SMA Wire Slack Measurement for Precise Actuator Positioning
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Solution Overview
Problem
Existing SMA actuator assemblies face challenges in accurately measuring and controlling the degree of slack in SMA wires, which affects the manufacturing process and the precision of actuator positioning, particularly in assemblies with plural SMA wires in opposed arrangements.
Innovation Solution
A method and apparatus for measuring the degree of slack in SMA wires by heating them to cause contraction and analyzing electrical characteristics such as resistance, and a closed-loop control method to compensate for slack during actuator operation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If SMA wire is held under tension during manufacture to accurately know its length and tension, then the length and tension can be accurately controlled, but the available stroke of the actuator is reduced
Solution Approach 1:
The patent applies preliminary action by pre-tensioning the SMA wire during the crimping process before final assembly. The wire is tensioned to a predetermined amount during manufacture, which establishes a known initial state. This preliminary tensioning allows the wire to be in a controlled state during assembly while still permitting full stroke operation when energized, as the wire can contract from its pre-tensioned state without being permanently constrained.
2Length of moving object
If SMA wire is held slack to increase available stroke, then the stroke is maximized, but the length and tension become difficult to measure and control
Solution Approach 1:
The patent applies parameter changes by using electrical resistance as a measurable parameter to determine the mechanical state of the SMA wire. During manufacture, the wire's resistance is measured to infer its length and tension state. This electrical parameter provides a non-contact, precise method to characterize the wire's mechanical condition without requiring the wire to be visibly taut or slack, enabling control while maintaining stroke capability.
Solution Approach 2:
The patent replaces direct mechanical measurement methods with electrical measurement. Instead of using mechanical gauges or visual inspection to determine wire length and tension, the system uses electrical resistance measurements. This substitution enables precise measurement of the wire's state even when held slack, as the electrical resistance changes predictably with the wire's mechanical condition, allowing control without mechanical constraint.
3Measurement precision
If tension is applied to pull SMA wire taught for measurement, then the length can be measured, but the tension itself affects the measurement accuracy
Solution Approach 1:
The patent applies self-service by using the SMA wire's own electrical resistance as the measurement mechanism. The wire's resistance naturally changes with its length, temperature, and mechanical state, providing self-indicating properties. By measuring the wire's resistance, the system uses the wire's inherent characteristics to reveal its state without requiring external measurement devices that would add complexity or introduce additional tension forces.
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
Enables accurate measurement and control of slack in SMA wires, improving manufacturing quality and actuator positioning precision, especially in assemblies with opposed SMA wires.
Implementation Method 1
heating the at least one length of shape memory alloy wire to cause contraction of the lengths of shape memory alloy wire
Implementation Method 2
The energy supplied to the lengths of SMA wire may be in the form of drive signal, e.g. electrical current, passing through the lengths of SMA wire, thereby effecting resistive heating of the lengths of SMA wire
Data Source
Figure 1~6D
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Figure 12~13
AI summary
A method of measuring the degree of slack of plural lengths of shape memory alloy wire where at least one of which is held slack, the method comprising: heating the lengths of shape memory alloy wire to cause contraction of the lengths of shape memory alloy wire; measuring an electrical characteristic of the lengths of shape memory alloy wire during said contraction of the lengths of shape memory alloy wire; and deriving a measure of the degree of slack of the lengths of shape memory alloy wire from the measured electrical characteristic.