Dynamic Locomotion of a Lower-Limb Exoskeleton Through Virtual Constraints Based ZMP Regulation

Robotic lower-limb exoskeletons have the potentials to assist individuals with paraplegia to perform normal ambulatory functions and to provide exceptional health benefits. While modern-day hardware for exoskeletons is becoming more powerful, there are still significant challenges in implementing a practical exoskeleton motion control framework that helps paraplegic individuals to complete regular ambulatory tasks stably, safely, and efficiently without the use of arm-crutches. Inspired by the current development in dynamic walking controllers for a bipedal robot, this paper proposes a Hybrid Zero Dynamics (HZD) based control approach for powered lower-limb exoskeletons to achieve dynamic hand-free locomotion. Due to the unmodelled dynamics and exerted forces from the user upon the exoskeleton, the model-based approaches such as Hybrid Zero Dynamics struggles when implementing on the actual hardware. In this paper, we systematically formulate a virtual-constraints-based regulation framework in order to robustly stabilize the system around a stable periodic gait within the HZD framework. This regulator is then used to regulate the zero moment point (ZMP) to improve the lateral stability of the bipedal robot by indirectly regulating the center of mass (CoM) position of the exoskeleton due to the lack of available force sensors at the bottom of the feet. The proposed approach presents a general structure with which the virtual constraints can be heuristically regulated to satisfy the stability condition imposed by the ZMP criteria without compromising the hybrid invariance of the walking gaits. The effectiveness of the regulators was demonstrated through stable walking of a powered lower-limb exoskeleton in simulation and experimentation.

New Robust Control Method Applied to the Locomotion of a 5-Link Biped Robot

SUMMARYThis paper proposes a new design of robust control combining feedback linearization, backstepping, and sliding mode control called FLBS applied to the locomotion of five-link biped robot. Due to the underactuated robot’s model, the system has a hybrid nature, while the FLBS control can provide a stabilized walking movement even with the existence of large disturbances and uncertainties by implementing smooth chatter-free signals. Stability of the method is proven using the Lyapunov theorem based on the hybrid zero dynamics and Poincaré map. The simulations show the controller performance such as robustness and chatter-free response in the presence of uncertainty and disturbance.

Bipedal Model and Hybrid Zero Dynamics of Human Walking With Foot Slip

Foot slip is one of the major causes of falls in human locomotion. Analytical bipedal models provide an insight into the complex slip dynamics and reactive control strategies for slip-induced fall prevention. Most of the existing bipedal dynamics models are built on no foot slip assumption and cannot be used directly for such analysis. We relax the no-slip assumption and present a new bipedal model to capture and predict human walking locomotion under slip. We first validate the proposed slip walking dynamic model by tuning and optimizing the model parameters to match the experimental results. The results demonstrate that the model successfully predicts both the human walking and recovery gaits with slip. Then, we extend the hybrid zero dynamics (HZD) model and properties to capture human walking with slip. We present the closed-form of the HZD for human walking and discuss the transition between the nonslip and slip states through slip recovery control design. The analysis and design are illustrated through human walking experiments. The models and analysis can be further used to design and control wearable robotic assistive devices to prevent slip-and-fall.