A novel characterisation of spatial manipulators based on their degrees-of-freedom

This paper deals with the nature of the degree(s)-of-freedom (DoF) of spatial manipulators. When such manipulators have less than six DoF, it is generally not clear as to how many of its DoF are rotational in nature and how many purely translational. The problem is further complicated by the fact that the split of the total DoF in these categories can change for the same manipulator at different subsets of its configuration space. This paper elaborates the concept of partitioning of DoF introduced in [1] and uses it to present a classification of spatial manipulators based on the nature of its motion at any given configuration. The identification of these classes in terms of the configuration and architecture of the manipulators can help substantially in their design. The impact of some changes of architecture on the instantaneous DoF of a 3-RPS spatial manipulator is presented to illustrate the theory developed in the paper.