Analytical study on increasing isotropy of intrinsic stiffness in manipulators through biarticular structure

When robots and humans interact in a task, stiffness is necessary for performance, while passive compliance is fundamental for safety. These two factors are the motivation for intrinsic compliant modulation in robots interacting with humans. Variable Stiffness Actuators (VSAs) allow for simultaneous position and stiffness control of a joint, and therefore have been implemented in the realization of intrinsically compliant and high performance manipulators. Most applications employ VSAs in a monoarticular structure, in which one actuator drives one joint. In the biological world however, biarticular muscles (muscles spanning two joints) play a fundamental role in motion control for humans, reducing link inertia and increasing isotropy of end effector force. In this work, a two-link planar manipulator actuated with VSAs in two different actuation structures (the traditional monoarticular and humanlike biarticular) is taken into account. The end effector stiffness in both actuation structures is calculated and analyzed. In comparison with the VSA monoarticular structure, the end effector stiffness in the VSA biarticular structure shows a higher isotropy in the region of the workspace favorable for executing dynamic tasks in contact with the environment.