3 Gyroscopes
Classification of Gyroscopes
The final group is distinct from the other three by having a freedom of movement in the plane of rotation and one more plane, at 90º to the first. This is called a rate gyro and can be used as a rate of turn indicator. A developed form of rate gyro is a rate-integrating gyro, and both have two planes of freedom.
To classify gyros, we speak of planes of freedom. The gyro itself rotates in one plane, about its spin axis. There are then two other planes that we can use to define the gyro, both of them at right angles to the plane of rotation, and at right angles to each other. Gyros are split into four main categories. The simplest is the free or space gyroscope that is completely free to move in all three planes in relation to its mounting system. Figure 3.2 shows a space gyro.
There exists a second form of classification of gyros by degrees of freedom; effectively the number of axes about which gyro displacement can be measured It does not count the axis of rotation of the gyro as one of these. Thus a two plane of freedom gyro is a single degree of freedom gyro.
Alignment of Gyroscopes Having classified gyros, we must define their orientation in space. This is done by specifying the alignment of the axis of rotation. Thus a vertical gyro has its spin axis in the Earth vertical. A horizontal gyro has its axis in the Earth horizontal, but as earth horizontal is a plane, not a line, this alone does not give sufficient information to fully define the axis. We have to add the azimuth information, and a full definition would be, for example, “a horizontal gyro with its axis aligned with true north”.
Figure 3.2 - Gyro in gimbal frame The next group is of a special form of space gyro called tied gyros. These retain freedom of movement in all three planes but there is now an external influence controlling the direction of the spin axis. An example of this would be the directional indicator (DI), where the spin axis is tied to the horizontal. This will be looked at later on. The third group is an even more specialised type of tied gyro where the spin axis is tied by the Earth’s gravity to remain in the Earth vertical. This is called an earth gyro, an example would be the artificial horizon. All of these, space, tied and earth gyros, have three planes of freedom.
3.2
Earth vertical is a line, formed at the intersection of all the vertical planes. Earth horizontal is a plane, made up of all the azimuth lines
v6.1.5
Instrumentation