Asymptotic observer of the link states of flexible joint robots with motor-side sensing

This paper proposes an approach for observing the link states, i.e. angular position and velocity, of the robotic joints with elasticities, when only the motor-side sensing is available. By separating the state-dependent nonlinearities of the rigid manipulator dynamics a linear sub-model of the elastic joint robot is obtained in an observable state-space form. The standard asymptotic Luenberger state observer is then designed for the given motor position signal which is measured by encoder and rectified from the nonlinear contribution. The configurable poles of observer allow shaping the estimated states dynamics to be fast enough in relation to the actual resonant joint behavior. Therefore, the join link states can be accurately reconstructed in the real-time, thus allowing the whole system state to be further used for the control purposes. The proposed method is experimentally evaluated on a single elastic joint setup under additional impact of gravity. It is shown that the observed pattern of oscillating joint torsion velocity coincides well with the measured one. The obtained joint torsion velocity can be used for stabilizing the elastic robot joints under control and reducing the related disturbing vibrations.