=,

Bj,,N_( i

Lithium

batteryor

solar' cell

-\ -I'liccl_c]ylo

Transmittecs LED orRr

t'lIIf

1....

__

_--"i!i

IIIIIII

---

IIlIllI IlllllI IliiIII

Data Transmission

Figure

1. Integrated

Electronics

Microgyro

_1

i

integrated

Electronics

Concept.

mechanical gyro structure would consist of a VLSI fabricated multi-layer polysilicon. The essential geometry is a circular disc rotating at 8 kHz and driven, suspended, rebalanced, and sensed completely electrostatically. This structure or set of structures will be at the center of and connected to a hybrid structure which includes the FET front ends of the capacitive sensor set. Drive electronics and the sensor filters and demodulators will be off chip. SensorFront Ends

Gyro I

ActuationElectr /

_.

[[[] / _------- _ L__2_-_') -, " "_--k

1

L____-

I i

i !

Sensor Filter/Demod

Figure

2. Configuration

of Prototype

DataHandling/PowerConditioning

Micro-Gyro.

A "rebalanced" gyroscope has a control system designed to hold the gyro in a constant position while it is subject to external forces. Since the controlled mass (the gyro rotor in this case) is held to a nearly constant position, the linearity of the sensors and actuators is improved, and the overall dynamics are simplified. A block diagram of the gyro system is shown in Figure 3. 514

Decomposition Electronics

-

Estimated ........... "'-

Rates

___,f

Input Rates

--1M----

Reference Position (Null)

Figure

Controller

Angular Sensors

rotations

dynamics

is sent

actuators,

system.

the

device

sensors,

macroscopic controlled

gyro

and the

rotor

by force

and

torque

control

rebalance

actuator/sensors

electrodes polysilicon

shared.

Capacitive

sensing

away from electronics.

actuating

active from

electrodes.

The

techniques,

we have

sandwiching

a thin

required control

above system

the

the

sensors

reduced insulating electrodes

requirements

complexity the rotor layer. placed as well

layers

are driven

structure

allows

the

drive, suspended

suspended

and

sensors.

directions

requiring

for upper

and

may

four

lower

be combined

is composed

electrodes

at 100 kHz

and

geometry

of three sensing

motor

sets of

radial so that

of structural silicon

placed

nitride

below

by SatCon

the

layer. rotor

designed

CONFIGURATION

the substrate.

as allow

error

to the

As with

by an insulating

all sensor

AND

A total

two

rotor

the

momentum,

position

These

of the

-

suspension.

radial

and

of the VLSI

over

dynamics

angular

actuator.

with

The

the resulting

of the actuator

and

and

the other conductive

SPECIFICATIONS In order to reduce

axial

layer

throughout

and

signals

by various

suspension driver

position

rotor.

displacements.

the appropriate

components

to produce

in both axial

to the

into rotor

signals.

sensed

conductive

is used

reference

for rebalance

and

- and the motor

separated

Pc_s;1_ns

to be applied

torques

sends

Knowledge control

by a motor

lower

torques

of electroquasistatic

actuators,

and

Each

and

Dynamics Gyroscopic

and

to the

system

actuators

be effected

for the upper

are

types

is spun

must

two

control

the actuator

non-rotary

the

cause forces

in relation The

of three

gyros,

Position

frame these

to a null position. from

consists

actuators,

support

transform

displacements

to a control

to be estimated

The position

of the gyro

these

,:.... _,_

Diagram.

of the rotor

returning

rates

s_.,_,_ Dynamics Actuator

Sensor Dynamics

Block

determine

signal

]-_,,,

I

3. Microgyro

mechanical

input

__{

utilize

well

to a two

polysilicon

developing

layer

and

three

This will reduce at higher

signal

processing

polysilicon nitride the

to noise

construction layers

complexity

are of the

ratios.

515

This placed

design

below

the

is shown rotor

over

conceptually the

in Figures

substrate

either

4, 5 and

6. All sensing

as an ion implanted

pattern

electrodes

forces

the

inner

rings

will provide

electrodes geometries

will will

will

detect

combined

couple

with

be included

tilting.

drive the

The

torque

large

area

upper

rotor

and radial electrodes

electrodes

actuation,

will

while

to provide

the overhanging

on the die.

