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A Simple Capacitive Displacement Measuring System

Report Number: AMRL TR 64-116
Author: Hoeft, Lothar O.
Corporate Author: Biophysics Laboratory
Laboratory: Biophysics Laboratory
Date of Publication: 1964-12
Pages: 22
Contract: Laboratory Research - No Contract
Project: 7232
Task: 723204
AD Number: AD0610286

This report presents the theory, development and performance of a capacitive displacement detector. The detector senses the displacement changes as changes in the capacitance of a probe located near the body whose displacement is being measured. These capacitance changes are converted into frequency changes by placing the capacitance probe in the tank circuit of an oscillator. The frequency changes are converted into voltage changes by standard FM techniques. The capacitance detector has a sensitivity of 30 volts/rho farad. When used with a 0.2 cm diameter probe, the displacement sensitivity was 4.78 volts/cm when the probe to surface distance was 0.1 cm. The noise level was low enough so that the system was able to measure displacements of the order of .00001 cm at frequencies below 1000 cps and displacements of the order of 5 x .000001 cm at frequencies above 1000 cps. The main advantages of this type of displacement detector over others is the ease of construction and adjustment.

RAF Centre of Aviation Medicine
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