Patent GB-994543-A (GB)
Title: Gyroscopic apparatus
IPC: G01C19/5635
Filed: 1963-10-15
Abstract: 994, 543. Vibrating string gyroscopic apparatus. NORTH AMERICAN AVIATION Inc. Oct. 15, 1963, No. 40700/63. Headings G1C and G1N. A vibrating string gyro comprises an electrically conductive string stretched between two facing diaphragms, means for imparting a vibration to the string and diaphragms, and means for providing a signal indicating the angle between the plane of vibration of the string and a reference plane parallel to the string and fixed in the gyro. A fine gold-plated quartz fibre 11 having a diameter of one to three mils and a length of one to two inches is stretched to the limit of practicable tension between diaphragms 12, 13 having a thickness of the order of 60 mils and formed integrally with a quartz supporting body 70 provided with a massive stiffening ring 71. The string is caused to vibrate by an electrostatic drive comprising an electrode 14 plated on the outer surface of diaphragm 13, an electrode 19 fixed on the case of the gyro, and a capacitive pick-off plate 15 also secured to the case and providing a signal indicating the amplitude and phase of the motion. This signal is fed to a signal amplifier 17, whose gain is controlled by a thermister 16 in its feedback circuit, and thence to a power amplifier 18 whose output is fed to the electrode 19. A D.C. bias voltage from a source 20 is applied to the electrode 14 to excite the driving circuit, and the thermister 16 ensures that the initial vibration of the diaphragm has an amplitude substantially greater than the normal operating amplitude. The apparatus for sensing the plane of vibration of the string comprises a magnet coil 31 wound on a core 30 and excited from an A.C. source 32 which has a frequency substantially different from the resonant string frequency and a non-integral multiple thereof. If the gyro is turned about the string axis so that the string is no longer vibrating in a plane parallel to the field of the magnet 30, 31, a current is induced in the string and is fed through an amplifier 34 to a first de-modulator 35 phase referenced by a signal having the frequency of the string vibration. This signal is provided by a pair of capacitive pick-off plates 36, 37 mounted adjacent the string and connected to opposite ends of the primary winding 38 of a transformer 39. A D.C. excitation voltage is supplied by a source 40. The output of demodulator 35 is fed to a second demodulator 50 phase referenced from the source 32, and the output of demodulator 50 may be fed in known manner to drive an appropriate servochannel of a stabilized platform on which the gyro is mounted. Apparatus is also provided for caging the gyro, and comprises a pair of magnetic cores 53, 54 having series wound coils 55, 56 which produce a field normal to the normal plane of vibration of the string and to the field of the magnet 30, 31. The coils 55, 56 can be connected by a switch 58 to a controllable signal derived, e.g., from a potentiometer 59 and thereby induce in the string a controllable current. A fixed D.C. reference field parallel to the field of the magnet 30, 31 is provided by a magnet 51, 52 connected to a constant D.C. source 49, and this exerts a force on the string tending to return it to its normal plane of vibration and proportional to the induced current in the string. Conveniently the gyro may be caged by feeding the output of the second demodulator 50 to the coils 55, 56 through the switch 58. Rapid caging can be provided if desired by connecting a relatively low resistance 65 across the string through a switch 66. The body 70 of the gyro is mounted within a low-reluctance sealed case 80 to shield the instrument from stray magnetic fields and the case 80 is secured to a fixed mounting plate 81 by means of a number of shock absorbers 82. Vibrations transmitted through the shock absorbers are cancelled out by reeds 76 (Fig. 5) formed by cutting away portions of the body 70, the reeds being so dimensioned as to resonate at the string frequency. The magnetic assemblies are mounted in a shell 85 adapted to be rotated within the body 70 by means of a pinion 89 meshing with teeth 88 out in the shell. A clamp screw 90 is provided so that the shell may be secured in any desired angular position. This position is selected so that in the chosen reference plane drift due to irregularities in the string is minimized. Alternatively, errors due to vibration from external sources may be minimized by driving the string in its second mode of vibration. Specification 885, 946 is referred to.