Purpose -The main purpose of the paper is to develop model basing on the modified and properly-adopted Fermi-Dirac equation which combines proper accuracy with adequate simplicity as well as to show how steady state and transient curves resulting from this model can be applied for solving design task. Design/methodology/approach -The standard Fermi-Dirac equation was modified and extended. Full performance cycle for the SMA actuator was characterized by double-valued function describing the actuator activation and the actuator deactivation. All these functions and parameters can be easily determined by analysis of measurement data or with use of Hooke-Jeeves optimization algorithm. Findings -SMA linear actuator can be used in mechatronic systems as a special non-standard drive when ultra-light mass and very simple mechanical construction of power feed system is required. The proposed steady-state and transient performance curves as well as operation diagram constitute sufficient base for effective designing SMA drive systems.Research limitations/implications -The greatest disadvantage of a SMA actuator is long time of deactivation resulting from slow self-cooling process. As far as efficiency is concerned as essential factor for choosing the most suitable linear actuator, there is no sense to take into account a linear SMA actuator because of its very low efficiency. Practical implications -Designer can use performance curve which determines proper length of SMA actuator and range of its motion. The proposed model can be implemented in SMA drive control unit for controlling position of the actuator. Originality/value -Similarities between change of martensitic phase during transition process and probability P of electron energy level distribution described by the Fermi-Dirac two-variable equation were taken into account. Such an approach seems to express in the most suitable way the physical nature of m-a transition. The authors decided to extend concept (proposed in Jayender et al.) and to adopt the Fermi-Dirac equation for describing behaviour of a SMA linear actuator.
In this article, changes in NiTi alloy (Flexinol) electrical resistance during cyclic stretching with small elongation were investigated. A dedicated test stand consisting of motorized vertical test stand, force gauge, and electric resistance measuring device with an accuracy of 0.006 Ω was developed. A dedicated control algorithm was developed using LabVIEW software. Changes in electrical resistance were investigated for the 0.1 mm Flexinol wire with length of 120 mm. Testing was performed in the elongation range between 0.25% and 1.5% in martensite phase. Tested samples were subjected to 30 stretching cycles with a movement speed of 10 mm/min. Obtained results show that the cyclic stretching of Flexinol wire reduces its electrical resistance with each stretching cycle. Moreover, it was noted that changes in Flexinol electrical resistance during cycling stretching depend on the assumed elongation and number of the already performed stretching cycles. The observed electrical resistance change decreases with each stretching cycle. Thus, the observed changes are greater during the first stretching cycles. For elongations exceeding 1%, the Flexinol electrical resistance in the first stretching cycle increases. In each subsequent cycle, electrical resistance decreases, as in the case of the smallest value of assumed elongation. In almost all tested cases (except in the case with 1.5% of assumed elongation), Flexinol electrical resistance after 30 stretching cycles was smaller than before the test.
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