On the stability of integral force control in case of contact with stiff surfaces

In this paper the robustness properties of explicit force control with respect to contact stiffness are investigated, considering different control laws applied to an industrial robot affected by joint compliance. A theoretical analysis, performed on the well-known linear fourth-order model of a single joint robot interacting with the environment, shows that, differently from the case of P or PD control, integral control ensures increasing stability with increasing contact stiffness. This result is believed new. In particular, it is shown that through the adoption of an integral control law, force control is actually feasible even in the case of an infinitely stiff environment, which is a quite common situation in. real industrial applications (metal working, deburring, etc.). It is also shown that the adoption of a feedback on motor velocity can greatly widen the range of the stabilizing integral gains as well as the achievable bandwidth. Experimental results, obtained with the industrial robot SMART 3S, are provided in order to substantiate the theoretical analysis.