Components of a mechanical gyroscope

The basic components of mechanical gyroscopes are as follows:

The instrument case: The case in which the other elements are housed and which provides the structure by which the instrument is mounted in a vehicle.

The rotor or inertial element: This is essentially a flywheel rotated at high angular velocity. The rotor usually has the majority of its mass at the outer edge, as the moment of inertia is the sum of the products of the individual masses and the square of their distance from the axis of rotation. This enables a high angular momentum to be achieved for a given angular velocity. This approach also allows a high angular momentum to be achieved with the lowest overall mass. A low rotor mass is desirable to minimise vibration and shock effects.

Gimbals: These are support frames on which the rotor or another gimbal is mounted to isolate the rotor from rotational motion, by allowing freedom of angular movement of these frames about the rotor. In the case of a two-gimbal sensor, the axes of rotation of the two gimbals and the rotor are arranged to be mutually.

Pick-off: This device is used to detect relative motion between the rotor and the gimbals or between the rotor and the instrument case. The pick-off produces an electrical signal, indicating the direction and amplitude of the motion from a reference position. There are three basic forms of pickoff technology commonly used with mechanical gyroscopes operating in torque re-balance mode:

- moving coil - using a small receiver coil and an a.c. excitation coil, so that any relative movement between the two modifies the flux sensed by the receiver coil;

- variable reluctance - the excitation and receiver coils are fixed to the case of the gyroscope, with a soft iron assembly attached to the moving component so that it is in the flux return path between the excitation and receiver coils. Motion of the soft iron components causes a change in its orientation in the excitation field, thereby modifying the return flux to the receiver coil;

- capacitive - there is a stationary plate close to the rotor, or moving component, whilst the rotor acts as the other plate of the capacitor. Movement of the rotor about its input axis, or axes for a two axis sensor, causes a change in separation between the two plates of the capacitor and hence there is a change in capacitance.

Torque motor or electromagnetic torque: When a gyroscope is used in a closed loop or torque re-balance mode, it is necessary to generate a torque on the rotor in order to return the rotor to the 'null', or zero, position. This is achieved using a torque generator, which usually takes one of two common forms:

- Permanent magnet - this type relies on the interaction between the field generated by a permanent magnet and that of an electromagnetic coil. Particularly with single-axis sensors, a coil 'cup' is fixed to the moving element and the permanent magnet attached to the case. This has several advantages such as reducing the sensitivity to external magnetic fields and allowing the magnet to be outside of the flotation fluid. However, it does require a pair of flexible leads to the coil which can generate error torques. In general, as a result of other constraints, dynamically tuned gyroscopes have the opposite configuration with the permanent magnet fixed to the moving element.

- Electromagnet - a soft iron component is attached to the sensing element and a coil is fixed to the case. When a current is applied to the coil, a magnetic field is produced that interacts with the soft iron producing a torque on the sensing element.

Re-balance loop: This is the term given to the electronic circuitry that receives and uses the signals from the pick-off assembly. It interprets these signals in terms of the current required in the torquer coils to return the inertial element to its 'null' position. The re-balance loop electronics can either be analogue or digital. In the case of the analogue re-balance loop, a continuously variable current is passed through the coil to return the inertial element to its 'null' position. When there is no displacement, then there is no current flow. A digital re-balance system generates precision current pulses of particular duration to force the inertial element back to its 'null' position.

Spin motor: This is the motor used to rotate the inertial element and give it the angular momentum that is vital for the operation of the mechanical gyroscope. Usually the spin motor is either a hysteresis motor or an inductive device. Some gyroscopes that have a short run-time use a blast of air or a small explosive charge to spin the inertial element, and for cheap and crude applications, a d.c. electric motor may be used.

Float: Rate-integrating gyroscopes have their rotor and spin motor sealed in a can that is immersed in a fluid to reduce the load on the gimbal bearings. This can, with its encapsulated components, is known as the float. Careful choice of the flotation fluid can reduce the load of the rotor assembly on the gimbal to zero. The centre of buoyancy is arranged to be close to the centre of gravity of the float and along the output axis in order to minimise acceleration sensitive errors.

Flotation fluid: This is the fluid in the gyroscope that gives buoyancy to the float in a floated rate-integrating gyroscope. It also provides damping of the motion of the float which gives rise to the integration function for the single-axis rate-integrating gyroscope.

Bearings: For the spin axis of the rotor, most gyroscopes use ball bearings in a race with a retainer, and are chosen to have low noise characteristics. This form of bearing needs a suitable lubricant with the following characteristics:

- it should not separate into solid and liquid components;

- it should have small or negligible change in viscosity over the temperature range of the sensor;

- it should not leak out from the bearing;

- it should retain its physical and chemical properties for the required shelf-life of the gyroscope.

Lubricants can severely limit the environmental performance and shelf-life potential of a sensor. An alternative form of bearing that overcomes the well-known problems of 'ball bearings' is the gas bearing. The drawbacks with gas bearings are the need for very tight tolerances in manufacture, of the order of a micrometer or better, and the use of very hard materials such as boron carbide as the two bearing surfaces touch and rub during both starting and stopping of the rotor. However, they are very low noise bearings and can last a very long time, particularly if the bearing runs continuously.