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

This manuscript presents control of a high-DOF fully actuated lower-limb exoskeleton for paraplegic individuals. The key novelty is the ability for the user to walk without the use of crutches or other external means of stabilization. We harness the power of modern optimization techniques and supervised machine learning to develop a smooth feedback control policy that provides robust velocity regulation and perturbation rejection. Preliminary evaluation of the stability and robustness of the proposed approach is demonstrated through the Gazebo simulation environment. In addition, preliminary experimental results with (complete) paraplegic individuals are included for the previous version of the controller.Comment: Submitted to IEEE Control System Magazine. This version addresses reviewers' concerns about the robustness of the algorithm and the motivation for using such exoskeleton

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

"I will never forget the emotion of my first steps […]," were the words of Fran?oise, the first user during initial trials of the exoskeleton ATALANTE [1]. "I am tall again!" were the words of Sandy (the fourth user) after standing up in the exoskeleton. During these early tests, complete paraplegic patients dynamically walked up to 10 m without crutches or other assistance using a feedback control method originally invented for bipedal robots. As discussed in "Summary," this article describes the hardware (shown in Figure 1) that was designed to achieve hands-free dynamic walking, the control laws that were deployed (and those being developed) to provide enhanced mobility and robustness, and preliminary test results. In this article, dynamic walking refers to a motion that is orbitally stable as opposed to statically stable

Advances in Feedback Control for High-dimensional Bipedal Models

The promise of bipedal machines brings with it the promise of advancements in both the work and personal sectors. The robots are coming to work alongside us at our pace. They are coming to work in environments which we deem too dangerous to send our fellow humans. They are coming to help us recover mobility when we suffer neural or muscular impairments. The machines being prototyped and built for these tasks are complex devices, with many links, joints, and actuators. Moreover, these bipedal machines are meant to exhibit diverse gaits, and many are designed with agility in mind. Taking full advantage of their mechanical capabilities requires advanced optimization and feedback design methods, which are the central theme of this dissertation. While bipedal machines typically refer to two legged robots, we also in our research consider full-assist exoskeletons as well. Our specific hardware targets are (1) an exoskeleton designed by Wandercraft that allows people with paraplegia to walk again without the use of crutches, and (2) a Cassie-series bipedal robot that can stand quietly in place, balance on a Segway, and walk at over a meter per second. The high-dimensional and hybrid natures of the dynamical models of these robots pose challenges in optimal gait design and gait stabilization that we identify and address in this dissertation. The first topic addressed in this dissertation is trajectory optimization. Specifically, we developed a toolset called C-FROST that speeds up the offline trajectory design process by a factor of six to eight times compared to using FROST. This allows the generation of 1000+ gaits for a 20 degrees of freedom (DOF) biped in under two hours. C-FROST allows FROST to generate a stand-alone C++ executable for running optimizations, thereby allowing for parallelization and easy deployment to the cloud. Benchmarking and practical examples demonstrating the capabilities of the C-FROST toolset are provided. The second topic is gait and feedback control design for the full-assist Wandercraft exoskeleton. This work was done early in the dissertation research and provided prototype software to the Wandercraft team. In addition to the exoskeleton’s model being high dimensional (with 36 state variables), its operation poses difficult challenges arising from workspace limitations required for patient safety and torque limits of the hardware. Feedback controllers are designed to meet these challenges using the method of Hybrid Zero Dynamics (HZD), gait libraries, and an extension of HZD by Xingye Da that is called Generalized Hybrid Zero Dynamics, or G-HZD for short. Work on the Wandercraft exoskeleton revealed a fundamental drawback in the original G-HZD approach, and that motivates the third main topic of the dissertation: a broad extension of the design and stability properties of the hybrid zero dynamics manifold in the G-HZD feedback design approach. In particular, in G-HZD, the manifold is built from trajectories that arise from a boundary-value problem, and thus specifying the proper boundary of the manifold is crucial to the success of the method. This dissertation proposes a novel method for the design of the boundary manifold. The new approach is subsequently demonstrated on an inverted pendulum on a cart and compared to the original approach. We then demonstrate how to implement the new approach on Cassie, an underactuated biped hybrid system, and report experimental results.PHDElectrical and Computer EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/171332/1/oharib_1.pd

First Steps Towards Translating HZD Control of Bipedal Robots to Decentralized Controlof Exoskeletons

International audienceThis paper presents preliminary results toward translating gait and control design for bipedal robots to decentralized control of an exoskeleton aimed at restoring mobility to patients with lower limb paralysis, without the need for crutches. A mathematical hybrid dynamical model of the human-exoskeleton system is developed and a library of dynamically feasible periodic walking gaits for different walking speeds is found through nonlinear constrained optimization using the full-order dynamical system. These walking gaits are stabilized using a centralized (i.e., full-state information) hybrid zero dynamics-based controller, which is then decentralized (i.e., control actions use partial state information) so as to be implementable on the exoskeleton subsystem. A control architecture is then developed so as to allow the user to actively control the exoskeleton speed through his/her upper body posture. Numerical simulations are carried out to compare the two controllers. It is found that the proposed decentralized controller not only preserves the periodic walking gaits but also inherits the robustness to perturbations present in the centralized controller. Moreover, the proposed velocity regulation scheme is able to reach a steady state and track desired walking speeds under both, centralized, and decentralized schemes

Towards Restoring Locomotion for Paraplegics: Realizing Dynamically Stable Walking on Exoskeletons

This paper presents the first experimental results of crutch-less dynamic walking with paraplegics on a lower-body exoskeleton: ATALANTE, designed by the French start-up company Wandercraft. The methodology used to achieve these results is based on the partial hybrid zero dynamics (PHZD) framework for formally generating stable walking gaits. A direct collocation optimization formulation is used to provide fast and efficient generation of gaits tailored to each patient. These gaits are then implemented on the exoskeleton for three paraplegics. The end result is dynamically stable walking in an exoskeleton without the need for crutches. After a short period of tuning by the engineers and practice by the subjects, each subject was able to dynamically walk across a room of about 10 m up to a speed of 0.15 m/s (0.5 km/h) without the need for crutches or any other kind of assistance

Towards Restoring Locomotion for Paraplegics: Realizing Dynamically Stable Walking on Exoskeletons

This paper presents the first experimental results of crutch-less dynamic walking with paraplegics on a lower-body exoskeleton: ATALANTE, designed by the French start-up company Wandercraft. The methodology used to achieve these results is based on the partial hybrid zero dynamics (PHZD) framework for formally generating stable walking gaits. A direct collocation optimization formulation is used to provide fast and efficient generation of gaits tailored to each patient. These gaits are then implemented on the exoskeleton for three paraplegics. The end result is dynamically stable walking in an exoskeleton without the need for crutches. After a short period of tuning by the engineers and practice by the subjects, each subject was able to dynamically walk across a room of about 10 m up to a speed of 0.15 m/s (0.5 km/h) without the need for crutches or any other kind of assistance