Calibration of Vibration Pickups at Large Amplitudes

Abstract

The use of the axial resonances of long rods and tubes makes it possible to calibrate vibration pickups accurately over a wide range of frequency and acceleration. Levels of several hundred g at a few hundred cps up to 12 kg at 22 kcps have been attained (1 g = acceleration of gravity taken as 9.81 msec−2). The rods and tubes are driven at one end by piezoelectric ceramic shakers chosen to give good motion at the frequency of vibration. Electrically, the shakers are capacitors and are tuned by inductors to resonance at the frequency of mechanical resonance of the rod. By using great care in tuning the shaker-conductor system and in the mechanical details of supporting the shaker, in coupling the rod to the shaker, and in supporting the pickup cable it has been possible to perform calibrations whose errors are estimated to be no greater than 2%. The motion of the surface to which the pickup was attached was measured by direct viewing with a high-powered microscope using stroboscopic light and by the method of interference fringe disappearance. More than fifty disappearances were necessary at some frequencies to provide adequate overlap between the methods. A number of peculiar phenomena occurring at high acceleration levels are described.