GYROSCOPES
Definition
Gyroscope, any device consisting of a rapidly spinning wheel set in a framework that permits it to tilt freely in any direction. i.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 anti-roll equipment on large ships, and in inertial guidance systems.
History
J.-B.-L. Foucault, a 19th.-century French scientist is responsible for giving the name gyroscope to a wheel, or rotor, mounted in gimbal rings, i.e. a set of rings that permit it to turn freely in any direction (Fig 1)
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. Anschutz-Kaempfe for use in a submersible.
In 1911 Elmer A. Sperry marketed a gyroscope in the United States, and one was produced in Britain not long after.
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 Anschutz Company in Kiel, Germany and in a Danish passenger ship in 1916.
A three-frame gyroscope (Fig.1) 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 gyrostabiliser to reduce the rolling of ships, thus minimising damage to cargo, reducing stress in the ships structure, and adding to the comfort of passengers.
The roll-reducing action of this type of gyrostabiliser was quite effective and independent of the speed of the ship. It had a number of disadvantages, including its excessive weight, cost and space requirements. Therefore it was not installed on later ships in part because of the introduction by Japanese shipbuilders of an underwater fin-type ship stabiliser in 1925.
Conventional three-frame gyroscopes are used in ballistic missiles for automatic steering together with two-frame gyroscopes (Fig 2) 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 pilot-less 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 stabilised 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 stabilised by gyroscopes to an extraordinary degree of precision.
It was not until the 1950s that this variety of platform was perfected, following work done in the design of air-supported bearings and flotation gyroscopes.
(National Engineering Laboratory, East Kilbride, Glasgow )
The three-frame gyroscope
If the base of a three-frame gyroscope (Fig. 1 ) 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 gyroscope inertia. If the speed of the wheel decreases, the gyroscope inertia gradually disappears, the rotor axle begins to wobble and ultimately takes up any convenient position. Rotors with a high speed and concentration of mass towards the rim of the wheel display the strongest gyroscopic inertia.
It is 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.
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 towards 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 Fig.3 where the spin axis is parallel to the horizontal plane and the end of the spin axis points due north. As the direction of the Earth's rotation is counter clockwise when seen from above the North Pole, the relative direction of this end will change through Northeast, East, Southeast, South etc..
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 top end is apparently tilting upward. 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 the top, 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 couple, i.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.
The unrestrained or free three-frame gyroscope has little practical use because its spin axis is subject to tilting and drifting owing to the rotation of the Earth. In the controlled state it is widely used. The term control of a gyroscope implies that the spin axis, by small continuous or intermittent application of torque (twisting force), is made to precess so that it oscillates around a mark fixed in relation to co-ordinates on the Earth rather than in relation to space.
Controlled gyroscopes
Controlled gyroscopes fall into three categories:
- The north-seeking gyroscope is used in marine applications. In the settling (or normal) position the spin axis is kept horizontal and in the plane of a meridian.
- The directional gyroscope is used in aircraft and is sometimes called a self-levelling free gyroscope corrected for drift. With its spin axis horizontal it has directional properties but does not automatically seek the meridian.
- The gyrovertical has its spin axis vertical and is used to detect and measure angles of roll and pitch.
These types of three-frame gyroscopes are called displacement gyroscopes because they can measure angular displacements between the framework in which they are mounted and a fixed direction-the rotor axis.
Two-framed gyroscopes
The following simulated experiments conducted on the two-framed gyroscopes such as shown in Fig.1 illustrate the basis of important applications.
If, with the rotor spinning and the spin axis in the horizontal plane, the base is rotated uniformly in the horizontal plane a definite resistance owing to gyroscopic inertia will be felt. At the same time, the spin axis will begin to precess in the vertical plane and will continue to do so until the axis is vertical and all gyroscopic inertia disappears.
If the experiment is then repeated as before, except that while the base is being turned in the horizontal plane, the precessional movement of the spin axis is stopped by the application of force to the end of the axle where it terminates in the gimbal ring, the resistance to the turning motion of the hand due to gyroscopic inertia will cease to exist.
In effect the precession process will have been reversed. A vertical downward force applied to the end of the rotor axle introduces a torque that makes the base precess at the same rate and in the same direction as the turning movement of the hand. The quicker the base is turned, the greater the force that must be exerted on the axle to stop the precession.