Cutaway View of Fully Suspended

Geometry

Showing Stator Electrode Pattern on Substrate

Axial Sense Electrode

_=

..... 1_Tilt-S2nsing Electrodes

Radial Positioning Sense Electrodes

Figure

4. Full

Suspended

Geometry

Showing

Sensing

Electrodes.

Cutaway View Showing Rotor and Overhanging Tilt Actuators

...'\

:- ......

Fully Suspended Rotor With Polysilicon Pattern for Axial and Radial Actuation

Figure

516

5. Fully

Suspended

Geometry

Showing

Rotor

Electrodes.

#0"

and the disc directions

be articulated.

tilt correction.

be

or a "polysilicon

layer over nitride. Sensors will detect capacitance between a pair of active electrodes underside of the rotor. The outer rings (Figure 4) will sense radial deflections in two while

will

A variety

Radial axial of

Upper Axial Actuator

/

Insulating Layers

[

"

[

' '!.\

Upper Radial Actuator/sensor Motor Actuator

Lower Radial Actuator/sensor

Implanted/DiffusedLower

Figure

6.

Cross

Section

Fully

Suspended

Electrodes

_:-

Geometry.

Sizing The without

target

significant

constrained

material

rpm

rpm).

be at least thinner substrate

was

pattern axial

the

stress

and

chosen

of this design

as the

buildup

electronics

largest

as a reasonable spin rate

requires

a total

(about

that could

fabrication.

in the

extension

ultimate

size

during

limits

for both the upper

be fabricated

The

motor

rotation

driver

of current

rpm)

rate

circuitry.

motor

10,000,000

for reasonable

actuation

rates

is

A rate

(about

determined

forces.

The

insulating

and

therefore

can be as thin as can be stably

gap

between

A smaller

raising the unstable unstable frequency.

gap

frequency

substrate would and

(polysilicon)

and rotor

unstable make

sensor

complicating

used

is nominally

frequency

on the rotor.

the radial

The

rotor

layer

layers

and

0.2 gm.

and

lowest

of three

actuators

say

capacitance

considerations.

rotor

below

the topology this,

gap

in the

chosen

by

limits.

1-2 p.m thick

than

The between

kHz) is well

fabrication

contains

200 _, was

due to residual

limits

(8.3 This

strength The

which

diameter,

warpage

by stress

of 500,000 20,000

rotor

The

drive

nitride

layer

for sensing

top structure

actuators

should

can be much

interacts

axially

with

fabricated. 2 #.

but is also

measurements

the control

and

This

represents

influenced more

problem.

accurate The

a tradeoff

by VLSI

fabrication

at the cost

2 g gap

gives

of

a 300

Hz

SENSORS

The pattern

position

in the

substrate

sensor

mechanism

provides

enough

is the electrostatic electrodes

detection

to discern

radial

of capacitance. position

and

The angle,

electrode tilt off 517

rotation

axis

and

macroscopic The

overall

devices,

basic

concept

inversely

with

gap. High capacitance

axial

and

has

is to drive

gap

position.

Capacitive

also

successfully

been

a constant

distance)

and

current

read the

position

across

resulting

via a hybrid

wires

to the sensing

construction

on the silicon

electrodes

will allow

an air gap

substrate

varies

proportional

good noise immunity. (about 10 14 farads),

range

to meet

to the

Though the the placement

in conjunction

required

in devices.

capacitance

is linearly

as preamplifiers

the dynamic

used

micromechanical

(whose

which

allow small

is commonly

to other

voltage

frequency modulation and demodulation of the microgyro axial gap will be very

FETs

sensing

applied

with

of guard

the resolution

requirements.

ELECTRONICS Sensor The

capacitance

of the position

sensor

Electronics varies

the target with detectable variations of the order leads will affect the tinearity of the measurement ability

to detect

by the

measurement

position

accurately

is also

Referring

gives R34,

wave and

to Figure

is the

time input

Pot (R2)

and

the sine

wave

voltage

for the

The and

the

other

side

Q6.

relationship gate.

zener

into U2. disc.

and

one of the

of a low input

used to implement differential source Q5 and

518

load

placed

on the

the

sensor

to

on the sensor guarded. The sensor

capacitance

Circuitry

is VCO/Function

The

point

Generator

diode,

IC with

triangle,

square,

The

input

source

follower's

gmR_/(l+gmR_).

R_ in this case

of I000.

from

of a differential

5 M_

resistor

source

amplifier

wave.

