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Transducers Testing by Optical

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Tutorial 1d

Christian Rembe
Polytec GmbH, GERMANY

Tutorial Description:

The complex mechanic designs of transducer systems make it more-and-more important to have precise measuring means to characterize mechanic properties. Sensor systems are usually characterized by comparing the system output with a defined system input. But this characterization method fails when the sensor characteristic does not explain reasons for objectionable behavior or even failure mechanisms. Optical nondestructive measurement techniques make possible to discover design or production imperfections.

This tutorial discusses different optical techniques to measure geometry and motions of mechanical structures in transducers. White-light interferometry is a relatively new method in nowadays industry to measure geometry parameters. Most important current developments of white-light interferometry address the standardization of this technique and the comparability to tactile stylus instruments. Other common optical methods such as holographic microscopy and confocal microscopy are also discussed in the tutorial. Vibration measurements by laser-Doppler vibrometry and stroboscopic video techniques with image processing are the next topics. Especially the prospects and limits of laser-Doppler vibrometry are introduced in detail. A laser-Doppler vibrometer measures the instantaneous velocity by the Doppler shift of light as broad-bandwidth signal. Mechanic disturbances can be isolated by the signal processing and narrow bandwidth measurements can be realized by FFT (Fast Fourier Transform) technique. Modern systems can measure vibrations with bandwidths up to the GHz range and with pm vibration-amplitude resolution. Several application examples demonstrate the performance capabilities of the different optical techniques.

Advanced testing methods for transducers are necessary to develop reliable, marketable devices. The main purpose for transducer testing is the realization of a feedback to the design-and-simulation process in an engineering development effort. This feedback should include geometric information, dynamic behavior, system parameters, and material properties. The determination of system parameters and material properties of MEMS-based sensors during production is the final topic of the tutorial.

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