Prosthetic Wrist Reduction Gear for Compact High-Torque Motion
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Solution Overview
Problem
Current prosthetic wrists are compromised by being either too heavy or long to achieve the necessary compactness and driving torque similar to a natural human wrist, while also failing to meet standards of silence and irreversibility, leading to limited functionality and user preference for passive devices.
Innovation Solution
A two-stage reduction mechanism comprising an epicycloidal and cycloidal transmission system, where the first stage is connected to the motor shaft for low driving torques and high speeds, and the second stage is connected to the terminal device for high driving torques and low speeds, utilizing smooth surfaces for friction-based force transmission and designed for compactness and high torque transmission.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Force
If a planetary gear transmission with multiple degrees of freedom is used to achieve high driving torque, then the driving torque increases, but the device becomes heavy and complex
Solution Approach 1:
The transmission system is divided into two independent stages: a first epicycloidal reduction stage for high-speed low-torque operation and a second cycloidal reduction stage for low-speed high-torque operation. Each stage is optimized for its specific operational regime, avoiding the need for a single complex multi-degree-of-freedom planetary gear system while achieving the required torque multiplication through sequential reduction
Solution Approach 2:
The clutch mechanism enables dynamic switching between the two reduction stages based on operational requirements. The system can engage only the necessary stage (first or second) depending on whether high-speed or high-torque conditions prevail, reducing overall system complexity and weight compared to a always-engaged multi-stage planetary system
2Volume of moving object
If the prosthetic wrist is made compact to fit anthropometric dimensions, then the size decreases, but the driving torque capability is reduced
Solution Approach 1:
The two reduction stages are arranged in a nested configuration where the second cycloidal reduction stage is positioned within or adjacent to the first epicycloidal stage. This nested layout maximizes torque multiplication within the smallest possible volume, fitting the compact prosthetic wrist within anthropometric dimensions while maintaining high driving torque capability through the combined reduction ratios of both stages
Solution Approach 2:
The patent replaces a single large planetary gear system with a combination of two smaller specialized reduction stages. The epicycloidal and cycloidal mechanisms provide equivalent or superior torque multiplication in a more compact arrangement, substituting the traditional single-stage planetary approach with a multi-stage specialized system that achieves better space utilization
3Object-generated harmful factors
If friction-based smooth surface transmission is used, then silence and smooth operation improve, but transmission efficiency may be reduced compared to toothed gears
Solution Approach 1:
The patent changes the fundamental transmission parameter from discrete tooth engagement to continuous smooth surface friction contact. This parameter change eliminates the impact and noise associated with tooth meshing while maintaining adequate transmission efficiency through optimized surface friction characteristics in the epicycloidal and cycloidal reduction stages
4Reliability
If the system provides irreversibility when motor is off, then position holding capability improves, but mechanism complexity increases
Solution Approach 1:
The cycloidal reduction mechanism inherently provides irreversibility through its geometric design, where the output shaft cannot drive the input shaft due to the non-conjugate tooth profiles and eccentric configuration. This self-locking property arises from the mechanism's geometry itself rather than requiring separate braking or locking components, achieving position holding capability without adding significant complexity
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 solution enables a compact, strong, and silent active wrist prosthesis capable of human-like pronation and supination movements within anthropometric dimensions, maintaining high torque and speed values while conserving energy during static loads, thus addressing the limitations of existing prosthetic wrists.
Implementation Method 1
the rolling of the satellite wheels on the outer ring gear is caused by the friction forces generated by radial interference between the central wheel and the satellite wheels and the outer ring gear
Data Source
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AI summary
A prosthetic wrist unit comprising a drive motor (3), suitable for being kinematically interposed between the distal end of an arm prosthesis and a prosthetic terminal device, for positioning the terminal device at desired orientations with respect to the arm prosthesis. The unit comprises a reduction mechanism comprising a first epicycloidal reduction stage (1) and a second cycloidal reduction stage (2), connected to each other, wherein the first stage (1) comprises a central wheel (10) connected to the motor shaft (30), one or more satellite wheels (11) rotatably placed around their own axis on a planet carrier (12), and a fixed outer ring gear (13), the satellite wheels (11) being in peripheral contact with the central wheel (10) and with the outer ring gear (13), and the planet carrier (12) being connected to the second stage. The central wheel (10), the satellite wheels (11) and the outer ring gear (13) have smooth mutually engaging surfaces, in such a way that the rolling of the satellite wheels (11) on the outer ring gear (13) is caused by the friction forces generated by radial interference between the central wheel (10) and the satellite wheels (11 ) and the outer ring gear (13).