TAILIEUCHUNG - Introductory Robotics P2

Consider how we cin join links together . At first sight there seem to be limitless ways of attaching one link to another while still allowing relative movement | Robot Anatomy 25 Now in this case p2 A p3 0 0 l r and so - r23 1 P2 -PÎ A P3 - pi r33 Notice that this also ensures that the rotation matrix satisfies the relation R TR I. That is the columns of the matrix are mutually orthogonal unit vectors. Putting all this together we have - 0 -1 0 1 0 0 0 0 1 The answer is a rotation about the z-axis together with a translation of one unit in each of the x y and z directions. Choosing nice points simplifies the calculations here. It is possible to find the rigid transformation from any three points as long as they do not all lie on a line. Essentially we must solve a system of linear equations for the twelve unknowns the rtj s and ti s. These equations will be linearly independent so long as the three points are not colinear. Hence we can keep track of links by following the progress of three points fixed to the link. Joints Consider how we can join links together. At first sight there seem to be limitless ways of attaching one link to another while still allowing relative movement. In the 1870s Franz Reuleaux a German mechanical engineer simplified things by defining lower pairs see fig. . A Reuleaux lower pair is a pair of identical surfaces one solid the other hollow. These surfaces fit together but can still move relative to each other while remaining in contact. Reuleaux found six such pairs and it can be shown that these are the only possibilities. Any surface of revolution gives a revolute or R-pair. Any helicoidal surface like the mating surfaces of a nut and bolt give a screw or H-pair. Any surface of translation like a prism results in a prismatic or P-pair. The surface of a cylinder is a surface of rotation and translation. Two cylinders form a cylindric or C-pair. A sphere is a surface of revolution about any diameter. A ball and socket are a spherical or S-pair. A plane is a surface of translation about any line in the plane and also a surface of revolution about any normal line. Two planes form a .

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