Control of Robot Interaction Forces Using Evolutionary Techniques

Abstract

For a robot manipulator to interact safely and human-friendly in an unknown environment, it is necessary to include an interaction control method that reliably adapts the forces exerted on the environment in order to avoid damages both to the environment and to the manipulator itself. A force control method, or strictly speaking, a direct force control method, can be used on those applications where the maximum or the desired force to exert is known beforehand. In some industrial applications the objects to handle or work with are completely known as well as the precise moment on which these contacts are going to happen. In a more general scenario, such as one outside a well-defined robotic workcell or when an industrial robot is used in cooperation with a human, neither the objects nor the time when a contact is ocurring are known. In such a case, indirect force control methods find their niche. These methods do not seek to control maximum or desired force, but they try to make the manipulator compliant with the object being contacted. The major role in the control loop is given to the positioning but the interaction is also being controlled so as to ensure a safe and clear contact. In case contact interaction forces have exceeded the desired levels, the positioning accuracy will be diminished to account and take care of the (at this moment) most important task: the control of the forces. Impedance control (Hogan (1985)) is one of these indirect force control methods. Its aim is to control the dynamic behaviour of a robot manipulator when contacting the environment, not by controlling the exact contact forces but the properties of the contact, namely, controlling the stiffness and the damping of the interaction. Moreover, the steady-state force can be easily set to a desired maximum value. The main idea is that the impedance control system creates a virtual new impedance for the manipulator, which is being able to interact with the environment as if new mechanical elements had been included in the real manipulator. First industrial approaches were focused on controlling the force exerted on the environment by a direct force feedback loop. A state-of-the-art review of the 80s is provided in (Whitney (1987)) and the progress during the 90s is described in (Schutter et al. (1997)). In many in22