Towards laser Doppler vibrometry from unmanned aerial vehicles

Ben J Halkon,Steve J Rothberg

doi:10.1088/1742-6596/1149/1/012022

Abstract

Laser Doppler vibrometers are technically well suited to general application but they offer special benefits in a variety of challenging measurement scenarios which are now well documented and accepted. An interesting and potentially powerful example of such a challenging measurement scenario is one where the laser vibrometer is mounted on/in an unmanned aerial vehicle in order that autonomous measurement campaigns can be undertaken in remote and/or harsh environments. One important challenge to overcome in such a scenario is the measurement sensitivity to vibration of the instrument itself or indeed of any steering optics used to point the probe laser beam toward the target of interest. In this paper, recently reported means by which this measurement sensitivity can be rectified by simultaneously obtained correction measurements will be developed. Specifically, this development is intended to lead towards laser Doppler vibrometry from unmanned aerial vehicles (UAVs) with correction of instrument motion being presented herein for the first time from a single, rather than a pair of, uniaxial accelerometers.

Highlights

Laser Doppler vibrometers (LDVs) measure target surface vibration velocity and are technically wellsuited to general application with benefits over traditional contacting transducers in a range of challenging measurement scenarios [1]
Until quite recently [2]-[4], little attention had been given to i) a quite fundamental aspect of operation which is that the measurement is of velocity relative to the instrument itself and ii) measurement scenarios in which interpretation of the data must consider instrument or laser beam steering optic motion
Practical application of LDV has typically either involved mounting of the instrument on a stable platform, i.e. on a tripod, or, in the case of significant levels of ambient (“base motion”) vibration, the use of passive or active anti-vibration mounting arrangements [5]. Often such measures will be sufficient to yield high quality outcomes but an alternative, in which the instrument vibration can be fully compensated for across a broad frequency range, would be clearly preferable. Such compensation would be beneficial where the measurement campaign involves the mounting of the instrument onto a platform in which the vibration level might be comparable with that of the target of interest, for example when making measurements from unmanned or other aerial vehicles

Summary

Introduction

Laser Doppler vibrometers (LDVs) measure target surface vibration velocity and are technically wellsuited to general application with benefits over traditional contacting transducers in a range of challenging measurement scenarios [1]. Practical application of LDV has typically either involved mounting of the instrument on a stable platform, i.e. on a tripod, or, in the case of significant levels of ambient (“base motion”) vibration, the use of passive or active anti-vibration mounting arrangements [5] Often such measures will be sufficient to yield high quality outcomes but an alternative, in which the instrument vibration can be fully compensated for across a broad frequency range, would be clearly preferable. For realistic vibration levels for both target and instrument/steering optic vibration, reductions of at least 37/47 dB, respectively, were demonstrated In both cases, real-world measurements were presented, in accordance with conventional noise and vibration engineering paradigms, showing that the required accelerometer compensation measurements yield a substantially enhanced understanding of the dynamic characteristics of the systems under test. The curves may be challenging to differentiate from one another when reproduced in greyscale; that, is the very point – that, for each of the two vibration levels shown, the (integrated) accelerometer signals are indistinguishable from that of the LDV

Electrodynamic shaker

Vibrating LDV

Conclusions