Trim

Pots

This R31,

-5 v. is one by UI,

is the

input

end

and

of a 10-

summed

electrostatic

with

levitation

to U3, which

drives

one

to the active Q2,

electrode

of the

"Cap

Gauge"

Q3, Q4, Q5, Q6, U4, and

U6 are

unity gain amplifier. Q2A and Q2B are the input impedances are two 200 micro-amp current sources voltage

greater

the product

is a current

source

with

follows

changes

gmR_, the

an output

so the source gate-to-source

side

U5.

gain amplifier.

The

1000 pmhos, the effective

output

U2 is the

is connected

unity

The

of the Pot is buffered

R2 circuit's

output

capacitance

generator.

is set up for -5 v.

wiper

Q1, Ul,

Summation

function

for a triangle

distortion.

which

The

sets the frequency

sine wave

sine wave

the low input capacitance followers. Their source

of the FETs is greater than in a thousand, which means by a factor

Sensor/Actuator

break

is the other.

U12

rotating

resistor

input

by any

E = I'_/C which

to minimize

ground

from

"_ from

to a diode

37 are adjusted

of a 5 M_

from

of 10 "17 F. Any stray capacitance unless the leads are appropriately

hampered

7 we see that U12

Q1 is a programmable turn

the distance

outputs. R34 and R35 determine the charge-up and charge-down currents into C35. and charge-from voltage levels are set by internal references in the ICL8038. This

a charge/discharge

triangle

with

electronics.

Electrostatic

and sine wave The charge-to

inversely

more

impedance should input

in gate

voltage

closely

the source

greater

than

(input)

2 M_

by the

follows and

Q4, the

the gm

follow the gate to better than 1 part capacitance is reduced, first order,

L

/ ,

C3

!

-r

_ i!

_-iI -:_

Figure

7. Schematic

of Capacitive

;

Sensing/Electrostatic

Actuation

Electronics.

519

The

sources

of Q3 are connected

to the drains

of Q2, and

its gates

are

sources of Q2. Gos of C2 is less than 2 p.mhos, so R_ for source followers k_ which means the drains of C2 will follow the gates of C2 to one part input gate to drain fF is achievable. Q2's U6,

sources

a high

inverting

capacitance.

output input)

With

are connected current

to the

buffer.

to implement

careful

The

unity

layout

inputs

output

gain

and

guarding

of U4, whose

a total

output

of U6 is fed back

for low capacitance

input

defined Mfl

by

the

resistor

amp

distance

sine

wave

U11,

U5.

frequency.

and

associated

carrier

ripple.

The

across

area

plate

capacitor,

and the return

plate.

to the U7 The

The

gyro

rectifier The

frequency

are the

same,

Drive

is a three-phase

i.e., produces

torque

it requires

is then of the

output

drives

the

flows

which

associated

5 by

to the

filter

centered

components) to a 4-pole

filter

the

measured

proportional

bandpass

subjected

is

through

is indirectly

inversely

4-pole

of

at

rectifies low-pass

is a DC signal

filter

with

less

Electronics

bipolar

only

U9, and

signal output

plate

voltage

and

input

(amplifier

the capacitor

of a second-order

(U8,

rectified

components).

wheel-motor

is synchronous,

input

return

This

to the

of Q2B

U6's

of 50

the summation output from output is connected to the

through

current

resistor.

to the defined

A full wave filter.

to the

to the

capacitance

is connected

amplifier.

that flows

the 5 M_

Motor

type

current

and the distance

of U5 is connected

of the Band-Pass

0.1%

area

a voltage

the active

output

the output than

plate

amp

US. It is proportional

between The

(U10,

active

to generate

differential

U12's

of differential

input

to the gate

active plate guard, and is fed back to the inverting input of U3, so that U3 maintains the summation at the cap gauge active plate. Also, U6's remaining

connected

Q3 is greater than 500 in 500, thus reducing

variable-capacitance

when

the rotation

a variable-frequency

drive

motor.

frequency

source.

Since

and

this motor

excitation

In addition,

the push-pull

excitation required for each of the three bipolar phases will require six high-voltage output stages. The motor is driven with balanced bipolar voltages so that the rotor will remain near zero potential so that the

axial

Motor ramp

actuators

start-up

will

up to the full-speed

voltage-to-frequency output and

will not require

of the

their

V/F

value

(V/F) converter

complementary

require

DC bias.

that the excitation of 25 kHz.

converter.

