A Simplified Variable Admittance Controller Based on a Virtual Agonist-Antagonist Mechanism for Robot Joint Control

In this paper, we propose a simplified variable admittance controller applied to robot joint control. It is based on a virtual agonist-antagonist mechanism (VAAM) consisting of contractile and parallel elements (CEs and PEs). ”Virtual” here means that every joint physically actuated by a standard servo motor can produce variably compliant motions as if it were driven by a pair of agonist and antagonist muscles. This makes it different from variable stiffness actuators (VSAs) with mechanically bulky and complex mechanisms. Moreover, the controller differs from other conventional PID admittance and variable admittance controllers since it only relies on force sensing at the end effector of robot rather than complex force/torque sensing of every joint. We have successfully implemented the controller on our hexapod robot which enables it to perform variable compliant behaviors, thereby reducing contact force and preventing leg damages when it is imposed with static or dynamical perturbation.