Orthotic Joint Drive With Variable Winch Ratio for High Torque
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Solution Overview
Problem
Existing active orthotic devices face challenges in providing high force without using ball screws or large output gears, while allowing free movement of patients and varying drive ratios during joint movement, which is essential for effective rehabilitation.
Innovation Solution
A compact actuator system utilizing a harmonic winch mechanism with a belt and spools, where the first spool pulls the belt a given length and the second spool feeds it less, allowing the output pulley to be pulled towards the first portion, providing a variable winch ratio that increases when the orthosis is bent and decreases when it is straight, enabling high torque at low speeds for tasks like stair ascent and higher speeds for walking.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Force
If a ball screw is used to provide high force and gear reduction, then the actuator can meet torque requirements, but the device size and weight increase significantly
Solution Approach 1:
The patent extracts the gear reduction function from a traditional heavy ball screw mechanism and implements it through a cable-driven pulley system. The cable is wrapped around a drum that rotates with the motor, providing mechanical advantage without requiring a heavy ball screw assembly. This separates the force multiplication function from the heavy mechanical components.
Solution Approach 2:
The patent replaces the traditional ball screw mechanical system with a cable-driven pulley system. Instead of using a ball screw to convert rotational motion to linear motion with gear reduction, the system uses a cable wrapped around a drum that rotates, creating a pulley-like mechanism that provides both motion transformation and mechanical advantage with significantly reduced weight.
2Force
If direct gearing is used to meet torque requirements, then high force can be achieved, but the device complexity increases and free movement is restricted
Solution Approach 1:
The patent implements a dynamic drive mechanism where the cable can be freely paid out or wound in by the motor. When assistance is needed, the motor winds the cable to create tension and provide force. When no assistance is needed, the cable remains slack and the joint moves freely without mechanical constraint. This dynamic on-demand engagement simplifies the overall mechanism compared to always-engaged direct gearing.
Solution Approach 2:
The cable acts as an intermediary element between the motor and the joint. It transmits force only when needed (when wound tight), while allowing free movement when slack. This intermediary approach replaces complex direct gearing with a simple cable-motor-pulley system that achieves the same functional goals with reduced complexity.
3Device complexity
If a fixed gear ratio is used, then the drive mechanism is simple, but it cannot accommodate varying torque and speed requirements during different phases of joint movement
Solution Approach 1:
The patent creates a variable mechanical advantage system where the effective gear ratio changes dynamically during operation. As the cable wraps around the drum, the radius from the drum center to the cable attachment point changes, creating a variable leverage ratio. This allows the system to provide higher torque at certain joint angles and higher speed at others, adapting to the varying requirements of rehabilitation exercises without complex multi-gear mechanisms.
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
This solution allows for efficient assistance during joint extension while allowing unassisted movement during flexion, providing a lightweight and compact system that meets the torque and speed requirements for various rehabilitation tasks without the limitations of traditional actuators.
Implementation Method 1
the first spool pulls the belt a given length and the second spool feeds it less than the given length so as to pull the output pulley towards the first portion
Data Source
AI summary
An orthosis includes a first portion and a second portion configured to attach on opposite sides of a joint. An actuator is configured to apply a force between the first and second portions. The actuator includes a first spool and a second spool rotatably mounted to the first portion. An output pulley is mounted to the second portion. A belt has a first end wrapped around the first spool, a second end wrapped around the second spool, and a mid-portion wrapped around the output pulley. The actuator is configured to rotate the first and second spools. The rotation of the first spool pulls the belt a given length, and the rotation of the second spool feeds the belt less than the given length, so as to pull the output pulley towards the first portion to pull the second portion towards the first portion.