This article presents the design equations for an on–off shape memory alloy actuator working against an external system of arbitrary constant forces. A binary shape memory alloy actuator is considered where a cursor is moved against both conservative and dissipative forces, which may be different during the push or pull phase. Three cases are analysed and differentiated in the way the bias force is applied to the primary shape memory alloy spring: using a constant force, a conventional spring or a second shape memory alloy spring. Closed-form dimensionless design equations are developed, which form the basis of a step-by-step procedure for an optimal design of the whole actuator

This work describes the conceptual design, the modelling, the optimization, the detail design and the virtual testing of a shape memory actuator purposely conceived to maximize torque and angular stroke while limiting overall size and electric consumption. The chosen design, achieved by means of a Quality Function Deployment approach, features a fully modular concept in which an arbitrary number of identical modules are assembled to produce the desired angular stroke and output torque. The basic module contains shape memory springs that actuate the device and also a conventional spring that reduces the torque ripple. Following the concept generation stage, a thermo-electromechanical model is developed and a numerical optimization performed, aimed at minimizing the electrical consumption of the actuator. Finally, the device is designed in detail and the actuator is tested virtually. Thanks to the proposed modular construction and the use of a conventional balancing spring, the device shows better performances than known rotary shape memory actuators in terms of rotation, torque and customization.

Shape memory alloys (SMAs) can be exploited successfully to reduce the complexity and the weight of actuators, but the main drawbacks that limit the use of SMAs are the low bandwidth, poor energetic efficiency, and unsatisfactory stroke. This article contributes to enhancing the mechanical, thermal, and electric performances of SMA actuators by providing analysis and design equations for helical springs with hollow round section. By emptying the inefficient material from its center, the hollow section features a lower mass, lower cooling time, and lower heating energy than its solid counterpart for given strength, stiffness, and deflection. The advantages of the hollow construction over solid springs are presented and discussed by means of dimensionless functions. A step-by-step procedure leading to the optimal design of hollow springs with minimum energy consumption is finally proposed.

An analytical procedure to design binary shape memory actuators is described. A generic actuator is considered where a cursor is moved against dissipative forces using an elastic system containing a primary shape memory spring and a bias (backup) element. Three typical cases are analysed and differentiated in the way the bias force is applied to the primary shape memory spring, using a constant force, a conventional spring, or a second shape memory spring. Dimensionless, closed-form relationships are developed, which form the basis of a step-by-step procedure for an optimal design of the whole actuator (primary active spring and bias element). Specific formulas regarding the detailed design of the shape memory elements of the actuator in the form of straight wires and wire helical springs are also presented.

Shape memory alloys (SMAs) are used in many applications as actuators. The main drawbacks that limit the use of the SMAs in the field of mechanical actuation are the low mechanical bandwidth (up to a few Hertzs) and the unsatisfactory stroke (several millimeters). This paper contributes to enhancing the performances of SMA actuators by proposing a new SMA helical spring with a hollow section. The hollow spring is modeled, then it is constructed, and finally it is tested in compression to compare its performances with those of a spring with a solid cross section of equal stiffness and strength. Emptied of the inefficient material from its center, the hollow spring features a lower mass (37% less) and an extremely lower cooling time (four times less) than its solid counterpart. These results demonstrate that helical springs with a hollow construction can be successfully exploited to build SMA actuators for higher operating frequencies and improved strokes.