Page 463 - The ROV Manual - A User Guide for Remotely Operated Vehicles 2nd edition

FIGURE 17.2
Mechanical gyro.
17.2.1.1 Slaved gyro
(Courtesy Graham Christ.)
17.2 Gyros 457
The slaved gyro is arranged to seek some type of reference point (such as True North). Unlike a magnetic compass, whose only orientation is to lines of magnetic flux from the Earth’s magnetic field, the gyrocompass senses its precession as it proceeds around the Earth and computes the axis of rotation. As the Earth only rotates in one direction, the pivotal point referencing to True North can then be deduced. For instance, in Figure 17.3, a gyro sitting on a table top located at Point A spins freely about its axis all day without precession as it is colocated on the North Pole (i.e., the pivotal point). A gyro at Point B maintains a constant orientation throughout the day as well since its rotation flows with the Earth’s, while Point C has the gyro perpendicular to the table at noon, parallel at 2100, perpendicular again (yet in the opposite orientation) at midnight, and parallel (but flipped from 2100) at 0900. The slaving mechanism measures precession about several axes, thus computing the precession over time to derive True North.
The gyrocompass’s (slaved gyro’s) function is based upon the following phenomena:
1. Gyroscopic inertia: the gyro’s conservation of angular momentum (i.e., natural resistance to change in orientation with regard to its inertial frame of reference)
2. Gyroscopic precession: the gyro’s measured deviation in local frame of reference to the gyro’s fixed frame of reference
3. The Earth’s rotation: the gyro is fixed in space while the gyro’s housing (termed the “gyro sphere”) is typically fixed to a base (termed the “phantom”) upon a floating vessel (which is fixed or floating with reference to Earth)
4. The Earth’s gravity: for basic orientation to the center of the Earth (for maintaining the gyro’s orientation with reference to the horizon—see Figure 17.4)