Patent GB-885946-A (GB)
Title: Gyro compass
IPC: G01C19/38
Filed: 1959-06-02
Abstract: 885,946. Gyroscopic apparatus; theodolites. NORTH AMERICAN AVIATION Inc. June 2, 1959 [June 6, 1958], No. 18772/59. Classes 97(2) and 97(3). A gyrocompass comprises a rotor 20 Fig. 3, spinning about axis 19 in a gimbal 23 which is free to rotate about a vertical axis 46 in a case 26, the arrangement being such that the space rate of rotation of the case 26 about the input axis 12, which is orthogonal to both the axes 19, 46, due to the rotation of the earth, produces a precession of the gyro about axis 46, so that when a pick-off 47 detects no such precession, the component of earth rate about axis 12 is zero which only occurs when the axis 12 is accurately aligned with the E-W direction. Such a gyrocompass may have a mirror 60 on which a theodolite 70, Fig. 4, orovided with an autocollimating eyepiece 79 is sighted to set the theodolite accurately in azimuth. Alternatively the theodolite may be mounted directly on the housing of the gyrocompass the theodolite azimuth scale being aligned with the input axis 12 of the gyroscope. After the E-W direction has been determined, the gyro case may be rotated through 90 degrees and the torque necessary * to prevent precession about axis 46 measured as an indication of the cosine of the angle of latitude, since the component of the earth rate along the axis 12 is proportional to it. T6 set axis 12 along the E-W direction, the screws 40, 41, 42 are adjusted so that axis 46 is vertical as shown by levels 44, 45, which may be highly accurate ones or only approximate ones, use then being made of the highly accurate levels in the theodolite associated with the gyrocompass by sighting on mirror 60. A "coarse bias adjustment" is made by energizing all portions of the gyrocompass. except the rotor 20, and opening a switch 57. Direct current from source 58, controlled by potentiometer 59 and resistor 61, then supplies power to a torquer 50, and the potentiometer 59 is adjusted until an indicator 52 reads zero, the indicator 52 being fed from the output of pick-off 47, energized by source 53, after being amplified at 48, demodulated at 49 and filtered at 56. In this condition the torque exerted by torquer 50 balances the internal torques acting on axis 46 due to fluid forces, electrical conductors, magnetic fields, and the like. The case 26 is then adjusted in movements about axis 27 by rotating an adjusting screw 33 which operates worm 32 and gear 31, the rotor 20 being energized and switch 57 closed, until the displacement detected by pick-off 47, and therefore indicator 52, is zero. Axis 12 is now approximately E-W Account must now be taken of the error torques acting upon the,gimbal with the rotor 20 energized e.g. those caused by vibration and thermal charges and this is done by modulating the rotor velocity, conveniently by changing its direction of rotation. So with the rotor 20 spinning in one direction, the case 26 is adjusted by screw 33 until the indicator 52 reads zero and the position of screw 33 with respect to scale 34 is noted. Rotor 20 is rotated in the opposite direction and the position of screw 33 with respect to scale 34 noted at which indicator 52 again reads zero and the average of the two positions of screw 33 is calculated and used. Axis 12 is now accurately aligned E-W. In a modification instead of a single torquer 50 being provided, and switch 57 opened or closed to make it a high or low resolution torquer, two separate torquers are provided having high and low resolutions respectively. In another form instead of averaging the two positions of screw 33, the error torque is measured with the rotor 20 spinning in the two different directions, and an average torque is applied directly to the axis 46 to cancel these error torques.