Stable, Robust Hybrid Zero Dynamics Control of Powered Lower-Limb Prostheses

Abstract

To improve the quality of life for lower-limb amputees, powered prostheses are being developed. Advanced control schemes from the field of bipedal robots, such as hybrid zero dynamics (HZD), may provide great performance. HZD-based control specifies the motion of the actuated joints using output functions to be zeroed, and the required torques are calculated using input-output linearization. For one-step periodic gaits, there is an analytic metric of stability. To apply HZD-based control on a powered prosthesis, several modifications must be made. Because the prosthesis and amputee are only connected via the socket, the prosthesis controller does not have access to the full state of the biped, which decentralizes the form of the input-output linearization. The differences between the amputated and contralateral sides result in a two-step periodic gait, which requires the orbital stability metric to be extended. In addition, because human gait is variable, the prosthesis controller must be robust to continuous moderate perturbations. This robustness is proved using local input-to-state stability and demonstrated with simulations of an above-knee amputee model.

Piecewise and unified phase variables in the control of a powered prosthetic leg

“…The heel-to-toe movement of the Center Of Pressure (COP) controlled the progression of the prosthetic stance period in [9], whereas the swing period was controlled by two impedance-based finite states. The horizontal hip position served as a phase variable in separate controllers for the stance and swing periods (i.e., two finite states for the gait cycle) in the simulations of [10]. Because the hip position is monotonic over the entire gait cycle, this “unified” phase variable enabled a single controller in the prosthetic leg simulations of [11].…”

Automatic tuning of virtual constraint-based control algorithms for powered knee-ankle prostheses

“…They have been successfully employed in several robotic locomotion control applications such as controlling underactuated biped robots [10], [11], powered prosthetic legs [12], [13], and biologically inspired snake robots [14]–[16]. In the context of powered prosthetic leg control, virtual constraints were used to unify the gait cycle control of the stance phase for the first time in [12].…”

“…Although progress has been made in model-based adaptive control of biped robots [17], such model-based approaches, which rely on input-output feedback line-arization, cannot be used in powered prosthetic applications. This limitation is due to the need for measuring socket interaction forces [13], requiring expensive multi-axis load cells, and the lack of exact knowledge of the prosthetic leg dynamical model parameters.…”

“…This section presents the human-prosthesis biped dynamical model from [13], [20], the necessary preliminaries from virtual constraint-based control, and the standard ESC architecture used in the paper. The planar model of a transfemoral amputee consists of seven leg segments and a point mass at the hip, as shown in Fig.…”

Characterizing and modeling the joint-level variability in human walking

“…Such dynamic models may aid in the design of assistive devices because interventions can be tested using realistic simulations (Martin and Gregg, 2015) prior to experimental testing with human subjects. Predictive simulations can also be used to investigate research questions that are impractical or impossible to answer via human subject testing, such as quantifying fall risk (Byl and Tedrake, 2009).…”