Feedback Control of an Exoskeleton for Paraplegics: Toward Robustly Stable Hands-free Dynamic Walking

“I will never forget the emotion of my first steps [...],” Françoise, first user during initial trials of the exoskeleton ATALANTE [1] discussed in this paper. “I am tall again!”, Sandy, the fourth user after standing up in the exoskeleton. In these early tests, complete paraplegic patients have dynamically walked up to 10 m without crutches or other assistance using a feedback control method originally invented for bipedal robots. This paper describes the hardware, shown in Figure 1, that has been designed to achieve hands-free dynamic walking, the control laws that have been deployed and those being developed to provide enhanced mobility and robustness, as well as the early test results alluded to above. In this paper, dynamic walking refers to a motion that is orbitally stable as opposed to statically stable. At present approximately 4.7 million people in the United States would benefit from an active lower-limb exoskeleton due to the effects of stroke, polio, multiple sclerosis, spinal cord injury, and cerebral palsy [2]. Moreover, by 2050, an estimated 1.5 million people in the United States will be living with a major lowerlimb amputation [3]. Such individuals expend up to twice the metabolic effort to walk at half the speed of ablebodied persons, experience higher-risk of falls, and have secondary pathological conditions such as osteoarthritis, back pain, and depression [4]–[6]. Lower-limb exoskeletons serve as assistive devices by providing support and balance to wheelchair users and enabling them to perform normal ambulatory functions such as standing, walking and climbing stairs. Lower-limb exoskeletons have also been utilized for gait training and rehabilitation purposes. More importantly, standing and walking with these assistive devices provides exceptional health benefits. For instance, for paraplegics the benefits include improvement of blood circulation, of respiratory, urinary, and intestinal functions, as well as positive psychological effects [7], fundamentally improving their quality of life. For spinal cord injury (SCI) patients, the benefits include Fig. 1: ATALANTE: An exoskeleton designed by Wandercraft for people with paraplegia.