  A new approach to the human-robot shared control of the Extra Robotic Legs
(XRL) wearable augmentation system is presented. The XRL system consists of two
extra legs that bear the entirety of its backpack payload, as well as some of
the human operator's weight. The XRL System must support its own balance and
assist the operator stably while allowing them to move in selected directions.
In some directions of the task space the XRL must constrain the human motion
with position feedback for balance, while in other directions the XRL must have
no position feedback, so that the human can move freely. Here, we present
Hybrid Open-Loop / Closed-Loop Control Architecture for mixing the two control
modes in a systematic manner. The system is reduced to individual joint
feedback control that is simple to implement and reliable against failure. The
method is applied to the XRL system that assists a human in conducting a
nuclear waste decommissioning task. A prototype XRL system has been developed
and demonstrated with a simulated human performing the transition from standing
to crawling and back again while coupled to the prototype XRL system.
