Variable-Stiffness Ankle-Foot Orthosis With Motorized Elastic Beam
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Solution Overview
Problem
Existing ankle-foot orthoses (AFOs) struggle to provide variable stiffness that adapts to individual user needs, with passive AFOs lacking adjustability and active AFOs being bulky, heavy, and energy inefficient, limiting their effectiveness in daily use.
Innovation Solution
A lightweight, variable stiffness ankle-foot orthosis system with a motor-driven elastic beam mechanism that adjusts stiffness levels based on user-specific data, allowing for customizable support and energy return during different phases of gait.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Use of energy by moving object
If passive AFOs with high elasticity are used, then energy storage during mid-terminal stance is improved, but adaptability to walking speed and inability to alter ankle stiffness worsens
Solution Approach 1:
The patent implements variable stiffness control by dynamically adjusting the pre-tension of the elastic element through a motor-driven spool mechanism. The stiffness parameter k_effective = k_elastic + (F_pre tension)/L changes in real-time based on walking speed detection, allowing the AFO to transition from a static passive device to a dynamic adaptive system that optimizes energy storage at different gait phases and speeds.
Solution Approach 2:
The system changes the physical parameter of elastic element pre-tension to achieve variable stiffness. By controlling the spool to wind or unwind the elastic element, the pre-tension force F_pre tension is adjusted, which directly modifies the effective stiffness parameter of the AFO. This parameter change enables adaptation to different walking speeds while maintaining energy storage capabilities.
2Adaptability or versatility
If active AFOs with actuators are used, then variable stiffness control is improved, but device weight and complexity worsens
Solution Approach 1:
The patent replaces traditional heavy motor actuators with a lightweight spool-based tensioning mechanism. Instead of using a motor to directly apply force to the ankle joint, the system uses a spool to wind and unwind an elastic element, converting rotational motion into tensile force. This mechanical substitution dramatically reduces actuator weight while maintaining variable stiffness control capability.
Solution Approach 2:
The elastic element serves as an intermediary between the lightweight spool actuator and the ankle joint. The spool controls the pre-tension of the elastic element, which then provides the actual stiffness control at the ankle. This intermediary mechanism allows a small lightweight actuator to control a much larger stiffness parameter, reducing the overall device weight.
3Force
If higher dorsiflexion stiffness is used, then support during stance phase is improved, but energy storage capacity and push-off power worsens
Solution Approach 1:
The system dynamically adjusts stiffness based on the gait phase and walking speed. During early stance when support is needed, the spool increases pre-tension to provide higher stiffness. During late stance when push-off power is needed, the spool reduces pre-tension to allow greater energy storage and power generation. This temporal dynamic adjustment resolves the contradiction between support and power.
Solution Approach 2:
The AFO implements periodic stiffness modulation synchronized with the gait cycle. The controller detects gait phase through sensors and periodically adjusts the spool position to create high stiffness during stance phase for support, then lowers stiffness during swing phase to prepare for the next cycle. This periodic action allows the system to provide both support and power at appropriate moments.
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 system provides adaptable stiffness support, enhancing energy efficiency and comfort, addressing the limitations of existing AFOs by reducing weight and complexity, and improving walking mobility for users with conditions like cerebral palsy and stroke.
Implementation Method 1
an elastic beam including a plurality of segments oriented with a first length in a radial direction that is orthogonal to the rotational axis
Implementation Method 2
a motor operatively connected to the plurality of segments. The motor is configured to adjust an effective bend length of each of the plurality of segments about the rotational axis
Data Source
AI summary
An apparatus is provided for variable stiffness in ankle-foot orthosis. The apparatus includes a first and second frame portion respectively attached to a shank and leg portion of an AFO frame. The second frame portion rotates relative to the first frame portion about a rotational axis. An elastic beam with segments is oriented in a radial direction. A motor is operatively connected to the segments to adjust an effective bend length of each segment about the rotational axis. A controller receives first data indicating a desired value of a level of stiffness of the apparatus and determines a desired value of the effective bend length of each segment based on the desired value of the level of stiffness. The controller transmits a signal to the motor to adjust the effective bend length to the desired value. A method employing the apparatus for variable stiffness in AFO is also provided.