Patent GB-608049-A (GB)
Title: Improvements in or relating to electrical phase-discriminative or sense-discriminative demodulating systems
IPC: G05D3/18
Filed: 1945-11-27
Abstract: 608,049. Automatic control systems for gyros. SPERRY GYROSCOPE CO., Inc. Nov. 27, 1945, No. 32061. Convention date, Oct. 2, 1942. [Class 38 (iv)] [Also in Groups XXXVIII and XI (c)] An electrical phase sense discriminator system comprises two devices, e.g. grid-controlled electric discharge tubes, having their output circuits fed with A.C. in selective anti-phase and their input circuits, which are biassed with A.C. of the same frequency also in anti-phase are fed with the A.C. signal voltages in like phase. As shown, for maintaining the vertical ring and the phantom ring 5 of a gyroscopic compass in alignment, the core 10 of a " pickoff " 9 is inserted on the phantom ring and co-operates with the armature 11 on the vertical ring. The primary winding 21<SP>1</SP> of the pick-off is supplied from a constant source of A.C. and the secondaries 22, 22<SP>1</SP> are connected in opposition across resistor 28 and are tuned to the supply frequency by condensers 25, 25<SP>1</SP>. Condenser 29 reduces the harmonic content of the output voltage, which is adjusted to zero, or minimum voltage, by the tapping 27 at the aligned position of the rings. The setting of potentiometer 31 determines the fraction of the output unbalance corresponding to the displacement of the rings applied to the primary of transformer 35, the secondary windings 36, 37 of which apply the displacement signal, in like phase relationship to the control grid of each of the similar double triodes 38, 39 in similar demodulators 20, 21. Bias resistors 42, 45 are arranged so as to cause at least partial rectification of the alternating input displacement signed in valves 38, 39. The output voltages of the demodulators 20, 21 are developed across the resistors 54, 65 respectively, being, by design, in like phase, and are applied to the grids of gas-discharge valves 85, 86. Transformers 75 and 88 are supplied, from the same A.C. source as the pick-up 9, with voltages variable relatively in phase by means of phaseshifting device 791. The secondaries 89, 90 of transformer 88 supply plate voltages to valves 85, 86 in anti-phase and the A.C. grid bias potentials therefor, similarly to anti-phase, are provided by secondaries 77, 78 of transformer 75. The plate currents of valves 85, 86 are applied through leads 92 to azimuth motor 7, which has a field 7<SP>1</SP> permanently excited. Misalignment of the vertical ring and phantom elements causes relative displacement of the armature 11 and core 10 and produces an unbalance voltage in windings 22, 22<SP>1</SP>, a portion of which is applied simultaneously to auxiliary demodulators 20, 21 through the transformer 35. The output of each auxiliary demodulator comprises an alternating component, of supply frequency, and of amplitude and sense corresponding to the misalignment, together with a demodulated component, of considerably lower frequency equal to that of the variation of the misalignment, of which the magnitude and sense again depend upon the misalignment. In auxiliary demodulator 20, the alternating component produces an alternating potential across resistor 54 and the demodulated component causes a reactive drop across choke 48, proportional to the rate of change of misalignment, which is modified by the differentiating circuit 53, 54 to produce a further voltage across resistor 54 proportional to the first and second time derivatives of the misalignment. The condenser 52 determines the ratio of the misalignment voltage to the rate voltage developed across reactor 54. The auxiliary demodulator 21 operates similarly to produce an A.C. displacement voltage and a rate voltage across resistor 65 which are respectively in the same and opposite senses to the corresponding voltages produced across resistor 54. As the field 7<SP>1</SP> of the motor 7 has a constant excitation, the motor rotates, in correspondence to the phase sense of the output of pick-up 9 which determines the rectified outputs from valves 85, 86, to neutralise a misalignment signal, hunting being prevented by the rate voltages. In a second embodiment (not shown), the outputs from a transformer identical to 35 of Fig. 2, are applied in anti-phase to the grids of a dual rectifier, biassed away from linearity, which is followed by a balanced dual amplifier. The in-phase outputs from the latter are applied respectively to the grids of gasdischarge valves, the A.C. biasses and plate potentials of which are phased as in the first embodiment, which control a correction motor driving both the phantom ring and a tachometer. The voltage applied from the latter, through a differentiating circuit, to the grids of the dual rectifier, functions as a second derivative control-bias simultaneously with the misalignment voltage and the system operates to correct misalignment without hunting. An integrating circuit may be substituted in place of the differentiating circuit to modify the tachometer output. Specification 483,635, [Group XXXVIII], is referred to.