Design of a Variable Stiffness Mechanism with Prescribed Behaviour Using a Variable Cam Pulley

Compliant robotics has the potential to further automate the agri-food industry by allowing delicate interaction with products of varying types and sizes. Compliant gripper designs often require non-linear springs to vary compliance/stiffness of the system. Springs with prescribed force-deflection curves are not readily available. This paper presents a variable stiffness mechanism based on a variable radius pulley placed in series with a linear spring. The mechanism is designed for use in a tendon-based gripper prototype of which previous non-linear springs did not perform well. An iterative, feedback-based method for generating a pulley profile following a desired force-deflection curve is constructed. Other symbolic and performance-metric based approaches are discussed. Mathematical approximations limit the expected accuracy of generated profiles. A proof-of-concept setup is built to test a range of generated profiles. Resulting measurements show strong correlation between desired and measured force-deflection curves. Error, likely resulting from mathematical approximations and material deformation, is mainly present at the higher non-linearity profiles, where the measurements deviate further from the desired values. Further focus on eliminating the mathematical approximations should result in a promising design that is easy to customize, simple to construct, and low friction.