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Experimental investigations of a recent fluxgate theory Carter, Matthew


A recent theory describes the fluxgate magnetometer as a modulated inductor. In that theory, hysteresis and demagnetization are implicitly incorporated in the sense-coil inductance, an easily measured quantity. In this thesis, the principle equation of that theory is experimentally tested. Expressions relating the open-circuit and short-circuit output from a fluxgate magnetometer to the magnetic field are derived from the principle equation. In order to test the proposed relations, the writer assembled a ring-core fluxgate a computer-controlled current source to drive the fluxgate, and circuits required to monitor the open-circuit and short-circuit output signals, initial tests showed that the integrated open-circuit output voltage from the fluxgate is proportional to the magnetic field. The constant of proportionality is simply the product of the length-to-turns ratio of the sense-coil and the maximum change in the sense-coil inductance caused the drive current. This result was correctly predicted by the aforementioned fluxgate equation. Test results from the short-circuit experiment were also correctly predicted by the fluxgate equation. Therefore, experimental data is provided that supports the validity of the fluxgate equation. The same fluxgate theory is used to predict specific values of drive current parameters that maximize the fluxgate output signal. The computer-controlled current source was used to generate a bipolar square-pulse waveform with an adjustable amplitude, frequency, and duty cycle. A sinusoidal drive waveform was also used. Experimental data confirm the validity of all the predicted relations, and thus, provide substantial support for theoretical work that has been recently published. As a final application, the fluxgate theory was used to quantify the behavior of a ring-core fluxgate immersed in a magnetic fluid. A fluxgate was put in magnetic fluid in an attempt to discover if the fluxgate responds primarily to the ambient flux density, and consequently, to determine whether the output signal could be enhanced by simply placing the sensor in a container filled with magnetic fluid, The experiment was terminated when inductance measurements taken on the immersed sensor showed that stray flux from the toroidal drive-coil significantly altered the permeability of the magnetic fluid, and thus altered the calculated values of flux density in the magnetic fluid.

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