Force tracking with unknown environment parameters using adaptive fuzzy controllers

The paper proposes an adaptive fuzzy control scheme for explicit force control of a robot manipulator in contact with an environment whose parameters are unknown and vary considerably. The scheme consists of three main components, a reference force model describing the desired behaviour of the force control system, a fuzzy force controller that determines the adjustment to the position control loop, and a fuzzy learning and adaptation mechanism that modifies the fuzzy force controller according to the difference between the actual and desired force responses. The modification is performed by shifting and contracting/expanding the membership functions of the fuzzy sets associated with the consequent rules of the fuzzy force controller. It is demonstrated through simulations of a manipulator that the scheme is capable of force tracking despite wide parameter variations, such as when the environment stiffness changes by several orders of magnitude