Shape Memory Alloys (SMA) produce actuation from temperature and/or applied-load induced transformation between the compliant martensite phase and the stiff austenite phase. Most applications use active electrical Joule heating to induce the actuation; there are few attempts to control the SMA actuation through the environmental temperature. Thermal hysteresis is one of the limitations which hinders the adaptation of passive SMA actuation due to the difference between the actuation trigger temperature and the restoration temperature.

Automotive grille shutters are used to reduce the aerodynamic drag of the vehicle when cooling requirements are low by blocking most airflow through the condenser, radiator, and fan module (CRFM); the lower aerodynamic drag improves fuel economy. A passively actuated SMA grille shutter was designed as a less complex and lower overall cost solution to current electro-mechanical solutions. The passive SMA actuator is mounted such that it senses CRFM exhaust air and is responsive to changes in thermal load of both the AC condenser and radiator.

This paper presents the design process for the automotive grille shutter SMA actuator using passive temperature control. Target opening and closing temperatures of the grille shutter system were determined using a combination of road and climatic wind tunnel testing to measure impact on fuel economy and air conditioning performance. The transformation temperatures of the SMA actuator were calibrated by varying stress on the SMA wires by means of the total cross-sectional area and/or the biasing force.

Road tests of the initial design indicated that the SMA actuation hysteresis band was too wide which sometimes led to an open shutter when the cooling requirement was at or below base air flow. A 5 °C smaller SMA actuation hysteresis band was required, while maintaining the start to open temperature to prevent impacting other performance metrics. A design of experiments to further tune the actuator was performed which investigated a design space of various cross-sectional SMA wire areas and biasing forces.  The wire area was varied by changing the number of 10 mil SMA wires used; the biasing force was changed by varying the number of leaf springs used.  Tests were performed in an environmental chamber to characterize the temperature-position behavior during heating and cooling cycles, the best candidate improved the hysteresis band by 2 °C.  To fully solve the undesired shutter open issue SMA wire with an additional 3 °C decrease in hysteresis would have to be fabricated. This study presents an industry need for the development of low hysteresis SMA wires/springs for passive applications, which can maximize the benefit of SMA actuation by utilizing the temperature-stress-strain coupling behavior of SMA.


Automotive Grille Shutter Using Passive Shape Memory Alloy Actuation

Division / Sponsors: 
Aerospace Division

Paper Proceedings: https://asmedigitalcollection.asme.org/SMASIS/SMASIS2020/volume/84027

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