Robotic perception of material: Experiments with shape-invariant acoustic measures of material type

Abstract

We present an active approach for discriminating different materials by impulsively contacting (hitting) them, and sensing and interpreting the resulting sounds. In theory, the angle of internal friction is diagnostic of material, but invariant over object shape. In our experiments, we observe that the angle of internal friction is not invariant over the frequency of the sound spectrum for which it is estimated. Hence, samples of different shapes, which exhibit power concentration at different frequencies, exhibit different values of the angle of internal friction. However, the results suggest that shape-invariance may be encoded in the functional form of the relation between the angle of internal friction and frequency.