Recently, Series Elastic Actuator (SEA) has been popularly used as a torque sensor thanks to its notable ability to calibrate the relation between torque and displacement. It has been applied to many robotic applications and used in a various industrial automation fields. However, most of the current SEAs have nonlinear torque-displacement characteristics which could not be easily alleviated. In order to be utilized as a feasible torque sensor, the wide linearity of a SEA in torque-displacement relationship is not an option. Also, adjustable compliance is needed to implement a mechanism with different stiffness, depending on the various cases where SEA can be applied. In this paper, we designed a Variable Stiffness Linear Series Elastic Actuator (VLSEA) mechanism that can achieve variable stiffness with a linear relationship between torque and displacement. At first, a design with a four-bar link was proposed for linear relations, but it was difficult to implement variable stiffness. We modified the design using the Scotch Yoke mechanisms for the model to have variable stiffness. Simulation of the designed model then verifies that the model can properly implement linearity and variable stiffness.

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