Development of a Bistable Fluid Valve Based on Shape Memory Alloys, Considering Production Using 3D Printing Processes

Actuator systems based on shape memory alloys (SMA) offer many advantages compared to conventional drives: a significantly higher work density and the resulting lightweight design potential, simple design and noiseless operation Due to these special properties, SMA actuators are predestined for valve applications and have been particularly successful on the market in pneumatic systems for seat comfort applications. In the field of fluid valves, the first commercial products have recently been announced, addressing the areas of medical engineering and the food industry. What all known solutions have in common is that the systems are generally only activated for a very short time and are therefore usually designed as normally open or normally closed valves. SMA valves that can switch between two different states and keep them without energy are not yet known from the state of the art. One application for such valves is for example in irrigation systems. Here it is necessary to switch off the valves from time to time for a few hours a day to ensure that crops are watered as required and to avoid overwatering.
This paper presents the development of an additively manufactured fluid valve using a bistable mechanism activated by an SMA wire actuator. For this purpose, the state of the art for SMA-based valves is presented first. Furthermore, various known bistable mechanisms are analyzed and discussed regarding their suitability for valve systems and their manufacturability using 3D printing technology. Based on this, an additively manufacturable drive mechanism with a high degree of integration is being developed and the switching distances and forces are investigated by measurement. This load characteristics can be used to design a wire against spring arrangement to realize the drive task. This enables the bistable mechanism to be switched sequentially between the open and closed positions with a single SMA actuator wire using electrical activation. After deactivation, the mechanism holds the valve in an open or closed position without electrical energy. Using measurement results from long-term measurements, we also show that the actuating mechanism can realize the required 3650 switching cycles. A subsequent investigation of additively manufactured test specimens of the kinematics using the extrusion layer process and the stereolithography shows considerable differences in the manufacturing accuracy, which has an impact on the functional performance of the valve. Based on these investigations, recommendations for the manufacturing of such valves using additive manufacturing processes are formulated and their limitations are discussed.