Variable impedance actuators: A review <s> Antagonistic Springs with Independent Motors. <s> Abstract This paper examines an actuation system, intended for use in a prosthetic arm, that mimics the ability of antagonistic muscles in biological systems to modulate the stiffness and position of a joint. The system uses two physical nonlinear springs arranged antagonistically about a joint to generate control of both stiffness and movement. To decouple the net joint stiffness from joint deflection, it is shown that the springs must generate a force that is a quadratic function of deflection. Large stiffness errors are shown to occur if the springs vary slightly from the quadratic form. A stiffness controlled region of the torque–deflection curves for the joint is found based on physical limitations of the spring deflections and the amount of space available for spring movement. Inside this region the stiffness is controllable, but beyond these limits the system acts as a rigid manipulator. Examination of the limits shows a relationship between the shape of the stiffness controlled region and the spring force functions. <s> BIB001 </s> Variable impedance actuators: A review <s> Antagonistic Springs with Independent Motors. <s> In this paper we introduce a classification of intrinsically compliant joint mechanisms. Furthermore, we outline design considerations for realizing such devices in order to match the requirements for robust and performant actuation. Based on this elaboration, a new design concept is presented, the DLR QA-Joint. Its performance is investigated by various experiments, covering velocity increase using the elastic energy, joint protection capabilities, and control performance. <s> BIB002
The main disadvantage of using antagonistic motors is that both motors need to work synchronously to either change the equilibrium position or the stiffness. This means the motors cannot be dimensioned for a specific task. For Antagonistic Springs with Independent Motors, the motors are arranged to (partially) decouple the control of equilibrium position and stiffness 4. For the Quasi-Antagonistic Joint (QA-Joint) BIB002 one motor (the link drive) adjusts the link side position, while the second motor (the stiffness drive) operates the stiffness adjustment. This is a partially decoupled system since when the stiffness is changed, the equilibrium position must be adjusted by the link side motor. Complete decoupling of the equilibrium position setting and the stiffness occurs when the endpoints of the two springs are mechanically coupled either by a lever arm or a pulley like e.g. the AMASC actuator  . Here the motor to set the equilibrium position is not on the joint but on the other side of the nonlinear springs. It is also possible to move this motor to the joint, making the design less complex. In this case the equilibrium position of both lever arms is horizontal, and the motor for the equilibrium position of the actual joint sets the relative position of the arm of the joint with respect to the lever arms BIB001 . With this design the angular joint range is extended, since it is not limited to the maximum extension of the springs.