Variable Stiffness Actuator (VSA)

An actuator whose joint “softness” can be mechanically adjusted on the fly, independent of position.

A variable stiffness actuator adds an elastic element between the motor and the joint, plus an extra mechanism — usually a second motor — that adjusts that spring's effective stiffness in real time, so joint position and joint softness can be controlled separately. It extends the series elastic actuator, whose stiffness is fixed, and has been studied systematically since the 2000s by groups including the University of Pisa, the German Aerospace Center (DLR), and the Italian Institute of Technology. The benefits: turning stiffness down reduces impact if the joint hits a person, making it safer, while turning stiffness up gives precise positioning when that's needed; the spring can also store energy and release it for explosive motions like throwing or jumping. The downside is a more complex mechanism with higher weight and cost, so it's mainly used on research platforms today — production robots more often get similar behavior from software-based impedance control instead.

ExampleDLR's Hand Arm System uses variable stiffness actuation in its arm and hand joints.

Also called

VSA, Variable Stiffness Joint

Related

Series Elastic Actuator (SEA) · Stiffness · Compliance · Impedance Control · Variable Impedance Control · Physical Human-Robot Interaction

Sources

Variable impedance actuators: A review (Robotics and Autonomous Systems, 2013)