SalCon Technology Corporation is developing a prototype electrostatically suspended micromechanical gyroscope in the Small Business Innovation Research (SBIR) program under Contracts 19282, and 19585 from NASA Langley Research Center. This paper presents the results of the Phase 1 feasibility study and the plans for fabrication in the upcoming Phase 2 program

Hawkey, Timothy J.

Torti, Richard P.

English

NASA Technical Reports Server

N94- 29789
Integrated Microgyroscope
Timothy J. Hawkey, Richard P. Torti
SatCon Technology Corporation
12 Emily Street, Cambridge, MA 02139
ABSTRACT
SatCon Technology Corporation is developing a prototype electrostatically suspended
micromechanical gyroscope in the Small Business Innovation Research (SBIR) program
under Contracts 19282, and 19585 from NASA Langley Research Center. This paper
presents the results of the Phase I feasibility study and the plans for fabrication in the
upcoming Phase II program.
1. INTRODUCTION
SatCon Technology has performed a study of micromechanical gyroscopes. The
goals of the Phase I SBIR program were to define a baseline system configuration and
establish technical feasibility. In the Phase II program, just beginning, a prototype, actuator
will be fabricated and tested. This paper presents a micromechanical device overvmw, a
summary of the Phase I results and plans for Phase II.
2. SPECIFICATIONS AND CONFIGURATION
As with all sensors, resolution, accuracy, large dynamic range, low power
consumption, small size and high bandwidth are desired. For this technology effort, the
obvious primary objective was small size with the goal of eventually developing a
complete two-axis rate gyro on a microchip. For this feasibility study, maxunum/minimum
angular rates of 10 radians/sec and 0.1 radians/sec and 200 Hz bandwidth were chosen as
specification goals to approximate what might be required for use on robotic arms. Other
performance specifications were determined by process controlled geometries and
parameters as discussed in Section 4.
2.1 Suspended gyro configuration
The initial micro-gyro configuration identified is shown in Figure 1. Configured as
a rebalance gym, this microfabricated machine consists of three types of electroquasistatic
components - the motor drive, position sensors, and the non-rotary actuators for rebalance
and suspension. Like suspended macroscopic gyros, the rotor is spun by a motor to
produce angular momentum, suspended and controlled by force and torque actuators, and
sensed by various position sensors. As discussed in Section 2, all the sub-components are
electric field devices. The term "rebalance" identifies the gyro mode of operation. When
acted on by external forces, the control system is designed to hold the gyro in a constant
position, rebalancing it. Knowledge of the actuator control signals and dynamics allows
the inputs to be determined. Since the controlled mass (the gym 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 system block diagram is shown in Figure 2.
The electrostatic force can be written in linearized form for a fixed potential as:
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https://ntrs.nasa.gov/search.jsp?R=19940025285 2020-06-16T14:19:38+00:00Z
F IV2B2CAx
where AX represents the excursion about the equilibrium position, and the coefficient is
positive. Since the force increases with decreasing distance, the motion is unstable and
without other compensation requires closed loop control. In the case of a fully suspended
rotor, this control must be effected in both axial and radial directions (five unstable degr_s
of freedom). There are, therefore, four sets of actuator/sensors - two for the upper and
lower axial suspension and two for upper and lower r_a!_re_b_ce actuator/sensors - and
the motor driver actuator, hence the multilayered configuration of the figure.
The fabrication of such a multi-actuated, multi-sensed device requires at least five
sets of alternating insulating/conducting layers on the rotor. For reasonable capacitive bias
currents and actuator forces, the facing areas must be at least 1-2 I.tm high with radial gaps
of the same order. The height of this rotor will accumulate to over 5t.tm_Present
technology, however, only makes feasible aspect ratios (rotor thickness to gap) of up to
2/1, with 1/1 being typical. This gap in fabrication technology will have to be bridged to
allow successful fabrication of a fully suspended micro-gyro. :
2.2 Motor configuration " : _ _ _
The electrostatic production of torque has ibng_n considered for motor drives.
