SMA thick wires for fast actuation

Abstract:
Usually Shape Memory Alloys (SMA) are adopted in industrial applications in the shape of thin wires (diameter ˂ 0,5 mm) fed by less than 12-24 Volts. Such voltage level allows quasi-static or low speed movement of the actuator. Providing high voltages for few milliseconds leads to high activation speed. Despite the other advantages reported in literature the use of high voltage for feeding the wire allow also fulfilling high speed activation on very thick wires. This work is aimed at showing the possibility to perform fast actuations on SMA thick wires (diameter ≥ 0.5 mm) achieving actuation speed in the order of pyrotechnic devices where activation times of few milliseconds are required. As written in US4091621A a pyrotechnic actuator can be described as “a piston, normally contained within a cylinder, is projected by electrical detonation of an explosive charge which is contained within the cylinder through which a detonation contact extends to the charge.” Common specifications for pyrotechnic actuators indicate a time of activation less than 10 milliseconds, a stroke in the range of 0,15 up to 2,5 inches and a load between 100 and 1700 lbs. Among the different applications of pyrotechnic devices motorcycle airbags have been investigated. For motorcycle airbags the standard prescribes an intervention time less than 200 ms and a duration time higher than 5 s. Motorcycle airbags can be divided into electronic and mechanical systems. The first ones use an electronic system to activate the pyrotechnic gas generator responsible to fill the bag. The mechanical ones use a mechanical trigger in order to activate perforation of a CO2 vessel. The pressure vessel is a steel cartridge with a sealing membrane of 0,4 mm of thickness. A simplified model to estimate the SmartFlex NiTi wire diameter and the actuation parameters has been developed. An experimental characterization to verify the model and understand the force required to cut a 0,4 mm sheet of steel has been carried out by changing the SmartFlex wire diameter, the cutter shape and size. To measure the activation time reached by the Smart Flex wire a high speed camera has been used. The latter is an iSpeed 220 Mono 4GB optioned with a video trigger system and a segmented memory. After the experimental characterization a prototype has been built using a commercial airbag [9] modified with the SmartFlex based perforation system. The prototype uses two commercial batteries of 9 volts for actuating the SmartFlex wire. The Smart Flex based actuator can cut the steel membrane in 2 ms while the bafg inflating time is around 160 ms. Reducing the intervention time could be possible by optimizing the fluido-dynamic profile but it was out of the authors’ purpose.