Problems and Solutions for Shape Memory Actuators in Automotive Applications

The increased automation of technical applications in automobiles requires the use of simple and effective actuator components. The interior of vehicles offer numerous opportunities where mechanical actuators can be or have already been replaced by electrical actuators, e.g. unlocking functions for cup holders or glove boxes. The growing application range of electrical actuators leads to new problems. The growing application range of electrical actuators leads to more weight, sounds and costs. With it the increased automation of technical applications in automobiles requires the use of simple and effective actuator components. Because of the high power density, the simple design and the noiseless actuation of shape memory alloys, they are a solution for this problem. Shape memory alloys are smart materials and have the ability to react to an exterior thermal or electrical stimulus. Besides they can combine sensor and actuator functions in one component. SMAs are rarely used by design engineers although there are numerous possibilities to use them in diverse automotive applications. One reason for this rejection is the effects of functional fatigue in SMA wires. There are no prognosis models or even simulation tools for these effects compared to the many studies on structural fatigue in other materials. After a certain number of cycles, the wire shows an elongation which slowly dislocates its working position. Moreover, with a growing number of cycles, it suffers a loss of effect which results in the reduction of its maximal displacement in technical applications. Another reason is that the transformation temperatures are too low for automotive applications. With it the requirements of a short reactivation time, of a high durability and for an operability in ambient temperatures between −40 and +85°C have been identified as critical for SMA actuators. Therefore, ways to increase these properties by technical means are presented in this study. The idea of “adaptive preload” for example pursues the aim to increase the transformation temperatures and to compensate the existing and inevitable fatigue.