Design and Modeling of a Module with Locally Linear Variable Stiffness

The Actuator with Manumotive Locally Linear Variable Stiffness (MLL-VSA) is an actuator which can independently control equilibrium position by one motor and tune stiffness manually. This paper describes the design and development of MLL-VSA which is an improved version of CompAct-VSA. The CompAct-VSA tuned the stiffness by controlling the location of a variable pivot axis actuated by a rack and pinion transmission system. Instead MLL-VSA regulates the position of the pivot axis by controlling location of a pair of antagonistic fine thread bolts and angle of deflection of a customized disc cam. As in the first implementation, the adjustable range of high stiffness is concentrated on a very small range of position of the pivot point. This realization results in a high nonlinear adjustment of stiffness, then it is hard to control its value precisely. While the MLL-VSA introduces a disc cam to solve the problem. The input for variable stiffness unit has a linear relationship with output of stiffness. Meanwhile it does not need an independently additional motor for keeping constant stiffness because of the self-locking function of fine thread bolts. As the torque amplification ratio can theoretically change from zero to infinitive, the level of stiffness can be tuned from very soft to complete rigid. Overall, this paper focus on the mechanics, principle of operation and modeling of actuator in detail. Finally, simulation results demonstrate the locally linear adjustment of stiffness and heavy load capacity achieved by the proposed MLL-VSA design.