A Compass Using a Thin Magnetic Film Component

The design and development of an allelectronic device which indicates bearing from local magnetic north is reported on. The device contains no moving parts and produces a d-c output voltage proportional to the bearing of a reference direction of the device from magnetic north. The sensing element of the compass consists of two thin magnetic films each of which contains a pumping coil and an output coil. When an a-c current is applied to the pumping coils an a-c voltage is developed in the output coil whose frequency is harmonically related to that of the pumping current and whose amplitude varies as H cos theta, where H is the magnitude of an externally applied magnetic field and theta is the angle between the external field and the axis of the output coil. By placing the two magnetic films in space quadrature and shifting the phase of the output voltages of the units so that the outputs are also in time quadrature, the output voltage of one unit will vary as H cos theta sin wt while that of the other will vary as H sin theta cos wt. The sum of these voltages will produce an a-c voltage whose phase shift depends upon theta, according to the trigonometric identity: cos theta sin wt + sin theta cos wt = sin (wt + theta). Detecting the phase shift of this signal will thus produce an output voltage proportional to the bearing of the magnetic field sensing element with respect to the external magnetic field. The method used in this device to detect this phase shift is to derive a series of pulses at the zero-crossings of the output voltage and apply these pulses to the "set" terminal of a bistable multivibrator. Another series of pulses derived from the zero-crossings of the pumping signal is applied to the "reset" terminal of the multivibratoro. The output of the multivibrator will then be a square wave whose duty cycle is proportional to the time interval between a "reset" pulse and the immediately preceding "set" pulse. Because these series of pulses are harmonically related, this time interval between the "set" pulse and the "reset" pulse will be proportional to theta, and thus the d-c component of the output of the multivibrator will be proportional to theta.