Patent GB-586506-A (GB)
Title: Improvements in gyromagnetic compasses for aircraft and the like
IPC: G01C19/36
Filed: 1945-02-13
Abstract: 586,506. Gyro - magaetic compasses. PRITCHARD, H. C., THOMAS, I. L., and ROBERTS, G. E. Feb. 13, 1945, Nos. 3630/45, 16093/45 and 2116/46. [Class 97 (iii)] [Also in Groups XXXVI and XXXVIII] Outputs corresponding to the components of the earth's magnetic field along reference axes, e.g. fore and aft and transverse in the supporting craft, are provided by flux-gate elements and applied to sine-cosine potentiometers, which are adjusted in accordance with a heading reference provided by a gyroscope, the outputs of the potentiometers being applied together to control means for adjusting the gyroscopic heading reference into correspondence with the magnetic heading reference provided by the flux-gate elements. As shown, pick-up windings 14, 15 on the magnetic detector elements 11, 12 are connected across the windings of sine-cosine potentiometers 16, 17, respectively, which are thus energised in accordance with the cosine and sine of the magnetic heading angle. Contact arms 23, 24, connected together at 28 through a direct current blocking condenser are disposed mutually at right angles on a common operating shaft 25, which is caused by the induction motor 32, amplifier 31 and autosyns 27, 30 to repeat the movements in azimuth of a gyro 26, so that factors proportional to the sine and cosine of the gyroscopic heading angle are introduced into the potentiometers 16, 17, respectively. Fixed centre taps 18, 19 on the potentiometers 16, 17 are connected in opposition to a phase discriminating rectifier from which the D.C. output passes via leads 33, 34 to a precessing coil 35 which is mounted on the vertical ring of the gyro and co-operates with arcuate permanent magnets 38, 39 on the inner gimbal. The departure of the gyro from the magnetic heading reference is indicated at 37. The shaft 25 is provided with an indicator 57 and a transmitter 58 for repeating the readings. In order to increaseaccuracy the windings 14, 15 may be supplied with D.C. so as to produce nulling field components which will be equal and opposite to the earth's field components only when the gyroscopic and magnetic heading references correspond. Additional contact arms 49, 50 at right angles to the contact arms 23, 24 respectively are provided and are connected at 53 through an A.C. filter 51, 52. The D.C. is supplied from the amplifier 22 to the centre taps 18, 19 of the potentiometers through leads 45, 46. A variable resistance 56 having three adjustments for tropical, temperate, and polar regions is included in the lead 45. The lead 46 passes through the primary of a transformer 47 which serves to isolate the A.C. input to the rectifier. The leads to the winding 14 include a variable resistance 54 (shunted by a by-pass condenser) and a variable resistance 55 to enable various constants to be adjusted. In a modification the potentiometer shaft is driven directly, by the gyroscope and each of the pick-up windings is connected to an amplifier including a phase discriminating rectifier, the D.C. outputs of which are passed to the corresponding potentiometers and to feed back coils on the detectors. Preferably the excitation current of the detector elements should have a D.C. component and a relatively small A.C. component so that the response is suppressed at alternate half cycles and outputs are obtained at the excitation frequency instead of at the second harmonic frequency. In a further modification the azimuth gyro is dispensed with and the output of a potentiometer adjusted by a rate of turn gyroscope is applied via an amplifier to an integrating motor driving the shaft of the sinecosine potentiometers. The output from the latter after amplification and rectification is applied either as an additional input to the amplifier of the integrating motor or to an additional motor coupled to the potentiometer shaft through a differential gear. When an azimuth gyro is used, it is preferably caused to settle with its axis in an E.-W. direction and incorporates a switch as described in Specification 586,507, to prevent adjustment of the gyro when the detectors are displaced about a N.-S. axis. The windings of the potentiometers may be angularly adjustable to compensate for magnetic variation. According to the first Provisional Specification, relays may be employed to introduce an auxiliary precessing coil when the gyroscopic departure is large.