Force control of rigid manipulators: a comparative study for non-rigid environments

This paper presents a comparative study between two force control strategies for robotic manipulators in the presence of nonrigid environments. The first approach combines two distinct control methodologies in the force control scheme: a model based predictive control (MBPC) and the impedance controller. The predictive controller generates, in an optimal way, the reference positions and velocities to the impedance controller which results in a desired force profile. The main advantage of this control strategy is to provide an easy inclusion of the environmental model in the controller design and thus to improve the global control system performance, especially in nonrigid environments. Moreover, since the impedance and the environmental models are linear, the solution of an unconstrained optimization procedure can be analytically obtained with moderate computational burden. The second approach is an impedance control scheme modified in order to obtain a force controller. The performance of both force control schemes is illustrated for a two degree-of-freedom PUMA 560 robot, whose end-effector is forced to move along a frictionless surface located on the horizontal plane. The simulation results obtained reveal an accurate force tracking performance for the predictive control approach in comparison to the impedance force controller in the presence of nonrigid time varying stiffness environments