The

and produces signal

a large

This

is accomplished

bipolar

three-phase

three

for driving

square-waves

the output

signal pair which develops the bipolar waveforms. drivers, one for each bipolar phase, and can deliver

CONTROL

One position, 520

of the goals

relative

of the controller

to the orientation

frequency

is to keep

of the "stator"

stages.

start

at the

sub-Hertz

with

a ramp

generator

takes

with

120 degrees

In addition,

The output circuit up to 120V.

level

generator the

and

single-phase

phase

the circuit

contains

and

difference generates

six high-voltage

DESIGN

the orientation frame

of the

of the

gyroscope.

rotor

fixed,

In addition,

in the null the

the

controller

must

the null

relative

objectives controller

accurate

orientation.

is desired is desired

performance strong

provide

and

measurement

As usual,

of the torque

the simplest

controller

in order to minimize hardware that can be easily implemented

stability

robustness

function

of the

operating

unstable

nature

of the

plant

required controller

from zero that will

tradeoffs,

speed

and

in particular

pendulum

speed to the full operational provide adequate performance

is required

to maintain

that can meet

the rotor

because

the

unstable.

plant

Because

at low speeds

dynamics

linear some

are a

of the open-loop

-- closed-loop

speed. The challenge, at all speeds.

in

the performance

complexity. In particular, a fixed-gain, in analog electronics. This will force

are open-loop

-- an inverted

that

then,

control

is to find

is

a fixed

gain

Modeling Three torque,

radial

sources of torque will be considered in this model, and the actively controlled torque produced by the

produces a stabilizing or restoring radial torque. stabilizing radial torsional spring. The torsional is expected to be _ -1 x 10 _3 Nm/rad. The voltage induced rotor

electrostatic

and

torque.

torque,

to the

associated

unstable (with

are voltage

linearized

an "unstable"

"stator"

the

motor

actuators The

model

spring

frame.

The

frequency

an associated

biased that

torsional

of 300 Hz)

is seen

frequency

spring

variation

of the plant

dynamics

a linear

relation

includes,

to the relative

caused

to dominate

actuator

stable

torsional

the

and

becomes

diagonal

the y-direction

identical

in this case,

tilt dynamics.

single-input,

from

of this

parallel

or diagonal

spring

produced

by the

produces

the

orientation

fiat low

(angle)

(SISO)

transfer

function

combination

frequency

with

the

actuator

magnitude

180 degrees

voltage of the

(with

an

spring

caused

by

Dynamics zero

speed

with no cross-coupling

At zero

single-output

input

to the

orientation

by the

to operational

speed

(8,333 Kahz) is dramatic. At zero speed, the plant is a two-degree-of-freedom The two by two plant transfer function matrix relating control voltage inputs output

between

in addition

of 71 Hz).

Open-Loop The

to provide actuators

is proportional

unstable

stable

torque, the motor actuators. The motor

For small angles, this can be linearized to give a spring constant for motor under operating voltages

of these

torque

the gravity electrostatic

speed,

therefore,

systems.

plant

can

torsional

between indicating

be treated

below

input

300

tilt dynamics as two

Hz, the

by the unstable

effects.

The

voltage

an unstable

rpm

unstable pendulum. to rotor orientation

the x-direction

to be dominated

motor

response of phase

the

At frequ¢ncies

are seen and

between

of 500,000

torsional

unstable

(torque)

spring.

dynamics spring

and

output

At frequencies

above the 300 Hz unstable frequency, the transfer function falls off with a slope of minus two, caused by the double integration of torque to angle, and scaled by the voltage to torque constant and the radial moment of inertia of the rotor. At full effects.

x orientation coupling

speed

Because and

transfer

(500,000

of the x-y

rpm

or 8,333

symmetry,

Hz) the plant

the two diagonal

y voltage

to y orientation)

functions

(x voltage

are the same.

to y orientation

dynamics or parallel

and

Similarly, y voltage

are dominated transfer the

functions

by gyroscopic (x voltage

two off-diagonal

to x orientation)

to

or cross-

are also

the 521

same.

The

cross

plant

is completely

(off-diagonal)

transfer

characterized

by two transfer

The design examine how

full-state

approach optimal,

feedback

compensators The various

first

step

operational

quadratic-regulator which

closely

is then

estimates

and

Design

implemented

These

(LQR)

approach.

as an output

of the velocity

states.

approach

to develop

in this design speeds.

matches

(diagonal)

to develop a fixed-gain controller for this speed varying plant was to full-state feedback controllers change with changing plant speed. This

controller

to provide

the parallel

functions.

