Shape Memory Actuator System for Passive Cooling Air Adjustment in Mechanical Design

Passive control of cooling processes is in designs best interest. Coolant medium flow to hot components must be kept at minimum acceptable level from lifting perspective to achieve maximum process efficiency. Required cooling of the hot components depends directly on engine power setting, which in general requires a relative complex system for monitoring critical parameters and adjusting coolant’s amount with engine load. Concerning the operation reliability, pseudoplastic shape memory alloys offer a high simplicity in the design of adjustment mechanisms with large operating displacements. As the shape memory effect is induced by temperature changes, the behavior of shape memory actuators and therefore the coolant’s amount can be adjusted to the load conditions of the engine by using appropriate shape memory materials. In this paper an actuator based on a shape memory membrane using the extrinsic two-way effect is presented to vary the cross-sectional area of a cooling air channel with respect to the engine operation. The reset of the mechanism after one temperature cycle of heating and cooling is realized by using a leaf spring element, which is in varing mechanical contact with the shape memory membrane depending on the hysteresis of the entire system. Maximum displacements of the system are attained for spring forces between the force generated by the shape memory membrane in the martensitic and austenitic state. Thus, the system mechanism exhibits two non-linearities of pseudoplastic shape memory characteristic and contact mechanics with friction. For this purpose experimental investigations were carried out to acquire the fundamental force displacement behavior of the shape memory membrane to design the optimal shape of the leaf spring element. The forces required to deform the shape memory membrane in the martensitic and austenitic state were measured with respect to the membranes displacement using a load cell and a linear variable differential transformer. The displacements of the membrane were introduced using a linear bearing system. The calculations for the design of an optimal leaf spring and especially its initial shape were carried out using a discrete multi body system consisting of beam elements and torsional springs. The leaf spring with the calculated optimal shape was fabricated and incorporated into the system. The displacement behavior of the system during heating and cooling was measured using an optical distance sensor. For the analyzed temperature range up to 100 °C, the paper describes the methodological appropriacy and relevance towards the application domain for evaluated temperatures.