A systematic analysis of spring symmetry on optimality of antagonistic variable stiffness actuation

We present a systematic co-design optimization framework that enables comparison of antagonist variable stiffness actuators (VSA) with and without spring symmetry constraints. The proposed framework promotes a fair comparative analysis by maintaining design continuity among system-optimal designs of symmetric and asymmetric VSA configurations. Through a case study of a VSA-powered knee prosthesis, we not only demonstrate that co-design of robots driven by VSAs can provide important performance benefits with respect to sub-system level control-optimal designs, but also provide evidence that relaxing symmetry constraints for certain periodic tasks can result in substantial advantages in performance. Our systematic comparative analysis justifies the use of asymmetric stiffness arrangements for antagonistic VSA-powered robotic systems performing periodic tasks.