Experimental characterization and modelling validation of shape memory alloy Negator springs

This article is aimed at the experimental characterization and modelling validation of shape memory alloy Negator springs. 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 main feature of a Negator springs is the nearly constant force-displacement behaviour in the unwinding of the strip, mounted on a rotating drum. Moreover, the stroke is very long, theoretically infinite as it depends only on the length of the initial strip. A Negator spring made of shape memory alloy is built and experimentally tested to demonstrate the feasibility of this actuator. The shape memory Negator spring behaviour is predicted both with an analytical model and with a finite element software. In both cases, the material is modelled as elastic in austenitic range while an exponential continuum law is used to describe the martensitic behaviour. The experimental results confirm the applicability of this kind of geometry to the shape memory alloy actuators, and the analytical model is confirmed to be a powerful design tool to dimension and predict the spring behaviour both in martensitic and austenitic ranges, as well as the finite element model developed.