handbook of modern sensors

316 8 Velocity and Acceleration
Fig. 8.12. Vibratory rate gyro concept. (From Ref. [7].)
One of the first such devices was a two-gimbal structure supported by torsional
flexures (Fig. 8.12). It is undercut and free to move in the active area. In operation, the
outer gimbal, or “motor”, is driven at a constant amplitude by electrostatic torquing
using electrodes placed in close proximity. This oscillatory motion is transferred to
the inner gimbal along the stiff axis of the inner flexures, setting up an oscillating
momentum vector with the inertial element. In the presence of an angular rotational
rate normal to the plane of the device, the Coriolis force will cause the inner gimbal
to oscillate about its weak axis with a frequency equal to the drive frequency and with
an amplitude proportional to the inertial input rate. Maximum resolution is obtained
when the outer gimbal is driven at a resonant frequency of the inner gimbal. The
readout of the output motion is accomplished by setting the differential change in
capacitance between the inner gimbal and a pair of electrodes. When operated in
an open loop, the angular displacement of the inner gimbal about the output axis is
proportional to the input rate; that is, the output angle is proportional to an inertia
ration term, the drive angle, φ 0 , the mechanical Q, and the input rate . It is inversely
proportional to the drive frequency ω n :
[ ]
Ix + I y − I z φ0 Q
=
. (8.15)
ω n
I x
In a practical application, the device is operated closed loop and the inner gimbal is
rebalanced to null in phase and in quadrature. A detailed description of the gyroscope
may be found elsewhere [7].
A more recent design that also belongs to the third category was developed by
British Aerospace Systems and Equipment along with its partner Sumitomo Precision