GYROSCOPE

any device consisting of a rapidly spinning wheel set in a framework that permits it to tilt freely in any directioni.e., to rotate about any axis. The momentum of such a wheel causes it to retain its attitude when the framework is tilted; from this characteristic derive a number of valuable applications. Gyroscopes are used in such instruments as compasses and automatic pilots onboard ships and aircraft, in the steering mechanisms of torpedoes, in antiroll equipment on large ships, and in inertial guidance systems. Figure 1: (Left) Three-frame gyroscope and (right) two-frame gyroscope J.-B.-L. Foucault, a 19th-century French scientist, is responsible for giving the name gyroscope to a wheel, or rotor, mounted in gimbal ringsi.e., a set of rings that permit it to turn freely in any direction (Figure 1, left). During the 1850s he conducted an experiment using such a rotor and demonstrated that the spinning wheel maintained its original orientation in space regardless of the Earth's rotation. This ability of a gyroscope to maintain its orientation suggested its use as a direction indicator, but it was not until 1908 that the first workable gyrocompass was developed by the German inventor H. Anschtz-Kaempfe for use in a submersible. In 1911 Elmer A. Sperry marketed a gyrocompass in the United States, and one was produced in Britain not long after. Figure 1: (Left) Three-frame gyroscope and (right) two-frame gyroscope In 1909 Sperry built the first automatic pilot using the direction-keeping properties of a gyroscope to keep an aircraft on its course. The first automatic pilot for ships was produced by the Anschtz Company in Kiel, Germany, and installed in a Danish passenger ship in 1916. A three-frame gyroscope (Figure 1, left) was used, also in 1916, in the design of the first artificial horizon for aircraft. This instrument indicates roll (side to side) and pitch (fore and aft) attitude to the pilot and is especially useful in the absence of a visible horizon. In 1915 the Sperry Company, employing a two-frame gyroscope, devised a gyrostabilizer to reduce the rolling of ships, thus minimizing damage to cargo, reducing stresses in the hull structure, and adding to the comfort of passengers. The roll-reducing action of this type of gyrostabilizer was quite effective and was independent of the speed of the ship. It had a number of disadvantages, including its excessive weight, cost, and space requirements, and it was not installed on later ships, in part because of the introduction by Japanese shipbuilders of an underwater fin-type ship stabilizer in 1925. Figure 1: (Left) Three-frame gyroscope and (right) two-frame gyroscope Conventional three-frame gyroscopes are used in ballistic missiles for automatic steering together with two-frame gyroscopes (Figure 1, right) to correct turn and pitch motion. German engineers made significant advances in this field during the 1930s, and their knowledge was later used in the design of guidance systems for the V-1 flying bomb, a pilotless aircraft, and the V-2 rocket, an early ballistic missile. In addition, the ability of gyroscopes to define direction with a great degree of accuracy, used in conjunction with sophisticated control mechanisms, led to the development during World War II of stabilized gunsights, bombsights, and platforms to carry guns and radar antennas aboard ships. Present-day inertial navigation systems for vehicles such as orbital spacecraft require a small platform that is stabilized by gyroscopes to an extraordinary degree of precision. It was not until the 1950s that this variety of platform was perfected, following work that was done in the design of air-supported bearings and flotation gyroscopes. Figure 1: (Left) Three-frame gyroscope and (right) two-frame gyroscope If the base of a three-frame gyroscope (Figure 1, left) is held in the hand with the rotor spinning and turned about any of the three axes, the rotor axle will continue to point in the original direction in space. This property is known as gyroscopic inertia. If the speed of the wheel decreases, the gyroscopic inertia gradually disappears; the rotor axle begins to wobble and ultimately takes up any convenient position. Rotors with a high speed and a concentration of mass toward the rim of the wheel display the strongest gyroscopic inertia. It is thus apparent that gyroscopic inertia depends on the angular velocity and the momentum of inertia of the rotor, or on its angular momentum. The rotor wheel is subject to the laws of rotational motion and inertia in that a freely rotating body will maintain a fixed direction in space, and the rotor tends to preserve its angular momentum, or spinning action, unless acted on by some external force. Figure 2: Apparent movement of the spin axis of a gyroscope The consequence of gyroscopic inertia is that to the observer on Earth the spin axis of a gyroscope makes an apparent movement over a period of time, although this apparent motion merely reflects the revolution of the Earth about its axis. There is one exception to this, that when the spin axis points toward the polar star, there is no movement of the spin axis with respect to the observer's surroundings, as the axis is parallel to the Earth's axis and points toward the celestial poles. This apparent movement is shown in Figure 2, in which, at position 1, the spin axis is parallel to the horizontal plane and the end of the spin axis, marked A, points due north. As the direction of the Earth's rotation is counterclockwise when seen from above the North Pole, the relative direction of the A-end will change through northeast, east, southeast, south (position 5), etc., and this clockwise movement will continue until, at the end of one period of rotation of the Earth (23 hours 56 minutes) the rotor and spin axis revert to their original position with respect to the observer on the Earth's surface. While this is taking place, the A-end is apparently tilting upward between positions 1 and 5 and tilting downward between positions 5 and 1. The change in azimuth (direction) of the spin axis is often referred to as drifting; sometimes tilting and drifting are collectively called apparent wander. If, while the rotor of a three-frame gyroscope is spinning, a slight vertical downward or upward pressure is applied to the horizontal gimbal ring at A or A, the rotor axle will move at right angles in a horizontal plane. But no movement will take place in the vertical plane. Similarly if a sideways pressure is applied at the same point, the rotor axle will tilt upward or downward. This second property is called precession. A precession or angular velocity in the horizontal plane is caused by the application of a couplei.e., parallel forces equal and opposite, in the vertical plane perpendicular to that of the rotor wheel. Precession is the tendency of the rotor's axis to move at right angles to any perpendicular force applied to it.