Gyroscope

I

Introduction

Gyroscope, any rotating body that exhibits two fundamental properties: gyroscopic inertia, or “rigidity in space”, and precession, the tilting of the axis at right angles to any force tending to alter the plane of rotation. These properties are inherent in all rotating bodies, including the Earth itself. The term gyroscope is commonly applied to spherical, wheel-shaped, or disc-shaped bodies that are universally mounted, so as to be free to rotate in any direction; they are used to demonstrate these properties or to indicate movements in space. A gyroscope that is constrained from moving around one axis other than the axis of rotation is sometimes called a gyrostat. In nearly all its practical applications, the gyroscope is constrained or controlled in this way. The prefix gyro is customarily added to the name of the application, as, for instance, gyrocompass, gyrostabilizer, and gyropilot.

II

Gyroscopic Inertia

The rigidity in space of a gyroscope is a consequence of Newton's first law of motion (see Mechanics), which states that a body tends to continue in its state of rest or uniform motion unless subject to outside forces. Thus, the wheel of a gyroscope, when started spinning, tends to continue to rotate in the same plane about the same axis in space. An example of this tendency is a spinning top, which has freedom about two axes in addition to the spinning axis. Another example is a rifle bullet, which, because it spins in flight, exhibits gyroscopic inertia, tending to maintain a straighter line of flight than it would if not rotating. Rigidity in space can best be demonstrated, however, by a model gyroscope consisting of a flywheel supported in rings in such a way that the axle of the flywheel can assume any angle in space. However, the model is moved about, tipped, or turned at the will of the demonstrator, the flywheel will maintain its original plane of rotation as long as it continues to spin with sufficient velocity to overcome the friction with its supporting bearings.

Gyroscopes constitute an important part of automatic-navigation or inertial-guidance systems in aircraft, spacecraft, guided missiles, rockets, ships, and submarines. The inertial-guidance instruments in these systems comprise gyroscopes and accelerometers that continuously calculate the exact speed and direction of the craft in motion. These signals are fed into a computer, which records and compensates for course aberrations. The most advanced research craft and missiles also obtain guidance from so-called laser gyros, which are not really inertial devices but instead measure changes in counterrotating beams of laser light caused by changes in craft direction. Another advanced system, called the electrically suspended gyro, uses a hollow beryllium sphere suspended in a magnetic cradle. The remainder of this discussion deals with the conventional gyro.

III

Precession

When a force applied to a gyroscope tends to change the direction of the axis of rotation, the axis will move in a direction at right angles to the direction in which the force is applied. This motion is produced jointly by the angular momentum of the rotating body and the applied force. A simple example of precession can be seen in the rolling hoop: to cause the hoop to turn a corner, guiding pressure is not applied to the front or rear of the hoop, as might be expected, but against the top. This pressure, although applied about a horizontal axis, does not cause the hoop to fall over, but to precess about the vertical axis, with the result that the hoop turns and proceeds in a new direction.

IV

Applications of the Gyroscope

The characteristic of gyroscopic inertia and the force of gravity can be used to make the gyroscope function as a directional indicator or compass. Briefly, if a gyroscope is considered mounted at the equator of the Earth, with its spinning axis lying in the east-west plane, the gyro will continue to point along this line as the Earth rotates from west to east. So the east end will rise in relation to the Earth, although it continues to point the same way in space. If a tube partially filled with mercury is attached to the frame of the gyro assembly, in such a way that the tube tilts as the gyro axle tilts, the weight of the mercury on the west or low side applies a force about the horizontal axis of the gyro. The gyro resists this force and precesses about the vertical axis towards the meridian. In the gyrocompass the controlling forces are applied automatically in just the right direction and proportion to cause the gyro axle to seek and hold the true meridian, that is, to point north and south.

Gyrocompasses are used in sea vessels all over the world. They are free from the vagaries of the magnetic compass; they indicate true, geographic north rather than magnetic north, and they have sufficient directive force to make practicable the operation of accessory equipment such as course recorders, gyropilots, and repeater compasses. The marine gyropilot has no gyroscope, but picks up electrically any divergence from the set course reference supplied by the gyrocompass; these signals are amplified and applied to the steering engine of the ship to cause the rudder to return the ship to its proper course.