handbook of modern sensors

8.6 Gyroscopes 317
Products Company Ltd. [8]. The design is based on a ring resonator that is micromachined
in silicon. Silicon has remarkable mechanical properties (see Section 18.1.1
of Chapter 18 for details); specifically, in its crystalline state, silicon has a fracture
limit of 7 GPa, which is higher than the majority of steels. Coupled with this is a
low density of 2330 kg/m 3 , resulting in a very robust material under its own weight.
The gyro resonator is etched out of the crystalline material. This ensures that the
properties of the resonator are stable over its lifetime and environment. The planar
vibrating-ring structure has all of the vibration energy in one plane. As such, under
angular rate, there is no coupling of vibration from one crystal plane to another, so
that the vibrating parameters are very stable over temperature.
In order for the resonator to function correctly, it must be supported in a way that
allows it to vibrate as freely as possible. The sensing element is shown in Fig. 8.11B.
The resonator comprises a 6-mm silicon ring, supported by eight radially compliant
spokes, which are anchored to a 10 × 10-mm support frame. Current-carrying conductors
are deposited and patterned onto the top surface only, and pads for wire bonding
are located on the outer support frame. The chip is anodically bonded to a supporting
glass structure which is thermally matched to the silicon. There are eight identical
conducting loops, each of which follows the pattern: bond pad→along length of support
leg→around 1/8 segment of ring→along length of next support leg→bond pad.
Each leg thus contains two conductors, one each from adjacent loops, in addition to a
third conductor, which lies between them, to minimize capacitive coupling. The silicon
substrate is also connected in order to provide a ground plane. The resonator may
be excited into vibration by any suitable transducers. These may function by means
of optical, thermal expansion, piezoelectric, electrostatic or electromagnetic effects,
for example. The excitation may be applied to the support structure which carries the
resonator, or directly to the resonator itself. The fundamental vibration mode is at 14.5
kHz. Figures 8.11C–8.11F show the effects of linear and angular acceleration on the
resonator. Figure 8.11C shows a side view of the resonator under conditions of no acceleration,
Fig. 8.11D shows the effect of z-axis linear acceleration, Fig. 8.11E shows
the effect of angular acceleration about the x axis, and Fig. 8.11F shows the effect of
angular acceleration about the y axis. Because the ring position changes with respect
to the frame, what is required is a combination of displacement pickup transducers
to detect a particular movement of the resonator. Resonator vibration may, for example,
be sensed by transducers working electromagnetically, capacitively, optically,
piezoelectrically, or by means of strain gauges. In this particular design, a magnetic
pickup is employed by pattern conductive loops along with a magnetic field which is
perpendicular to the plane of the ring. The magnetic field is provided by samarium
cobalt and the entire structure is housed in a standard hermetic metal integrated circuit
can package.
8.6.3 Optical Gyroscopes
Modern development of sensors for guidance and control applications is based on
employing the so-called Sagnac effect, which is illustrated in Fig. 8.13 [9]. Two
beams of light generated by a laser propagate in opposite directions within an optical