Fig. 4
Overview of the control structure of the exoskeleton. The control architecture is divided into three parts: high-level control, low-level control and safety functions. The high-level control is replaying trajectories for the exoskeleton joint positions and the stiffness setpoint. The individual tasks have differring trajectories grouped in modes. The modes can be selected by the user pressing buttons on the crutches or by an operator with an external computer. The trajectories are executed by a low-level position control loop for each joint. The exoskeleton state is supervised by safety functions that stop the exoskeleton if, e.g., the redundant sensing disagrees or the motors receive a position request that is outside of the allowed range of motion. φ rl , φ rp , φ rh designate the reference joint angles, defined by the trajectories (stiffness for φ rp and walking, inclines or stairs respectively for φ rl and φ rh ). φ l , φ p and φ h are the angles measured with the position sensors that are fed back to the low-level controller and evaluated in the safety functions of the exoskeleton. I l , I p , I h designate the current sent to the motor. l refers to the lever, h to the hip and p to the pretension motors