A vibration pick-up which does not load the system being examined
S K Rushforth, A. Selwood · Journal of Scientific Instruments · 1958
The present device measures the amplitude, and indicates the phase of vibration of a structure without touching or loading it in any way. The transducer may be positioned anywhere adjacent to the surface of the structure so as to measure its amplitude of vibration at any given point. The apparatus is only suitable for the measurement of continuous simple harmonic oscillations such as appear in resonance testing A moving-coil vibrator is held close to the structure and is electrically driven so that the vibrator oscillates in sympathy with the structure. This sympathy of movement is detected by measuring the electrical capacitance between two plates, one formed by the structure itself, and one mounted at the tip of the vibrator shaft. The amplitude and phase of the current which drives the vibrator are adjusted until this capacitance becomes constant. The vibrator then moves with the same amplitude and phase as the structure and any type of vibration amplitude pick-up may be permanently fixed to the vibrator shaft. Whatever mechanical power is absorbed by this measuring pick-up comes from the supply to the vibrator and not from the structure itself. In the present apparatus, used for resonance testing a small wind-tunnel model, vibration amplitudes up to 0.030 in. may be measured with good linearity and reproducibility, with a resolution of about 0.0001 in. It is essential that the structure under examination be metallic, or, if not, covered with an electrically conducting layer.