Compliance is a key requirement for safe interactions with the environment for any robot. It has been well established that the human body exploits various arrangements of compliance such as series compliance (musculo-tendon units) and parallel compliance (joint capsules and ligament complex) to achieve robust and graceful interaction with the environment. Mechanical compliance can be similarly arranged in robotic joints in series or parallel to actuators. The effects of such arrangements on the closed loop properties of robotic joints such as stability, disturbance rejection and tracking performance have been analyzed separately, but their combined effects have not been studied. We present a detailed analysis on the combined effects of series and parallel arrangements of compliance on low inertia robotic joints. Our analysis shows the stability limitations of achievable joint stiffness due to series compliance and the subsequent increase in the stable upper limit of achievable joint stiffness by addition of parallel compliance. We provide guidelines towards designing compliance to improve the stability and performance of low-inertia robotic joints, which can be applied to the improvement of robotic hands performing grasping and manipulation tasks. We validate our analysis by means of an experimental platform and discuss the various characteristics and the effects of both arrangements of compliance on robotic hands.

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