STF Twisted String Actuator for Fast, Variable-Stiffness Wearables
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Solution Overview
Problem
Existing wearable robots, particularly soft exoskeletons, face limitations in drive speed, response time, and power support for human joint movement, and existing muscle-like actuators lack the ability to mimic the force-length-velocity curve of human muscles effectively.
Innovation Solution
Development of a twisted string actuator impregnated with shear thickening materials (STF) that mimics human muscle properties by providing variable stiffness and auxiliary pulling force, utilizing a small-size DC motor to twist the string, which increases stiffness and output force when the critical shear rate is reached.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If rigid exoskeletons are used to provide solid support and maintain joint concentricity, then stability and support capability are improved, but the device becomes bulky, heavy, and uncomfortable for users
Solution Approach 1:
The patent replaces rigid exoskeleton structures with soft robotic actuators that use flexible materials and thin-film components. The soft actuator comprises a flexible housing, flexible circuit board, and battery pack that can conform to body contours, eliminating the need for bulky rigid linkages while maintaining support capability through controlled mechanical deformation.
Solution Approach 2:
The patent substitutes traditional rigid mechanical transmission systems with direct-drive soft actuators that generate force through electromagnetic or piezoelectric mechanisms integrated within flexible structures. This eliminates complex mechanical linkages, joints, and concentricity requirements while maintaining effective force transmission to human limbs.
2Ease of operation
If soft exoskeletons are used to improve flexibility and comfort, then ease of movement and user comfort are improved, but the drive speed and response time become limited with insufficient power
Solution Approach 1:
The patent employs smart materials within the soft actuator that can dynamically change their physical parameters in response to electrical stimuli. The flexible circuit board controls multiple actuators with variable stiffness and force output, enabling the system to adapt power delivery to match task requirements while maintaining flexibility and comfort.
Solution Approach 2:
The soft actuator utilizes composite construction combining flexible polymers, electromagnetic or piezoelectric elements, and rigidifying structures that can selectively stiffen during actuation. This composite approach enables the actuator to maintain flexibility at rest while generating sufficient power and response speed during active movement assistance.
3Reliability
If multiple linkages are attached to human limbs to maintain firm connection, then connection stability is improved, but user comfort deteriorates due to tight fixing straps
Solution Approach 1:
The patent uses soft robotic actuators with flexible housings that can be directly worn against the body without requiring multiple rigid linkages or tight fixing straps. The flexible construction naturally conforms to body movements while maintaining stable connection through distributed contact forces across the wearable interface.
Solution Approach 2:
The patent extracts and eliminates the need for complex linkage systems and multiple attachment points by integrating the actuation mechanism directly into a single wearable unit that moves with the body. This reduces the number of components requiring attachment to human limbs while maintaining connection stability.
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 STF-based twisted string actuator achieves high twisting speed, improved stiffness, and increased pulling force, effectively supporting human joint movements with a response time of around 0.1 s, mimicking the functionality of human muscles.
Implementation Method 1
increasing the twisting speed can increase the shear rate of fibers of string acting on STF and thus the elastic modulus of STF. The TSA features a high twisting speed, and when the twisting speed reaches the critical shear rate of liquid-solid transition of STF, the overall stiffness of the string suddenly increases
Data Source
AI summary
A muscle-like actuator comprises a motor with a rotatable drive shaft and a string with a shear thickening fluid (STF) embedded therein. One end of the string is attached to the drive shaft and the other end is connected to a load to form a twisted string actuator (TSA). By controlling the speed and current of the motor, the characteristics of the actuator can be changed. Multiple strings may be located in a flexible soft tube to improve the mechanical properties of the actuator.