Modular quasi-passive mechanism for energy storage applications: towards lightweight high-performance exoskeleton

Quasi-passive exoskeletons have emerged as a solution that avoids the high energy requirements that negatively affect the efficiency of exoskeletons. These exoskeletons do not deliver positive mechanical work to the joint, but accumulate and deliver energy in a viscoelastic element that is actively placed or removed parallel to the user's muscles. This paper seeks to address the problem of bulky actuators in quasi-passive exoskeletons by experimentally evaluating the proposed quasi-passive mechanism consisting of a pneumatic cylinder, acting as an elastic element, and a solenoid valve replacing a mechanical clutch. The main advantage of the proposed mechanism is that the elastic element can be turned on and off at any time and position, with a high switching frequency, which improves the possibility to harvest the energy. Our aim was to investigate whether, with the proposed actuation, it is possible to achieve the force that has previously been shown to have a positive effect on ankle plantar flexion. Furthermore, we analyzed how the timely activation of the solenoid valve affects the force characteristics. We built the testbed equipped with sensors that allow measurements of torque, pressure, and angular deflection. The obtained measurements are in line with the theoretical model, where the force achieved is within the required range. In addition, it was shown that the time for the actuator to switch off from the peak force is about 100 ms, without major air leaks and energy bursts. The presented results highlight that the actuation of this type is a good candidate for designing a lightweight high-performance quasi-passive exoskeleton.