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Unformatted text preview: Introduction to Robotics, H. Harry Asada 1 Chapter 3 Robot Mechanisms A robot is a machine capable of physical motion for interacting with the environment. Physical interactions include manipulation, locomotion, and any other tasks changing the state of the environment or the state of the robot relative to the environment. A robot has some form of mechanisms for performing a class of tasks. A rich variety of robot mechanisms has been developed in the last few decades. In this chapter, we will first overview various types of mechanisms used for generating robotic motion, and introduce some taxonomy of mechanical structures before going into a more detailed analysis in the subsequent chapters. 3.1 Joint Primitives and Serial Linkages A robot mechanism is a multi-body system with the multiple bodies connected together. We begin by treating each body as rigid, ignoring elasticity and any deformations caused by large load conditions. Each rigid body involved in a robot mechanism is called a link, and a combination of links is referred to as a linkage. In describing a linkage it is fundamental to represent how a pair of links is connected to each other. There are two types of primitive connections between a pair of links, as shown in Figure 3.1.1. The first is a prismatic joint where the pair of links makes a translational displacement along a fixed axis. In other words, one link slides on the other along a straight line. Therefore, it is also called a sliding joint. The second type of primitive joint is a revolute joint where a pair of links rotates about a fixed axis. This type of joint is often referred to as a hinge, articulated, or rotational joint. 1 (a) Prismatic joint (b) Revolute joint Figure 3.1.1 Primitive joint types: (a) a prismatic joint and (b) a revolute joint Combining these two types of primitive joints, we can create many useful mechanisms for robot manipulation and locomotion. These two types of primitive joints are simple to build and are well grounded in engineering design. Most of the robots that have been built are combinations of only these two types. Let us look at some examples. Robot mechanisms analogous to coordinate systems One of the fundamental functional requirements for a robotic system is to locate its end- effecter, e.g. a hand, a leg, or any other part of the body performing a task, in three-dimensional space. If the kinematic structure of such a robot mechanism is analogous to a coordinate system, 1 It is interesting to note that all biological creatures are made of revolute type joints; there are no sliding joints involved in their extremities....
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- Fall '05