Increasing stroke and output force of linear shape memory actuators by elastic compensation

Shape memory actuators are a class of very interesting actuators due to the high power to weight ratio, plus the fact that they can work in harsh environments and can be easily constructed. The main defects of this technology are the short strokes and the non-uniformity of the useful force over the stroke. This paper aims to limit these two problems by introducing a passive system of elastic compensation. We first develop a functional design procedure of the active elements and of the compensation system in order to obtain the force and stroke desired. We also show two compensation mechanisms that are able to execute the laws required, and we provide expressions for the kinematic design. A numerical example for an actuator with a single shape memory element shows that, all other conditions being equal, the elastic compensation produces increases in stroke (for equal useful force) or useful force (for the same stroke) that are more than 2.5 times greater. A proof-of-concept actuator including a rocker-arm compensating mechanism is also built and tested to confirm the theoretical predictions.

Research highlights
► We develop a general theory for elastic compensation of SMA actuators.
► A compensated actuator has greater stroke and output force than uncompensated ones.
► We propose two general mechanisms to provide elastic compensation to any actuator.
► We build a compensated actuator which more than double the stroke for given output force.