1Force Control Beyond the Passivity Interaction Paradigm

Several interaction control algorithms have been proposed for physical human-robot interaction (pHRI) with the aim of assisting human movements. Some example are “path control” [4], “virtual model control ” [7], “clime ” [9] or gravity com-pensation [5], [3]. This class of algorithms is often built upon a force/torque controller that is supposed to be ideally fast. However when the algorithm is implemented the force variable cannot be commanded instantaneously and its transients de-pend both on robot and human dynamics. In particular, using a traditional controller we can have slow force/torque responses when the human displays a low mechanical impedance and force/torque overshoots when the human has a high impedance configuration. These effects are undesirable because they can generate disturbances that interfere with the rehabilitation strategy and even potentially lead to instability and injuries