Chaotic Response in Robot Joint Mechanism Due to Backlash Component

Estimation of modal parameters of mechanical structures is usually carried out by utilising the Frequency Response Function (FRF) for linear systems, skeleton identification such as Hilbert transform, or describing function method for nonlinear systems. However, these techniques could be applied only when the output of a system is periodic for a periodic input. However, under certain excitation conditions in nonlinear systems, chaotic behaviour might occur so that the response is aperiodic. In that case, chaos quantification techniques, such as Lyapunov exponent, are proposed in the literature. For experimental validation of this phenomenon, a robot joint mechanism developed at KULeuven/PMA was used. In this setup, backlash was introduced in the connection of the harmonic drive to the shaft. Chaotic response, which appears under certain excitation conditions and could be used as backlash signature, is dealt with both by a simulation study and by experimental signal analysis after application of appropriate filtration techniques.