Four classes of electroquasistatic motor actuators have emerged which have analogues with
magnetically operated devices - variable capacitance, electrostatic induction, permanent
electret, and electric hysteresis. In the work by Bart, induction and variable capacitance
motors were studied in detail with attempts made to fabricate both axial gap and radial gap
variable capacitance motors. Radial gap variable cap_itance _tors have _n
successfully built and tested at both MIT and Berkeley. Our system therefore baselined a
radial gap motor as fabricated at MIT2.
2.3 Sensor configuration
The baseline sensor configuration chosen was capacitive position sensing using a
superposition of high frequency signals between the axia!__0_trol ¢l__Qdes gndthe rotor.
Decomposition of the four sensor Signals-(0ncf_aTxi=_stator_Te, c_ti:6de-)_vides :the
signals needed to control the rotor. Capacitive position sensing is commonly used in
macroscopic deviceS, and has also been successfully applied to other micromechanical
devices. The basic concept is to drive a constant current across an air gap (whose
capacitance varies inversely with gap distance) and read the resulting voltage Which is
linearly proportional to the gap. High frequency modulation and demodulation allow good
noise immunity. Though the capacitance of the microgyro axial gap will be very small
(about 3 x 10-1,* farads), the placement of FETs (field effect wansistors) on the silicon
substrate as preamplifiers as done by Sch_dfs_ould _ow reasonable measurements to be
made3.
3. ELECTROMECHANICAL DESIGN
The design of the motor drive and rebalance actuator is based on the specifications
discussed in Section 3, many of which were derived from known fabrication constraints
258
and experience. Dimensions were generally conservatively chosen given the uncertainty in
fabricating devices with this technology. In practice, some or all of these dimensions may
be changed in the prototype development effort. These sizing numbers, however,
represent what would be required for engineering development of a useful instrumentation
sensor.
3.1 Sizing
The overall rotor diameter, 200 It, was chosen as the largest size that could be
fabricated without significant warpage due to residual stress buildup during fabrication.
The rotation rate is constrained by stress limits in the rotor and electronics limits in the
motor driver circuitry. A rate of 500,000 rpm (8.3 kHz) was chosen as a reasonable
extension of current motor rates (about 50,000 rpm). This is well below the ultimate spin
rate (about 10,000,000 rpm) determined by material strength limits and is limited by motor
drive electronics. The required electronics frequency, 25 kHz, is attainable without
excessive noise problems. The motor height was set to 2.2 it, the same height as currently
fabricated micromotors. The maximum value of total rotor thickness is limited by
fabrication technology to about 2 times the radial gap of 1.5 it. The axial gap between
substrate and rotor was set at 2 it. This represents a tradeoff between gap capacitance and
rotor unstable frequency. A smaller gap would make sensor measurements more accurate
at the cost of raising the unstable frequency and complicating the control problem. The 2 It
gap gives a 300 Hz unstable frequency.
3.2 Sensor/Actuator
The four segment rebalance sensor/actuator electrode design is similarly derived
from the specifications. While it was not attempted to optimize the pattern for torque,
depositing the conducting region from 0.7 R to 1 R gives adequate rebalance _rque.s _d
adequate capacitance for sensing inclinations as small as 0.008 °. A possible difficulty is
the actually attainable depth and uniformity of the implanted electrodes since the thermal
noise limit is sensitive to circuit resistance. The sensor capacitance between each of the
four electrodes and the rotor is approximately 3x10-14 F. With a typical commercial
oscillator frequency of 100 kHz, the sensor current is about 15 nA for operation at 0.8v.
Since commercial devices sense capacitance in the same range (although with larger
electrode and gaps) it should be possible to use standard techniques for angle information.
259
3.4 Summary
The tables below give a summary of the dimensions and parameters predicted for
the two degree of freedom microgyro with specifications as discussed in Section 2.
