The stability of position and force control of an uncertain multiple-robot system is presented. The main objective is to obtain the bounded stable tracking performances for position and force simultaneously when the dynamic system is not exactly known. To perform tasks in a system of multiple redundant robots, the coordination among the robots may be incorporated within the position and force controls of the robots, and the constraints among the end-effectors and the rigid object grasped by the end-effectors. When the physical parameters of the robots are known, control laws which are linear in position/orientation errors may be applied to achieve asymptotic stability in both position/orientation of the object, and interactive force/torque among the object and end-effectors. When the physical parameters are not exactly known, robust control laws may be applied to compensate for the errors caused by the uncertainties. The bounds of the errors are dependent on the bounds of the uncertainties and some design factors that can be specified.>

Two different control schemes, position model referenced control (PMRC) and force model referenced control (FMRC), are developed for a simplified two robots model with one interacting axis. The dynamic stability of the position (or force) control under the FMRC (or PMRC) is analyzed. Results indicate that PMRC, although providing better dynamic performance of the position control, may degrade the dynamic performance of the force control. However, FMRC can provide better dynamic performance of both position and force control.>

When two robots execute coordinated motion in industrial application such as assembly, not only is the specification of the desired motion of each robot's end effector required, but also that of the desired contact force exerted on the end effectors. The involvement of the forces is the motivation for a consideration of compliance. The compliant coordination control scheme of two moving robots is presented. One robot is equipped with a force/torque sensing wrist, which provides force feedback signals and permits the fine coordinated motion control of complex assembly tasks. A control strategy is derived so that the measured force/torque can be used to modify the trajectory of one of the robots. This control scheme can be realized in real time by using the proposed computational structure. The system stability is related to the force-displacement conversion gain in the forward path. An experiment that screws a moving nut onto a moving bolt has been carried out as an illustration.>

The authors present a solution to the optimal force distributions for two redundant coordinating robots. Two coordinating robots whose end-effectors firmly grasp a common inertial load form a closed-chain structure. In these closed kinematic chains, the two robots must observe a set of constraint conditions on the end-effectors' positions, orientations, linear and angular velocities and the forces and torques inserted onto the end-effectors by the load. To achieve the maximum load capability, the optimal force distribution under the known posture is developed. If both robots are redundant, joint solutions may be obtained if some optimization criterion such as manipulability is imposed. The singularity, manipulability, and force ellipsoid are investigated for the two coordinating redundant robots in terms of the two manipulator Jacobian matrices. The manipulability is shown to correspond to the cascade structure while the force ellipsoid corresponds to the parallel structure in terms of their manipulator Jacobian matrices. Null-space contributions towards singularity escapement and mechanical advantage are found.>