Dimensional synthesis of the optimal RSSR mechanism for a set of variable design parameters

This paper deals with the dimensional synthesis of the RSSR mechanism, also known as spatial four-bar linkage (R and S stand for revolute and spherical kinematic pairs respectively). To univocally describe the geometry of the RSSR mechanism a specific set of geometry parameters is necessary. Generally speaking, in a synthesis problem a subset of these parameters, defined as design parameters, is usually considered as assigned whereas the remaining ones, defined as design variables, have to be found by the synthesis procedure. That is, the goal of the synthesis procedure is to find the values of the design variables, while satisfying both functional requirements of the mechanism and constraints on the design parameters. In this paper each design parameter is assigned as variable within a given range rather than being assigned as a single value. In general, varying a design parameter means obtaining a different mechanism which has different functional performances as a consequence. This feature gives raise to a novel synthesis problem, which has not been treated in the literature yet. In particular, the RSSR mechanism synthesis presented in this paper takes the optimization of the force transmission as an objective function, while referring to a given range of values of each design parameter. In addition, prescribed constraints on given extreme angular positions for both the input and the output links, on the upper and lower bounds for the transmission ratio, and on the upper and lower bounds for the design variable values have to be satisfied. The synthesis problem, set as a constrained minimization problem, is solved numerically in two steps by means of a Genetic algorithm followed by a quasi-Newton algorithm. As an example of application, a dimensional synthesis of an RSSR mechanism used in a hand exoskeleton is reported.