Adaptive friction compensation in robot manipulators: low-velocities

The problem of modeling and compensation of friction velocities close to zero is analyzed. A model, linear in parameters, which captures the downward bends at low velocity is used to adaptively compensate for friction. The need for this type of model is mainly motivated by instability phenomena that can be caused by overcompensation when simple models (such as Coulomb friction models) are used as a basis for the friction compensation. This model, in combination with an adaptive computed torque method, is tested experimentally in the last link of a robot manipulator. The prediction error e_k, which directly gives the accuracy of the predicted friction, remains within the preestablished threshold of ±0.8 Nm. The experiments were realized using an IBM PC-compatible computer, leading to a sampling time of 13 ms