  While existing literature encompasses exoskeleton-assisted sit-stand tasks,
the integration of energy recovery mechanisms remains unexplored. To push these
boundaries further, this study introduces a portable pneumatic knee exoskeleton
that operates in both quasi-passive and active modes, where active mode is
utilized for aiding in standing up (power generation), thus the energy flows
from the exoskeleton to the user, and quasi-passive mode for aiding in sitting
down (power absorption), where the device absorbs and can store energy in the
form of compressed air, leading to energy savings in active mode. The absorbed
energy can be stored and later reused without compromising exoskeleton
transparency in the meantime. In active mode, a small air pump inflates the
pneumatic artificial muscle (PAM), which stores the compressed air, that can
then be released into a pneumatic cylinder to generate torque. All electronic
and pneumatic components are integrated into the system, and the exoskeleton
weighs 3.9 kg with a maximum torque of 20 Nm at the knee joint. The paper
describes the mechatronic design, mathematical model and includes a pilot study
with an able-bodied subject performing sit-to-stand tasks. The results show
that the exoskeleton can recover energy while assisting the subject and
reducing muscle activity. Furthermore, results underscore air regeneration's
impact on energy-saving in portable pneumatic exoskeletons.
