Electromechanical Termination of Shape Memory Alloy Wire
Shape Memory Alloy (SMA) Wire Actuation Devices use a specific alloy of Nitinol (Nickel Titanium) material, which contracts 4% when heated. Heat generation can be accomplished using electrical current or thermal energy. Compared with conventional actuation methods, such as motors or solenoids, SMA devices require less energy, and enable much lighter and more compact designs.
SMA wires cannot easily be terminated by many standard joining techniques, such as welding, soldering, and mechanical methods. These methods are often not practical due to (1) metallurgy of the wire, (2) the small sizes that is often used in complex actuation applications, and (3) the need to terminate the SMA (Shape Memory Alloy) wire under tension. These connections must endure a variety of electrical, mechanical and environmental conditions demanded by the application.
Here we will discuss:
How crimping technology solves these challenges to provide highly reliable and repeatable assemblies in high-volume manufacturing environments.
How these difficult-to-handle assemblies can be efficiently packaged for further automation to integrate them into final assemblies.
Crimping is perhaps the most widely used and most effective joining technique for nitinol wire, and it can be done via a variety of techniques, including the crimping of a sleeve over the joint. The combination of stamping strip material properties and the geometrical configuration of the crimp provides the required mechanical grip around the nitinol wire.
Crimp Force Monitor Systems can closely monitor crimp height by analyzing and processing crimp signatures and detecting acceptable ranges for the crimping forces. Process Capability Indices (Cpk and Ppk) for retention force can be used to monitor crimping reliability.
Autosplice has become a leader in design and manufacturing of custom products using Shape Memory Alloy (SMA) wire that incorporate our patented crimp designs. Examples include the ‘book fold’ and ‘splice’ designs.
Packaging of SMA Actuators
It is very important to make sure that the nitinol wire is not overstretched, will not tangle, and is easy to use during the assembly process of installing the SMA actuator subassembly inside of the actuator mechanism. Reliable performance of the device requires precision positioning of the electromechanical connectors relative to the nitinol wire and maintenance of specific dimensions between the connectors, with narrow tolerance schemes under closely-controlled wire tension.
The automated mass production of SMA assemblies requires a complex quality control system to validate and confirm the critical parameters of SMA assembly, such as minimum pull force of the crimps, length of SMA wire between crimps under controlled wire tension, amount of actuation, and wire return.
Our inspection system consists of fixtures to position and actuate the SMA assembly, and then use calibrated power supplies and PLC control to provide the precise energy impulse to the SMA wires. Precision optical microscopes, such as the OGP SmartScope® Flash™, are used to measure all required control dimensions in automated mode.
CONCLUSION
Crimping is the effective joining technique for nitinol wire, and requires continuous monitoring of crimping forces and crimp geometry to ensure a reliable electromechanical connection.
Autosplice provides highly repeatable, high volume reliable assemblies for various applications. The assemblies often employ efficient packaging for further automation to integrate SMA devices into customer assemblies.
Statistical Process Control is used to ensure process stability.
