Mobile robotic manipulators can augment the strength and dexterity of human operators in unstructured environments. Here, the control system for a six degree-of-freedom heavy-lift mobile manipulator for lifting and inserting payloads on the deck of a ship is described. The robotic hardware and the application present several control challenges, including structural resonances, high joint friction that varies with time, limited sensors for measuring the joint friction, complex interaction with the environment, tight tolerances for the insertion tasks, lack of bilateral force feedback of the contact forces, and ship motions. The control system enables an operator to perform insertion tasks using feedback of tactile clues of the manipulator position, and reduces the effects of friction with a combination of sensor-based, adaptive, and model-based methods of friction compensation. The control architecture is validated in simulation and on a laboratory manipulator.
