Variable stiffness actuator (VSA)

An actuator capable of adjusting its own output stiffness independently of its position, either by changing the preload on a spring element or by reconfiguring the mechanical structure. VSAs allow a joint to be stiff during free-space motion (for accuracy) and compliant during contact (for safety), adapting in real time. They add mechanical complexity but resolve the stiffness-versus-compliance tradeoff that is otherwise fixed at design time. (see Topic 7.2) for the servo motor foundation that VSA designs build on.

An actuator design, an active area of ongoing robotics research as of 2026, that allows the effective stiffness of a compliant joint to be adjusted during operation rather than fixed at the time of manufacture, aiming to combine the tunability associated with active compliance with more of the fail-safe, physics-based robustness of passive compliance. This remains an emerging research direction rather than a widely deployed production design.