Dynamics of Digital Force Control Applied in Rehabilitation Robotics

Controlled structures are often required to keep desired contact forces between some of its elements. A classical example is the controlled interaction of a robot with its environment when the control of the contact force between the robotic actuator and the workpiece is prescribed. Experiments often call the attention to the destabilising digital effects, like sampling, in these systems. In this paper the stability of a newly developed force based teaching-in method is analysed. The method is applied in rehabilitation robotics. The stability limits are presented in the parameter space of the sampling time, control gains and mechanical parameters of the robot. The least force error and the fastest settling force signal are calculated. The influence of the elasticity of the force sensor is analysed as well as the possible bifurcations. Real parameter case study confirms the analytical predictions.