Simulation based Design of a rotatory SMA-Drive

The design and optimisation of a rotatory drive powered by Shape-Memory-Alloy (SMA) actuators is described in this paper. SMA-actuators used in technical applications are parameterised by the use of trial-and-error methods, because there is a lack of computer-aided design tools for this active material. A numerical modelling approach was developed to design and optimise the geometry and the load and heating conditions of SMA actuators in a technical system to achieve a good dynamic and a high reliability. The Shape-Memory Effect used in most technical Systems is the extrinsic two way effect (2WE). This effect can be simulated with the numerical model which was implemented in MATLAB/SIMULINK^©. The focus of the model is on the activation behaviour of the SMA actuator, which defines its rate of heating and cooling. Different load conditions and 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 obtain an optimal system performance. Furthermore fatigue parameters of the SMA actuator are considered in the numerical model. The model was used to design a rotatory SMA-drive system, which is based in the moving concept of a wave drive gear set. In opposite to a conventional system, which is driven by an electric motor, the SMA-Drive consists of a strain wave gear and SMA-wire-actuators that are applied circularly to generate the rotatory movement. Special characteristics of this drive system are a high torque density and a high positioning accuracy.