DESIGN OF SMA HELICAL ACTUATORS: AN EXPERIMENTAL STUDY

The Shape Memory Alloy (SMA) helical actuation architecture overcomes the typical strain limitations of straight wire SMA actuators by producing larger stroke in a small package size. SMA Helical actuators also provide design tailorability where the tightness of the coil can be used to make tradeoffs between force and displacement, along with the coupled tradeoff between package length and diameter. These are very attractive for industrial settings such as the automotive industry, but require models and design processes to support the needs of the quick, early design cycles typically required. This paper presents an experimentally-based parameter study from which a streamlined design process is extracted. SMA helical actuators with a range of wire diameters and coil diameters were fabricated and experimentally characterized. The functional dependence of performance metrics such as austenite stiffness, force level of the martensite plateau, and recoverable strain were evaluated. A two-step decoupled design procedure is presented based on the resulting empirical model where the kinematic design is first undertaken to select the spring index which provides the required strain within the available package space, and then the non-dimensionalized force is used to scale the design such that the required actuation force is achieved. The streamlined, systematic design framework presented in this paper provides the means for the design of compact, large stroke helical actuators in industrial settings.