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10.1.3 Precession The tilting or turning of the gyro axis as a result of applied forces. When a deflective force is applied to the rim of a stationary gyro rotor the rotor moves in the direction of the force. However when the rotor is spinning, the same force causes the rotor to move in a different direction, as though the force had been applied to a point 90 0 around the rim in the direction of rotation. A gyro will resist any force that attempts to change the direction of its spin axis. However, it will move (precess) in response to such force; NOT in the direction of the applied force, but at right angles to it, as illustrated in Figure 50. The figure shows the application of a force which gives rise to a couple about the torque axis. The resulting turning movement about the axis of precession causes the rotor to move to a new plane of rotation, as the spin axis attempts to align itself with the axis about which the torque is applied.
Figure 50: Illustration of gyroscopic precession.
These rules apply to all spinning gyros: 1. A gyro rotor will always precess about an axis at right angles to both the torque axis and the spin axis. 2. A gyro rotor always precesses in a direction so as to align itself in the same direction as the axis about which the torque is applied. 3. Only those forces tending to rotate the gyro rotor itself will cause precession. 4. Precession continues while torque is applied and remains constant under constant torque. 5. Precession ceases when the torque is removed or when the spin axis is in line with the torque axis (the axis about which the force is applied). 10.1.4 The application of the precession principle The principle of precession can be exploited to provide a very accurate measure of angular rotation or rotation rate. Since a spinning wheel, or rotor, will only precess if a torque is applied to it, a rotor suspended in an instrument case by gimbals will maintain its spin axis in a constant direction in space. Changes in the angles of the gimbals will then reflect any changes in orientation of the case with reference to the spin axis direction. Alternatively, if controlled torques are applied to the rotor to keep its spin axis aligned with a direction defined by the case of the instrument, then the measurement of these torques will provide measurements of the angular velocity of the instrument, and hence of the angular velocity of any body to which the instrument is attached. Various sensor configurations have been developed over the years based on the principles described above. Attention is focused here on the dual-axis gyroscope, the type of sensor used in Gyrodata survey tools.
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