Compliant motions allow alignment of workpieces using naturally occurring interaction forces. However, freefloating objects do not have a fixed base to absorb the reaction forces caused by the interactions. Consequently, if the interaction forces are too high, objects can gain momentum and move away after contact. This paper proposes an approach based on direct force control for compliant manipulation of free-floating objects. The objective of the controller is to minimize the interaction forces while maintaining the contact. The proposed approach achieves this by maintaining small constant force along the motion direction and an apparent reduction of manipulator inertia along remaining Degrees of Freedom (DOF). Simulation results emphasize the importance of relative inertia of the robotic manipulator with respect to the free-floating object. The experiments were performed with KUKA LWR4+ manipulator arm and a two-dimensional microgravity emulator (object floating on an air bed), which was developed in-house. It was verified that the proposed control law is capable of controlling the interaction forces and aligning the tools without pushing the object away. We conclude that direct force control works better with a free-floating object than implicit force control algorithms, such as impedance control.

The purpose of this paper is the development of an inverse dynamic model of a two degree of freedom electrohydraulic actuator. The actuator is to be incorporated at the base of each of three fingers of a nine degree of freedom mechanical hand, currently under development. Motion in the proposed actuator is facilitated about intersecting pitch and yaw axes, thus creating spherical actuation. The dynamic model incorporates frictional and hydraulic losses, commonly overlooked sources of energy dissipation. The model is to be used in the control scheme of the mechanical hand and in the optimal synthesis procedure of the actuator. The latter application, briefly described here, takes into account specified motion and torque requirements, pressure, peak input force and size constraints. Particular attention is paid to traditional performance indices, such as mechanical advantage.

The purpose of this article is to develop an inverse dynamic model of a two-degree-of-freedom electrohydraulic actuator. The actuator is to be incorporated at the base of each of three fingers of a nine-degree-of-freedom mechanical hand, currently under development. Motion in the proposed actuator is fa cilitated about intersecting pitch and yaw axes, thus creating spherical actuation. The dynamic model incorporates frictional and hydraulic losses, which are commonly overlooked sources of energy dissipation. The model is to be used in the control scheme of the mechanical hand and in the optimal synthesis procedure of the actuator. The latter application, briefly de scribed here, takes into account specified motion and torque requirements, pressure, peak-input force, and size constraints. Particular attention is paid to traditional performance indices, such as mechanical advantage.