Design and Analysis of Two Types of Variable Stiffness Actuator Based on Adjustable Moment Arm Mechanism

This paper introduces the design and analysis of two types of novel inherently compliant actuators. The two actuation systems are characterized by the property that the output apparent stiffness can be varied independently from the position. The two proposed actuators can regulate the stiffness through the adjustable moment arm mechanism with minimum energy consumption. In the first VSA, this behavior is realized by implementing a lever arm mechanism with variable pivot point position. Compared with the existing VSA that based on the lever arm principle, the way of the spring compressed is more convenient to be implemented by using the tendon and pulley mechanism. Furthermore, the kinematic structure for spring compression is more beneficial for dealing with the unexpected dynamic collision situation. The internal elastic elements of the VSA use the fitted quadratic spring, which improve the characteristics of actuating torque and output stiffness. The stiffness regulation of the second VSA is realized by implementing the connecting rod and slider mechanism. This is a equivalent model of the lever arm mechanism with a variable length moment arm. The stiffness of the VSA is only related to the effective working radius for linear spring configuration, but not to the applied external torques or the deflection angle. This feature makes the stiffness control of the VSA more convenient. The working principles of the two actuators are elaborated and the characteristics of torque and stiffness of the VSA are presented. The mechanical solutions of the two actuators are described.