Angular Momentum-Based Control of an Underactuated Orthotic System for Crouch-to-stand Motion

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 orthosis-dummy 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.