Allocation of dimensional tolerances for a parallel robot

The errors in the kinematic parameters of robots, due to the machining and to the assembly of the mechanical links, affect the positioning accuracy of robots. To obtain good accuracy these errors must be limited. In order to limit the positioning error it is important to carefully choose dimensional tolerances. Thus to obtain an optimum design of robots it is important to relate the positioning accuracy to the design tolerances, and then to manufacturing cost. This paper presents a method to synthesize the design tolerances of spatial robots, using a parameter that settles a relation between the cost and the positioning error. This method was implemented and tested on the Delta robot. Delta is a four degrees of freedom (dof) robot with a parallel structure, conceived for the manipulation of lightweight objects at fast working rhythms. After a statistical analysis of the effects of manufacturing tolerances and of the angular position of drives on the position accuracy, the method was applied obtaining optimal design and assembly tolerances to reach an assigned positioning accuracy.