Patent GB-734076-A (GB)
Title: Improvements in indicating apparatus, direction transmitters and in particular magnetic compasses and their compensation
IPC: G01C17/38
Filed: 1951-06-05
Abstract: 734,076. Electric follow - up systems. BELLON, F. B. June 5, 1951 [June 6, 1950], No. 13285/51. Class 40 (1). In a magnetic compass repeater system in which the deflections of the magnetic sensitive element are detected by a secondary magnetic element magnetically coupled thereto, the secondary element comprises a movable magnet with in some cases one or more fixed magnets in addition, and is so disposed that the torque exerted by the whole of the secondary element on the sensitive element remains substantially zero whereas the movable magnet of the secondary element is deflected sufficiently to actuate the repeaters. As shown, Fig. 1, the secondary element comprises a pair of normally parallel magnets 2, 3, not necessarily identical, disposed on opposite sides of the compass magnet 1, and so dimensioned and disposed in balanced relationship as to exert substantially zero torque thereon in any position of the compass. When the craft heading alters, the compass element 1 rocks the magnet 2 through a small angle to close a contact a or b which controls a motor to drive the framework carrying the magnets 2, 3 to follow the movements of the compass element 1. The magnet 2 is normally in unstable equilibrium so that the repeater motor oscillates about the rest position and tends to produce a smaller oscillatory movement of the compass magnet 1. The oscillations of the magnet 2 may be sustained by mechanical or magnetic means if desired. Alternatively, hunting of the repeater motor may be prevented, if desired, by opposing the torque produced by the magnet 2 as it shifts from one contact to the other, e.g. by moving the magnet 3 so that it is always parallel to the magnet 2 or by magnetic or electric means. The detector magnet 2 may alternatively carry a mirror co-operating with a light source and two photo-electric cells connected in a bridge circuit for controlling the follow-up motor, or the magnet 2 may carry an electrode c, Fig. 2, co-operating diffeientially with fixed electrodes a1, bl in the motor control circuit. The speed of the follow-up motor may be controlled proportionally to the angular displacement of the compass by producing a counter-torque on the detector magnet 2 by means of a further magnetic field, the intensity of which varies with motor speed. In an alternative arrangement the detector magnet 2, instead of controlling a follow-up motor, energizes through a time delay circuit a coil producing a counter torque on the detector magnet, so that the coil current gradually increases to a value proportional to sin #, when the deflecting torque is balanced by the counter torque. # is the magnetic heading of the craft and by using two detector systems disposed at 90'degrees to each other currents proportional to sin #, and cos # may be derived and used to actuate remote heading indicators of the moving iron or C.R.T. type or a course recorder. Other arrangements Figs. 3, 4, 5 (not shown) of the detector magnets are described. When the compass magnet is a sinusoidal magnet, the secondary element may consist of the movable magnet only, the poles of which are arranged to be on a common equipotential line or surface of revolution due to the sensitive element. Arrangements are described, Fig. 12 (not shown), for applying correcting fields electromagnetically to the compass element proportional to the sine of the tilt angle, of the craft carrying the compass, about two mutually perpendicular axes, e.g. pitch and roll axes. Transmitter construction, Fig. 6. The " sinusoidal " compass magnet 21 carrying the compass card 22 is mounted in an eddy current copper-damping cylinder 23 above the detector magnet 25 and a further magnet 24. The contacts a, b control a motor in receiver 26 to drive the elements 24, 25 into alignment with the sensitive element 21. Semicircular error corrector magnets 27, quadrantal error corrector magnets 28, and dip-error corrector magnet 29 are provided. Circuit arrangements. In Fig. 7 closure of contact a or b by the detector magnet 25 energizes a corresponding half of a relay 31 to energize the follow-up motor 33 in the appropriate sense to drive a transmitter 34 to which receivers 26, 35 are connected. In Figs. 8, 9 (not shown) closure of contact a or b causes the appropriate one of a pair of normally blocked valves to pass current and reverse the phase sense of the current in the control phase of a two-phase follow-up motor. The contacts a, b may be readily replaced by the differential condenser of Fig. 2, in the Fig. 8, 9 circuits. The detector element 25 may, alternatively, vary a capacitor 40, Fig. 10, to vary the frequency of an oscillator 41, the output of which after amplification at 42 is fed to a frequency discriminator 43, the D.C. output of which operates the servomotor 44 which drives the plate a1 to restore the frequency of the oscillator to that of the discriminator. In Fig. 11 the differential condenser of Fig. 2 controls differentially the current flow in transformer primaries 45, 46. The field coils of the followup motor 44 are energized from the respective transformers through diodes 48, 49. Specification 674,074, [Group XXXVI], is referred to.