NiTi Shape Memory Alloy Actuator Research Articles

Liquid metal-induced corrosion of nickel-titanium alloys by gallium alloys for liquid metal-enabled shape memory applications

Liquid gallium-indium eutectic has seen an expansion of applicability, such as a multifunctional electro-thermal fluid within a NiTi shape memory alloy actuator. Various NiTi alloys and elemental Ni, Ti, and Cu were exposed to liquid gallium alloys from 220 °C to 950 °C for up to 1200 h. NiTi was additionally thermally cycled through martensite-to-austenite for 270 cycles. SEM and X-ray spectroscopy revealed corrosion morphology and reaction layer chemistry. Each tested NiTi alloy and Ti did not exhibit liquid metal-induced corrosion until a threshold between 400 °C to 600 °C, while Ni and Cu are attacked at temperatures as low as 220 °C.

Effect of process parameters and electropolishing on the surface roughness of interior channels in additively manufactured nickel-titanium shape memory alloy actuators

Recently, laser-powder bed fusion (L-PBF) has been utilized to produce a NiTi shape memory alloy actuator with embedded channels for liquid metal forced fluid convection to increase actuator heat transfer rates. To enable further increases in performance, it is critical to characterize and control the surface quality of fully interior channels which have higher surface roughness compared to exterior top surfaces. This work utilizes a design of experiments methodology by varying laser power, scan speed, hatch space, scan pattern, channel orientation, and channel diameter on the as-fabricated surface roughness of the overhangs and walls of interior channels in NiTi. To enable post-process increases in surface quality, the channels are subjected to an electropolishing treatment and further characterized. Internal channel surfaces are characterized using optical profilometry and SEM imaging. It is concluded that channel orientation plays a prominent role in determining the surface roughness of as-fabricated interior channels, and a lower laser energy density results in the highest reduction in surface roughness after an electropolishing treatment.

The Analysis of Superelasticity and Microstructural Evolution in NiTi Single Crystals by Molecular Dynamics.

Superelasticity in shape memory alloys is an important feature for actuators and medical devices. However, the function of the devices is typically limited by mechanical bandwidth and fatigue, which are dominated by the microstructures. Thus, in order to correlate the mechanical response and the microstructures, the microstructural evolution in NiTi single crystals under the compression, tensile, and shearing tests is simulated by molecular dynamics (MD) in the current study. Then, the martensite variant identification method, which identifies the crystal variants/phases of each lattice based on the transformation matrix, is used to post-process the MD results. The results with the detailed information of variants and phases reveal many features that have good agreement with those reported in the literature, such as X-interfaces and the transitional orthorhombic phase between the austenite and monoclinic phases. A new twin structure consisting of diamond and wedge-shaped patterns is also discovered. The macroscopic behavior, such as stress-strain curves and the total energy profile, is linked with the distribution of dislocation and twin patterns. The results suggest that the loading cases of shear and compression allow a low critical strain for the onset of martensitic transformation and a better superelasticity behavior. Therefore, the two loading cases are suitable to apply to the NiTi actuators. The current work is expected to provide insight into the mechanical responses and design guideline for NiTi shape memory alloy actuators.

Experimental Characterization and Control of a Force Actuator Based on Shape Memory Alloy Wire

This paper presents an experimental design of a force and position controller for a Ni–Ti shape memory alloy actuator. The focus of this work is a measurement device for the characterization of a SMA spring together with the fuzzy controller for position and force that can be quickly and easily implemented. This device makes it possible to develop a mechanism for robotic applications with slow actuation speed, e.g., to mimic a robotic finger. The contribution of our work to the scientific literature is an easy control of force and position, as well as weight reduction when compared to a similar actuator controlled by a traditional electric motor. Ni–Ti wire was winded up to form a coil; an electric current passing through the actuator changes its temperature, which leads to a phase transformation resulting in a length change of the wire. Thus, the actuator behaves as a spring, whose force exerted can be adjusted by an electric current. A closed-loop fuzzy logic controller was programmed in the LabVIEW environment, allowing the control of the force exerted by the actuator, as well as the position of an attached load. The actuator performance was evaluated experimentally before and after a training session.

