Gyroscopes 3
Gyroscopic Wander
friction. A perfect gyro with no external forces acting on it will not suffer from real wander.
The rigidity of a gyro system will tend to keep the spin axis fixed in space. Any movement away from this fixed direction is called wander. Gyro wander can be either drift or topple. These are earth references. Gyro drift occurs when the spin axis turns in the Earth horizontal plane, gyro topple occurs when the axis tilts in any earth vertical plane. Figure 3.3 shows examples of drift and topple. Notice that a gyro with a vertical axis can initially only topple and not drift.
Apparent Wander Having said that perfect gyros do not suffer from real wander there are many occasions when they appear to, always because our orientation in space has changed while the gyro’s orientation has not. This is apparent wander.
Drift and topple are references to earth alignment, not to space
Figure 3.4 Figure 3.4 shows five gyroscopes, A, B, C, D and E and the apparent effect on them as they are carried around on the rotating earth. Horizontal gyro B, at the equator with its axis aligned to the local meridian shows no apparent drift as it is carried round on the rotating earth. Horizontal gyro E at the north pole is showing an apparent drift of the full 15º per hour as the Earth rotates under it. The apparent drift, zero at the equator and the full 15º/hr at the poles, is a function of latitude:
Figure 3.3 Drift and topple are references to earth alignment, not to space There is another looser meaning of topple. Topple is also used to describe what happens when a gyro in a gimbal system meets its mechanical limit stops and precesses rapidly in random directions.
Apparent drift = 15 x Sin(latitude) degrees per hour. Gyro C, at the equator, which began as a vertical gyro, appears to become a horizontal gyro then becomes a vertical gyro again. It is showing apparent topple at a rate of 15º per hour. Vertical gyro D at the north pole is showing no apparent topple. The apparent topple, zero at the poles and the full 15º/hr at the equator, is also a function of latitude:
Real Wander Whenever the gyro spin axis moves away from its initial defined orientation in space the gyro is said to suffer from real wander. Real wander can either be deliberately induced by applying an external correcting force, as in alignment of tied gyros, or can be caused by imperfections in the gyroscope, unbalanced gimbals or bearing Instrumentation
Apparent topple = 15 x Cos(latitude) degrees per hour. v6.1.5
3.3