Abstract
Assist-as-needed (AAN) algorithms for the control of lower extremity rehabilitation robots can promote active participation of patients during training while adapting to their individual performances and impairments. The implementation of such controllers requires the adaptation of a control parameter (often the robot impedance) based on a performance (or error) metric. The choice of how an adaptive impedance controller is formulated implies different challenges and possibilities for controlling the patient's leg movement. In this paper, we analyze the characteristics and limitations of controllers defined in two commonly used formulations: joint and end-point space, exploring especially the implementation of an AAN algorithm. We propose then, as a proof-of-concept, an AAN impedance controller that combines the strengths of working in both spaces: a hybrid joint/end-point impedance controller. This approach gives the possibility to adapt the end-point stiffness in magnitude and direction in order to provide a support that targets the kinematic deviations of the end-point with the appropriate force vector. This controller was implemented on a two-link rehabilitation robot for gait training—the Lokomat®Pro V5 (Hocoma AG, Switzerland) and tested on 5 able-bodied subjects and 1 subject with Spinal Cord Injury. Our experiments show that the hybrid controller is a feasible approach for exoskeleton devices and that it could exploit the benefits of the end-point controller in shaping a desired end-point stiffness and those of the joint controller to promote the correct angular changes in the trajectories of the joints. The adaptation algorithm is able to adapt the end-point stiffness based on the subject's performance in different gait phases, i.e., the robot can render a higher stiffness selectively in the direction and gait phases where the subjects perform with larger kinematic errors. The proposed approach can potentially be generalized to other robotic applications for rehabilitation or assistive purposes.
Highlights
Exoskeletons for gait rehabilitation or walking assistance in subjects with neurological injuries have flourished in the last decades (Esquenazi et al, 2017)
The resulting stiffness ellipses are described in terms of size, shape, and orientation (Mussa-Ivaldi et al, 1985), whereby size indicates the length of the major axis of the ellipse, shape the ratio between the major and minor axis of the ellipse, and orientation the angle between the major axis and the x axis
If we examine one of the critical points of the late swing phase, i.e., right before heel strike, in further detail (Figure 12), it becomes apparent that, especially in the subject with Spinal Cord Injury (SCI), the hybrid controller generates an end-point stiffness ellipse rotated in the direction of the error
Summary
Exoskeletons for gait rehabilitation or walking assistance in subjects with neurological injuries have flourished in the last decades (Esquenazi et al, 2017) These devices seek to control the leg segments of the user and try to restore a gait pattern that is both physiological (i.e., following kinematic characteristics observed in non-impaired individuals) and safe. One way to achieve the latter is through adaptation of the robotic support based on the user’s capabilities (Cai et al, 2006; MarchalCrespo and Reinkensmeyer, 2009) This concept is known as Assist-As-Needed (AAN) (Emken et al, 2005). In this paper we analyze the implications of using joint or end-point space formulation for the control of lower limb exoskeletons. We model these systems as two-segment exoskeletons with a shank and a thigh segment. For further details on the calculation of stiffness ellipses and force field, see the Appendix 1
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