A traceable dynamic force transducer

A traceable dynamic force measurement system built at the National Physical Laboratory (NPL) for the frequency range of 0.1 to 100 Hz, is described. The strain on a load-cell element was measured by two independent methods, both calibrated in the static force standard at the NPL. All instrumentation and the measurement system for determination of the dynamic amplitudes were calibrated dynamically against reference sine waves. The load cell was then tested dynamically in a servohydraulic machine and the two force outputs compared. The two methods of strain measurement chosen were the strain gage and a capacitance gage, designed to measure the strain in as diverse a way as possible from the strain gages. Two load cells were built, with steel and aluminum elements. The strain-gage bridge was d-c excited, amplified, filtered and digitally sampled. The capacitance gage was a-c excited from a commercial capacitance bridge, which was calibrated by the injection of reference signals at the sideband frequencies. This technique is in principle applicable to any a-c excited instrumentation. The measurement system consisted of a digital storage oscilloscope, coupled to a computer. The force amplitudes were measured by cross correlation against sine waves generated in software at the frequency of the applied force. Comparison of the two methods of strain measurement and detection of systematic errors caused by the dynamic response of the capacitance electrodes lead to the determination of the uncertainty of dynamic force measurement. This was calculated to be ±0.4 percent over the frequency range of 0.001 to 100 Hz. The thermoelastic effect was also visible at frequencies below 0.1 Hz. Paper was presented at the 1989 SEM Spring Conference on Experimental Mechanics held in Cambridge, MA on May 28–June 1.