Patent GB-850692-A (GB)
Abstract: 850,692. Correcting flux gate compasses. SPERRY RAND CORPORATION. June 17, 1957 [July 10, 1956], No. 19019/57. Addition to 850,596. Class 40 (1). Means for compensating for the errors introduced into a flux gate compass system on an aeroplane due to the magnetic field of the plane itself, Coriolis acceleration, tilt of the flux gate coils from the horizontal, &c., comprises a regulated D.C. source which is selectively fed via potentiometers to the coils of the flux gate to produce compensating magnetic fields of the correct magnitude and direction.- As shown, a flux gate assembly 1 (Fig. 2) consisting of three coils 12, 13, 14, wound one on each of the three limbs of a Y- shaped, high permeability, nickeliron core 10 is pendulously mounted on an aircraft so that the windings sense only the horizontal component of the earth's magnetic field. The central portion of the core 10 is energised by a vertical winding 15, whereby second-harmonic voltages are generated in the windings 12, 13, 14 which are fed via leads 16, 17, 18 (Fig. 1) to a selsyn receiver 2 whose rotor signal is used to monitor a directional gyroscope 3. The compensating currents are fed to the flux gate coils 12, 13, 14 via leads 16, 17, 18, and are determined separately for each source of error, as follows. Errors due to the field produced by the iron and the electrical currents in the aircraft itself.-If the plane is horizontal only the horizontal component of this field is sensed by the flux valve, and is neutralised by producing an equal and opposite field, obtained by feeding a regulated D.C. voltage from a source 50 through a potentiometer 21 to winding 12 of the flux gate and also through potentiometers 25, 26 to windings 13, 14, the sliders of potentiometers 25, 26 being ganged and moving in opposite directions. By adjustment of the sliders a compensating field of any magnitude and direction can be obtained to neutralise exactly the field due to the plane itself. If the plane tilts in pitch or roll, the vertical component of its field, multiplied by the sine of the angle of pitch or roll, also affects the flux gate, the effect of the horizontal component being unchanged for small angles of tilt. Correction is introduced automatically by pitch and roll angle signals obtained from a vertical gyroscope 57, which are added to the previous signals. Errors due to tilt produced by Coriolis acceleration are shown to be proportional to the aircraft velocity and the square of the sine of the aircraft position in latitude. The former is set up on each of the sliders of ganged potentiometers 35, either manually by knob 34, or automatically, and is fed through each of two ganged potentiometers 36 wound in accordance with a sine-squared function, whose sliders are set in accordance with the latitude. These signals are fed to windings 13, 14 to produce a field which eliminates the error due to Coriolis acceleration. Errors due to tilt produced by craft acceleration are similarly eliminated by obtaining signals proportional to the product of (i) the rate of turn, linear velocities and angle of dip, for lateral acceleration, and (ii) linear acceleration, air speed and latitude for longitudinal acceleration.