Patent GB-911842-A (GB)
Title: Improvements in vertical sensing devices
IPC: G01C19/44
Filed: 1959-02-10
Abstract: 911,842. Measuring electrically. ABBEY, A. (American Bosch Arma Corporation). Feb. 10, 1959, No. 4669/59. Class 40 (1). In a device for removing errors caused by various accelerations from the indications of a vertical-sensing pendulum, an additional force proportional to the time derivative of the accelerations is applied to the pendulum by mechanical means and a signal proportional to the force is subtracted from the output of the signal producing pick-off. The Specification contains a mathematical discussion which shows that if T/f 0 <SP>2</SP> is large and #, the sensitive plane of the sensing device relative to the north is constant, where T is the ratio of the damping coefficient to the weight of the pendulous mass and f 0  is the natural frequency of the pendulum then in the special case of a slowly moving vehicle the time derivative of the horizontal acceleration and hence the force to be applied is proportional to 1/Tg [V N  cos #+VE sin #] for one of a pair of perpendicularly mounted pendulums and l/Tg[VN cos (90 + #) + VE sin(90 + #)] or 1/Tg [VE cos # - VN sin #] for the other and signal voltages equal to this should be subtracted from the pick-off output. Here V N  and VE are the northerly and easterly components of vehicle speed. As shown in Figs. 1 and 2 a vertical spin axis gyro 10 is mounted in gimbals 13, itself supported in frame 14 which is mounted for rotation in a vertical gimbal 21. Gimbal 21 is supported from the deck 35 of the craft by a Cardan suspension in which the shaft is supported by gearing 28 and motor 34. Frame 14 carries two vertical sensing devices 38, 39 at right angles to each other, each consisting of a pendulum damped to make the term T/f 0 <SP>2</SP> referred to above large, and suspended from the rotor of a pick-off 36, 37, the stator of which is attached to the frame and to torque motors 43, 44, the stators of which are also carried by frame 14. The gyro constitutes a stable vertical. Displacement of shaft 22 from the spin axis of the gyro is detected by perpendicular pick-offs 17, 19, the outputs of which energize resolver 25. This resolver supplies voltages to roll and pitch gimbal motors 34, 29 which then bring the axis of shaft 22 parallel to that of the gyro 10. Pendulums 38, 39 detect the tilt of frame 14 and therefore of the gyro out of the vertical and energized torque motors 18, 20 so as to precess the gyro spin axis into the vertical. To produce the quantities referred to above resolver 50 is fed with a voltage proportional to craft speed while its rotor is positioned in accordance with the output (C 0  - #) of a differential C 0  being the craft's course. The output of winding 49 is then S 0  cos (C 0  - #) or V N  cos #+VE sin #. This is fed through a sealer 59 which multiplies it by 1/Tg and applies it to the control winding of torque motor 43 so as to apply torque proportional to it to pendulum 38. At the same time this value is subtracted from the output of pick-off 36. Similarly, the output of winding 58 is 1/Tg [VE cos # - VN sin #] and this is applied to pendulum 39 and subtracted from the pick-off output. The two corrected outputs are fed to torque motors 18, 20. To ensure that the sensitive planes 0 of the pendulums do not change, shaft 52 drives the rotor of selfsynchronous device 62, the stator of which is fixed relative to the deck. A signal (Co - 0) is induced in the stator windings indicating the displacement of the pendulum from the fore and aft line of the craft. The stator windings are connected to the stator windings 64 of a self-synchronous transformer, the rotor of which energizes the control field winding of motor 24 and is driven by a shaft 22 supporting frame 14. If the supports for motor 34 and the opposite bearing 33 are aligned fore and aft, motor 24 is energized to drive frame 14 into the position where vertical sensing device 36 is sensitive to tilts in the vertical plane displaced # degrees from the north. The Specification states that for rapidly moving craft certain of the above assumptions are not valid. The accelerometers must be rotated in azimuth at a constant rate d #/dt also an additional force proportional to RWE cos # sin # must be applied to the sensing device and the voltage proportional to this force subtracted from the indication of the vertical sensing device, where R is the mean radius of the earth, W e  is the earth's sidereal rate of rotation and # is the instantaneous latitude. In the arrangement shown in Figs. 4 and 5, gimbal 14 is extended to accommodate a horizontal spin axis gyro. Initially, the axis of the gyro is aligned to the meridian by making gyro 70 act as a compass. To this end the output of pick-off 76 is applied in series with and RW e  sin # voltage developed at member 90 to control motors 79 and 82. This causes the gyro to settle on the meridian when switch 83 is operated to disconnect motor 82. Gyro 70 is now free to rotate in azimuth, but its spin axis is kept horizontal by torque motor 79 and pick-off 76. The gyro rotates in azimuth at the required rate d #/dt and frame 14, which follows it, rotates at the same speed. The additional item RWE cos # is introduced into the system by a voltage RWE applied to resolver 84, the rotor of which is set at # and the Specification describes in detail how the total correcting force and voltage are derived.