  Balance loss is a significant challenge in lower-limb exoskeleton
applications, as it can lead to potential falls, thereby impacting user safety
and confidence. We introduce a control framework for omnidirectional recovery
step planning by online optimization of step duration and position in response
to external forces. We map the step duration and position to a human-like foot
trajectory, which is then translated into joint trajectories using inverse
kinematics. These trajectories are executed via an impedance controller,
promoting cooperation between the exoskeleton and the user.
  Moreover, our framework is based on the concept of the divergent component of
motion, also known as the Extrapolated Center of Mass, which has been
established as a consistent dynamic for describing human movement. This
real-time online optimization framework enhances the adaptability of
exoskeleton users under unforeseen forces thereby improving the overall user
stability and safety. To validate the effectiveness of our approach,
simulations, and experiments were conducted. Our push recovery experiments
employing the exoskeleton in zero-torque mode (without assistance) exhibit an
alignment with the exoskeleton's recovery assistance mode, that shows the
consistency of the control framework with human intention. To the best of our
knowledge, this is the first cooperative push recovery framework for the
lower-limb human exoskeleton that relies on the simultaneous adaptation of
intra-stride parameters in both frontal and sagittal directions. The proposed
control scheme has been validated with human subject experiments.
