Shape Memory Alloy Microactuator Research Articles

Micromachining of functional Ni-Mn-Ga single crystals using femtosecond pulsed laser

Ni-Mn-Ga-based magnetic shape memory (MSM) alloy microactuators have a high potential in miniaturized devices for which conventional mechanisms or materials are no longer feasible. However, manufacturing single-crystal-based Ni-Mn-Ga micro actuators is challenging due to their high brittleness, which precludes conventional machining methods. The present work introduces and develops a novel manufacturing method – femtosecond pulse width laser (FPWL) ablation micromachining – for the manufacture of a functional actuators from oriented Ni-Mn-Ga single crystals. Notably, using a green laser (wavelength 515 nm) permits micromachining in the martensitic phase due to the low heating effect of the process. Optimized FPWL parameters are here used to machine a functional actuator from a five-layered modulated (10 M) martensite Ni-Mn-Ga single crystal. The actuator develops a fully reversible and repeatable magnetic-field-induced strain of 6.5% without any post-processing.

Micro-actuation in bi-layered shape memory alloy structures

The use of shape memory alloys to generate actuation is analyzed in structures generated by attaching at high temperature shape memory alloy films to a substrate. In particular, the actuation of the free end of a cantilever is modelled for the case where the film that is deposited on a substrate is not a homogeneous one, but one that consists of two layers. The difference between the layers can be – in one case – a compositional one or – in the second case – can be one that is produced from the same target, but with one layer deposited using one set of conditions and the other one using another set of conditions. For the second case, the simplest analysis starts from using two different deposition temperatures. The current work details the results obtained by modelling the actuation of the free end of cantilever type architectures with shape memory alloy films deposited as bilayers. The results show that the slope of the actuation on heating and on cooling can be modeled, a result that enlarges the spectra of application for shape memory alloy micro-actuators.

Hierarchical Integration of Thin-Film NiTi Actuators Using Additive Manufacturing for Microrobotics

Shape memory alloy (SMA) actuators can provide significant advantages for small-scale robotics given their robustness, energy density, and low voltage actuation. However, NiTi thin films typically found in SMA microactuators do not often provide useful forces and displacements for microrobotic applications. This work presents a fabrication process in which NiTi thin film actuators are integrated with two-photon polymerization (TPP) 3D printing to scale these actuators up for use in mesoscale systems. Individual unimorph actuators are characterized with respect to uniformity across many actuators so that actuators can be arrayed together for even larger forces or combined toward the operation of complex mechanisms. The resulting actuators are fast to prototype, reliable and stable (up to 5000 cycles), and can utilize complex geometries that are otherwise challenging to achieve with conventional MEMS microfabrication techniques. A 2D positioner is demonstrated by combining six individually controlled actuators with conventional mm-scale fabrication techniques (3D stereolithography printing, wire bonding and PCB assembly). The actuators are controlled by a commercial microcontroller and powered using a standard Lithium polymer battery. [2020-0208].

Wearable Microsystem for Minimally Invasive, Pseudo-Continuous Blood Glucose Monitoring: The e-Mosquito

This paper presents a wearable microsystem for minimally invasive, autonomous, and pseudo-continuous blood glucose monitoring, addressing a growing demand for replacing tedious fingerpricking tests for diabetic patients. Unlike prevalent solutions which estimate blood glucose levels from interstitial fluids or tears, our design extracts a whole blood sample from a small lanced skin wound using a novel shape memory alloy (SMA)-based microactuator and directly measures the blood glucose level from the sample. In vitro characterization determined that the SMA microactuator produced penetration force of 225 gf, penetration depth of 3.55 mm, and consumed approximately 5.56 mW·h for triggering. The microactuation mechanism was also evaluated by extracting blood samples from the wrist of four human volunteers. A total of 19 out of 23 actuations successfully reached capillary vessels below the wrists producing blood droplets on the surface of the skin. The integrated potentiostat-based glucose sensing circuit of our e-Mosquito device also showed a good linear correlation (R2 = 0.9733) with measurements using standard blood glucose monitoring technology. These proof-of-concept studies demonstrate the feasibility of the e-Mosquito microsystem for autonomous intermittent blood glucose monitoring.

A novel shape memory alloy microactuator for large in-plane strokes and forces

This paper describes a novel concept for in-plane actuators based on a thin free-standing shape memory alloy (SMA) film. The presented guidelines can be used to design a variety of actuators that can provide a combination of large strokes and forces for linear and rotary motions. A prototype actuator demonstrated a displacement of 45 μm related to 4.5% of the SMA film length, and a force of up to 115 mN related to a stress of 230 MPa in the SMA film without plastic deformations. These capabilities allow the actuator to work against the stiff springs that are essential for the devices’ ability to sustain vibrations, impacts, and accelerations.

