A set-point-generator for indirect-force-controlled manipulators operating unknown constrained mechanisms

In this paper we propose a set-point-generator (SPG) for indirect-force-controlled (IFC) manipulators, interacting with mechanisms, which impose narrow bilateral kinematic constraints on their end effector, like doors, cabinets and drawers. These mechanisms could also have dynamic properties, due to their inertia, friction and gravity, which demands to consider applied forces when choosing a set-point for the IFC. Neither the type of the constraint (linear, circular), nor required manipulation forces are assumed to be known. The proposed SPG consists of two parts: i) estimating the single direction of possible motion based on filtering of the measured end effector velocities; and ii) choosing an appropriate set-point for the underlying IFC, resulting in an effective operation of the mechanism. A major aspect of our approach is to explore the kinematic constraints with the manipulators desired set-point, avoiding direct force control, as the required interaction force is unknown and hence there is no definite reference force. The presented approach is a generalization of our previous work on constrained manipulation of unknown mechanisms and extends the applicability to a wider class of manipulators by considering joint-level IFC and taking into account the applied forces in yielding a robust and effective controller. The approach is evaluated in various experiments on a manipulator, providing joint space compliance.