Adaptive SMA actuator priming using resistance feedback

Abstract

Shape memory alloys (SMAs) are a group of alloys which demonstrate the unique ability ofreturning back to a previously defined shape or size if subjected to the appropriate thermalconditions. They have been implemented as actuators—where heat is controlledvia applied current—in a wide range of applications spanning several fields suchas robotics, aeronautics, automotive and medicine. SMA manufacturers specifywhat they refer to as the ‘safe current’ which is the maximum current that canbe applied to the SMA wire indefinitely without damaging it by overheating.However, this current is typically specified at room temperature under naturalconvection conditions. The objective of this work is to develop controllers for SMAactuators in automotive applications and this requires predictable and consistentfunctionality across a wide range of ambient temperatures, typically from − 40 to85 °C. Consequently, applying the safe current in cold ambient temperatures may not actuatethe SMA whereas it could potentially over-heat the SMA at high ambient temperatures. Inthis paper, we use a novel approach involving resistance feedback to achieve moreconsistent actuation across a range of ambient temperatures and compare experimentalresults for several different control strategies. The results show that controller designs usingan adaptive current to actuate the SMA wire achieved more consistent results across thedesired range of ambient temperatures compared to using the fixed safe current. Of thesedesigns, a controller strategy dubbed Minus 4.5% achieved the most consistent actuationresults and was a significant improvement over conventional control strategies.