Thermomechanical behavior of bulk NiTi shape-memory-alloy microactuators based on bimorph actuation

Shape-memory-alloy (SMA) has attracted considerable attention in recent years as a smart and efficient material, due to its unique properties. SMA microactuators became one of the potential solutions for unresolved issues in microelectromechanical systems (MEMS). This paper presents a thermomechanical behavior analysis of bimorph SMA structure and studies the effect of varying the SMA layer thickness, the type of stress layer and its thickness, and the processing temperature on the displacement of the microactuator. Furthermore, the analyzed results were verified by experimental work, where the fabrication of the SMA microactuators followed the standards of the MEMS fabrication process. SiO2, Si3N4 and Poly-Si were used as stress layers. The fabrication results showed that the bimorph SMA structure achieved maximum displacement when SiO2 was used. The SMA structure with dimensions of 10 mm (length) × 2 mm (width) × 80 µm (thickness), had maximum displacement of 804 µm when 4.1 µm of SiO2 layer was deposited at a temperature of 400 °C.

Thermo-mechanical characterization of optical thin films filters deposited onto shape memory alloy micro-actuators

Different optical thin films filters have been deposited by plasma techniques onto shape memory alloy micro-actuators controlled by the heating of laser irradiations. Thermomechanical cycles have been performed to investigate the stability of these optical filters. Excellent properties have been obtained with amorphous hydrogenated silicon/silica and amorphous hydrogenated silicon/plasma polymerized methacrylic acid-based filters; no degradation was observed after up to 50 000 cycles. Other silver/plasma polymerized methacrylic acid-based filters, which have a better selectivity and permit the use of more laser sources of different wavelengths for optical actuation, showed lower stability. The optical-filtering properties are lost during the thermomechanical cycles, due to a rearrangement of the silver atoms with heating. Thus, there is a trade-off between wavelength selectivity and the stability of the structures used for the optical control of the micro-actuators.

Mechanical properties identification and design optimization of nitinol shape memory alloy microactuators

Microactuators are essential elements of MEMS and are widely used in these devices. Microgrippers, micropositioners, microfixtures, micropumps and microvalves are well-known applications of microstructures. In this paper, the design optimization of shape memory alloy microactuators is discussed. Four different configurations of microactuator with variable geometrical parameters, generating different levels of displacement and force, are designed and analysed. In order to determine the optimum values of parameters for each microactuator, statistical design of experiments (DOE) is used. For this purpose, the Souza et al constitutive model (1988 Eur. J. Mech. A 17 789–806) is adapted for use in finite element analysis software. Mechanical properties of the SMA are identified by performing experimental tests on Ti-49.8%Ni. Finally, the specific energy of each microactuator is determined using the calibrated model and regression analysis. Moreover, the characteristic curve of each microactuator is obtained and with this virtual tool one can choose a microactuator with the desired force and displacement. The methodology discussed in this paper can be used as a reference to design appropriate microactuators for different MEMS applications producing various ranges of displacement and force.

Digital Fabrication of “Smart” Structures and Mechanisms - Creative Applications in Art and Design

This paper describes the design and fabrication of novel “soft” structures and mechanisms employing “smart” shapechanging materials. These structures and mechanisms incorporate shape memory alloy (SMA) micro-actuators, enabling them to exhibit lifelike movement when stimulated by the application of electric current. Fabricated by 3D printing in a soft elastomer material, their design includes internal channels into which the SMA actuators are easily mounted. Other design features allow flexibility of movement and facilitate cooling of the SMA actuators.A tentacle-like active structure is described, which incorporates an antagonistic pair of SMA micro-actuators, allowing it to exhibit two-way motion. Results are presented for the speed and range of motion of the tentacle-like structure.The paper goes on to describe a creative arts application for smart active structures and mechanisms which exploits the technologies under investigation: an interactive puppet which exhibits lifelike, expressive movement. This research in digital fabrication and smart materials has implications for the fields of interactive and robotic art and design, soft robotics and physical computing.