Two important conclusions may be drawn from this experiment:
- There is resistance to the turning motion of the base if, and only if, the spin axis precesses.
- The force needed to stop the precession is directly proportional to the rate of turning of the base.
This force can be exerted by a spring arrangement in which the gyroscope (Fig.4) measures the rate of change of azimuth and is used in aircraft and ships as a rate-of-turn indicator.
The gyroscope illustrated in Fig. 4 is used on ships to measure the rate of roll, i.e. the angular roll velocity. The spin axis is positioned at right angles to the fore-and-aft line and the rate of roll is measured about this line.
These gyroscopes are called velocity or rate gyroscopes as distinct from displacement gyroscopes. The sensitive or input axis of a rate gyroscope is at right angles to its spin axis, while with a displacement gyroscope the spin axis is directly equivalent to the sensitive or input axis. A north rate gyroscope combined with a north displacement gyroscope, therefore, have their spin axes at right angles to each other.
A wide variety of gyroscopic devices have been developed. Some of the more familiar types are discussed here.
Stabiliser for ships
The main components of a ship stabiliser are a set of fins and the gyroscopes. The fins protrude from the ship's hull and are so operated that the forward motion of the ship produces a tilt in one direction on one fin and in the opposite direction on the other fin.
When properly controlled, therefore, these fins oppose the rolling motion. The gyroscopes sense the vertical angular displacement and the roll velocity and provide the proper control for the fins.
Inertial navigation systems
Neither position nor velocity can be sensed directly by an inertial system. Acceleration (change in velocity), however, can be detected by an accelerometer and this can be used to determine the position of a ship, aircraft, or space vehicle.
Basically this navigational system comprises three components: the platform, the gyroscopic frame and the computer. The accelerometers, mounted with their input axes mutually at right angles, are carried on a platform. Two accelerometers measure acceleration in the horizontal plane - the requirement for surface navigation.
For space navigation an additional accelerometer measures acceleration in the vertical plane. Each of the acceleration signals can be converted into distance travelled by determining, firstly, the total change in velocity which, added to the known initial velocity, gives the vehicle velocity; and second, the total change in position that, added to the known initial position, yields the present vehicle position.
The gyroscope frame is responsible for the stabilisation off the platform. Three rate gyroscopes are fitted in the frame with their input axes mutually perpendicular. Two of the gyroscopes provide the horizontal alignment of the platform - an essential requirement to eliminate the influence of accelerations due to gravity - while the third is responsible for the north-south alignment. Pitch, roll and yaw are detected by the three gyroscope input axes.
The gimbal deflection of each of the gyroscopes is converted into a signal voltage that, when amplified, drives a servomotor via a gear train to rotate the frame back to its original position.
The gyroscope frame also detects tilting and drifting due to the Earth’s rotational movement (Fig 3). If the platform is to be kept horizontal and north-south stabilised, torque signals must be applied to the roll and pitch servomotors to offset the precssion caused by the tilting, as well as to the azimuth servomotor to eliminate the precession caused by drifting.
The rate gyroscopes are not spring-restrained. Instead, flotation gyroscopes in which the precession is opposed by the viscous drag of a liquid are employed. The opposing torque is therefore proportional to the precession rate, instead of the precession displacement, as in a spring-restrained gyroscope.
The computer performs the necessary calculations. Specifically, it applies certain corrections to the acceleration, integrates acceleration to velocity and velocity to distance, computes latitude and longitude, and converts geocentric latitudes into geographical latitudes.
If the inertial system is used for inertial guidance in space navigation, then the computer also compares the vehicles position with the destination or target position to provide steering commands and compares the vehicles velocity (both direction and magnitude) with the programmed velocity vector to provide rocket steering and engine cut-off commands.
Stabilised platforms and gunsights
The inertial type platform is extremely small and must be stabilised to an extraordinary degree of precision, but the method of stabilisation used for gun platforms is essentially the same. The gyroscopes that detect platform displacement are not as accurate as the flotation type.
The gyroscopic gunsight revolutionised aerial gunnery. The sight fitted on the gun contains a gyroscope capable of measuring angular velocities in two lanes at tight angles to each other.
