Triaxial magnetic heading sensing apparatus having magnetaresistors and nulling coils

摘要

A triaxial magnetic heading sensing apparatus includes nulling coils and netoresistive sensors having inputs and outputs. The nulling coils are arranged in parallel relation to three orthogonal planes intersecting at a common center point. The sensors are disposed along orthogonal axes in parallel relation to the orthogonal planes and in close proximity to the center point. Each sensor has a bridge configuration balanced in the absence of an applied external magnetic field and unbalanced in the presence one. Also, a signal generator is connected to the inputs of the sensors for exciting the sensors with a bipolar voltage waveform exhibiting alternating positive and negative voltage polarities punctuated by "off" states of zero volts. Further, a voltage integrator and low pass filter are connected in series with the output of a respective one of the sensors for generating an error signal representative of an imbalance due to the presence of an applied external magnetic field and for coupling the input and output of each nulling coil and of each sensor into a servo control loop such that each sensor is driven by the error signal to a magnetic nulling condition with zero steady state error. A pick-off resistor is connected to the output of the low pass filter for outputting a representation of the nulling current to provide a measure of the applied external magnetic field.