chapter9 - Introduction to Robotics, H. Harry Asada 1...

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Introduction to Robotics, H. Harry Asada 1 Chapter 9 Force and Compliance Controls A class of simple tasks may need only trajectory control where the robot end-effecter is moved merely along a prescribed time trajectory. However, a number of complex tasks, including assembly of parts, manipulation of tools, and walking on a terrain, entail the control of physical interactions and mechanical contacts with the environment. Achieving a task goal often requires the robot to comply with the environment, react to the force acting on the end-effecter, or adapt its motion to uncertainties of the environment. Strategies are needed for performing those tasks. Force and compliance controls are fundamental task strategies for performing a class of tasks entailing the accommodation of mechanical interactions in the face of environmental uncertainties. In this chapter we will first present hybrid position/force control : a basic principle of strategic task planning for dealing with geometric constraints imposed by the task environment. An alternative approach to accommodating interactions will also be presented based on compliance or stiffness control . Characteristics of task compliances and force feedback laws will be analyzed and applied to various tasks. 9.1 Hybrid Position/Force Control 9.1.1 Principle To begin with let us consider a daily task. Figure 9.1.1 illustrates a robot drawing a line with a pencil on a sheet of paper. Although we humans can perform this type of task without considering any detail of hand control, the robot needs specific control commands and an effective control strategy. To draw a letter, “A”, for example, we first conceive a trajectory of the pencil tip, and command the hand to follow the conceived trajectory. At the same time we accommodate the pressure with which the pencil is contacting the sheet of paper. Let o-xyz be a coordinate system with the z -axis perpendicular to the sheet of paper. Along the x and y axes, we provide positional commands to the hand control system. Along the z -axis, on the other hand, we specify a force to apply. In other words, controlled variables are different between the horizontal and vertical directions. The controlled variable of the former is x and y coordinates, i.e. a position, while the latter controlled variable is a force in the z direction. Namely, two types of control loops are combined in the hand control system, as illustrated in Figure 9.1.2. F z O y z x Figure 9.1.1 Robot drawing a line with a pencil on a sheet of paper Department of Mechanical Engineering Massachusetts Institute of Technology
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Introduction to Robotics, H. Harry Asada 2 Force Control + Position Control + Robot Position Reference Controlled Variables Force Reference Figure 9.1.2 Position and force control loops The above example is one of the simplest tasks illustrating the need for integrating different control loops in such a way that the control mode is consistent with the geometric constraint imposed to the robot system. As the geometric constraint becomes more complex and
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chapter9 - Introduction to Robotics, H. Harry Asada 1...

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