Real-time path tracking with torque limits by using a disturbance observer

When the minimum-time trajectory of a manipulator along a geometrically prescribed path is planned, considering the manipulator´s dynamics and actuators´ torque limits, at least one of the joints is at the torque limit. The execution of such a trajectory by a conventional feedback scheme results in torque saturation. Consequently, the tracking error cannot be suppressed and the manipulator may deviate from the desired path. We propose a feedback control method for path tracking which takes the torque saturation into account. A “path coordinate system” is defined, which is composed of the component along the desired path and the components normal to the path. Control of the components normal to the path is given priority in order to keep the motion of the manipulator on the path. A disturbance observer is used to estimate nonlinear dynamics instead of calculating them directly. This method can reduce real-time computations and can also reject unmodeled disturbances. The simulations of a two-degree-of-freedom manipulator show the effectiveness of the method