Akihiro Oota

Calibration of vibration pick-ups with laser interferometry: part IV. Development of a shock acceleration exciter and calibration system

Measuring shock acceleration with peak accelerations ranging from 200 to 5000 m s?2 is a significant concern for mechanical or electrical applications in industries. To precisely calibrate accelerometers using shock acceleration, we developed a shock acceleration calibration system at the National Metrology Institute of Japan. In the calibration system, the shock acceleration exciter generates shock acceleration using rigid-body collisions between three metallic bars inside an air bearing. The pick-up to be calibrated is fixed on an edge surface of the third metallic bar so that the pick-up moves together with the metallic bar. The displacement of the pick-up is measured by a He?Ne laser interferometer with traceable voltage, length and time standards. The sensitivity of the pick-up is evaluated by analysing signals from the accelerometer and laser interferometer. We describe the calibration procedure, the specification of the shock acceleration calibration system and the uncertainty in the shock-acceleration calibration. The shock acceleration calibration result has the expanded uncertainty of roughly 1.0% with a coverage factor 2 and is comparable to the vibration acceleration calibration result with an En value below 0.3.

The methods for the calibration of vibration pick-ups by laser interferometry: part V. Uncertainty evaluation on the ratio of transducer's peak output value to peak input acceleration in shock calibration

This study describes the uncertainty of measurement about accelerometer sensitivity in shock calibration based on ISO 16063-13. The typical shock approximates to a sine-squared waveform with peak acceleration in the range from 200 to 5000 m s−2 and a pulse width of several milliseconds. The shocks are generated by a transient collision motion on contact with a rubber surface between two rigid bodies. A laser interferometer with a He–Ne laser measures the displacement change that is input to the accelerometer to be calibrated. The accelerometer sensitivity is evaluated in the time domain and defined by the ratio of the accelerometers output peak value to the input acceleration peak value. The individual uncertainty components are assessed to formulate an uncertainty budget that includes an expanded uncertainty with a coverage factor of k = 2 at approximately 6.5 × 10−3.

Effect of demodulator unit on laser vibrometer calibration

In this study, the effect of demodulator unit characteristics on the calibration of a laser vibrometer is investigated. For this purpose, two commercial available laser vibrometers with analogue demodulator units are used in the experiments. The demodulator units are electrically calibrated using simulated frequency-modulated signals, which are equivalent to output signals obtained from laser optics during laser vibrometer calibration. The calibration results of the demodulator units show extremely similar characteristics to laser vibrometer calibration results carried out in accordance with the new proposed draft (ISO16063-41). Although both calibration results had a large deviation of more than 0.5 % from the nominal sensitivity, a smaller deviation within 0.5 % was obtained by correction on the basis of the demodulator calibration results. The calibration results for both commercially availbale laser vibrometers indicate same amount of deviation after correction.. Most of the large deviation in the laser vibrometer calibration is due to the demodulator characteristics. In ISO16063-41 draft, laser vibrometer calibration is carried out by applying actual vibration to the laser vibrometer. However, the acceleration amplitude range applicable for calibration is limited due to the capability of the vibration exciter. Therefore, the measurable dynamic range of the laser vibrometer is not always sufficiently covered in the calibration. To overcome this problem, our investigation suggests the applicability of a combination of individual component calibrations.

Preliminary implementation of primary calibration system for laser vibrometer

In this paper, we outline a preliminary implementation of primary calibration for a laser vibrometer (LV) on the basis of newly proposed first ISO draft. The sine approximation method is applied to our calibration system. Under some different measurement conditions, primary calibration is preliminarily carried out for achieving reliable measurements. From these results, the effects of acceleration amplitude stability, mechanical distortion, position of measuring beam, and spot diameter of measuring beam have been discussed. Some conditions required for achieving the reliable calibration is indicated on the basis of the discussion. Additionally, the strict regulations of the distance between spots and such amount of mechanical distortion would be indispensable for guaranteeing a higher reliability of such calibration within an expanded uncertainty (k = 2) of less than 0.5 %.