Page 131 - An Introduction to Microelectromechanical Systems Engineering

110 MEM Structures and Systems in Industrial and Automotive Applications

field induces a Lorentz force. Its radial component is responsible for the oscillation of
the ring in the plane of the die at approximately 14.5 kHz—the mechanical resonant
frequency of the ring. The sensing mechanism measures the voltage induced around
one or more loops in accordance with Faraday’s law: As the ring oscillates, the
area of the current loop in the magnetic flux changes, generating a voltage. Two
diametrically opposite loops perform a differential voltage measurement. One can
simplistically view an actuating and a sensing loop as the primary and secondary
windings of a transformer; the electromagnetic coupling between them depends on
the ring vibration pattern and thus on the angular rate of rotation.
Closed-loop feedback improves the overall performance by increasing the band-
width and reducing the system’s sensitivity to physical errors. Two separate feed-
back loops with automatic gain control circuits maintain a constant oscillation
amplitude for the primary mode of resonance and a zero amplitude for the secon-
dary resonance mode. The feedback voltage required to null the secondary mode is a
direct measure of the rate of rotation.
The fabrication of the sensor is relatively simple. A silicon dioxide layer is depos-
ited on a silicon wafer, then lithographically patterned and etched. The silicon diox-
ide layer serves to electrically isolate the current loops. A metal layer is sputter
deposited, patterned, and etched to define the current loops as well as the bond pads.
A layer of photoresist is spun on and patterned in the shape of the ring and support
flexural beams. The photoresist then serves as a mask for a subsequent DRIE step to
etch trenches through the wafer. After removal of the photoresist mask, the silicon
wafer is anodically bonded to a glass wafer with a previously defined shallow cavity
on its surface. Little is available in the open literature on the packaging, but it is clear
from the need to include a permanent magnet that the packaging is custom and spe-
cific to this application.
The specification sheet of the CRS03-02 gives an output scale factor of
20 mV/(º/s) with a variation of ±3% over a temperature range from –40° to +85ºC.
The noise is less than 1 mV rms from 3 to 10 Hz. The nonlinearity in a rate range of
±100 º/s is less than 0.5 º/s. The operating current is a relatively large 50 mA at a
nominal 5-V supply.

Angular-Rate Sensor from Daimler Benz
The sensor from Daimler Benz AG of Stuttgart, Germany [31], is a strict implemen-
tation of a tuning fork using micromachining technology (see Figure 4.25). The tines
of the silicon tuning fork vibrate out of the plane of the die, driven by a thin-film pie-
zoelectric aluminum nitride actuator on top of one of the tines. The Coriolis forces
on the tines produce a torquing moment around the stem of the tuning fork, giving
rise to shear stresses that can be sensed with diffused piezoresistive elements. The
shear stress is maximal on the center line of the stem and corresponds with the opti-
mal location for the piezoresistive sense elements.
The high precision of micromachining is not sufficient to ensure the balancing of
the two tines and the tuning of the two resonant frequencies—recall from the earlier
discussion that the vibration modes of a tuning fork are not degenerate. An imbal-
ance in the tines produces undesirable coupling between the excitation and sense
resonant modes, which degrades the resolution of the device. A laser ablation step
precisely removes tine material and provides calibration of the tuning fork. For this