Mechanical Design and Development of a Continuous Rotational Variable Stiffness Actuator

One of the major issue concerning the coexistence of a human being and a robot is the interaction between a high stiffness object and a softer body part, especially if the impact is characterized by a high energy transmission. Expected or fortuitous collisions can be either managed using control strategies or mechanical elements such as SEA (Serial Elastic Actuators) or VSA (Variable Stiffness Actuators). This article deals with the design of an original prototype of a Continuous Rotation Variable Stiffness Actuator (CR-VSA). The solution proposed is made of two main functional subsystems: motor-reducer group and a variable stiffness equipment. The latter is formed by four steel foils whose length is defined by an actuated slider; the longer the beam, the lower the stiffness. The foils connect the rotation of the output to the one of the speed reducer, a cycloidal drive; encoders measure the angular coordinates to compute the position of the output shaft and the torque applied. A working prototype of this CR-VSA has been manufactured using rapid prototyping and low-cost solutions; ROS (Robot Operating System) has been used to implement the control strategy. This paper shows the design phase and the results achieved by the prototype built.