An international review of laser Doppler vibrometry: Making light work of vibration measurement

The fundamental mechanism by which speckle noise is generated in laser vibrometry before describing a new numerical simulation for prediction of speckle noise level in a real measurement is considered. Factors within the simulation include rate of change of phase within individual speckle transitions, low-pass filtering to match the frequency range of experimental data with which comparison is to be made, a track-and-hold facility for periods of low signal amplitude, and wavefront curvature effects. The simulation data provide real insight into the phase and amplitude modulation of the Doppler signal, and good agreement is found in the final comparison with experimental data from a measurement on a rotating target.

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Numerical simulation of speckle noise in laser vibrometry

Laser vibrometers and contacting transducers, target rotation and six degree-of-freedom vibration: what do we really measure?

It is readily accepted that a laser vibrometer measures target velocity in the direction of the incident laser beam, but this measured velocity must be considered in terms of the various components of the target velocity. This paper begins with a review of the theoretical description of the velocity sensed by a single laser beam incident in an arbitrary direction on a rotating target undergoing arbitrary vibration. The measured velocity is presented as the sum of six terms, each the product of a combination of geometric parameters, relating to the laser beam orientation, and a combination of motion parameters?the 'vibration sets'. This totally general velocity sensitivity model can be applied to any measurement configuration on any target. The model is also sufficiently versatile to incorporate time-dependent beam orientation and this is described in this paper, with reference to continuous scanning laser Doppler vibrometry. For continuous scanning applications, the velocity sensitivity model is shown formulated in two useful ways. The first is in terms of the laser beam orientation angles, developing the original model to include time dependency in the angles, whilst the second is an entirely new development in which the model is written in terms of the mirror scan angles, since it is these which the operator would seek to control in practice. In the original derivation, the illuminated section of the rotating target was assumed to be of rigid cross section but, since continuous scanning measurements are employed on targets with flexible cross sections, such as beams, panels and thin or bladed discs, the theory is developed in this paper for the first time to include provision for such flexibility.

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Vibration measurements using continuous scanning laser Doppler vibrometry: Theoretical velocity sensitivity analysis with applications

This paper begins with a review of the fundamental mechanism by which speckle noise is generated in Laser Vibrometry before describing a new numerical simulation of speckle behavior for prediction of noise level in a real measurement. The simulation data provides real insight into the phase and amplitude modulation of the Doppler signal as a result of speckle changes. The paper also includes experimental data looking at the influence of speckle noise in measurements on rotors with a selection of surface treatments and in scanning and tracking configurations.

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Laser vibrometry meets laser speckle

A two-channel laser Doppler vibrometer system is described for studying spacing modulation of the head/disk interface in a magnetic recording disk file. Three types of sliders, namely a standard two-rail slider, an experimental negative-pressure slider, and a cross-cut slider, are studied. In comparing the results from the single-channel and dual-channel laser Doppler systems, excellent agreement is observed for the resonant frequencies of the air bearing. The single-channel system, however, exhibits problems with offset in the velocity and displacement signals that are absent with the two-channel system. >

Measurement of head/disk spacing modulation using a two channel fiber optic laser Doppler vibrometer

The dynamic response of low flying sliders is investigated using a laser Doppler vibrometer. The motion of sliders excited by an artificial defect is studied in both the time and frequency domain and the dynamic response of different types of sliders is examined, including narrow rail, ‘bleed-slot’, and negative pressure sliders. In addition, suspension loading and disk velocity effects on the slider dynamics are investigated. A simple spring/mass model describing the dynamics of the slider is discussed.

T.A. Riener

Papers

Measurement of head/disk spacing modulation using a two channel fiber optic laser Doppler vibrometer

Dynamics of magnetic recording sliders investigated using laser Doppler interferometry