Force and position control of robot manipulators: Learning and repetitive control approach

When a robot performs the same task repeatedly, a learning or repetitive controller can enhance the performance of the system significantly. Learning or repetitive control, however, has not been studied in the force control of a robot manipulator as extensively as in the position control of a robot manipulator. In this dissertation, learning control is applied to hybrid force and position control of robot manipulators. Also, repetitive control is applied to the control of a spring loaded end effector.;When the geometry and position of the constraint surface is known, the hybrid force and position controller and the feedforward compensator can be designed in the constraint coordinates. When the operation is periodic, the learning hybrid force and position control enhances the control performance as the feedforward compensator is updated in each cycle by the force and position error in the preceding trials. This scheme is proved to be stable in the sense of Lyapunov.;In the experiments, a two degree of freedom SCARA-type direct-drive robot manipulator is used to test the feasibility of the learning hybrid force and position control. The deburring tool mounted on the upper link of the robot could follow a flat, tilted flat, and curved 1/4" aluminum plate with a desired contact force of 10 N (within the root-mean-square force error of 1.95 N) and with desired tangential velocity. Considering the loss of contact observed at the initial trial, the performance of the system improved significantly.;A spring loaded end effector is useful in assembly operations such as mounting an electronics package on a board. With the known stiffness of the spring, the desired contact force tracking is accomplished by controlling the spring displacement. Uncertainties in the environment such as the stiffness of the board make it difficult to preprogram the required control force. As the operation repeats, the tracking errors for the spring displacement, i.e., the contact force, are compensated by the repetitive control. Repetitive control is robust under the presence of parametric uncertainties. Unmodeled dynamics make it necessary to modify the repetitive controller to gain stability robustness at the expense of tracking performance at high frequencies.;The compliance of the contact mechanism plays an important role in the stability of the force control. When the compliance is not known in advance, the lumped stiffness is estimated, and the feedback controller gains change according to the stiffness estimation. When the position of the constraint surface has uncertainty, a method of estimating both the stiffness and the position of the constraint surface is suggested.