BAe gets silicon vibes with micromachined gyroscope

BAe gets silicon vibes with micromachined gyroscopeRichard Ball
First developed as a toy some 80 years ago, the gyroscope has become an important part of any inertial guidance or sensing system.  
  Hi-tech pencil sharpener… The silicon gyro is combined in a single device with a mixed-signal Asic which includes a phase-locked loop, gain control, amplitude detector, filter and demodulator.
However, the mechanical gyroscopes used today for motion and attitude sensing are expensive to make and bulky.
British Aerospace Systems & Equipment (BASE), working with Sumitomo Precision Products of Japan, has started production of what it claims is the first commercially available micromachined ring gyroscope.
Solid state devices have been successfully used in military applications, but are now being demanded by commercial systems such as vehicle ABS, airbag and navigation systems.
To lower cost – down to $50 in high volumes – and improve reliability, BASE turned to silicon micromachining.
Conventional gyros use a rotating ring, which is not possible with a solid state structure. Therefore BASE uses a vibrating ring, which is acted upon by the Coriolis force when the ring is turned.     What is the Coriolis effect?
The force acts on a moving element in a rotating body, adding a perpendicular movement to the element. The simplest demonstration is to hold a garden hose (remember to turn on the water) and turn in a circle. The Coriolis effect makes the water lag behind the turn. Contrary to popular belief and pseudo-science, it doesn’t make water spiral down the sink one way in the Northern hemisphere and the other South of the equator. Once the water starts to spiral in one direction, it continues to do so. The Coriolis effect alone would only make the water rotate once every 24 hours, like storms around low pressure areas.
The firm started work on Coriolis sensing elements in the early ‘90s. Ceramic cups were eventually replaced by a vibrating metal ring, in turn replaced by today’s silicon ring.
Creating the silicon ring structure involves a process known as deep trench etching. This results in the ring being supported by eight ‘Z’ shape legs (see pictures). The shape of the legs allows the ring to vibrate.
The vibration is the simplest mode possible. The ring becomes an ellipse in one direction, then an ellipse at 90?. Thus the four points at 45?, 135? and so on are at rest.
If the ring is turned or rotated in its own plane, then the Coriolis force comes into play. The vibrations lag the turn, creating a secondary vibration which means that the points on the ring at the 45? angles are displaced. This displacement is measured, and is proportional to the rate of turn.

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