Computer-Aided Development and Simulation Tools for Shape-Memory Actuators
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Shape-Memory Alloys (SMAs)—Smart and Lightweight Actuators
SMAS exhibit the remarkable property to recover a previously imprinted shape after a deformation. Both the thermal and the superelastic transformation effects rely on the martensitic phase transformation.[1] While heating, the transformation from the low-temperature phase B19 (martensite) into B2 (austenite) initiates by exceeding the austenite start temperature (As). This is linked to a shape change ending at the austenite finish temperature (Af). During subsequent cooling, the reconversion of the material is provoked by the retransformation B2 into B19 from martensite start temperature (Ms) to martensite finish temperature (Mf). The effect can be used to generate mechanical forces and displacements that are known as actuations in the field of mechatronics. The most commonly used shape-memory materials are binary nickel titanium alloys (NiTi) consisting nearly of a 50 pct ratio of both elements. The ratio is the crucial factor for setting up the transformation temperatures. Shape-memory alloys have certain characteristics that are unique compared with other actuating principles. A striking advantage of SMA is the significantly higher working capacity compared with conventional actuators.[2] A shape-memory HORST MEIER, Professor, and ALEXANDER CZECHOWICZ, Research Assistant, are with the Chair of Production Systems, Institute of Product and Service Engineering, Ruhr-University Bochum, 44801 Bochum, Germany. Contact e-mail: Czechowicz@lps. rub.de Manuscript submitted March 31, 2011. Article published online December 7, 2011 2882—VOLUME 43A, AUGUST 2012
wire with a diameter of 2 mm can lift a weight of 120 kg by a self-weight of just 25 g. Furthermore, shapememory actuators make noiseless actuation possible, which is desired in automotive comfort applications. Despite these advantages, the assertion of SMA is hindered by the limited knowledge about the development process and a lack of prototyping experience in this certain field. This often leads to trial-and-error development processes of shape-memory actuators, which induce high costs and time-consuming projects. The engineering problem has several solutions. On the one hand, a simplification of the engineering process can be achieved by using generic shape-memory components. As described in further publications, standardized shape-memory actuators comparable to standardized electromagnetic devices are easier to handle regarding system implementations.[3] On the other hand, a numerical simulation of the shape-memory element behavior can provide essential data for the development process. The advantage of a numerical simulation toward a finite-element model is the simple manageability and the consideration of boundary elements of a shape-memory actuator system (e.g., ambient temperatures, mechanical connection, controllers, etc.) Figure 1. B. Adaption of a Development Process for Mechatronic Shape-Memory Alloy Systems Referring to standards in the engineering process, the V-Model[4] describes the development proces
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