Parallel NiTi SMA Actuator with Preload Spring
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Solution Overview
Problem
Existing shape memory alloy (SMA) actuators face challenges in predictability and durability due to unpredictable actuation behavior under varying loading conditions, especially in applications requiring high forces and limited space, such as orthopedic devices, where the strain and fatigue life of NiTi elements depend heavily on stress levels and prestress values.
Innovation Solution
The development of an SMA actuator that incorporates a bundle of individual wires or rods made of NiTi, electrically connected in parallel and mechanically arranged in parallel, with a preload spring system that allows for optimal prestress and simultaneous heating of all elements to prevent non-uniform deformation, enhancing force output and cycle life while maintaining predictability.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Force
If a single NiTi element is used in the actuator, then the structure is simple, but the force output and cycle life are insufficient
Solution Approach 1:
The actuator divides the single NiTi element into multiple parallel elements (at least two NiTi elements arranged in parallel). Each element contributes to the total force output while maintaining independent operation. This segmentation increases the force capacity and cycle life without requiring a completely new design approach, simply by multiplying the basic functional unit.
Solution Approach 2:
Multiple NiTi elements are merged into a single actuator assembly where they work together to produce combined force output. The parallel arrangement allows the elements to share the mechanical load and experience similar stress cycles, thereby extending the overall cycle life while achieving higher force output than a single element could provide.
2Force
If high stress levels are applied to NiTi elements to increase force output, then the force capability improves, but the fatigue life decreases
Solution Approach 1:
By segmenting the load across multiple NiTi elements, each element experiences a reduced stress level compared to a single element design. This stress distribution allows the actuator to achieve high total force output while individual elements operate within safer stress ranges that preserve fatigue life.
Solution Approach 2:
The actuator optimizes the stress and prestress parameters by using multiple elements in parallel. This changes the operational parameters from high stress on single element to distributed stress across multiple elements, achieving a favorable balance between force output and cycle life.
3Force
If individual NiTi wires are electrically connected in series, then the voltage requirement is reduced, but the heating uniformity and force output are compromised
Solution Approach 1:
The electrical connection is segmented into parallel connections for each NiTi element. This allows each element to receive full heating power independently, ensuring uniform heating across all elements and maximizing the force output from each element while maintaining consistent temperature distribution.
Solution Approach 2:
Multiple NiTi elements are electrically merged in parallel to receive combined heating power. This merging approach ensures that all elements heat uniformly and simultaneously, producing uniform contraction and maximizing the overall force output of the actuator.
4Reliability
If no preload spring is used, then the structure is simpler, but the actuation behavior becomes unpredictable and the converter may be damaged by excessive preloading forces
Solution Approach 1:
A preload spring is introduced to apply a controlled preliminary force to the NiTi elements before actuation. This preliminary action ensures that the elements are always operated within safe stress ranges and that the actuation behavior remains predictable and repeatable across multiple cycles.
Solution Approach 2:
The preload spring acts as a cushioning mechanism that prevents excessive forces from damaging the NiTi converter. By providing a controlled baseline force, the spring protects the converter from harmful stress spikes while maintaining predictable actuation characteristics.
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 actuator achieves higher strain outputs and extended cycle life by optimizing stress and prestress combinations, ensuring predictable performance and increased durability, particularly suitable for orthopedic applications where high forces are needed with limited space and sensor control is difficult.
Implementation Method 1
upon the at least one converter undergoing thermally induced phase transition which makes it contracted
Implementation Method 2
upon the at least one converter undergoing phase transition caused by temperature change
Implementation Method 3
a preload spring... upon the at least one converter undergoing phase transition caused by temperature change and enhanced by the preload spring
Data Source
Figure 1~2
Figure 3
Figure 4(a)~4(b)
AI summary
The present invention is about an actuator comprising at least one shape-memory-alloy based converter in a housing and at least one preload spring. The actuator is configured to cause a motion of at least one movable member. The optimal structures along with corresponding methods for improving an actuator are claimed in the independent claims. Preferable embodiments are presented in the dependent claims.