Transitions in the velocity pattern of lower mobility parallel manipulators

Lower mobility parallel manipulators are designed to perform tasks that require less than six degrees of freedom. The operational mode is given by the type of screw system spanned by the end-effector’s twist and it is configuration dependent. A change in the dimension of such screw system occurs at singularities; a loss of freedom of the end-effector will make the operation not feasible while a gain of freedom will produce uncontrolled motions. In this contribution the authors show that there are also poses in which, being no quantitative alteration of the freedom of the moving platform, there are changes in the type of its screw system. These changes are viewed as transitions in the velocity pattern of the end-effector. Such transitions can alter the operational motion and produce a problem that should be avoided, but it can be also a useful new operational mode. Because in many industrial applications the operational mode is viewed in terms of the pure translations and the rotations available in the end-effector, the authors have used such concepts to denote the type of motion. In this paper the authors present a procedure to analyze these transitions and present an example.