05.11.2015
The number of measuring systems that can detect more than one component of the vector physical quantities "force" and "torque" is increasing. Consequently, there is an increasing demand for traceability in the field of multi-component measurements. By means of an additional facility for torque generation, it becomes possible to generate – in addition to the realized force in the range from 20 kN to 1 MN – a precise torque in the range from 20 N·m to 2 kN·m. This measuring facility is part of the 1 MN force standard machine (1-MN-K-NME). In this way, measuring systems can be investigated and characterized which require combined loads of axial forces Fz and torques Mz, e.g. friction coefficient sensors used in the screw industry, or wheel-load sensors.
For this additional facility, a specific measurement uncertainty budget has been elaborated. This budget encompasses a model which takes physical and geometric influence factors into account. This includes, among other things, environmental influences, geometric characteristics, or the mass stacks. The influence of the different possible factors on the signal stability (e.g. friction influence due to the air bearing or to the pendulum oscillations of the mass stack of the 1 MN force standard machine) has been investigated. For application in practice, also the realignment process of the mass stacks, the flatness errors of adaptation parts and angular deviations are taken into account. The model therefore encompasses a consideration of the system according to the vector components and, thus, also an estimation of the acting disturbances. The quantities considered as disturbing quantities are the shearing force Fy, an additional axial force Fz, and the bending moments Mx and My. From the additional comparison measurements carried out with four torque transfer transducers at the 20 kN·m-Dm standard machine of Working Group 1.22, the expanded measurement uncertainty currently yielded (k = 2) lies in the range of 3·10-4 [1].
For the characterization of industrial sensors, a measurement uncertainty < 1·10-3 already represents considerable progress. A workshop organized at PTB at the beginning of the year gathered members of the Deutscher Schraubenverband (DSV) and set the course for a research cooperation project. Within the scope of this cooperation project, comparison measurements will be carried out on various friction coefficient sensors at the end of the year. The objective is to characterize the crosstalk behaviour and to develop analytical assessment methods. That there is a need for such research activities becomes evident in Fig. 1. Shown in this figure is the signal change as a function of the different load combinations for Fz and Mz. It can be seen that the signal change is directly coupled to the two quantities and leads to a maximum deviation of 3.7 % here. When calibrating individual quantities, this influence would be completely neglected. The logical consequence therefore consists in elaborating a calibration procedure for combined loads. The new measuring facility provides the metrological capacities required to develop and validate new calibration procedures.
Figure 1: Left: Members of the Deutscher Schraubenverband visiting the additional facility in the Gauss Building. Right: Characterization of the crosstalk behaviour for a 2-component sensor under combined load situations.
Reference:
[1] S. Baumgarten, D. Röske, R. Kumme, “COMPLETION AND MEASUREMENT UNCERTAINTY BUDGET OF THE MULTI-COMPONENT MEASURING DEVICE FOR FORCE UP TO 1 MN AND TORQUE UP TO 2 KN·M”, Proc. of the XXIth IMEKO World Congress, August 30 – September 4, 2015, Prag, Czech Republic
Contact:
Sebastian Baumgarten, Dept. 1. 2, WG 1. 22, e-mail: 
sebastian.baumgarten(at)ptb.de