Spindle vibration and sound field measurement using optical vibrometry

Abstract: Mechanical systems often produce a considerable amount of vibration and noise. To be able to obtain a complete picture of the dynamic behaviour of these systems, vibration and sound measurements are of significant importance. Optical metrology is well-suited for non-intrusive measurements on complex objects. The development and the use of remote non-contact vibration measurement methods for spindles are described and vibration measurements on thin- walled structures and sound field measurements are made. It was shown that by making the surface of the spindle optically smooth, both harmonic speckle noise and crosstalk between vibration components could be avoided in laser vibrometry measurements. The radial misalignment and the out-of-roundness of the spindle could also be determined from the signal. Furthermore the technique was also used for measuring the vibrations of a tool during milling of an aluminium workpiece. The cutting vibrations were determined from the laser vibrometry signal and were compared to the measured cutting force and to the spindle head vibration. Measurement of radial vibrations along a line on a rotating polished shaft was made using digital holographic interferometry. This technique enables full field vibration measurements in two or more directions simultaneously. This method also provides mode shapes directly and may be helpful in vibration testing. Modal analysis of a thin-walled workpiece fixed in the milling machine table has been carried out for different stages of machining using scanning laser vibrometry. The result has been used for obtaining the correct boundary conditions of a finite element model of the workpiece. The finite element model together with the measured tool response obtained by laser vibrometry has been used as input parameters for predicting machining stability. Laser vibrometry measurements on a violin excited by a rotating disc were performed. The chain of interacting parts of the played violin was studied: the string, the bridge and the plates as well as the generated sound field. The measurements on the string showed stick-slip behaviour and the bridge measurements showed that the string vibrations were transmitted to the bridge both in the horizontal and the vertical direction. Measurements on the plates showed complex operational deflection shapes. The sound fields were measured and visualized for different harmonic partials of the played tone. However, the measured sound field is a two-dimensional projection of a three-dimensional sound field. This projection effect is illustrated by measurements of a sound field emitted from several ultrasound transducers from different projection angles. It was shown that by making a sufficient number of laser vibrometry measurements, the three-dimensional sound field could be reconstructed using a tomography algorithm. The idea is to apply the measurement method in rotating machines, where near-field acoustic measurements may provide additional information about a rotating machine part. The measurement methods that are developed and used provide increased understanding of the dynamics of complex structures such as thin-walled or rotating spindles. This may be utilized in the optimization of the machines currently available and in the development of machine parts.