Spin Angular Speed
Spin Moment of Inertia
Precession Moment of Inertia
Minimum Angular Rate
Thermal Sensor Noise
Angular Sensitivity
Sensor Electrode Capacitance
Sensor Operating Current (0.8 V)
Nominal Drive Actuator Potential
Drive Torque Per Pole
Rotor Radius
Radial Gap
Vertical Gap
Rotor Thickness
# Stator Poles
# Rotor Poles
500 kRPM
1.5x10-18 kgm-m2
8x10-19 kgm-m2
0.01°/s
<0.4x10-3 w_ :=
.. L
<0_008- : _ :
3x10-14F
15 x10-9 A
0.5 v
10-11 N-m
501am
1.5 ).tm ----= :
2 _m
2.2 l.tm
12
8
4.1
4. CONTROL AND ELECTRONICS DESIGN
Control System
This section presents the controller design philosophy. For a more detailed
description of the controller, the reader may _fer to ReTerenc_4._h0wn _ Figure 2the
system block diagram consists of the actuator and plant (gyroscopic) dynamics, the
sensors, the controller, and the decomposition electronics. One of the goals of the
controller is to keep the orientation of the rotor fixed, in the null position, relative to the
orientation of the "stator" frame of the gyroscope. In addition, the controller must provide
accurate measurement of the torque that is required to maintain the rotor in _e null relative
orientation. As usual, the simplest controller that can meet the performance objectives is
desired in order to minimizehardware C6rnp]exity_ Vmparticular, a fixed:gain, linear
controller is desired that can he easily implemented in analog electronics. This will force
some performance and stability robustness tradeoffs, in p_cular because the plant
dynamics are a strong function of the operating speed and are open-loop unstable. Because
of the open-loop unstable nature of the plant -- an inverted pendulum at low speeds --
closed-loop control is required from zero speed to the full operational speed. The
challenge, then, is to find a fixed gain controller that will provide adequate performance at
all speeds.
The design approach to develop a fixed-gain controller for this speed varying plant
was to first examine how optimal, full-state feedback controllers change with changing
plant speed. These full-state feedback controllers assume knowledge of the position and
velocity of the rotor in both radial directions. Based on the behavior of these paran_ter-
varying, full-state feedback controllers and the addition of some physical insight, a fixed-
gain, full-state feedback controller can be chosen that provides reasonable performance
over the full speed range. This full-state feedback controller is then implemented as an
output feedback controller using lead-lag compensators to provide estimates of the velocity.
r
260
4.2 Electronics
4.2.1 Sensor electronics
The capacitance of the position sensor varies inversely with the distance from the
sensor to the target. Over the specified measurement range, the sensor varies in capacitance
from approximately 0.027 picofarads (pF) to 0.033 pF. Any stray capacitance on the
sensor leads will effect the linearity of the measurement unless the leads are appropriately
guarded. The ability to detect position accurately is also hampered by any load placed on
the sensor capacitance by the measurement electronics.
A block diagram of the sensor electronics is shown in Figure 3. The sensor is
driven by a 100 kHz current source so that the resultant voltage is proportional to the
sensor impedance. The AC voltage produced across the sensor is buffered by a "guard"
loop, full-wave rectified, and low-pass filtered to produce a DC output voltage directly
proportional to distance. The effects of stray capacitance on the sensor leads are greatly
reduced by driving the shield of the sensor cable with a "guard" loop. The guard loop
drives the shield with a voltage identical to that across the sensor, and thus no current flow
is possible. The impedance of the shield is a capacitance greater than 200 pF to ground. In
order to drive this load, the voltage across the sensor is buffered by the FET input stage, a
high-bandwidth differential op-amp, and a high-current buffer. The bandwidth of this
follow-up loop must be high to reduce any effects of the guard capacitance upon the sensor
capacitance. To reduce the input capacitance of the sensor electronics, a Field Effect
Transistor (FET) input stage is used. This input stage presents a very high resistance and
low capacitance load to the sensor. The FET input stage is configured as a source-follower
where the source signal will exactly follow the gate (input) signal. The source terminals of
the lETs are each loaded with a transistor current source. The high impedance of the
current sources reduces the effect of any gate-to-source capacitance. The drain of the FETs
are capacitively coupled to the buffered sensor voltage to reduce the effects of any gate-to-
drain capacitance. Stray capacitance on the circuit board would he reduced by placing the
buffer-loop circuitry on a copper-clad board with the copper clad driven by the guard
voltage. The output of the sensor guard-loop is bandpass filtered to eliminate both DC drift
and high-frequency noise effects. The signal is then full-wave rectified and low-pass
f'dtered to produce a DC output voltage.
