Calibration of transducers for vibro-acoustic power measurements

  • Ohlrich, Mogens, (Project Manager)

    Description

    The piezoelectric transducer principle used in accelerometers and force transducers generally provides good linearity and fine overall characteristics with respect to frequency and dynamic range. Another quality of this measurement principle is that the deviations from the ideal (flat) amplitude and phase characteristics are small and of little importance in most vibration measurements. However, the deviations cannot be ignored in measurements of complex mobilities and in measurements of the vibratory power that sources inject into structures, or the power transmitted between connected structures. It has been found that very strict requirements are needed for such vibro-acoustic power measurements, especially with respect to the phase accuracy at off-resonance conditions.
    The purpose of this project (initiated in the ASPEN project mentioned above) is to investigate the possible causes of uncertainties in measurements of vibratory power with accelerometers and force transducers. Simple analytical models and experiments have revealed that the structural damping of the transducers is responsible for small phase errors. Tests of a number of accelerometers (of B&K types 4344 and 4375) and force transducers (of B&K type 8200) have shown that their absolute phase errors are typically about 1.2° and 0.5°, respectively. This corresponds to a relative phase error of 0.7° between the measured transducer signals of acceleration (or velocity) and force, and this is what matters in power measurements. It has also been found that instrumentation phase errors of transducer preamplifiers may contribute significantly, giving an overall bias error in power measurements of up to 1.5° at mid and high frequencies, and considerably more at low frequencies. Fortunately, this error between measurements channels can be compensated for at fixed settings by performing a broadband, absolute calibration with an ideal mass.
    StatusFinished
    Effective start/end date01/06/199630/12/1998