Analytical and Numerical Modelling of Shape Memory Alloy Negator Springs for Long-Stroke Constant-Force Actuators

Analytical and Numerical Modelling of Shape Memory Alloy Negator Springs for Long-Stroke Constant-Force Actuators PDF Andrea Spaggiari and Eugenio Dragoni SMASIS2012-7964 pp. 353-361; 9 pages doi: 10.1115/SMASIS2012-7964 This paper explores the merits of shape memory Negator springs as powering elements for solid state actuators. A Negator spring is a spiral spring made of strip of metal wound on the flat with an inherent curvature such that, in repose, each coil wraps tightly on its inner neighbour. The unique characteristic of Negator springs is the nearly-constant force needed to unwind the strip for very large, theoretically infinite deflections. Moreover the flat shape, having a high area over volume ratio, grants improved bandwidth compared to any solution with solid wires or helical springs. The SMA material is modelled as elastic in austenitic range while an exponential continuum law is used to describe the martensitic behaviour. The mathematical model of the mechanical behaviour of SMA Negator springs is provided and their performances as active elements in constant-force, long-stroke actuators are assessed. The SMA Negator spring is also simulated in a commercial finite element software, ABAQUS, and its mechanical behaviour is estimated through FE analyses. The analytical and the numerical prediction are in good agreement, both in martensitic and in austenitic range.