Negative Stiffness in Compliant Shell Mechanisms

Abstract

Present stroke rehabilitation devices for the arm are often difficult to use by the patient himself and cannot be used at home. A compliant shell mechanism could overcome the shortcomings of the current available devices. By the use of monolithic shell mechanisms a simple to use device can be designed, which can be made wearable. It would make it easier for stroke rehabilitation patients to do repetitive rehabilitation exercises at home. The focus lies on balancing the gravity during the lifting of the upper-arm. To achieve that a negative stiffness in the compliant shell mechanism is necessary.
Negative stiffness arises when tape springs are bent and buckle for a short range of motion. Tape springs are thin-walled beams with a curved cross section. The short range of the negative stiffness limits the use for static balancing over a longer range of motion. In this project an analysis is presented on how the range of the negative stiffness can be increased by changing the geometry. The addition of longitudinal curvature to the tape spring results in a more gradual negative stiffness for a longer range of motion. It is shown why the addition of longitudinal curvature results in a more gradual and longer range of negative stiffness during the bending of shell.