Control loops with detection of inner electrical resistance and fatigue-behaviour by activation of NiTi -Shape Memory Alloys

Shape memory alloys are smart materials. Due to their ability to change into a previously imprinted actual shape through the means of thermal activation, they are suitable as actuators for microsystems and, within certain limitations, macroscopic systems. To apply these smart materials to a wide range of industrial applications, a simple method of controlling the actuator effect is required. The detection of inner electrical resistance allows to gauge the actuator movement. By usage of a microcontroller a smart system without any hardware sensors can be realized. Changing outer boundary conditions can be compensated by software. Furthermore an analysis and a description of the functional fatigue, affecting the control loop, is of particular importance. A fatigue calculator dependent upon duty cycle is subjoined to the existing actuator simulation implemented in MATLAB/ SIMULINK. The focus of the simulation-model is on the activation behaviour of the SMA actuator, which defines its rate of heating and cooling. These parameters describe the dynamical characteristics of the actuator and the complete SMA powered system. Different load conditions, various actuator geometries and shapes, e.g. wire or spring actuator are simulated by the calculation of the energetic balance of the whole system. The numerical model can be used to simulate time variant heating currents in order to achieve an optimal system performance for a defined time response of the actuator. 1. Shape memory actuators and their importance for mechatronical systems Shape memory alloys (SMAs) exhibit the remarkable property to recover a previously imprinted shape after a deformation. That effect can occur either at constant temperatures (mechanical memory, pseudoelasticity), or changing temperatures (thermal memory). Both effects rely on the martensitic phase transformation [1]. (martensite) into B2 (austenite) initiates by exceeding the austenite start temperature. This is linked to a shape change. During subsequent cooling the reconversion of the material is provoked by the . This work focuses on the one way effect in combination with restoring force devices, the so called extrinsic two way effect [2]. Most commonly used shape memory materials are binary nickel titanium alloys (NiTi), consisting nearly of a 50 % ratio of both elements. The ratio is the crucial factor for setting up transformation temperatures. Shape memory alloys have certain characteristics which are unique in comparison to other actuating principles. competitiveness of microand mechatronical systems is determined by precision, cost, quality and simplifications. Systems driven by shape memory elements can be used for smart actuators, which include aspects of flexibility and mass decimation in addition to the above-qouted. The aspect of simplification is related to memory actuator generally consists of fewer parts than conventional ones. The reduction of parts often enhances the reliability of actuating components. Besides the mass of shape memory systems, as compared to other actuators, is lower.). A striking advantage of SMAs is the significant higher working capacity in comparison to conventional actuators[3]. In terms of designing smart actuator systems, SMAs provide the possibility to create controller systems without sensor hardware. The detection of inner electrical resistance allows to steer the actuator movement [4]. For example a simple valve component with NiTi drive activated by external heating can be used as well as an electrically activated valve or a controlled proportional valve. This also decreases costs as the volume of systems variety can be reduced. Nowadays many researchers focus their work on applying NiTi to actuator applications, but a e-mail: [email protected] ESOMAT 2009, 05006 (2009) DOI:10.1051/esomat/200905006 © Owned by the authors, published by EDP Sciences, 2009 This is an Open Access article distributed under the terms of the Creative Commons Attribution-Noncommercial License (http://creativecommons.org/licenses/by-nc/3.0/), which permits unrestricted use, distribution, and reproduction in any noncommercial medium, provided the original work is properly cited. Article available at http://www.esomat.org or http://dx.doi.org/10.1051/esomat/200905006 fundamental problems for their commercial use still remain. This includes control paradigms and functional fatigue effects [1]. Hence the present work deals with concepts, resolving these problems by integration of an appropriate calculation into a numerical simulation of the activation behavior of SMA wire actuators. For that purpose analysis and modeling of electrical resistance behavior aims for enhancement of an SMA numerical simulation of controlled systems. Furthermore a life cycle calculation based on a fatigue simulation is implemented in the model, too.