Variable Spool Transmission for Passive Prosthetic Grip Scaling
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Solution Overview
Problem
Prosthetic hands face challenges in achieving high speed and strength while being cost-effective, as they rely on expensive precision motors, and existing variable transmission systems are limited by size, complexity, and material requirements, making them unsuitable for compact and custom prosthetics.
Innovation Solution
A 3D-printed passively variable transmission system using elastomeric materials that dynamically adjust the spooling radius based on tendon tension, allowing for high speed at low load and increased force at high load, integrated into a tendon-driven prosthetic hand.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Power
If expensive precision motors are used to achieve high speed and force, then performance is improved, but cost increases significantly
Solution Approach 1:
The patent changes the effective radius parameter of the spool dynamically. When the cord is pulled, the spool's effective radius decreases, which mechanically amplifies the force output. This parameter change allows a low-cost motor to achieve high grip forces without requiring expensive high-performance motors.
Solution Approach 2:
The transmission system dynamically adjusts the spool's effective radius during operation. The spool transitions from a larger effective radius during retraction to a smaller effective radius during gripping, optimizing performance throughout the motion cycle. This dynamic adjustment enables cost-effective motors to deliver variable force and speed characteristics.
2Power
If jointed mechanisms are used for dynamic gearing adjustment, then speed and force are improved, but size and complexity increase
Solution Approach 1:
The transmission system is self-regulating. As the cord tension increases during gripping, the spool's effective radius automatically decreases, providing mechanical advantage without requiring external control mechanisms. The system uses the load itself to adjust its transmission ratio, eliminating the need for complex jointed mechanisms or active control systems.
Solution Approach 2:
The patent extracts the dynamic adjustment function from complex mechanical mechanisms and embeds it directly into the spool's geometry and material properties. The spool is designed with a conical or frustoconical shape that inherently provides variable radius without requiring additional joints, links, or actuators.
3Force
If traditional variable transmission systems are used, then force multiplication is improved, but they are too large and complex for compact prosthetics
Solution Approach 1:
The patent merges the spool and transmission functions into a single integrated component. The spool itself performs the force multiplication through its variable effective radius, eliminating the need for separate transmission mechanisms. This consolidation dramatically reduces the size and complexity of the prosthetic hand while maintaining high grip forces.
Solution Approach 2:
The spool is made from elastomeric material, which allows it to deform elastically under load. This flexibility enables the spool to change its effective radius dynamically without requiring complex mechanical structures. The elastomeric material provides both structural integrity and the necessary compliance for force multiplication.
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 achieves a ~3x increase in grip force without compromising flexion speed, enabling the prosthetic hand to close fingers in 0.5 seconds and apply maximum fingertip forces of ~32 N, while being lightweight and cost-effective, with a mass of ~399 grams and a material cost of less than $500.
Implementation Method 1
a constrictable outer member disposed around a circumference of the spool... a greater tension force causes the constrictable outer member to constrict more than a lower tension force
Data Source
Figure 1A~1C
Figure 1D~1E
Figure 2A~4
AI summary
A transmission is presented, including a spool having a bore. An outer member disposed on the spool, and a cord is configured to at least partially wrap around the outer member. The outer member is configured to constrict and unconstrict to a force applied to the outer member. In this way, rotation of the spool causes a tension force to be applied to the cord, and a greater tension force in the cord will cause the outer member to constrict more than a lower tension force in the cord.