Optimizing Link Flexibility for Safety and Dynamic Performance for Collaborative Manipulators

Cobots are designed to work in close collaboration with humans. To ensure safety in shared workspaces, ISO/TS 15066 imposes limits on the collaborative robot speeds to limit the impact forces during collisions. These limits depend on robot design, including inertia. Flexible link manipulators combine the advantage of lightweight design and the introduction of an elastic element, which is promising in the context of collaborative robots. However, reduced stiffness affects dynamic performance. As trade-offs between link flexibility for safety and dynamic performance remain unclear, this paper investigates how undesired side effects of flexibility relate to the positive aspects of compliance and lightweightness. To investigate the effect of flexibility on both safety towards a human and controlled manipulator dynamics, a generic simulation framework was designed. Safety is assessed by a simulation study on the influence of varying geometrical parameters on impact force at collision with a human head, after which the corresponding maximum allowable end-effector velocities per ISO/TS 15066 threshold were derived. To analyze control performance, a generic controller was implemented, consisting of a PID with direct strain vibration control, with gains optimized for each link design. The resulting end-effector tracking performance during point-to-point motion is evaluated. Based on these results, a co-design methodology was proposed to optimize link geometry and control gains for the fastest possible, yet safe motion for a given task. The effectiveness of the method was experimentally validated to assess whether optimal link stiffness and controller gains yield the desired dynamic performance while respecting ISO safety thresholds.