Decoupling the Feedback and Feedforward Components of Impedance Controllers: Theory and Experiments in Lower-Limb Prostheses
Year: 2025
Type: preprint
Abstract: Robotic prostheses commonly use joint-space impedance controllers parameterized by stiffness, damping, and an equilibrium angle to create desired behaviors. Although these controllers are often interpreted as equilibrium angle tracking controllers, their parameters are chosen such that ground and user interactions cause a different kinematic pattern to emerge, complicating their design, tuning, and interpretation. To address this challenge, we introduce an alternative formulation of the impedance controller comprising both a feedback position control term and feedforward torque control term. This equivalent form clarifies how the impedance parameters shape both the nominal and perturbed behaviors of the controller. In both theory and experiments with an above-knee amputee participant, we demonstrate that controllers with appropriately designed feedforward torque components can produce identical nominal behaviors despite differences in stiffness and damping, which primarily govern how the system responds to perturbations. Our findings offer important insights for prosthesis controller design and tuning: 1) our decoupled parameterization allows independent prescription of an impedance controller’s nominal and off-nominal behaviors; 2) tuning stiffness and damping based on nominal walking alone is insufficient; and 3) even non-impedance paradigms can benefit from applying impedance concepts to achieve robust real-world behavior.
Authors: T. Kevin Best, Elliott J Rouse, Robert D Gregg
Subfield: Biomedical Engineering
Field: Engineering
Domain: Physical Sciences
Open Access status: gold
OpenAlex ID: W4415602575
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