A bistable magnetically enhanced shape memory microactuator with high blocking forces

A novel approach of combining nonmagnetic shape memory alloy (SMA) microactuators with electroplated magnetic layers (ML) is presented, which enables the use of two intrinsic actuation principles in a single SMA-ML component allowing considerable gain in functionality. In the present work, we demonstrate local electroplating of Ni-81.75at.%Fe discs onto the surface of Ti-49at.%Ni microbridge structures. Antagonistic SMA-ML microactuators are fabricated by mechanically coupling two SMA-ML microbridges in their center by a spacer and prestraining them with respect to each other. The SMA-ML microactuators show bistable performance in the presence of an inhomogenous magnetic field generated by permanent magnets near the magnetic discs. Compared to antagonistic SMA microactuators fabricated without electroplated discs, a considerable improvement of mechanical performance is achieved. Magnetically enhanced SMA-ML microactuators of 7×7×4mm3 overall size show a blocking force and bistable stroke of about 33 mN and 84μm, respectively, while corresponding conventional SMA microactuators show no blocking force and a considerably smaller bistable stroke of 15μm.

Shape-memory alloys: effective 3D modelling, computational aspects and design of devices

The employment of shape-memory alloys (SMA) in a large number of applications in many fields of engineering, among which biomedical engineering, has been the motivation for an increasing interest in the direction of a correct and exhaustive modelling of their macroscopic behaviour in order to construct reliable simulation tools, which can be successfully used in the design procedures of SMA devices. In this paper, we review a robust three-dimensional model giving a good description of pseudo-elastic and shape-memory behaviours and we use it for the development of a new design procedure of SMA micro-actuators. Starting from a SMA micro-device proposed in the literature, we then adopt such a procedure to design a new effective variant of micro-gripper.

Transfer bonding technology for batch fabrication of SMA microactuators

Currently, the broad market introduction of shape memory alloy (SMA) microactuators and sensors is hampered by technological barriers, since batch fabrication methods common to electronics industry are not available. The present study intends to overcome these barriers by introducing a wafer scale transfer process that allows the selective transfer of heat-treated and micromachined shape memory alloy (SMA) film or foil microactuators to randomly selected receiving sites on a target substrate. The technology relies on a temporary adhesive bonding layer between SMA film/foil and an auxiliary substrate, which can be removed by laser ablation. The transfer technology was tested for microactuators of a cold-rolled NiTi foil of 20 μm thickness, which were heat-treated in free-standing condition, then micromachined on an auxiliary substrate of glass, and finally selectively transferred to different target substrates of a polymer. For demonstration, the new technology was used for batch-fabrication of SMA-actuated polymer microvalves.

Shape memory alloys for microsystems: A review from a material research perspective

One important challenge of microsystems design is the implementation of miniaturized actuation principles efficient at the micro-scale. Shape memory alloys (SMAs) have early on been considered as a potential solution to this problem as these materials offer attractive properties like a high-power to weight ratio, large deformation and the capability to be processed at the micro-scale. This paper reviews various attempts made to introduce SMAs in microsystems as well as design principles for SMA microactuators. It presents the status of current research and potential developments from a material research perspective.

QUASIPASSIVITY-BASED ROBUST NONLINEAR FLAP POSITIONING CONTROL USING SHAPE MEMORY ALLOY MICRO-ACTUATORS

This paper proposes a quasipassivity-based robust nonlinear control law ensuring position control of a rotary flap by means of an antagonist-type shape memory alloy microactuator. The control system employs variable-structure control to obtain robust performance, phase-lead compensation to quasipassivate the shape memory alloy dynamics and quasipassivity-based analysis to warrant robust ultimate boundedness of system trajectories. The feedback connection of the two paths leads to ultimate boundedness of tracking error trajectories of the plant despite uncertainties in the dynamic loads affecting the leading edge flap and in the friction found in the actuator. Since accurate numerical simulations and development of new concepts of microactuators based on shape memory alloys require a tractable, constitutive law accurately describing the relationship between force, displacement and temperature in the material, the paper also presents a hybrid micro-macro-mechanical shape memory alloy constitutive model. This model is based on a combination of structural modeling on a microscopic scale and transformation kinetics modeling on a macroscopic scale. The proposed control law and hybrid micro-macro-mechanical model are placed in closed-loop by means of numerical simulations that demonstrate the validity of the nonlinear control scheme.

Bending Model of an Integrated SMA Micro-Actuator

In this paper, we give a model of a Shape Memory Alloy (SMA) thin blade working in bending. This work is based on the general study and modeling of an integrated SMA micro-actuator called the “ module”, controled by thermoelectric effect. This module was developed in two versions. The first one is composed of a NiTi thin blade on which are placed two copper elements which play the role of heat sinks. Its temperature is controlled by Joule effect. In the second version, Bismuth Telluride ingots are used instead of copper elements. This second configuration allows the temperature control of the SMA blade by Peltier effect. The model of the bending behavior is based on a general macroscopic description of the thermomechanical behavior of SMA. The self-accommodating (pure thermal effect) and oriented (stress-induced) martensites are taken into account. The thermal boundary conditions used in this paper correspond to the first version of the micro-actuator. The numerical resolution by finite differences leads to the comprehension of phase transformation for pseudo-elastic and thermal cycling bending tests.

