Motor and load velocity estimation for digital servo drives: an application to robots with elastic joints

This paper deals with the problem of estimating motor and load velocities of a positioning servomechanism, affected by torsional elasticity at the transmission, using only a motor position transducer. A fifth order observer is adopted, which includes an integrator to estimate steady-state disturbances. The procedure to identify observer model parameters is presented. Observer design and sensitivity of estimation to perturbations on model parameters are discussed. The estimated velocities are compared with the measures of velocities achieved with additional sensors on an industrial robot. Very good agreement is found between estimated and measured rotor velocities, while it is apparent that the multivariable nature of a robot arm, as well as the “sticktion” phenomenon, should be considered for a good estimation of the load velocities