Comparison of measurement systems for free fall tests and calculations of the coefficient of restitution

The following article presents an overview of two measurement techniques for determining the coefficient of restitution (CoR) of multiple impacts of granules on a rigid plate. Experiments were performed with two different kinds of granules, which were dropped freely on an impact plate starting from a defined height. The impacts were recorded by an acoustic sensor as well as with a high-speed camera. The evaluation of the measured data shows slight differences between the CoRs arising from the two measurement techniques, which can be explained by the difference in accuracy between the two techniques. Furthermore, the influence of drag on the impacts is derived, and the necessity of a precise calculation method is discussed. Finally, a thorough description and a summary of the advantages and disadvantages of the measuring techniques and the calculation methods are given and it is shown when it is possible to estimate the CoR by uniform acceleration and when it is necessary to calculate the CoR considering the drag on the granule.