Modelling and identification
From a mechanical viewpoint, a robotic system is in general constituted by a locomotion apparatus (legs, wheels) to move in the environment and by a manipulation apparatus to operate on the objects present in the environment. It is then important to disti
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Modelling and identification From a mechanical viewpoint, a robotic system is in general constituted by a locomotion apparatus (legs, wheels) to move in the environment and by a manipulation apparatus to operate on the objects present in the environment. It is then important to distinguish between the class of mobile robots and the class of robot manipulators. The mechanical structure of a robot manipulator consists of a sequence of links connected by means of joints. Links and joints are usually made as rigid as possible so as to achieve high precision in robot positioning. The presence of elasticity at the joint transmission or the use of lightweight materials for the links poses a number of interesting issues which lead to separating the study of flexible robot manipulators from that of rigid robot manipulators. Completion of a generic task requires the execution of a specific motion prescribed to the manipulator end effector. The motion can be either unconstrained, if there is no physical interaction between the end effector and the environment, or constrained if contact forces arise between the end effector and the environment. The correct execution of the end-effector motion is entrusted to the control system which shall provide the joint actuators of the manipulator with the commands consistent with the desired motion trajectory. Control of end-effector motion demands an accurate analysis of the characteristics of the mechanical structure, actuators, and sensors. The goal of this analysis is the derivation of mathematical models of robot components. Modelling a robot manipulator is therefore a necessary premise to finding motion control
C. C. de Wit et al. (eds.), Theory of Robot Control © Springer-Verlag London Limited 1996
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CHAPTER 1. MODELLING AND IDENTIFICATION
strategies. In order to characterize the mechanical structure of a robot manipulator, it is opportune to consider the following two subjects. • Kinematic modelling of a manipulator concerns the description of the manipulator motion with respect to a fixed reference frame by ignoring the forces and moments that cause motion of the structure. The formulation of the kinematics relationship allows studying both direct kinematics and inverse kinematics. The former consists of determining a systematic, general method to describe the end-effector motion as a function of the joint motion. The latter consists of transforming the desired motion naturally prescribed to the end effector in the workspace into the corresponding joint motion. • Dynamic modelling of a manipulator concerns the derivation of the equations of motion of the manipulator as a function of the forces and moments acting on it. The availability of the dynamic model is very useful for mechanical design of the structure, choice of actuators, determination of control strategies, and computer simulation of manipulator motion. It is worth emphasizing that kinematics of a manipulator represents the basis of a systematic, general derivation of its dynamics.
Both kinematic and dynamic model
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