Modelling and control of robotic joints based on sliding pairs

In this paper, the investigation on the friction phenomena acting at joint level in the robotic hand developed within the DEXMART project is reported. This side effect is given by the adoption of joints based on sliding pairs to reduce the complexity of the mechanism and to ease the assembly procedure. The joint friction has been modelled by means of a specialised version of the LuGre friction model in which the friction coefficients vary with the normal force. The estimated friction has been used in the joint position control loop as a feed-forward term for reducing the position error, as shown by experimental results. The developed friction model has been also used in the experiment to reconstruct the effects of the contact force applied to the joint by the external environment by means of a suitable observer.