Fig. 3

Schematic of the Variable Stiffness Actuation (VSA) unit and its expected stiffness range. The VSA (inspired from the MACCEPA and MARIONET systems) is illustrated on the left. The lever motor (M _lever ) situated in the lever unit controls the lever position relative to the shank. The lever unit is connected to the thigh through the spring k, which can be pretensioned (by the pretension motor M _pretension ). Varying pretension, which changes spring length x, results in a change of the stiffness. The stiffness also varies with the deflection α, describing the deflection of the lever unit from its equilibrium position. Stiffness in function of x and α is shown on the right. The mechanically available stiffness modulation range is indicated as a grey area. Holding a pretension continuously is limited by the motor’s continuous current limit indicated with the 100% line (yellow). The relative angle between thigh and shank (knee angle) β therefore depends on the lever’s equilibrium position, the load applied to the joint and its stiffness