Text 1b. Base Motion Isolation

The principal advantage of our spinning wheel is that it's lazy: It wants to stay where it is in space.

This means that if the base surface turns around the outer gimbal axis or around the inner gimbal axis, the gyro spin axis stays put. In other words, the gimbal system isolates the rotor from the base rotations.

This last statement deserves a word of explanation. Let us start with the base surface horizontal and with the spin axis pointing straight up. If the base turns around the outer gimbal axis, or if the base turns about both axes at the same time, the spin axis still points straight up. The spin axis is stabilized in space, or has been isolated from base motion.

If some kind of angle measuring gadget called the pickoff (a potentiometer, synchro, E-pickoff) is used to measure the rotation of the base surface around the inner gimbal axis and/or the outer gimbal axis, we can then determine how far the base surface has moved from a stabilized reference line (the gyro spin axis).

A Bit of History

The ability of the gyro spin axis to stay fixed in space was utilized in 1850 by Jean Foucault (1819-1369) of France, in order to demonstrate the rota­tion of the earth. The spin axis stays fixed in space while the base surface,

attached to the earth, rotates in space. Therefore, after a period of time, the spin axis looks as if it had rotated with respect to the earth. Foucault coined the word gyroscope from two Greek words: gyros, meaning revolution, and skopein, meaning to view. Thus, gyroscope means to view revolution or

rotation.

In more recent times, the base surface of the gyroscope may be attached to an airplane or missile. Suppose the stabilized reference line has been given some particular orientation. The gimbal pickoffs will tell us how far the vehicle has deviated from the predetermined position. These pickoff signals can be used to actuate control circuitry, to place the vehicle back in the proper position.

This is pretty much how an autopilot works. Gyroscopes used in this manner were first demonstrated in 1914 by Lawrence Sperry (son of gyro pioneer Elmer Sperry). He won 50,000 francs in Paris, France, by flying an airplane low over a judges' stand with his hands off the controls, and with a mechanic walking out on the wings.

Answer the questions to the text.

1.What is the origin of the word gyroscope?

2.Describe the principal advantage of the gyro’s spinning wheel.

Text 1C. Classification of Gyros

Remember the following definitions:

Single degree of freedom gyro – гироскоп с одной степенью свободы

Two degree of freedom gyro – гироскоп с двумя степенями свободы

Quadrature axis – поперечная ось

Vertical gyro - гировертикаль, гирогоризонт

Two-Degree-of-Freedom Gyro

The stabilized reference line may be given some particular orientation. If this orientation is up and down along the earth's vertical, and kept that way by means of some kind of plumb bob pendulous reference the device is called a vertical gyro.

If the stabilized reference line is horizontal, the unit is called a directional gyro. A gyro which is free to move about both the inner and the outer gimbal axis, but-where the spin axis has no particular orientation, is called a free gyro.

The general class of units we have been talking about so far covers the two-degree-of-freedom gyros. Their base surface can rotate about two quad­rature axes (axes at right angles to each other), the inner gimbal axis and the outer gimbal axis without disturbing the stabilized reference line.

Single-Degree-of-Freedom Gyro

It is also possible to have single-degree-of-freedom gyros. Here, the base surface is rotated about only one axis without disturbing the stabilized reference line. For example, the spin axis is stabilized against rotation around the gimbal axis, but would be disturbed by rotations about the quadrature axis.

There are two special single-degree-of-freedom gyros: the integrating gyro, which measures angular position (somewhat similar to the gyros mentioned thus far) ; and the rate gyro, which measures rate-of-change of angular position. This rate-of-change of angular position is called angular velocity.

Three-Degree-of-Freedom Gyro

A three-degree-of-freedom gyro can also be built. However, either two two-degree-of-freedom gyros (usually a combination of a vertical gyro and a directional gyro), or three single-degree-of-freedom gyros (usually integrating gyros) are needed. It appears impossible to build a three-degree-of-freedom gyro using only one rotor.

A three-degree-of-freedom system is called a stable platform. Pickoffs of a stable platform measure deviation of the vehicle about three quadrature (mutually perpendicular) axes. These axes form a space stabilized co­ordinate system. The stable platform will be considered in much more detail in the pages following.

Degree-of-freedom is a common way to classify gyroscopes. But in determining the degree-of-freedom, the freedom to rotate about the spin axis is not counted. For example, on a two-degree-of-freedom gyro, the spin axis (or stabilized reference line) is not disturbed by rotations about the outer or inner gimbal axis. At first glance, it might appear that the spin axis is also not disturbed by rotations about itself. Actually, no stabilization is achieved about the spin axis. We could not, for example, place a pickoff between the inner gimbal and the rotor and be able to read out how much the inner gimbal (or base surface) has rotated about the spin axis. However, a pickoff between the outer gimbal and the base surface will measure base rotation about the outer gimbal axis.

Gyroscopes have also been classified as being instrument or non-instrument units. Generally, instrument gyros are used as transducers or sensors, while non-instrument gyros are used to do work. A vertical gyro is an example of an instrument gyro, and a ship stabilizer is an example of a non-instrument gyro.