4.1 Introduction

Gyroscopes are used in various applications to sense either the angle turned through by a vehicle or structure (displacement gyroscopes) or, more commonly, its angular rate of turn about some defined axis (rate gyroscopes). The sensors are used in a variety of roles such as:

• stabilisation,

• autopilot feedback,

• flight path sensor or platform stabilisation,

• navigation.

It is possible with modern gyroscopes for a single sensor to fulfil each of the above tasks, but often two or more separate clusters of sensors are used.

The most basic and the original form of gyroscopes makes use of the inertial properties of a wheel or rotor spinning at high speed. Many people are familiar with the child's toy which has a heavy metal rotor supported by a pair of gimbals [1]. When the rotor is spun at high speed, the rotor axis continues to point in the same direction despite the gimbals being rotated. This is a crude example of a mechanical, or conventional, displacement gyroscope.

Examples of mechanical spinning wheel gyroscopes used in strapdown applications are the single-axis rate-integrating gyroscope and twin axis 'tuned' or flex gyroscopes. An alternative class designation for gyroscopes that cannot be categorised in this way, is not surprisingly called unconventional sensors, some of which are solid-state devices. The very broad and expanding class of unconventional sensors includes devices such as:

• Rate transducers which include mercury sphere and magneto-hydrodynamic sensors;

• Vibratory gyroscopes;

• Nuclear magnetic resonance (NMR) gyroscopes;

• Electrostatic gyroscopes (ESGs);

• Optical rate sensors which include ring...