A novel mechanism for changing the stiffness of a series elastic actuator over a wide range is briefly explained. The series elastic actuator's mathematical modeling, control method and physical limitations are addressed in this work as extension of our previous work. Two remaining issues of the previous work have been studied in this paper: 1 - A new nonlinear compensator is developed to improve the force tracking performance of the actuator. 2- Performance of the control system is studied by substitution of the saturation operator with equivalent gain. Finally, simulations are performed to show the ability of the controller to track desired force commands.

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