Lasers turn to rock and roll

A laser measurement system has helped developers to measure vibration in rotating machines

Researchers at Loughborough University have developed the first non-contact method of quantifying the bending and torsional vibration of a rotating component. The breakthrough could be very significant for those involved with quantifying the rotational properties of equipment such as high speed turbomachinery or the vibration of tools in milling and turning machinery. The technology could also help oil and gas engineers to optimise drilling procedures, as well as automotive engineers to measure shaft torsional vibration.

This notoriously difficult measurement is a further development of Laser Doppler Vibrometry (LDV). It provides an accurate measure of shaft torsional vibration in situations where measurements of torsional vibration have previously shown sensitivity to bending vibration. Bending vibrations include a change in direction of the component shaft’s rotational axis as well as solid body motions such as an engine block rocking in its mounts.

In the past, only contact methods such as piezoelectric accelerometers attached to an engine block could be used to measure bending vibrations.

The new approach solves the problem by taking two laser torsional vibrometers (LTVs) and arranging them symmetrically about the normal to the shaft’s rotation axis. LTVs rely on the Doppler shift in light backscattered by a rotating component. A single LTV directs two parallel beams onto the shaft. The shaft scatters the light and the beams then recombine to produce a beat on a photodetector. The beat frequency equals the difference frequency between the two beams.

Demodulation of this photodetector output then produces a voltage analogue of the target rotation speed. Torsional vibration causes fluctuations in the rotation speed of the illuminated region and therefore in the output voltage.

The Loughborough team has tested its new approach on a four-cylinder, two litre diesel engine. It measured the bending vibration of the crankshaft while the engine was running – a major source of noise in cars.

The technique could be built into robust optical `point and shoot’ instruments. Because only optical access would be needed, downtime could be saved.