The measurement of vibrational power transmission using laser technology

Abstract

Control of noise and vibration in complex, built-up structures is a common problem for the vibration engineer. Reduction of vibration levels through extensive application of damping materials and vibration isolation techniques is time-consuming and can involve expensive 'down time'. The power flow measurement technique overcomes this problem by allowing quantitative comparison of the net, vibration energy transmission amplitude through different elements of a structure, and thus identification of the primary vibration transmission pathways. Vibration control techniques can then be applied in the most efficient and least costly manner.

To date, implementation of the power flow measurement technique has concentrated on the use of accelerometers. Accelerometers, however, as a contacting transducer, mass-load the structure and cause the measured vibration signal to be reduced in amplitude and shifted in phase. This leads to power flow measurement errors, whose magnitude increases with transducer mass. In contrast, laser Doppler vibrometers offer a remote, non-contacting means of vibration measurement. Practical power flow measurement results obtained using two I.S.V.R. laser vibrometers compare favourably with those acquired simultaneously employing the two accelerometer method. This laser vibrometer is not sufficiently sensitive for many practical structural vibration problems, however, and a new, high-sensitivity, PZT laser vibrometer has been developed to overcome this problem. Two PZT laser vibrometers are employed to take low velocity amplitude, (10 μm/s), power flow measurements over the frequency range 0 to 2.5 kHz. Measured amplitudes are found to differ by only a small percent from those obtained using the two accelerometer method.

Dynamic speckle noise is a feature of all laser vibrometer measurements taken on diffuse, target surfaces. Its effect has been investigated and it is shown how spurious, optical noise, caused by speckle motion, can reduce the vibrometer sensitivity to normal-to-surface vibration. Further to this, it is shown how these effects can introduce pseudo-vibration signals into the laser vibrometer output. Typical noise signal amplitudes are determined.
To minimise the effect of inter—transducer response errors, an optical configuration consisting of a new, laser velocity gradient transducer and a PZT laser vibrometer is utilised to make practical power flow measurements. This is shown to be a viable alternative to the use of accelerometers since it not only has comparable accuracy, but also, in enabling remote, non-contact measurements, increases the rate at which measurements can be made, and extends use of the technique to surfaces where accelerometers cannot be employed, e.g. hot, lightweight and inaccessible surfaces.

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