Behavioral Model and Experimental Validation for Spool-Packaged Shape Memory Alloy Linear Actuators

Shape memory alloy actuators are attractive for many industries because of their high energy density and moderate strokes and forces, which can enable low cost, light weight, high performance devices. SMA wire actuators are particularly advantageous because of their simple form, well-developed manufacturing and quality control, actuation with simple electrical circuits, and faster heating and cooling rates. Unfortunately, packaging the long lengths of SMA wire needed to produce required deflections in many applications is an ongoing technical challenge. This paper investigates spooling as a packaging approach to provide more compact actuator footprints and explores the accompanying design tradeoff between packaging and performance. A predictive analytical model is derived that relates the linear output motion to key design parameters (geometric and frictional properties, SMA material properties, and externally applied loads) within model limits originating from binding due to accumulation of friction across the spool. An experimental study is presented which exercises the model with respect to applied load, wrap angle and spool position within the actuator system, and the results correlate well with the model prediction in both form and magnitude. The study provides insight into the impact of key design parameters and exposes the design tradeoff between the packaging approach and performance losses. Based on the resulting spooling model, actuators can be designed with minimized losses and improved packaging footprints, positioning SMA wire actuators as a viable alternative to conventional industrial actuators.