Variable-stiffness actuator

Stiffness is adjusted mechanically#

Mechanical stiffness describes how much actuator torque changes for a given output deflection. A variable-stiffness actuator changes that relationship by moving a spring attachment, altering a lever arm, changing spring pretension, or using an antagonistic arrangement. One mechanism may set the equilibrium position while another changes stiffness.

This differs from commanding a lower feedback gain on a rigid actuator. Both can change apparent interaction behaviour, but a VSA stores energy in a physical elastic element and retains mechanical compliance even when sensing or control is delayed.

Robots can change behaviour with the task#

Low stiffness can absorb shocks and reduce transmitted contact forces. Higher stiffness can resist disturbance or support more accurate motion under load. For a legged robot, elasticity can also store and return energy during a step.

The variable-length leaf-spring actuator is one researched design for legged locomotion. Its authors report prototype hopping experiments and identify size, mass, stiffness range, adjustment speed, and energy used for adjustment as design concerns. A separate differential spiral joint study demonstrates a tendon-driven mechanism intended to partly separate position and stiffness commands. These are results for particular prototypes, not evidence that every VSA improves a humanoid.

Added mechanics create trade-offs#

A VSA normally needs extra springs, transmissions, or motors. These add mass, volume, friction, backlash, and control variables. The useful stiffness range and the speed at which it can change may be limited. Some designs also couple stiffness adjustment to position or stored energy.

A series elastic actuator has an elastic element in series with the drive, but its physical stiffness may be fixed. A VSA specifically makes mechanical stiffness adjustable. Neither term alone states the actuator's bandwidth, torque density, efficiency, or safety under impact.