Model of a NiTi shape memory alloy actuator

A model of motion of an agonistic–antagonistic shape memory alloy actuator is developed in this article. The model shows that the dynamics of the actuator can be well described by a nonlinear motion model of human muscle with nonlinear damping. To complete the model, a method for the determination of the damping properties of the actuator system is suggested. To this end, an analytical expression for the damping coefficient of the system was developed. The experimental verification of the proposed model of motion was conducted through a comparison of the experimentally measured and numerically simulated step responses. The simulated step responses were in close agreement with their experimentally measured counterparts on the shape memory alloy actuator prototype.

Application of Ni-Ti shape memory alloy actuators in a walking micro-robot

As walking robots are requested to perform tasks in rough terrain, the development of actuators capable to flexibly adapt to the unstructured environment becomes more and more necessary. The conventional mechanisms with stiff joints make the robots more complex, heavy, large and expensive. Shape Memory Alloys are a category of artificial muscles which can be used as actuators for a walking robot. Even if they can exhibit large changes in shape when heated and cooled, only one part of their de-formation can be used, if we want to maximize the actuator life. This is why smart mechanisms that can convert the small strain of the wire into large motion are necessary. In this paper, an example of using Shape Memory Alloys as actuators for a hexapod walking micro-robot is presented. A leg mechanism that can convert the small strain of these actuators in large motion is also discussed. DOI: http://dx.doi.org/10.5755/j01.mech.20.1.3531

Application of trained NiTi SMA actuators in a spatial compliant mechanism: Experimental investigations

This contribution presents the experimental investigations on the open loop behavior of a spatial compliant mechanism using trained NiTi Shape Memory Alloy (SMA) actuators. The mechanism consists of a central superelastic pillar on which a moving platform is mounted and actuated by three SMA wires. The SMA actuators have been trained by thermal cycling with constant bias force before fixing them on to the mechanism. The optimal power requirements of the actuators have been determined experimentally. Open loop time responses have been obtained by applying various voltage inputs and duty cycles to the actuators for establishing a suitable working point. Models representing the reverse transformation (heating) and forward transformation (cooling) have been obtained from experiments. The relationship between the actuation force and the tilt of the moving platform obtained with trained and untrained SMA actuators are compared. The experiments demonstrate that the results obtained with trained actuators are in close agreement with the analytical results.

The Durability of "Smart" Composites Containing Shape-Memory Alloy Actuators

This paper presents the results from an experimental study into the actuation response of laminates containing Shape-Memory Alloy (SMA) actuators. Unidirectional glass-fibre reinforced epoxide laminates were constructed containing Ni-Ti SMA actuators orientated parallel to the glass-fibre direction. Two volume fractions of actuator were employed (0.5 and 1 vol%) with actuation strain levels of 2 and 5%. These laminates were designed to exhibit a macroscopic shape change during electrical activation of the SMA. This work has addressed a major durability issue associated with the operation of such laminates. This is the instability in both shape and deflection which occurs during the early operational cycles. It is shown that such changes depend critically on the initial shape-memory training of the actuators during manufacture, and on the subsequent in-situ retraining of the memory effect within the embedded SMA.

A novel differential shape memory alloy actuator for position control

SummaryThis paper discusses a novel design for a rotary differential NiTi shape memory alloy actuator, which incorporates a mobile heat sink. The new mechanism is described in detail, and its performance in a position control system is compared with that of the same actuator without heat sink. Experimental results are presented which show an improvement in actuator response time, as well as closed-loop bandwidth. The design of the mechanism gives enhanced performance, without the increased power consumption associated with fixed heat sinks.

The Actuation Response of Model SMA Hybrid Laminates

This paper presents the results from an experimental study on the actuation response of composite laminates containing Shape-Memory Alloy (SMA) actuators. Unidirectional glass-fibre reinforced epoxide laminates were constructed containing Ni-Ti SMA actuators orientated parallel to the glass-fibre direction. The volume fraction of actuator was varied in the range 0 to 2.5 vol%. These laminates were actuated electrically and were designed to exhibit macroscopic shape changes during actuation. The work presented in this paper was a preliminary appraisal of the durability issues associated with the use of such hybrid composites. This has shown that actuator fraction plays a significant role in controlling both the shape change obtained from these materials and the hysteresis associated with their forward and reverse actuation. It has also been shown that during cyclic operation hybrid laminates can suffer from instabilities in shape change due to the accumulation of internal damage. However, by careful selection of the maximum actuation strain and actuator fraction it is possible to achieve useable and stable cyclic shape changes