This paper presents a balance control for an active lower limbs exoskeleton based on the instantaneous capture point concept and human balance strategies. Our goal is to implement it on a real underactuated exoskeleton EMY. The control, inspired from biomechanic studies, aims at assisting an able-bodied person while preserving his comfort and his safety to the maximum. We first present briefly how the machine can balance itself according to disturbances which can come from the operator, and how it can imitate human balance strategies. We then show how we tackle the reduction of the number of actuators problem which leads to an underactuation/overactuation problem.

This paper presents an open-loop balance control for an active leg exoskeleton based on human balance strategies, and how the machine can balance itself according to perturbations. The control is designed to balance the exoskeleton with a view to assist a well and able operator that leads the movements of the coupled system {operator+exoskeleton}. It is inspired by biomechanic works showing that human balance relies on three strategies : the displacement of the center of mass, the contribution of each leg to produce efforts and stepping. We assimilate the exoskeleton to a Linear Inversed Pendulum model to describe its global behavior, and we use its capture point to identify a loss of balance situation possibly caused by the operator and adapt the reaction of the machine. Thanks to capture point's dynamics regarding to the center of mass and the center of pressure, we are able to control the machine and bring it back into a stable situation.

Lower-limb exoskeletons are a promising option to increase the mobility of persons with leg impairments in a near future. However, it is still challenging for them to ensure the necessary stability and agility to face obstacles, particularly the variety that makes the urban environment. That is why most of the lower-limb exoskeletons must be used with crutches: the stability and agility features are deferred to the patient. Clinical experience shows that the use of crutches not only leads to shoulder pain and exhaustion, but also fully occupies the hands for daily tasks. In November 2020, Wandercraft presented Atalante Evolution, the first self-stabilized and crutch-less exoskeleton, to the powered exoskeleton race of the Cybathlon 2020 Global Edition. The Cybathlon aims at promoting research and development in the field of powered assistive technology to the public, contrary to the Paralympics where only participants with unpowered assistive technology are allowed. The race is designed to represent the challenges that a person could face every day in their environment: climbing stairs, walking through rough terrain, or descending ramps. Atalante Evolution is a 12 degree-of-freedom exoskeleton capable of moving dynamically with a complete paraplegic person. The challenge of this competition is to generate and execute new dynamic motions in a short time, to achieve different tasks. In this paper, an overview of Atalante Evolution system and of our framework for dynamic trajectory generation based on the direct collocation method will be presented. Next, the flexibility and efficiency of the dynamic motion generation framework are demonstrated by our tools developed for generating the important variety of stable motions required by the competition. A smartphone application has been developed to allow the pilot to choose between different modes and to control the motion direction according to the real situation to reach a destination. The advanced mechatronic design and the active cooperation of the pilot with the device will also be highlighted. As a result, Atalante Evolution allowed the pilot to complete four out of six obstacles, without crutches. Our developments lead to stable dynamic movements of the exoskeleton, hands-free walking, more natural stand-up and turning moves, and consequently a better physical condition of the pilot after the race compared to the challengers. The versatility and good results of these developments give hope that exoskeletons will soon be able to evolve in challenging everyday-life environments, allowing patients to live a normal life in complete autonomy.