Finite element simulation of SMA microactuators with large deflection

For finite element simulation of shape memory actuators, a two-phase macromodel combined with a large deflection procedure is implemented in a FEM program for 3D mechanical analysis. The program is part of a tool for coupled electrical, thermal and mechanical problems. Details of the solution procedure are discussed. The model is used to investigate the performance of a shape memory alloy (SMA) microgripper. The microgripper is made of a monolithic SMA device of 2 x 5.8 x 0.1 mm 3 size comprising various integrated functional units like two microactuators for active closing and opening the gripping jaws and an optical slit. The simulated results are in quantitative agreement with experimentally determined data.

A novel actuation mechanism on the basis of ferromagnetic SMA thin films

A novel actuation mechanism is presented, which makes use of the antagonism of intrinsic magnetic and shape recovery forces acting on a ferromagnetic shape memory alloy (SMA) microactuator in a magnetic field. This mechanism is associated with large actuation and small biasing forces in each actuation direction resulting in a large stroke. Since no additional biasing elements are required, extremely compact designs are possible. As a demonstrator, an optical microscanner of 7 mm×3 mm×5 mm size is presented showing large scanning angles up to 120° in off-resonance mode. The scanning performance keeps frequency independent below a critical frequency, which is determined by heat transfer times.

High Transition Temperature Shape Memory Alloys for Micro-actuator Systems

ABSTRACTA new generation of thin film shape memory alloy (SMA) for MEMS micro-actuator has been developed, in which film structure and chemistry are optimized, for enhanced higher transition temperature, higher strain recovery rate as well as reduced actuation time by improving the heat transfer rates. Thin film TiPdNi was produced using Ion Beam Assisted Deposition (IBAD) technique both by in-situ heat treating during deposition and followed by post processing heat treatment. Films deposited on unheated substrates were found to be highly amorphous with minimal B2 austenite crystallization, while films deposited on heated substrates produced a highly crystallized twinned B19 martensitic structure through the bulk of the film. For films deposited on heated substrates, a 70 nm thick transition layer was found to exist between the bulk film and silicon substrate. Severe delamination and oxidation as a result of post heat treatment on IBAD deposited samples made in-situ heat treatment most suitable for processing thin film SMAs for MEMS applications. The desire to introduce this innovative technology to the field of SMA micro-actuators is based on two primary advantages of IBAD process over existing technology used to apply thin film SMAs. First, the chemical composition and grain size of the applied coating can be precisely controlled over a wide range of values. Second, the SMA can be deposited as thin films ≤ 2 μm thick with smaller grain size, much denser than films applied using sputter deposition technology. The effects of various processing parameters, and post processing heat treatment, on properties of the thin film SMA were studied.

Residual stress-loaded titanium–nickel shape-memory alloy thin-film micro-actuators

Residual stress in titanium–nickel thin films has been exploited as a force load for thin-film shape-memory alloy micro-actuators, thus eliminating the need for providing ‘external’ force load for device training and operation. Thermal cycling during device operation is accomplished using electrical current-induced Joule's heating for temperature ramp-up and thermal conduction/convection-induced cooling for temperature ramp-down. In response to thermal cycling, hysteresis loops in both the displacement and electrical resistance of the micro-actuator have been observed—thus demonstrating the existence of shape-memory effect in the micro-actuators. Elimination of manual training of individual devices makes it possible to operate large arrays of shape-memory alloy micro-actuators realizable using micro-fabrication techniques.

An integrated shape memory alloy micro-actuator controlled by thermoelectric effect

Abstract The integrated shape memory alloy (SMA) micro-actuator, which is presented here, has millimetre length size. The originality is the use of a thermoelectric phenomenon (i.e. the Peltier effect) to control the temperature of the SMA. A SMA thin blade, working in flexure, is connected with a thermoelectric material, bismuth telluride (N an P doped Bi 2 Te 3 ). To our knowledge, it is one of the first investigation with a real measurement of the motion of the SMA actuator using a built-in thermoelectric system. Using this system, we are able to control the temperature of the SMA blade in a range of −10 to 90 °C. Thermal simulations have shown that the time response, of a similar system heated by Joule effect, is about five times greater than the time response of the Peltier actuator. Moreover, this system is able to work in a confined environment. The actuator will be described and the experimental results will be given.