Compliant robotics has the potential to further automate the agri-food industry by allowing delicate interaction with products of varying types and sizes. Many compliant gripper designs require non-linear springs to vary the 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 method for generating a pulley profile following a desired force-deflection curve is constructed. Other Symbolic and optimization-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. The resulting measurements show a 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 could result in a promising design that is easy to customize, simple to construct, and low friction.