Shape memory alloy (SMA) actuation is becoming an increasingly viable technology for industrial applications as
many of the technical issues that have limited its use are being addressed (speed of actuation, mechanical connections,
performance degradation, quality control, etc.) while increasing production capacities drive costs to practical levels.
Shape memory alloys are often selected because of their high energy density which can lead to compact actuators;
however, wire forms with small cross-sectional diameters tend to be long (10 to 50 times the length of required stroke).
Spooling the wire can be used for compact packaging, but as the spool diameter decreases performance losses and
fatigue increase due to bending strains and stresses. This paper presents a simple, design-level model for spooled SMA
wire actuators with linear motion outputs that includes the effects of friction and wire bending and accounts for the
actuator geometry, applied load, and material friction and constitutive properties. The model was validated
experimentally with respect to the ratio of mandrel to SMA wire diameter and agrees well in both form and magnitude
with experiments. The resulting model provides the framework for the analysis and synthesis of spooled SMA wire
actuators to guide the selection of design parameters with respect to the tradeoffs between performance and packaging.
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.