Dynamic calibration of forces now possible

Industrial applications require dynamic force calibrations, e.g. for material- testing machines or test benches in the automotive and aviation industries. Whereas high-precision procedures have been available for several decades for the static calibration of force transducers allowing traceable calibration by means of deadweight force standard machines, the dynamic calibration of such force transducers is not as advanced.

PTB's new method is essentially based on the same principles as static calibration: the force is generated by the loading of deadweights. According to Newton's laws, force = mass × acceleration, with gravitational acceleration acting.

Dynamic calibration becomes possible by inducing sinusoidal vibrations on the force transducers using an electrodynamic shaker. The displacement amplitude of the shaker essentially depends on the frequency and lies in the range from a few micrometres up to several centimetres. The force transducer to be calibrated is loaded with an additional mass whose acceleration is then measured with a laser vibrometer. The product of acceleration by mass yields the acting dynamic force. The calibration result is the dynamic sensitivity as a quotient from the electric signal of the force transducer and of the dynamic force as a function of the frequency. Using a laser scanning vibrometer allows the acceleration to be measured not only in one single point, but over the whole surface of the additional mass. This allows certain parasitic influences, such as, e.g., wobbling displacements of the additional mass, to be detected and to be taken into account when indicating the uncertainty.

This procedure allows relative measurement uncertainties of 0.5 % to 1.0 % to be attained below the resonance frequency of the measuring set-up – which depends on the size of the additional mass – and of a few per cent above this frequency. PTB has recently started offering services based on this new technique.