Force Sensor Characterization Under Sinusoidal Excitations
The aim in the current work is the development of a method to characterize force sensors under sinusoidal excitations using a primary standard as the source of traceability. During this work the influence factors have been studied and a method to minimise their contributions, as well as the corrections to be performed under dynamic conditions have been established. These results will allow the realization of an adequate characterization of force sensors under sinusoidal excitations, which will be essential for its further proper use under dynamic conditions. The traceability of the sensor characterization is based in the direct definition of force as mass multiplied by acceleration. To do so, the sensor is loaded with different calibrated loads and is maintained under different sinusoidal accelerations by means of a vibration shaker system that is able to generate accelerations up to 100 m/s2 with frequencies from 5 Hz up to 2400 Hz. The acceleration is measured by means of a laser vibrometer with traceability to the units of time and length. A multiple channel data acquisition system is also required to simultaneously acquire the electrical output signals of the involved instrument in real time.
INVESTIGATIONS FOR THE MODEL-BASED DYNAMIC CALIBRATION OF FORCE TRANSDUCERS BY USING SHOCK FORCES
To investigate and validate the approach of a model-based dynamic calibration of force transducers by using shock excitations, numerous measurements were performed at the 20 kN primary shock force calibration device of PTB. The tests included several strain gauge force transducers of greatly differing structural design, size, weight and mechanical coupling. Previous studies proved modal oscillations of the measurement setup of various origins that limit the usable bandwidth. Their effect on the modal-based calibration method with its data analysis procedures to be developed is further investigated and discussed in this paper.
Interferometric evaluation of the sensitivity of accelerometers to differential heating
Laser interferometry is the preferred method currently used by National Metrology Institutes for primary calibration of accelerometers. Highly accurate calibrations require well controlled environmental conditions. Laboratories usually focus on the control of ambient air temperature and humidity. Another important source of influence is the vibration exciter, which besides uniaxial motion can generate undesirable secondary effects. This paper will treat the problem of differential heating of an accelerometer under calibration, when caused by a rise in temperature of the vibrator moving element. A simple device used to evaluate the change in the charge sensitivity of an accelerometer due to heating through its mounting base is presented. The sensitivity to differential heating can then be determined by the use of accurate interferometric methods. The knowledge of this sensitivity enables the calibration laboratory to correct the calibration results obtained with vibration exciters, which exhibit significant temperature variations. For instance, some commercial models can present variations higher than 20 °C, when the Bessel function J1 minimum-point method is applied. The experimental setup, which has been developed at INMETRO to evaluate this characteristic is described and some measurement results are presented.