Abstract
This paper deals with the specific hysteretic effects of a multi-antagonistic shape-memory alloy (SMA) actuation system in which each wire can be thermally activated individually (one or more at a time). A planar system with six SMA wires organized in a ternary rotational symmetry is studied numerically, via finite element calculation software. The objective is to analyze the functional characteristics of such mechanism, whose response during a thermal activation sequence is multi-antagonistic and hysteretic. Important points are highlighted, such as the avoidance of buckling and plasticity, the possibility of locking a configuration without energy input, and the “attraction” effect of any heating step on the following steps. The feasibility of reaching a given target in the workspace is also illustrated. Finally, the analysis shows the necessity to consider these multi-antagonistic and hysteretic aspects of the actuation in the future design and control of such type of mechanisms. The latter could be of interest as hollow shaft rotary actuators with additional (small) translation degrees of freedom in applications requiring long-term and stable positioning at ambient temperature.
Highlights
New types of actuation are currently being studied to develop the generation of mechanisms, in particular by employing smart materials [1,2,3]
A so-called one-way memory effect consists of heating the martensitic sample to activate the M A transformation, leading to the recovery of the memorized shape
The mobile platform was considered to be made of steel (Young’s modulus of 200 GPa) with a cross-section of 150 mm2 for each of its three branches, which makes it effectively rigid compared to the shapememory alloy (SMA) wires
Summary
New types of actuation are currently being studied to develop the generation of mechanisms, in particular by employing smart materials [1,2,3]. Note that a two-way memory effect at zero force or stress is possible after thermomechanical “training”, consisting of numerous thermal cycles at fixed strain or stress, or from constrained aging [16, 17] These special SMAs require microstructural/metallurgical optimization to achieve sufficient stability of the effect with respect to the applications [18]. We aim to analyze the hysteretic properties of a six-wire prestressed planar mechanism with ternary symmetry This architecture is chosen to have redundant actuation for three independent DOFs. The two-way memory effect is used to create multidirectional actuation thanks to the antagonistic arrangement of the six SMA wires. The following notation is used for the symbols throughout the paper: italic letters are scalars and bold letters are vectors in space
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