Establishing and Maintaining a Fixed Position
You now know that a free gyro maintains a fixed position in space. Because of this property, a free gyro can be used to establish a stable, unchanging reference, in any plane (horizontal, vertical, or any specific position in between). The gyro-erecting system has the function of positioning the gyro to the desired position and helping to keep it there.
Any gyro-erecting system must meet the following requirements:
The system must provide torques (forces) of sufficient magnitude and direction to precess the gyro so that its spin axis is brought to the desired position after the rotor is spinning at its operating speed. The system must provide torques to precess the gyro back to the required position at the proper rate and direction to cancel the effects of apparent and mechanical drift.
Erection may be done mechanically or electrically, depending on the type of power available. Specific erection systems are many and varied. We will briefly discuss only two of them.
Mercury Erecting System
One of the common erection systems used for vertical gyros uses mercury as the element for sensing gyro position with respect to vertical. Mercury also provides the force to precess the gyro toward vertical when the gyro drifts.
This system consists of two tanks of mercury fastened to opposite sides of the gyro case and connected by a small mercury tube as shown in figure 3-12. A small air tube is also connected between the tanks to prevent a vacuum from forming. If the spin axis tilts away from the vertical, as shown, the mercury will flow from one tank to the other. The added weight in the left tank provides a torque which causes the gyro to precess. At this point, if you were to apply the rule for precession, you would see that the precession would be 90° away from the desired direction.
Figure 3-12. - Mercury erecting system.
To overcome this difficulty, the point where the torque is applied must be moved. The torque point is moved by causing the gimbal assembly to slowly and continuously rotate in the proper direction. This is done in the following manner. With a small mercury tube, the mercury will take nearly a second to find its level. At the same time the mercury is flowing, a small motor is rotating the gimbal supporting the gyro about 18 times a minute. Therefore, during the time that it takes the mercury to flow into the low tank, the entire gimbal assembly has rotated 90°. The torque will now be applied at a point which causes the gyro to precess in the proper direction to maintain the gyro spin axis in a vertical position.
Mercury Ballistic Erecting System
The erection system used in many horizontal gyros is very similar to the vertical gyro system just discussed. It is called the mercury ballistic erection system. The mercury ballistic system has the added feature of maintaining the spin axis not only in the horizontal plane, but also with the spin axis aligned North-South.
There are many different methods of causing free gyros to precess to either the vertical or the horizontal plane. All such systems use the forces of gravity to sense variation from the desired position: all systems also use the principles of precession to position the gyro property.