There are some exoskeletons providing assistance based on
passive springs or pneumatic cylinders – systems with a physical
impedance and compliance – controlled only by means of the
activation and/or enrolling of these elements. Distinguished from
the predefined trajectory control (Section 3.2), in which the system
continuously tracks a pre-recorded joint trajectory, this assistive
strategy controls the device to act synchronously with expected
gait events; for example, the pneumatic actuator on/off switch
in Pneumatic active gait orthosis by G. Belforte (3.6.1) and in
Power assist wear by D. Sasaki (3.6.2), the position of Bowden
cable in Soft Exosuit (3.6.3), and the damper mechanism on/off in
MIT Exoskeleton (3.6.4). The desired assistance from these devices
depends not only on the delivery time of the control command, but
also on the characteristics of the elastic elements (stiffness, inertia,
damping).