According to the end-effector mobility, PRs are commonly classified into those with complete mobility (six degree of freedom) and those with lower mobility (less than 6-DOF). Generally speaking, lower mobility PRs consist of less number of limbs and therefore simpler and cheaper, comparing to those with complete mobility. Since many applications do not need a complete 6-DOF motion, low-mobility PRs have recently attracted a lot of attention in the robotics research community.
Based on the motion transmission characteristics, PRs are categorized into decoupled and un-decoupled ones. A decoupled PR has an architecture allowing specific motion axes to be driven in a certain order by the designated actuators independently. The decoupled motion will result in simpler displacement calculation, singularity analysis, workspace evaluation, static analysis, and motion planning and control. For an un-decoupled PR, its output motion DOF is always coupled together, driven by one or multiple actuators. Therefore, its kinematic analysis and static analysis tend to be more difficult, so are the motion planning and control.
For successful applications, the determination of a PR has to be depending on the specific task requirements, aiming to maximize its advantage aspects while minimizing disadvantage aspects.