The gyroscope sight can be thought of as the three-frame gyroscope shown in Fig.1, constrained by horizontal and vertical springs to the inner and outer gimbol, respectively.
Instead of a mechanical spring arrangement, variable-strength magnetic fields are used to constrain the rotor axle in azimuth and elevation. The field coils for producing the horizontal component of this magnetic field are coupled to the range finder. The current through the vertical field coils is adjusted so that the field depends on the drop of the projectiles due to gravity.
The sensitivity of the gyroscope in the horizontal plane is a function of the sighting range; in the vertical it is a function of the gravity drop. In operating this gunsight, often called a predictor sight, the gunner holds the image of a central dot over the target while the gun is automatically aimed by the gyroscope at the place where the target will be at the expiration of the time of flight of its projectile.
Aircraft instruments
The three primary gyroscopic instruments fitted to he flight panel are a rate-of-turn indicator, a directional gyroscope, and an artificial horizon. Such gyroscopes may be driven by electric motors or by air jets. The directional gyroscope forms a standard reference for the pilot and navigator. It is a three-frame gyroscope with its spin axis in the horizontal plane. As soon as tilt develops, a switch is closed between the gyroscope housing and the vertical gimbol ring and a motor introduces a torque in the horizontal plane that causes the gyroscope to precess back towards the horizontal.
The artificial horizon displays the rolling and pitching motion of the aircraft. It consists basically of a three-framed gyroscope with its spin axis vertical and automatic correction devices to counteract the apparent motion of the spin axis around the celestial pole and any other random precessions.
Other applications
The gyroscope principle has been utilised in many other applications, such as the gyrocompass, gyropilot, and in non-rotating gyroscope devices.
A compensated magnetic compass, free from external accelerations, indicates magnetic north, which varies from true north from place to place on the Earth's surface. A gyrocompass however, when properly adjusted, can be made to indicate true north.
The marine gyrocompass is a three-frame gyroscope with its spin axis horizontal. To achieve the north-seeking and actual location (or meridian settling) properties of a gyroscope, use is made of the tilting effect of the spin axis when it is not pointing true north. As soon as tilt develops, a pendulum type device introduces torques that precesses the spin axis towards the meridian, causing it to describe a spiral with an ever-decreasing radius.
When stabilised the spin axis is maintained in the meridian plane by a precession equal but opposite to the drift at the particular latitude. When there is no tilting effect the marine gyrocompass will lose its directional properties and become useless. This is the case at the poles and also when a vehicle moves due west with a speed equal to the surface speed of the Earth. Because the latter condition can easily exist in an aircraft in the middle and upper latitudes, it cannot be used for air navigation.
Aircraft gyrocompasses are on automatically monitoring directional gyroscopes in which the monitoring device senses the direction of the meridian and ensures that the gyroscope axis is maintained in this direction. The monitoring device consists of a magnetic sensing unit called the flux valve, and allowance is made for variation in the direction of the Earth's magnetic field.
The gyropilot, commonly called an automatic pilot, consists basically of three devices, each of which detects disturbances of the aircraft in one plane and corrects these disturbances by moving the appropriate control: the rudder control for azimuth and sudden change in heading (yaw) disturbances, aileron control for roll disturbances, and elevator control for pitch disturbance.
In modern gyropilots rate detection forms the principle reference and displacement detection plays a secondary role. In such designs, a rate gyroscope detects yaw disturbance and the change in heading is detected by the associated gyrocompass. The two signals are added electronically and cause corrective rudder control to be applied to the rudder servomotor. The roll disturbance is detected by a roll rate gyroscope and by a roll angle pendulum, which senses displacement.
The aileron servo applies corrective action. A pitch rate gyroscope and pitch pendulum detects pitch disturbance. The elevator servo applies corrective action. Extensive use is made of two-frame gyroscopes to measure a vehicle's rate of turn.
Rate gyroscopes are also mounted on theodolites used for orientation of field artillery and for surveying. Theodolites mounted on gyrocompasses are used in mine exploration, since metal deposits would disturb magnetic compasses. Vertical three-frame gyroscopes with pen recorder attachments often are used to analyse rolling and pitching movements of ships and rocking of trains.
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