Kinematic issues in 6R cuspidal robots, guidelines for path planning and deciding cuspidality

A cuspidal serial robot can change inverse kinematic solutions (IKS) without crossing singularities because it has multiple IKS in a singularity-free region. This property of robots has been researched for over thirty years but has not been taken seriously when designing new robots. The presented work points out issues related to nonsingular change of IKS and path planning specific to the cuspidal robots present in existing commercial robots used in various applications. The multiple IKS at the initial end-effector pose allows the user to choose an initial IKS that may lead to a continuous and repeatable path. We analyze in detail how the initial IKS choice affects the prescribed path’s feasibility and repeatability. Cuspidal robots can be used safely if the workspace is analyzed, considering the cuspidality property. For these reasons, we propose a path testing and planning methodology that considers different path scenarios. Given the rise of unconventional designs in 6R robots, the identification of cuspidal properties in the design phase of a robot is of paramount importance. We recall all the known criteria for cuspidality and propose new methods to decide if a given 6R robot is cuspidal. Accordingly, a practical guideline is proposed for deciding the cuspidality of a generic 6R robot.