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.