Due to recent technology advancements and increased component availability, exoskeleton research is now a hot topic. Numerous exoskeletons already exist [1]. Among these, passive exoskeletons offer some advantages over active solutions, e.g., lower weight (no big motors or batteries). They use springs to store energy passively instead of using motors and batteries to actively input power into human joints.

Our work focuses on research of an exoskeleton that will assist the user in a quasi-passive way that uses motors only to change the state of the device (e.g. to close a clutch). In this case, by utilizing an elastic element in parallel with the human calf with a clutch [2], so that the device can assist the calf muscles and reduce their efforts, thus reducing the human flat terrain walking effort.

Our first prototype [3] utilized a completely mechanical clutch (inspired by the device in [2]). We tested it in a quantitative cross-over study comprised of 4 young individuals under laboratory conditions. The users showed a reduced walking effort if the spring of correct stiffness was used and if the clutch operated smoothly with a correct timing. By measuring the oxygen consumption, an effort reduction of 10% was observed in the best case, compared to normal walking. Still, we found several areas of improvement like timing issues and non-smooth clutch operation which all reduce the devices effectiveness. Both problems were improved in the second prototype (shown in Fig 1) where a small servo motor was added to the clutch to help (dis-)engage the spring. The timing is dictated by a heel-down switch activation. The second prototype was tested in qualitative crossover study [4] comprised of 7 people. Each user used the exoskeleton for a time period under different conditions and expressed his observations in a qualitative oriented questionnaire. Its results positively graded the exoskeletons operation and showed that most user did seem to feel an effort reduction.

One way to improve the current design (2nd) is by weight reduction. Its current weight is around 1.2 kg which stems mainly from the stainless steel frame. Its comfort is also not yet high enough which was pointed out in the questionnaire. Both problems also mask the feeling of effort reduction on the user. These remarks, including findings from both studies and the development experience gained, will serve as a basis for the development of an improved third version of the quasi-passive ankle exoskeleton.