# Gyroscope

A disc with a heavy rim mounted in such a way that its axis of rotation can adopt any position. Once the disc is spinning, the rotation axis remains fixed with reference to fixed stars, which makes the gyroscope useful for determining movement away from a fixed course.

The image shows a CAD model of a toy gyroscope. The disc is shown with the green rim and this is attached to an axle with a bearing at each end. The cage around the disc supports the bearings and gives some protection for fingers when the disc is spinning. The gyroscope is spun up by firstly winding a length of string around the axle and then pull the string hard in one continuous action until it realeases. A high rotational speed will be imparted on the disc in this way.

Mechanical gyroscopes spun electrically with a motor and allowed to rotate freely in a gimble were used in navigation systems until they were gradually replaced by solid state devices.

Three gyroscopes rotating about perpendicular axes help to maintain the orientation of a spacecraft in space by detecting rotation about any of the axes and initiating a mechanism to correct it.

### Caging

The process of orienting and mechanically locking the spin axis of a gyro to an internal reference position.

### Capturing

The use of a torquer to restrain the spin axis of a gyro to a specified position relative to the spin reference axis.

### Cat Whisker

A fine wire pickoff.

### Degree of Freedom

Number of orthogonal axes about which the spin axis is free to rotate.

### Drift

The angular deviation of the spin axis of a gyro from a fixed reference in space.

### Gimbal

A support which provides the spin axis with a degree of freedom.

### Gimbal Freedom

Maximum angular displacement about the output axis of a gimbal.

### Gyro Horizon

An artificial horizon or an attitude gyro.

### Gyro Pickoff

A device which produces a signal, generally a voltage, as a function of the angle between two gyro gimbals or between a gimbal and the base.

### Gyroscopic Inertia

The property of a rotor of resisting any force which tends to change its axis of rotation.

### Nutation

The oscillation of the axis of any rotating body, as a gyroscope rotor.

### Topple

The vertical component of precession or wander, or the algebraic sum of the two.

### Topple Axis

That horizontal axis, perpendicular to the (horizontal) spin axis of a gyroscope, around which topple occurs.

### Tumble

To precess suddenly and to an extreme extent as a result of exceeding its operating limits of bank or pitch.

Subjects: Physics