Patent GB-947436-A (GB)
Title: Stable reference apparatus
IPC: G01C19/5635
Filed: 1962-01-18
Abstract: 947,436. Automatic control of tension. NORTH AMERICAN AVIATION Inc. Jan. 18, 1962 [March 17, 1961], No. 1829/62. Heading G3R. [Also in Division G1] In a gyro of the type in which a string 10, Fig. 1, is maintained vibrating nominally in a plane, due to external vibration as well as asymmetry and anisoelasticity of the string ends, the string path of vibration is actually an ellipse and the major axis thereof tends to advance in the same direction as the motion of the string along the ellipse. By increasing the frequency or decreasing the amplitude of vibration, however, the major axis may be made to precess in the opposite direction and between these operating conditions there is a critical operating point wherein the elliptical path, and thus the precessional drift, is a minimum. This point may be detected by measuring the A-C component of tension which passes through a null and phase reversal at the critical point. As shown the quartz wire 10 is provided with a conducting coating such as gold or chromium 13 and attached by its enlarged end portions 11,12 to a driving disc 17 and discs 40, 41 forming a resilient mounting, respectively. The disc 17, which may be resonant at the drive frequency, and is secured to the housing 14 by an annular sleeve 18, co-operates with pick-off electrode 30 and electrostatic drive electrode 32. A closed loop oscillator is formed by pick-off 30, lead 33, amplifier 38, phase adjuster 39, amplitude controller 34, power amplifier 35 and drive electrode 32. A source of direct current 36 and resistor 37 ensures that the capacitor operates at basically constant charge, so that the voltage on plate 30 is proportional to the deflection of string end 12. The resilient movement of discs 40,41, is detected by a fixed electrode 42, the signal passing through lead 63 to amplifier 64 being proportional to the tension in the string 10. A biasing source of direct current 62 is provided. The output of amplifier 64 is fed to a demodulator 65, phase referenced by a signal from pick-off 30 amplified at 66 and, if necessary, phase adjusted at 68. The tension variation signal is fed through servo compensation network 67 to amplitude controller 34 to vary the amplitude of the drive signal fed to driving electrode 32 so as to maintain the string 10 vibrating at the critical point. A permanent magnet 52 carrying pick-off capacitor plates 53, 54 defines a reference direction with respect to which the angular position of the plane of vibration of the string is measured. The angle output signal is obtained at 51. The plates 53, 54 are electrically connected by slip rings 55, 56, and brushes 57, 58, to external leads so that the magnet 52 may be rotated by pinion 59 meshing with gear teeth, the magnet being locked in position by set screw 60. In a modification the wire 10 forms one arm of a strain gauge bridge tension detector. In another form tension is detected by a piezoelectric crystal. In further forms the tension variation signal controls the bias of a variable gain valve or transistor or controls the heating of a resistor located close to a thermistor in the feedback loop of a drive amplifier. The tension variation signal may control the frequency of a variable frequency drive oscillator instead of varying the amplitude of the drive. The string 10 instead of being conductively coated may be in the form of a capillary having an internal electrical conductor.