1984-02-01

A Technique for Non-Contact Position Sensing 840304

A noncontact true position sensor has been developed for use in automotive applications. The sensor detects eddy current induced changes of phase in a magnetic circuit. Work has been done in the areas of EGR control, low tire sensing, throttle position sensing, power steering control, pressure sensing and various other areas using a position measurement. Speed and torque measurements are other applications of this sensor and can be performed concurrently with position measurement. This paper discusses the design and application of eddy current phase shift based position sensors in providing a reliable, accurate, simple and low cost sensor.
The basic operating principle involves inducing high frequency eddy currents in a conducting plate or other geometric shape such as a cone. This paper describes the mechanism of eddy current generation and resulting phase shift of current in a transducer coil.
Methods of detecting phase shift and thereby determining position are discussed. One such method uses simple, low power CMOS circuitry. The relationship of position to phase shift can be either linear or non linear depending on the geometric shape of the conductor in which the eddy currents are induced.
Techniques for temperature compensation and rejection of errors due to undesired motion in various applications are illustrated. Laboratory measurements have demonstrated temperature drifts of less than ±1½% over -40°C to +85°C. Finally, the paper discusses various applications and results of measurements.

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