Rapid measurement of tension in multi-wire arrays using free damped oscillations

Abstract

We explore the possibility of using free (as opposed to driven) oscillations to verify wire tension in large, open-frame multiwire planes. Using finite-element simulation we predict the signal when two wires are set in motion by applying and holding a DC voltage between them (electrostatic excitation). The resulting current signal is marginal. However, the program relates voltage, oscillation amplitude and signal, and it is easy to get enough amplitude (of order microns) by gently tapping the wire frame. We perform an end-to-end experimental test. Four 1.5m wires, at various tensions around 5N, are mounted in an aluminum channel. They are connected to a low-noise, balanced current-to-voltage preamp designed for good common-mode rejection. The 150V bias, from 9V batteries in series, is built into the preamp. Data are taken with a Tektronix 1202B scope, stored on a USB flash drive, and transferred to a laptop computer for analysis. Ten measurements, each with 2500 samples spanning 1s, are taken for each of the three wire pairs. The wires are excited by tapping the frame with a small mallet. Because the wire tensions are different, the waveforms exhibit beats. The data are analyzed by a program which decomposes the waveform step by step to extract the base and beat frequencies and thereby, two resonant frequencies per wire pair. Finally, those frequencies are disambiguated to obtain the frequency for each wire, which is then converted to a tension. The four tensions are then found mechanically by measuring the g/mm required to deflect each wire at its midpoint. The two values of tension agree well. We conclude that the method verifies tension to approximately 2% (1 sigma) in a little over 1s for 1.5m wires, and would be faster for shorter wires.