this strategy, the controller usually relies on an inverse dynamic
model of the exoskeleton; the force exerted by the wearer on
the exoskeleton is set on a positive feedback loop of the controller
and could be scaled down by an amplification parameter
[25]. So this controller is distinguished from a traditional
model-based controller which calculates the desired joints positions
or torques directly. When the exoskeleton could accurately
shadow the wearer’s movement, the force exerted by the user will
approach to zero. However, the controller could also amplify an external
disturbance force, which makes the system become unstable.
In case of destabilization occurring, the user has to promptly
move him/herself in order to create a new stable condition of the
whole system [26]. Moreover, this assistive strategy requires a high
accuracy of the inverse dynamic model.