Stiction Compensation in Agonist-Antagonist Variable Stiffness Actuators

Abstract

Abstract-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.

Differential Noncircular Pulleys for Cable Robots and Static Balancing

“…The variation of l FV during the rotation, i.e. ∆l FV , must therefore be compensated by the elongation of a spring (as in [3,5,8,15]) or by the motion of a guided body to which the cable is connected, such as the end-effector of a cable-suspended robot, see [10]. 2.…”

“…One of the main advantages of using noncircular pulleys for this specific application is the reduced weight and increased distance between the joints that one can achieve. However, the most common usage of noncircular pulleys one can find in the literature so far appears to be the design of nonlinear rotational springs for which the rotation angle vs. torque relationship is nontrivial and determined by the pulley profile [3][4][5]. Most noticeably, one can use this type of springs with the aim of statically balancing mechanisms, as for instance in the pioneer work presented in [6].…”