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Model-based shock calibration of force transducers

29.09.2014

Within the scope of the European research project EMRP IND09 "Traceable Dynamic Measurement of Mechanical Quantities", the approach of the model-based dynamic calibration of force transducers by using shock excitation has been investigated. The dynamic behaviour of force transducer and measuring device is described by models whose characteristic parameters are to be identified from the measurement data.

For this purpose, force transducers of different design, size and mechanical coupling have been experimentally investigated at PTB's 20 kN shock force calibration device. The 20 kN shock force calibration device uses two airborne impact masses of approx. 10 kg which are brought to collision with the force transducer mounted in-between.

As an example, Figure 1 shows one of the investigated force transducers of a nominal load of 2.2 kN, mounted to the (removed) reaction mass. The impacting mass body of 10 kg, accelerated by a drive, hits the force transducer and generates a force pulse of a duration of approx. 0.7 ms which is followed by a pronounced ringing of approx. 3.7 kHz (cf. Figure 3, top). 

Figure 1: Mounted force transducer

To provide traceability for the shock force calibration, the inertial forces of both mass bodies are measured by means of laser interferometry. The observed dynamic behaviour can be understood with the aid of a model. In the case of this model-based calibration, both the force transducer and the calibration device are described as a series arrangement of spring-mass-damper elements. The model parameters of the force transducer, which are characteristic for the dynamic behaviour, its masses, stiffnesses and dampings, are to be determined. Fitting of measured and modelled shock forces allows the identification of the sought parameters from the measurement data.

For the verification of suitable models and the development of suitable methods for the analysis of measurement data and the parameter identification, models of different complexity were compared. Figure 2 shows three models with 3, 4 or 5 model masses which describe the stiffness of the force transducer and also its coupling stiffness to the reaction mass (4 and 5 model masses).

 

Figure 2: Models of the shock force calibration device with mounted force transducer (blue components).

The result of the parameter identification is illustrated by the example of the transducer shown in Figure 3. The figure compares the measured shock signals with the simulated shock signals. It can be seen that the coupling stiffness of the force transducer must be described by the model to understand the observed behaviour. The clarification of the still remaining deviations requires further investigation.
The investigations presented here were carried out in close cooperation with PTB's Working Group 8.42 "Data Analysis and Measurement Uncertainty". Detailed information can be found in [1].

Figure 3: Comparison of modelled and measured shock signals: force transducer signal (top), acceleration of the reaction mass (bottom).

 

Literature:

[1] M. Kobusch, S. Eichstädt, L. Klaus, T. Bruns, "Investigations for the model-based dynamic calibration of force transducer by using shock forces", Proc. of Joint IMEKO International TC3, TC5 and TC22 Conference, Cape Town, South Africa, 2014. online at: Opens external link in new windowwww.imeko.org/publications/tc22-2014/IMEKO-TC3-TC22-2014-005.pdf

Michael Kobusch, Department 1.7, WG 1.73, e-mail:Opens window for sending emailmichael.kobusch(at)ptb.de
Sascha Eichstädt, Department 8.4, WG 8.42, E-Mail:Opens window for sending emailsascha.eichstaedt(at)ptb.de