Motion and force control
In this chapter we deal with the motion control problem for situations in which the robot manipulator end effector is in contact with the environment. Many robotic tasks involve intentional interaction between the manipulator and the environment. Usually,
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Motion and force control In this chapter we deal with the motion control problem for situations in which the robot manipulator end effector is in contact with the environment. Many robotic tasks involve intentional interaction between the manipulator and the environment. Usually, the end effector is required to follow in a stable way the edge or the surface of a workpiece while applying prescribed forces and torques. The specific feature of robotic problems such as polishing, deburring, or assembly, demands control also of the exchanged forces at the contact. These forces may be explicitly set under control or just kept limited in a indirect way, by controlling the end-effector position. In any case, force specification is often complemented with a requirement concerning the end-effector motion, so that the control problem has in general hybrid (mixed) objectives. In setting up the proper framework for analysis, an essential role is played by the model of the environment. The predicted performance of the overall system will depend not only on the robot manipulator dynamics but also on the assumptions made for the interaction between the manipulator and environment. On one hand, the environment may behave as a simple mechanical system undergoing small but finite deformations in response to applied forces. When contact occurs, the arising forces will be dictated by the dynamic balancing of two coupled systems, the robot manipulator and the environment. On the other hand, if the environment is stiff enough and the manipulator is continuously in contact, part of its degrees of freedom will be actually lost, the motion being locally constrained to given directions. Contact forces are then viewed as an attempt to violate the imposed kinematic constraints. Three control strategies are discussed; namely, impedance control, parallel control, and hybrid force/motion control. Impedance control tries
C. C. de Wit et al. (eds.), Theory of Robot Control © Springer-Verlag London Limited 1996
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CHAPTER 4. MOTION AND FORCE CONTROL
to assign desired dynamic characteristics to the interaction with rather unmodelled objects in the workspace. Parallel control provides the additional feature of regulating the contact force to a desired value. Hybrid force/motion control exploits the partition of the task space into directions of admissible motion and of reaction forces, both arising from the existence of a rigid constraint. All strategies can incorporate the best available model of the environment; however, the achieved performance will suffer anyway from uncertainty in the location and geometric characteristics (orientation, curvature) of the contact surfaces. Moreover, for impedance and hybrid force/motion control, the measure of contact forces mayor may not be needed in nominal conditions. Thus, the main differences between these two strategies rely in the control objectives rather than in the implementation requirements of a specific control law . On the other hand, parallel control is aimed at closing a force control loop ar
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