Calibration of vibration pickups by the reciprocity method

Abstract

The reciprocity theory for the relationship between mechanical force and velocity and electric current and voltage is presented for a linear electrodynamic vibration-pickup cali­ brator having a driving coil, a velocity-sensi ng coil, and a mounting table. The theory takes account of flexibility in the calibrator structure and in the coils, electric coupling be­ tween different parts of the same coil and between parts of one coil and parts of the other, and flexibility in the magnet structure. The theory shows what measurements are required in using a linear electrodynamic calibrator for the absolute calibration of vibration pickups. A description is given of the mechanical arrangement and electric circuitry used at the National Bureau of Standards in calibrating calibrators by the reciprocity method. A typical velocity-sensing-coil calibration curve is presented showing the effect of pickup mass on the calibration factor. Typical examples of measurement of calibration factor and me­ chanical impedance of pickups are also presented. Practical limitations of the reciprocity method due to nonlinearity resulting from lack of tightness of mechanical joints, resonance effects, amplitude effects, etc., are discussed.