A Magnetic Null-Balance Displacement Measuring System Using Radio-Frequency Modulation

Magnetic null-balance and a unique magnetic method of radio-frequency modulation combine to relate an electrical output precisely to the displacement of a small permanent magnet probe. The magnetic null-balance eliminates undesirable effects of magnetic material characteristics, and the use of a radio-frequency carrier provides large stabilizing gain with simple circuitry. The electrical analog output is derived by demodulation and filtering so that it contains only the frequency components of the measured displacement. Precise linearity between the displacement and the output is insured by the large amount of over-all dc negative feedback and by adherence to geometrical criteria developed for the displacement sensor. Static and dynamic tests of several of the measuring systems designed to produce 0-4 ma signals in response to 0-0.100-inch displacements have shown gratifyingly small errors, verifying the design expectations.