4.2.3 Motor drive electronics
The gym wheel-motor is a three-phase bipolar variable-capacitance motor. Since
this motor type is synchronous, i.e., produces torque only when the rotation frequency and
excitation frequency are the same, it requires a variable-frequency drive source. 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 ground potential. Any voltage induced on the rotor will cause it
to be strongly attracted to the grounded substrate because of its large surface area.
Motor start-up will require that the excitation frequency start at the sub-Hertz level
and ramp up to the full-speed value of 25 kHz. This will be accomplished with a ramp
generator and voltage-to-frequency (V/F) converter. The bipolar three-phase generator
takes the single-phase output of the V/F converter and produces three square-waves with
120 degrees phase difference and their complementary signal for driving the output stages.
In addition, the circuit generates the signal pair which develops the bipolar waveforms.
The output circuit contains six high-voltage drivers and can deliver up to 120V.
261
5. FABRICATION
Fabrication is a major challenge with micromechanical systems. Overall system
feasibility and functionality are more closely tied to fabrication methods than with macro
scale systems. For this program, it is planned to use the microsystems fabrication facilities
at MIT and base the motor design on those developed there. The program will be broken
into three technology demonstration steps aimed at verifying component technologies
before integration into a complete system.
The first step will be the fabrication of a capacitive sensor demonstration unit. This
will consist of a microfabricated cantilever beam and a differential field effect transistor
pair. The FETs will be connected to an electrode below the cantilever and serve as the
capacitive sensor pre-amplifier as discussed in Section 5. Optical methods will be used to
independantly verify the position of the cantilever. ......
The next technology demonstration will be a non-rotating closed loop suspension of
microfabricated plate. These prototypes will be used to develop specific mechanical,
electrical and control designs and fabricaion sequences for the integration of sensor
technology into a micro-suspension system. Testing will verify actuator and control
characteristics and allow the development of a rotating micro-suspension
Finally, after the fin'st two tasks, the rotating, suspended microgyro will be
developed. This will extended the non-rotating suspension with a motor drive and
additional electronics. It is anticipated that substantial process development may be
necessary for the successful completion of this task. =
6. CONCLUSIONS
The results of the feasibility study were very positive. Though extensions of
fabrication technology will be required for fabrication of the prototype device, the potential
for successful development is very good. An operational micromechanical gyroscope
would have many applications in commercial, aerospace, and military sectors.
7. REFERENCES
1. Howe, et all, "Silicon Micromechanics: Sensors and Actuators On A Chip", IEEE
Spectrum, July 1990.
2. Bart, S.F., Modeling and Design of Electroquasistatic Microactuators, Ph.D. Thesis,
MIT,Septembef 1990. -= : :=-= _:: :: - ..... ! .... _ :==
3. M. Schmidt, et all, "Surface Micromachining of Polyimide/Metal Composites For a
Shear Stress Sensor", in IEEE Micro Robots and Teleoperators Workshop, Hyannis MA
1987.
: :7 7:_E ; :: ;
4. Hawkey, T, and Toni, R. "An Integrated Microgyroscope", Final Report, contract
NAS[- 19282, SatCon Technology Corporation, Cambridge, MA July 1991.
262
8. FIGURES
Axial Actuator/_ ASpacer
Sensor ",_ / /Motor
_ Sensor Plate
t _L1 --
V" i_ :,l,,:J_,:.............._:.:<._i=_K Side
_, ! ,_-,___ View
Silicon Substrate
Radial Actuator Actuator/Sensor
Electrodes
200 p -- _"
Radial Actuator Top
View
Rotor
Actuator Electrodes
Axial Actuator/Sensor
Top View, Bottom View
Sensor Electrodes
Figure 1. Suspended Microgyroscope Configuration
263
InputRates
Reference
Position -_,
(Null)
__ DecompositionElectronics
f_
:_)__ Controller t .,_ Actuator _ Gyroscopics_ Dynamics Dynamics
I Sensor Dynamics l_
Pol_t_rli
Estimated Rates
Figure 2. Microgyro Block Diagram
Current Source I_ Sine
Source
Follower
wave I
_-_ Amplifer CurrentDriver
1 I
Band Passl _1 Full Wave
Filter _ Rectifier
Low Pass
Filter
Figure 3. Sensor Electronics Block Diagram
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Integrated microgyroscope

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