Powered Ankle-Foot Prosthesis Actuator Design
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Solution Overview
Problem
Conventional passive ankle-foot prostheses fail to provide net positive work during the stance period, leading to inefficient locomotion and pathological gait patterns in amputees, due to their inability to match the size and weight of the human ankle while delivering sufficient power and torque, and lack of effective control systems that mimic human ankle biomechanics.
Innovation Solution
A powered ankle-foot prosthesis incorporating a series and parallel elasticity actuator system with a force-controllable actuator and unidirectional parallel spring, designed to mimic the quasi-static stiffness and torque source of the human ankle, providing both spring behavior and active push-off during stance, and employing a finite-state controller to manage impedance and position control.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Weight of moving object
If conventional passive ankle-foot prosthesis are designed to match the size and weight of human ankle, then the prosthesis structure becomes compact and lightweight, but the power and torque delivery capability becomes insufficient
Solution Approach 1:
The patent replaces conventional passive mechanical spring mechanisms with an active powered actuator system that includes a motor, transmission, and control electronics. This substitution enables the prosthesis to generate sufficient power and torque while maintaining a compact form factor, as the active system can deliver required forces without the excessive mechanical leverage arms needed in passive designs.
Solution Approach 2:
The patent implements a dynamic control system that adjusts the actuator output in real-time based on gait phase detection and force feedback from load cells. This dynamic adjustment allows the prosthesis to optimize power delivery during different phases of the gait cycle, delivering high torque when needed (during push-off) while maintaining a lightweight structure that would be overly heavy if designed for peak static loads.
2Device complexity
If conventional passive ankle-foot prosthesis use simple spring mechanisms, then the device complexity is reduced, but the ability to mimic human ankle biomechanics and provide net positive work deteriorates
Solution Approach 1:
The patent incorporates load cells that measure the forces acting on the prosthesis and feed this information back to the control system. This feedback enables the controller to adjust actuator output to accurately replicate human ankle torque-angle-velocity relationships, providing net positive work during the gait cycle. The feedback mechanism allows complex biomechanical mimicry without requiring overly complex mechanical structures.
Solution Approach 2:
The control system automatically detects gait phase transitions and adjusts actuator behavior accordingly without requiring external intervention. The system self-regulates to provide appropriate impedance control during stance phase and position control during swing phase, accurately mimicking human ankle behavior while maintaining manageable system complexity through autonomous operation.
3Power
If powered ankle-foot prosthesis incorporate force-controllable actuators with series and parallel elasticity, then the power delivery and biomechanics mimicry are improved, but the device complexity and manufacturing difficulty increase
Solution Approach 1:
The patent divides the actuator system into distinct functional modules: a series elastic element (spring) for force control, a parallel elastic element for stability, a motor, and a transmission system. This segmentation allows each component to be manufactured and tested independently using conventional processes, then assembled into the complete powered prosthesis. The modular approach manages manufacturing complexity while enabling sophisticated power delivery characteristics.
Solution Approach 2:
The patent employs composite elastic elements combining series and parallel spring mechanisms to achieve desired force-displacement characteristics. These composite mechanical structures can be manufactured using conventional spring fabrication techniques and assembled into the actuator system, providing complex biomechanical behavior without requiring overly complex manufacturing processes for individual components.
4Productivity
If powered ankle-foot prosthesis implement finite-state controller for impedance and position control, then the gait efficiency and walking economy are improved, but the control system complexity increases
Solution Approach 1:
The patent implements a finite-state controller that operates in periodic cycles corresponding to the gait cycle, alternating between stance phase (impedance control) and swing phase (position control). This periodic control strategy matches the natural rhythm of human walking, improving gait efficiency and walking economy. The state-machine approach manages control complexity by organizing control logic into discrete, repeating phases rather than continuous complex calculations.
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 prosthesis enhances walking economy and reduces metabolic energy expenditure in amputees by delivering net positive work and mimicking natural ankle behavior, improving gait symmetry and efficiency.
Implementation Method 1
incorporating a series and parallel elasticity actuator system with a force-controllable actuator and unidirectional parallel spring
Implementation Method 2
unidirectional parallel spring, designed to mimic the quasi-static stiffness and torque source of the human ankle
Data Source
AI summary
A powered ankle-foot prosthesis, capable of providing human-like power at terminal stance that increase amputees metabolic walking economy compared to a conventional passive-elastic prosthesis. The powered prosthesis comprises a unidirectional spring, configured in parallel with a force-controllable actuator with series elasticity. The prosthesis is controlled to deliver the high mechanical power and net positive work observed in normal human walking.