Cable-Driven Ankle Exoskeleton Torque Control With Series Elasticity
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Solution Overview
Problem
Designing effective lower-limb exoskeletons that can comfortably apply high torques at high bandwidth to assist gait while minimizing metabolic energy cost and interference with natural motion, and accommodating varying user anthropometry.
Innovation Solution
Development of tethered ankle exoskeletons with strong lightweight frames, comfortable three-point contact, series elastic elements, and modular design for improved torque control, featuring off-board motor actuation and real-time torque control using proportional feedback and damping injection, with torque sensors and adjustable components to fit different users.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If series elastic elements are added to improve torque control and decouple human from motor inertia, then torque control and comfort are improved, but device complexity increases
Solution Approach 1:
A series elastic element (spring) is introduced as an intermediary component between the motor and the ankle joint. This spring acts as a mechanical mediator that decouples the human user from the motor inertia, improving torque control and comfort while maintaining system functionality.
Solution Approach 2:
The stiffness of the series elastic element is optimized to balance torque control performance with device complexity. By carefully selecting the spring stiffness parameter, the system achieves effective torque modulation without excessive complexity.
2Force
If forces are applied far from the ankle joint to increase lever arm, then the magnitude of applied force is reduced, but device envelope and width increase
Solution Approach 1:
The force application point is moved from a lateral extension (increasing width) to a longitudinal extension along the leg (increasing lever arm in the direction of motion). This dimensional shift reduces the device envelope width while maintaining the mechanical advantage of a longer lever arm.
3Reliability
If the cable is configured to provide torque in one rotational direction only, then unidirectional torque control is achieved, but bidirectional torque capability is lost
Solution Approach 1:
The torque control function is segmented into two independent systems: a cable-driven mechanism for plantarflexion (one-way) and a separate dorsiflexion mechanism. This segmentation allows precise control in the primary direction while providing limited capability in the opposite direction through alternative means.
Solution Approach 2:
A ratchet mechanism acts as an intermediary that allows the cable to provide unidirectional torque control while permitting controlled motion in the opposite direction. The ratchet mediates between the one-way cable force and the two-way ankle motion requirement.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The exoskeletons demonstrate high peak torque and bandwidth, enabling comfortable and efficient robotic assistance with reduced metabolic energy cost and minimal interference with natural motion, suitable for a range of users through adjustable design.
Implementation Method 1
the cable is configured to provide the torque by exerting a first force on the lever and a second force on the frame
Implementation Method 2
Series elasticity improves torque control and decouples the human from the inertia of the motor and gearbox
Data Source
AI summary
This document describes an exoskeleton device that includes a cable, a lever that is connected to the cable, a frame comprising a strut that redirects the cable toward the lever, wherein the frame is coupled to the lever by a rotational joint; and a motor that is connected to the cable and configured to cause the cable to provide a torque about the rotational joint, wherein the cable is configured to provide the torque by exerting a first force on the lever and a second force on the frame, and wherein the cable is further configured to provide the torque in a first rotational direction and is prevented from applying the torque in an opposite rotational direction to the first rotational direction.