Controller

first

functions,

was

full-state

the regulator

feedback

full-state

controllers

This controller type

feedback

is the

performance

were

"optimal"

goals

controller

using

feedback

controllers

designed

using

initial

condition

of the controller

for the

lead-lag

for

the

linear-

regulator, micro-

gyroscope. Both problems

the

zero-speed,

when

used

slow decay time becomes unstable tradeoff

can

involved

over

full-speed

state range.

feedback The

be effected

between

trying

linear

high-speed

performance

combinations

suffer

from

state-feedback

to yield a state-feedback compensator minimum decay time over the whole

with speed

fulI_-speed

a factor

bandwidth

was

resulting

keep

"best"

under

compensator

and low-speed

of the zero-speed

are the ratio of zero-speed to full-speed controlled by the cost-on-control-weighing

The

compensators

zero-speed

exhibits

compensator however, a

stability.

compensator.

feedback gains, parameters.

performance

compensator

and low-bandwidth at full-speed. The full-speed state-feedback at low speeds. By using a combination of these compensators,

iteratively

parameters bandwidths

and

the full speed

This The

procedure

design

and the compensator These parameters were

iterated

a minimum of 1000 Hz bandwidth and the largest range. In addition, the ratio between zero-speed and of 10.

uses

less parallel-proportional

gain

than

the zero-speed

design and less cross-proportional gain than the full-speed design. The resulting closed-loop rootlocus versus rotational speed is shown in Figure 8 along with the zero-speed case. The "best" compensator

has

improved

damping

and decay

Sensor A set of structures a single nitride, two are shown in Figures response

of capacitive

actuation

voltages.

manipulating,

522

and

compatible

with

time

compared

to the zero-speed

Testbed

the first

multi-user

run I at MCNC

layer polysilicon process. Masks for key structures 9 and 10. The rotor geometries will allow testing sensing

Additionally, evaluating

circuitry they

and

the effectiveness

will serve

micromechanical

case.

to acquaint structures.

of guarding us with

was

designed

around

and a view of the results of the accuracy and and the

the techniques

response of driving,

to

105 Full Speed _- 8.3 kHz 10 4

Stability Boundary 103 (_ = 320 Hz

I

o

10z c o

E

10_

Full Speed _- 8.3 kHz

10o Zero Speed 10-1 -4.00

-300

Zero Speed

-200

i

i

-100

0

IOO

200

300

400

ReoiPort(Hz)

Figure 8.

Root-locus

of Plant

Eigenvalues

versus

Rotational

Speed.

_;_

Ir,;i];

-

' t'i ;

L:':',_: ,:,j ,,,, A4;.._.

Figure

9. Composite

Mask

of Bushed

,2:

Motor.

523

Figure

524

10. SEM

of VLSI

Fabricated

Motor.

Performance A summary Table

1.

of system Microgyro

Spin

parameters Performance

is given

Specifications

in the table.

Specifications

Angular

Speed

8.3 kHz (500 kRPM) 1.5xl 0 18 kgm-m 2

Spin Moment of Inertia Precession Moment of Inertia Minimum Thermal

Angular Rate Sensor Noise

8xl 0 "19 kgm-m 2 O.Ol°/s < 0.4x10 3 v

Angular

Sensitivity

< 0.008 °

Thermally Sensor

Limited Electrode

Rebalance

Rate

Capacitance

Sensor Operating Current (0.5 V) Nominal Drive Actuator Potential Drive

Torque

Per

Pole

100 Hz -3x10 14 F -0.4xl

0 "9 A

5v 10 11 N-m

Viscously Induced Torque Spindle Friction Torque Rotor Radius

< 10 _1N-m 0.2x10 12 N-m

Radial

2.0 lam

100 _tm

Gap

Vertical Gap Rotor Thickness

2.0 p.m

# Stator

Poles

2.0 _tm 6

# Rotor

Poles

4

Nominal

Gap

1. DARPA sponsored delivered in April.

0.5 v

Voltage

at Microfabrication

Center,

Research

Triangle

Park,

NC.

Dies

were

525

Session

9b - Rotating

Machinery

Chairman: National

Cheng

and Energy

Chin

E. Lin

Kung

University

6-_"_

Storage

_ _,_s

527