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204 Inertial Sensors

distinguish the two different output modes, a different voltage modulation fre-
quency (200 and 300 kHz) is used for each sense electrode pair. The reported per-
formance was 1°/sec in a 25 Hz bandwidth. The natural driving frequency of the
rotor is about 25 kHz. Similar to single-axis devices a high quality factor can be used
to amplify the output motion. In a 60-mTorr vacuum Junneau et al. report a quality
factor of about 1,000. Electrostatic tuning of the different resonant frequencies can
be used. Cross-coupling between the two output modes is a major problem and was
measured to be as high as 15%. This implies that for a commercially viable version
more research has to be done for such a dual-axis gyroscope.
A conceptually similar implementation was reported by An et al. [68]. The
authors reported a higher resolution, of 0.1°/sec, which was mainly due to a thicker
proof mass (7 µm).

8.3.3 Commercial Micromachined Gyroscopes
Silicon Sensing Systems (a joint venture between BAE SYSTEMS and Sumitomo Pre-
cision Products [69]) is producing a very successful commercial gyroscope based
upon a ring-type sensing element. It uses magnetic actuation and detection, which
may prove to be problematic for further device size reduction. The ring has diameter
of 6 mm and is connected by eight radially compliant spokes to a support frame with
the dimensions of 10 × 10 mm. It is fabricated by deep reactive ion etching of a
100-m-thick silicon wafer. Current-carrying conductor loops are deposited on the
surface of the ring structure. These loops, together with the magnetic field, set up by
the permanent magnet provide the signal pick-off and primary oscillation mode
drive. This gyroscope has a resolution of 0.005/sec, a bandwidth of 70 Hz, and a
noise floor of 0.1/sec in a 20-Hz bandwidth. A picture of the sensor is shown in
Figure 8.28. Currently, they are developing a capacitive sensor without a permanent
magnet, thereby allowing for further size reduction [70].
Analog Devices has recently released the ADXRS family of integrated angular
rate-sensing gyroscopes, which contains the ADXRS300 (with dynamic range of
±300 mº/sec) and the ADXRS150 (with dynamic range of ±150º/sec). It is the first
fully integrated commercial gyroscope. A picture of the chip is shown in Figure
8.29(a). It operates from 5V supply over the industrial temperature range of –40°C

Figure 8.28 Commercial micromachined gyroscope from Silicon Sensing Systems. (From: [69].
© 1997 BAE Systems. Permission obtained from BAE Systems, who are a part of SSS.)