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Chapter 9 — Force Control

Luigi Villani and Joris De Schutter

A fundamental requirement for the success of a manipulation task is the capability to handle the physical contact between a robot and the environment. Pure motion control turns out to be inadequate because the unavoidable modeling errors and uncertainties may cause a rise of the contact force, ultimately leading to an unstable behavior during the interaction, especially in the presence of rigid environments. Force feedback and force control becomes mandatory to achieve a robust and versatile behavior of a robotic system in poorly structured environments as well as safe and dependable operation in the presence of humans. This chapter starts from the analysis of indirect force control strategies, conceived to keep the contact forces limited by ensuring a suitable compliant behavior to the end effector, without requiring an accurate model of the environment. Then the problem of interaction tasks modeling is analyzed, considering both the case of a rigid environment and the case of a compliant environment. For the specification of an interaction task, natural constraints set by the task geometry and artificial constraints set by the control strategy are established, with respect to suitable task frames. This formulation is the essential premise to the synthesis of hybrid force/motion control schemes.

Recent research in impedance ontrol

Author  Unknown, Case Western Reserve University, Cleveland

Video ID : 684

Experimentacl research on impedance control done in 1991 at Case Western Reserve University in Cleveland, Ohio. The demonstrations involve three scenarios: stiffness control without force sensing; impedance control based on a wrist force sensor; and impedance control based on joint torque sensing. This work was published in the ICRA 1991 video proceedings.

Integration of force strategies and natural-admittance control

Author  Brian B. Mathewson, Wyatt S. Newman

Video ID : 685

When mating parts are brought together, small misalignments must be accommodated by responding to contact forces. Using force feedback, a robot may sense contact forces during assembly and invoke a response to guide the parts into their correct mating positions. The proposed approach integrates force-guided strategies into Hogan's impedance control. Stability of both geometric convergence and of contact dynamics are achieved. Geometric convergence is accomplished more reliably than through the use of impedance control alone, and such a convergence is achieved more rapidly than through the use of force-guided strategies alone. This work was published in the ICRA 1995 video proceedings.

Experiments of spatial impedance control

Author  Fabrizio Caccavale, Ciro Natale, Bruno Siciliano, Luigi Villani

Video ID : 686

The videod results of an experimental study of impedance control schemes for a robot manipulator in contact with the environment are presented. Six-DOF interaction tasks are considered that require the implementation of a spatial impedance described in terms of both its translational and its rotational parts. Two representations of end-effector orientation are adopted, namely, Euler angles and quaternions, and the implications for the choice of different orientation displacements are discussed. The controllers are tested on an industrial robot with open-control architecture in a number of case studies. This work was published in A. Casals, A.T. de Almeida (Eds.): Experimental Robotics V, Lect. Note. Control Inform. Sci. 232 (Springer, Berlin, Heidelberg 1998)

Compliant robot motion: Control and task specification

Author  Joris De Schutter

Video ID : 687

The video contains work developed in the PhD thesis of Joris De Schutter, where the concept of compliant motion based on external force feedback loops and on the task frame formalism to specify interaction tasks were introduced. The video was recorded in 1984. The references for this video are 1. J. De Schutter, H. Van Brussel: Compliant robot motion II. A control approach based on external control loops, Int. J. Robot. Res. 7(4), 18-33 (1988) 2. J. De Schutter, H. Van Brussel: Compliant robot motion I. A formalism for specifying compliant motion tasks, Int. J. Robot. Res. 7(4), 3-17 (1988)

COMRADE: Compliant motion research and development environment

Author  Joris De Schutter, Herman Bruyninckx, Hendrik Van Brussel et al.

Video ID : 691

The video collects works on force control developed in the 1970s-1980s and 1990s at the Department of Mechanical Engineering of the Katholieke Universiteit Leuven, Belgium. The tasks were programmed and simulated using the task-frame-based software package COMRADE (compliant motion research and development environment). The video was recorded in the mid-1990s. The main references for the video are: 1. H. Van Brussel, J. Simons: The adaptable compliance concept and its use for automatic assembly by active force feedback accommodations, Proc. 9th Int. Symposium Indust. Robot., Washington (1979), pp.167-181 2. J. Simons, H. Van Brussel, J. De Schutter, J. Verhaert: A self-learning automaton with variable resolution for high precision assembly by industrial robots, IEEE Trans. Autom. Control 27(5), 1109-1113 (1982) 3. J. De Schutter, H. Van Brussel: Compliant robot motion II. A control approach based on external control loops, Int. J. Robot. Res. 7(4), 18-33 (1988) 3.J. De Schutter, H. Van Brussel: Compliant robot motion I. A formalism for specifying compliant motion tasks, Int. J. Robot. Res. 7(4), 3-17 (1988) 4. W. Witvrouw, P. Van de Poel, H. Bruyninckx, J. De Schutter: ROSI: A task specification and simulation tool for force-sensor-based robot control, Proc. 24th Int. Symp. Indust. Robot., Tokyo (1993), pp. 385-392 5. W. Witvrouw, P. Van de Poel, J. De Schutter: COMRADE: Compliant motion research and development environment, Proc. 3rd IFAC/IFIP Workshop on Algorithms and Architecture for Real-Time Control. Ostend (1995), pp. 81-87 6. H. Bruyninckx, S. Dutre, J. De Schutter: Peg-on-hole, a model-based solution to peg and hole alignment, Proc. IEEE Int. Conf. Robot. Autom. (ICRA), Nagoya (1995), pp. 1919-1924 7. M. Nuttin, H. Van Brussel: Learning the peg-into-hole assembly operation with a connectionist reinforcement technique, Comput. Ind. 33(1), 101-109 (1997)

Robotic assembly of emergency-stop buttons

Author  Andreas Stolt, Magnus Linderoth, Anders Robertsson, Rolf Johansson

Video ID : 692

Industrial robots are usually position controlled, which requires high accuracy of the robot and the workcell. Some tasks, such as assembly, are difficult to achieve by using using only position sensing. This work presents a framework for robotic assembly, where a standard position-based robot program is integrated with an external controller performing with force-controlled skills. The framework is used to assemble emergency-stop buttons which had been tailored to be assembled by humans. This work was published in A. Stolt, M. Linderoth, A. Robertsson, R. Johansson: Force controlled assembly of emergency stop button, Proc. Int. Conf. Robot. Autom. (ICRA), Shanghai (2011), pp. 3751–3756