Rotatable cam-based variable-ratio lever compliant actuator for wearable devices

This paper presents a novel variable stiffness mechanism based on the combination of a rotatable cam and a variable-ratio lever principle. The proposed configuration results in i) a pseudo-linear torque/deflection characteristics, where ii) the stiffness is varied rotationally and perpendicularly to the external load. The first is convenient for easier mechanism dimensioning and offers a more conventional control design. The second allows for a more energy efficient stiffness variation, thereby enabling the use of a less powerful stiffness variation motor and a reduction of the overall actuator weight and size. The presented mathematical model derivation and theoretical evaluation support both desired mechanism characteristics. Practical use of the mechanism is evaluated through the application of the developed prototype in a fully sensorized variable stiffness actuator intended for a wearable application. The results of the experiments and a comparison to other variable stiffness mechanisms show that the mechanism geometry is convenient for light and compact design.