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| Speaker: | Guillermo Silva - Pineda |
| Title: | Primary calibration of vibration transducers by laser interferometry and trends |
| Topic Group: | News from the South: Recent Metrology Activities at CENAM |
| E-mail: | gsilva@cenam.mx |
| Co-Authors: | Guillermo Silva-Pineda |
| Abstract: |
Nowadays a common and accurate technique to measure displacement is the laser interferometry. The leading National Metrology Institutes use to develop its own reference measurement systems in order to achieve the high accuracy required by the National Measurement Standards. At CENAM, homodyne and heterodyne laser interferometers have been under development for the last decade in order to calibrate both magnitude and phase shift of vibration transducers according to International Standards. In this paper an analysis of the homodyne laser interferometer is presented. The international standard ISO 16063-11:1999 “Methods for the calibration of vibration and shock transducers” describes three different methods to calibrate vibration transducers, specifically fringe disappearance, minimum-point and sine approximation. Only the sine approximation method can be used to calibrate the complex sensitivity (sensitivity modulus and phase shift) of the vibration transducer under calibration. A quadrature laser interferometer (QLI) is used to calibrate different vibration transducers in the frequency range of 1 Hz to 20 kHz. The measurement errors and non-linear behavior of the QLI limit the accuracy of the final results depending on the frequency and level of displacement. Experimental corrections of the laser interferometer set-up, signal conditioning and signal processing are discussed in order to achieve the best accuracy of the QLI. Specifically the importance of the optical adjustment of the phase quadrature output from the interferometer, electrical matching of the two photodetectors output and an implementation of the zero phase forward and reverse digital filtering compared with the sine approximation algorithm in the signal processing of the QLI are presented. Mechanical distortions like transverse and rocking motions from the vibration exciter is also evaluated in the error analysis presented. Calibration results of standard vibration transducers, i.e. standard laboratory accelerometers, and laser vibrometers are analyzed and their accuracy improvements when the experimental corrections are implemented. Some of recently developed MEMS accelerometers have been evaluated using the QLI, the results of short and long term stability are presented. The optical phase quadrature condition in the output signal of the QLI against the Heidemann correction is evaluated and their effect on the final calibration uncertainty is presented. |
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