Constrained motion control of hydraulic manipulators using impedance control

One of the most difficult problems in robotics is the control of manipulators in constrained motion. Constrained motion is defined as any motion where the manipulator must work in contact with the surrounding environment. The difficulty is the contact force applied to the robot end-effector changes the dynamics of the system. Typically, position controllers are used to move a manipulator along a prescribed trajectory. However, a position controller views the reaction force as a disturbance and attempts to reject the disturbance. Instead of rejecting the reaction force the force should be accommodated. An impedance controller accommodates the reaction force by matching the robot dynamics to a desired set of dynamics.; The desired set of dynamics given in an impedance controller are second order. A second order set of dynamics ignores the actuator dynamics assuming the actuators are ideal. However, if the robot is hydraulically driven the dynamics are third order and the nonlinear actuator dynamics should be compensated. This research focuses on reformulating impedance control to include the actuator dynamics by two different methods. The first method proposes using a third order set of desired dynamics in the impedance control formulation. The second method suggests that a second order impedance be used, but the reaction force is estimated by pressure feedback instead of a force sensor. Both controllers are initially developed for a single degree of freedom manipulator and then extended to multiple degree of freedom manipulators. A Routh-Hurwitz analysis shows both controllers decouple the single degree of freedom hydraulic manipulator from the environmental stiffness. Impedance controllers are typically used in conjunction with the computed torque method to cancel the nonlinear dynamics. Modifications are made to the computed torque method to handle the third order dynamics of a hydraulic manipulator.; The two proposed control laws are tested and compared on a two degree of freedom manipulator that is driven by hydraulic actuators through a constrained motion task. Comparisons are made on the basis of tracking ability, complexity and actuator power output.