Definition and measurement of precision: operation-performance relationship
It is difficult to establish exact definitions in absolute terms for the accuracy with which the positioning and orienting of the end effector, or tool holder, of a robot is carried out.11 The error vectors dP and dø depend not only on a large number of variables relating to the robot (eg geometrical defects, mechanical deformations), the operating conditions (eg temperature, load), the situation vectors and corresponding velocity vectors, but also on the time history of the situation. This is imposed by the operation to be carried out. In Chapters 3–7 the components of these possible errors are characterized and identified so as to establish models. These are presented either in parametric form, as functions of the variables describing the state of the robot and its environment, or in non-parametric form (eg tables of points, hypersurfaces). It is useful, however, to be able to express robot performance in a condensed form, on condition that the measurements taken are consistent with the operation under consideration. For this reason, a list of definitions of the types of precision is given below, followed by an explanation of the classes of precision of industrial robots as a function of their working range (ie the volume in which any point may be accessed by the reference point of the tool holder) and of the load (ie maximum weight of the terminal device and the load held by the manipulator arm).
KeywordsMachine Tool Spot Welding Industrial Robot Tool Holder Geometrical Performance
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