Adaptive resetting of SMA actuators

Shape memory alloys (SMAs) have essential advantages compared with conventional actuators, in particular their high-power density and their silent mode of operation. However, this material has not yet gained acceptance in technical applications. The main reasons are the missing simulation tools and a lack of knowledge of materials as well as the companies’ uncertainty as to how to handle SMA. The resetting of the SMA element to generate a repeatable movement is often a defined problem. In this context, reset springs made of steel are conventional solutions, although their characteristics are a disadvantage. To reach a high level of power output and hence a high degree of efficiency, a reduction of the preload is necessary. A solution for this problem is an adaptive resetting. One main possibility to generate an adaptive resetting is given by the agonist–antagonist principle where two SMA elements work against each other. Here, the reset force can be applied if necessary. The advantage of this type of design is that a conventional return spring or a mechanical brake for clamping the position (electrically operated) is not necessary. Another possibility for adaptive resetting is to change the spring characteristics of a pseudoelastic SMA element by heating. The aim of this publication is to sum up the different possibilities of adaptive resetting of shape memory actuators. It also provides methods and the knowledge to support the development process of such resetting principles. The development of these methods is based on the analysis of different designs and requirements. Based on the experimental results, a conclusion of the possibilities is given.