Behavioral Model and Experimental Validation for a Spool-Packaged Shape Memory Alloy Actuator

Shape memory alloy (SMA) based actuators have the potential to be lower mass, more compact, and more simplistic than conventional based actuators (electrical, hydraulic, etc); however, one of the key issues that plagues their broad use is packaging since long lengths of wire are often necessary to achieve reasonable actuation strokes. Spooling the wire around pulleys or mandrels is one approach to package the wire more compactly and is useful in customizing the footprint of the actuator to the available application space. There is currently a lack of predictive models for actuator designs with spooled packaging that account for the variation of stress and strain along the wire's length and the losses due to friction. A spooling model is a critical step toward the application of this technique to overcome the packaging limitations on SMA actuators. This paper presents the derivation of an analytical predictive model for rotary spooled SMA actuators that accounts for general geometric parameters (mandrel diameter, wire length, wire diameter, and wrap angle), SMA material characteristics, loss parameters (friction), and the external loading profile. An experimental study validated the model with good correlation and provided insight into the effects of load and wrap angle. Based upon the model and experimental results, the main limitation to this approach, binding, is discussed. The analytical model and experimental study presented in this paper provide a foundation to design future actuators and insight into the behavioral impact of this packaging technique.