Variable Stiffness Joint Based on Pneumatic Relocation Mechanism

The concept of storing and reusing energy has emerged in quasi-passive exoskeletons as a solution to weight reduction and energy savings. These types of exoskeletons only store and reuse energy, but do not generate net positive mechanical work. In order to store different amounts of energy, the quasi-passive mechanism needs to be extended with the variable stiffness. In this paper, we present a newly designed variable stiffness joint that can change the geometry by exploiting the pneumatic artificial muscle (PAM). In addition to the PAM, the joint consists of a pneumatic cylinder and three fast-switching air valves. The PAM is the component that contracts in length when filled with compressed air and is inherently compliant. By filling the PAM with air from the cordless portable air pump, the pneumatic cylinder’s effective length changes, resulting in a variation in torque, stiffness, and energy capacity. The joint is quasi-passive, meaning it only uses electricity to power the pump and the fast-switching valves. This implies that the joint can only store and reuse the energy, but not generate the torque. The article gives a system overview with the mathematical model, after which the experiment was performed to validate the hypothesis, and the results are discussed.