Shape memory alloy (SMA) wires, typically in the form of Nickel-Titanium (NiTi) can be plastically deformed at low temperatures and return back to their original shape when heated above their transition temperature. This is based on a phase transformation from martensite to austenite and can be achieved in actuator applications via joule heating. Because of their high energy density SMA wires are especially suitable for small and lightweight actuator systems, such as valves, latching mechanism and robot end-effectors. For designing these systems, it is important to know the characteristics and behavior of the actuator wire. As the manufacturers’ datasheets give only basic information, engineers have to do much experimental work and use their experience when designing a new system. Unknown parameters are for example the exact load elongation diagram, the change of behavior after several cycles and the maximum stroke given by the biasing system. Also crucial for many applications is the influence of changing ambient temperatures, as well as the control of the current source for heating the wire.
For the prediction of SMA behavior in certain applications and a better general understanding of the NiTi characteristics, a high-end test rig is designed and presented in this work. With this test rig, thermo-mechanical and electrical behavior of SMA wires with a length of 100 mm and diameters from 25 to 100 µm can be determined. Via hardware-in-the-loop, arbitrary load scenarios such as springs, masses and bistable elements can be simulated depending on the application. All measurements occur in a closed climate chamber, where the wire is shielded from uncontrolled air flow. The maximum ambient temperature that can be reached is 100 °C. Some key elements of the setup are the custom designed clamping mechanism guided on air bearings, a linear drive with sub micrometer resolution and a high resolution load cell.