Stiction Compensation in Agonist-Antagonist Variable Stiffness Actuators

In the last decade new actuator designs have been presented trying to introduce at mechanical level the advantages of compliance. Ranging from serial elastic actuators to different designs of variable stiffness actuators, various prototypes have been proposed and implemented on robots, thus allowing performance of novel and challenging tasks. Nevertheless some of these new devices often are affected by the drawbacks related to friction. In particular, static friction due to its discontinuous nature, can produce undesired behaviors that are rather difficult to compensate. In this paper we present a novel kind of passive variable stiffness actuator based on agonist-antagonist configuration. The specific design we adopted improves the capability of the system in mechanically compensating the external disturbances, but on the other hand intensifies the effect of stiction during the cocontraction of the agonist and antagonist side of the actuator. The consequence is the appearance of a set of neutral equilibrium configurations of the output joint that we named “dead-band”. This issue is tackled analytically investigating the propagation and the distribution of the stiction components within the whole system. The result is a condition over the spring potential energies that is exploited to properly design the new non-linear springs. Eventually experimental tests are conducted on the real actuator, showing the effectiveness of our analytical approach.