Angular momentum-based control of an underactuated orthotic system for crouch-to-stand motion
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Angular momentum-based control of an underactuated orthotic system for crouch-to-stand motion Curt A. Laubscher1 · Ryan J. Farris2 · Jerzy T. Sawicki1 Received: 8 October 2019 / Accepted: 20 July 2020 © Springer Science+Business Media, LLC, part of Springer Nature 2020
Abstract This paper presents an angular momentum-based controller for crouch-to-stand motion of a powered pediatric lower-limb orthosis. The control law is developed using an underactuated triple pendulum model representing the legs of an orthosisdummy system where the hip and knee joints are actuated but the ankle joint is unpowered. The control law is conceived to drive the angular momentum of the system to zero, thereby bringing the system to a statically balanced upright configuration. The parameters of the dynamic model of the orthosis-dummy system are experimentally identified and used to synthesize the momentum-based controller. Control parameters are selected using closed-loop pole placement of the linearized system via numerical optimization to ensure local closed-loop stability with adequate damping and satisfactory response time without too large controller gains. The controller is applied in simulation to determine the region of viable initial conditions resulting in no knee hyperextension or loss of balance, as determined from a zero-moment point analysis. The controller is then implemented in experiment showing feasibility of the control strategy in practice. Results are compared against a similarly-synthesized linear-quadratic regulator. Keywords Underactuated pendulum · Angular momentum-based controller · Crouch-to-stand · Orthosis
1 Introduction Currently available powered lower-limb orthoses on the market, or described in the literature, such as the Indego (Farris et al. 2011) and ReWalk (Esquenazi et al. 2012) exoskeletons, often require the user to manage balance aids such as crutches, walkers, or hand rails. Other devices such as the Rex exoskeleton (Kwak et al. 2015) manage balance through large platform feet and control which ensures that the center of pressure (CoP) of the system never leaves the area of the foot platform. However, managing balance in this fashion comes with weight, size, and speed penalties. Development of a control law capable of balancing the exoskeleton wearer
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Jerzy T. Sawicki [email protected] Curt A. Laubscher [email protected]
1
Department of Mechanical Engineering, Cleveland State University, Cleveland, OH, USA
2
Human Motion and Control Division, Parker Hannifin Corporation, Macedonia, OH, USA
without the need for balance aids would provide significant benefit to exoskeleton usability and user acceptance. Given the importance of balance in orthosis-assisted gait, quantitative assessment of whole-system balance continues to be an important area of research. The bulk dynamics of a human-orthosis system can be represented using a robot model, which can derived using, for example, the Newton–Euler method based on first principals from rigid-body dynamics or the Euler–Lagrange method b
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