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218 MEMS and Microstructures in Aerospace Applications

Excitation of the microgyro dynamics is achieved by applying a potential to the
two drive electrodes. The drive electrodes and sense electrodes are suspended by
silicon springs above matching electrodes on the base plate. The post adds inertia to
the system which boosts the sensitivity to rotational motion. The electrical potential
between the drive electrodes and their respective base plate electrodes creates an
electrostatic force that, ideally, rocks the cloverleaf assembly about the y-axis. The
amplitude of the rocking motion can be maximized by driving the electrodes at the
natural frequency of this DoF, known as the drive mode. If the device is rotated
about the z-axis, then the rocking about the y-axis is coupled into rocking about the
x-axis via Coriolis acceleration in the x–y frame fixed to the gyro. The rocking
about the x-axis is referred to as the sense mode and the x-axis response is related to
the angular rate of rotation about z. The operating principles of the PRG microgyro,
fabrication details, and preliminary performance results have been extensively
documented.
Other gyroscopes, such as the ones designed by BEI Sensor and Systems
Company, use tuning fork designs, where the result of the Coriolis force is a
vibration mode orthogonal to the standard in plane tuning fork mode. 30,37
Also, as previously discussed, the Draper Laboratory MEMS TFG design
has been optimized for infusion in the NASA NMP ST6 ISC flight hardware.
Similar to the JPL PRG, the principle of operation for the Draper Laboratory
MEMS TFG is fundamentally based upon the Coriolis force. The resonant structure
is composed of two proof masses which are each driven electrostatically with
opposite oscillatory phases. Alternating voltages applied to the outer motor drive
electrodes create electrostatic forces between the interlocking tines of the motor
electrode and proof mass, which results in lateral (in the plane of the wafer)
oscillatory motion. The proof masses are driven in a tuning-fork resonance mode.
In response to an angular rate, V, being applied about the input axis, perpendicular
to the velocity vector of the masses, a Coriolis acceleration is produced which
forces the masses to translate in and out of the plane of oscillation. This resultant
antiparallel, out-of-plane motion is measured via the capacitive pick-off, providing
an output signal proportional to the rate input. Closed-loop control is employed to
maintain the proof mass at constant amplitude and the rate sensing is conducted in
an open loop manner.
The successful operation of this device depends on the electronics that controls
the mechanism motion and senses the rate output. Each gyro axis requires an analog
ASIC and a supporting field programmable gate array (FPGA), both on ball grid
arrays. The gyro electronics requires only power and needs no direction from the
microprocessor board except for requests for information. Gyro rate information
is currently sampled at a fixed rate of 600 Hz, and the resultant information is
communicated digitally through a low-voltage differential signaling (LVDS) inter-
face to the microprocessor. The gyro electronics and the packaged gyro sensors are
placed on printed wiring boards and can be assembled by standard pick and place
assembly equipment.

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