Nonlinear friction estimation for digital control of direct-drive manipulators

Control algorithms for precise motion control require a complete robot dynamic compensation, including also nonlinear friction phenomena occurring at very low velocities. Several models, including both static and dynamic friction, have been proposed in the last few years, the most used being the LuGre model and its modifications. Unfortunately, a good estimation of the dynamic friction parameters is often quite difficult to be achieved in practice. In this paper, the parameters of the static part of this friction model are estimated for a doublearm direct-drive planar manipulator, and the identified model is used for torque reconstruction during an assigned motion, by considering the nominal values of the robot inertial parameters. The results are compared with those obtained by estimating the manipulator inertial parameters, together with the parameters of a simplified third order polynomial friction model. Experiments confirm the validity of the available, nominal, inertial values, and show that, at least in our case, similar results are obtained by the two friction models.