Admittance Shaping Controller for Lower-Limb Exoskeletons
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Solution Overview
Problem
Existing exoskeleton systems face challenges in delivering arbitrary assistive torque profiles to users, as they often exhibit finite mechanical impedance, limiting their mobility and requiring complex estimation of muscle torques and motion intent, which can be difficult to accurately predict.
Innovation Solution
The exoskeleton system employs an admittance shaping controller that models leg dynamics as a linear time-invariant system, replacing the leg's admittance with a coupled system admittance by generating target DC gain, natural frequency, and resonant peak, allowing the exoskeleton to behave as a source of assistance by actively modifying its port impedance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Force
If the exoskeleton applies controlled forces to assist user movement, then the assistive torque is improved, but the mechanical impedance limits mobility and requires complex muscle torque estimation
Solution Approach 1:
The patent changes the control parameter from torque-based control to impedance-based control. By defining the exoskeleton's mechanical impedance characteristics (mass, damping, stiffness parameters) instead of directly controlling torque, the system achieves assistive functionality without requiring complex muscle torque estimation. This parameter transformation simplifies the control architecture while maintaining effective assistance.
Solution Approach 2:
The patent replaces the complex biological control system (which would estimate muscle torques and motion intent) with a simplified mechanical impedance model. The exoskeleton behaves as a damped spring-mass system with predefined impedance characteristics, eliminating the need for sophisticated sensors and algorithms to predict user intent while still providing natural and effective assistance.
2Stability of the object's composition
If the exoskeleton exhibits finite mechanical impedance, then the system stability is improved, but the user mobility is reduced
Solution Approach 1:
The patent makes the mechanical impedance dynamic rather than static. The exoskeleton's impedance parameters (particularly damping and stiffness) are adjusted in real-time based on the phase relationship between exoskeleton motion and user leg motion. When motions are in-phase, the system provides assistance with optimized impedance characteristics, thereby improving mobility while maintaining stability through adaptive control.
Solution Approach 2:
The patent dynamically changes the impedance parameters based on operating conditions. By monitoring the phase difference between exoskeleton and user leg motions, the controller adjusts damping and stiffness parameters to optimize the balance between stability and mobility. This allows the system to provide low impedance (high mobility) when needed while maintaining sufficient stability for safe operation.
Data Source
AI summary
The control method for lower-limb assistive exoskeletons assists human movement by producing a desired dynamic response on the human leg. Wearing the exoskeleton replaces the leg's natural admittance with the equivalent admittance of the coupled system formed by the leg and the exoskeleton. The control goal is to make the leg obey an admittance model defined by target values of natural frequency, resonant peak magnitude and zero-frequency response. The control achieves these objectives objective via positive feedback of the leg's angular position and angular acceleration. The method achieves simultaneous performance and robust stability through a constrained optimization that maximizes the system's gain margins while ensuring the desired location of its dominant poles.