A Methodology for the Development, Production, and Validation of R-Phase Actuators

The R-phase transformation has interesting features with potential for applications that need a small temperature hysteresis and good dynamic behavior, such as thermostatic valves. The aim of this article is to show the development, production, and validation process of different R-phase shape memory alloy (SMA) actuators, starting with a semi-finished wire and concluding with a finalized R-phase spring actuator. This study focuses mainly on the calculation, the thermomechanical treatment, and experimental validation of the designed actuators. The first section of this article presents a mathematical dimensioning tool for different R-phase actuators, especially for extension SMA springs. The second part shows specific parameters on the R-phase transformation during thermomechanical treatment. The parameters Ni-content and annealing temperature are being varied to achieve different transformation behavior of the R-phase. The third section relates to the production process of calculated SMA spring actuators based on the R-phase transformation. In the fourth and last section of the article, the performance of selected actuators will be characterized in functional tests, and the results will be compared with the calculated results of the mathematical model.     

NiTi形状记忆合金的热电行为

研究了形状记忆合金线的热电行为。当合金线被加热到温度高于其相变温度时,由于相的转变而产生一个大的机械力。将 SMA 线用作驱动器,并研究了不同的参数及它们之间的关系。在不同的压力水平下,这些变化的参数分别是弹性应变(位移)、温度磁滞和电阻。通过传递热模型得到安全的加热电流并预测了相变温度。在自然空气对流的条件下,采用680 mA的电流加热、冷却合金线796 s。在43 MPa的应力水平下,应变恢复率为4.33%,相应的电阻变化为11.2%。在加热、冷却循环过程中,电阻变化与位移和电流分别呈线性关系。该研究有助于精确控制有、无外部传感器反馈的SMA线驱动器。     

Strategies for Self-Repairing Shape Memory Alloy Actuators

Shape memory alloys (SMAs) are thermally activated smart materials. Due to their ability to change into a previously imprinted actual shape by the means of thermal activation, they are suitable as actuators for microsystems and, within certain limitations, macroscopic systems. A commonly used shape memory actuator type is an alloy of nickel and titanium (NiTi), which starts to transform its inner phase from martensitic to austenitic structure at a certain austenite start temperature. Retransformation starts at martensitic start temperature after running a hysteresis cycle. Most SMA-systems use straight wire actuators because of their simple integration, the occurring cost reduction and the resulting miniaturization. Unfortunately, SMA-actuators are only seldom used by constructors and system developers. This is due to occurring functional fatigue effects which depend on boundary conditions like system loads, strains, and number of cycles. The actuating stroke does not reduce essentially during the first thousand cycles. Striking is the elongation of the wire while maintaining the stroke during cycling (walking). In order to create a system which adjusts and repairs itself, different concepts to solve this problem are presented. They vary from smart control methods to constructive solutions with calibration systems. The systems are analyzed due to their effective, life cycle, and system costs showing outstanding advantages in comparison to commonly used SMA actuators.     

Functional Characterization of a Novel Shape Memory Alloy

A novel shape memory alloy (SMA) has been developed as an alternative to currently available alloys. This alloy, commercially known by its proprietary brand SMARQ, shows a higher working range of temperatures with respect to the SMA materials used until now in actuators, limited to environment temperatures below 90 °C. SMARQ is a high temperature SMA (HTSMA) based on a fully European material technology and production processes, which allows the manufacture of high quality products, with tuneable transformation temperatures up to 200 °C. Both, material and production processes have been evaluated for its use in space applications. A full characterization test campaign has been completed in order to obtain the material properties and check its suitability to be used as active material in space actuators. In order to perform the functional characterization of the material, it has been considered as the key element of a basic SMA actuator, consisting in the SMA wire and the mechanical and electrical interfaces. The functional tests presented in this work have been focused on the actuator behavior when heated by means of an electrical current. Alloy composition has been adjusted in order to match a transition temperature (As) of +145 °C, which satisfies the application requirements of operating temperatures in the range of ?70 and +125 °C. Details of the tests and results of the characterization test campaign, focused in the material unique properties for their use in actuators, will be presented in this work. Some application examples in the field of space mechanisms and actuators, currently under development, will be summarized as part of this work, demonstrating the technology suitability as active material for space actuators.     

Analysis and Evaluation of the Dynamic Performance of SMA Actuators for Prosthetic Hand Design

It is widely acknowledged within the biomedical engineering community that shape memory alloys (SMAs) exhibit great potential for application in the actuation of upper limb prosthesis designs. These lightweight actuators are particularly suitable for prosthetic hand solutions. A four-fingered, 12 degree-of-freedom prosthetic hand has been developed featuring SMA bundle actuators embedded within the palmar structure. Joule heating of the SMA bundle actuators generates sufficient torque at the fingers to allow a wide range of everyday tasks to be carried out. Transient characterization of SMA bundles has shown that performance/response during heating and cooling differs substantially. Natural convection is insufficient to provide for adequate cooling during elongation of the actuators. An experimental test-bed has been developed to facilitate analysis of the heat transfer characteristics of the appropriately sized SMA bundle actuators for use within the prosthetic hand design. Various modes of heat sinking are evaluated so that the most effective wire-cooling solution can be ascertained. SMA bundles of varying size will be used so that a generalized model of the SMA displacement performance under natural and forced cooling conditions can be obtained. The optimum cooling solution will be implemented onto the mechanical hand framework in future work. These results, coupled with phenomenological models of SMA behavior, will be used in the development of an effective control strategy for this application in future work.     

Design of a Telescopic Linear Actuator Based on Hollow Shape Memory Springs

Shape memory alloys (SMAs) are smart materials exploited in many applications to build actuators with high power to mass ratio. Typical SMA drawbacks are: wires show poor stroke and excessive length, helical springs have limited mechanical bandwidth and high power consumption. This study is focused on the design of a large-scale linear SMA actuator conceived to maximize the stroke while limiting the overall size and the electric consumption. This result is achieved by adopting for the actuator a telescopic multi-stage architecture and using SMA helical springs with hollow cross section to power the stages. The hollow geometry leads to reduced axial size and mass of the actuator and to enhanced working frequency while the telescopic design confers to the actuator an indexable motion, with a number of different displacements being achieved through simple on-off control strategies. An analytical thermo-electro-mechanical model is developed to optimize the device. Output stroke and force are maximized while total size and power consumption are simultaneously minimized. Finally, the optimized actuator, showing good performance from all these points of view, is designed in detail.     

Continuous Rotary Motor Actuated by Multiple Segments of Shape Memory Alloy Wires

An attempt has been made in this study to develop a continuous rotary motor actuated by multiple segments of shape memory alloy wires (SMA motor). In order to meet the requirements of continuous rotation, large output torque, and stall torque, a unidirectional rotary actuator was designed, and five rotary actuators were fixed on one shaft in series. When every actuator is actuated in turn, the SMA motor can realize a continuous rotation. In order to simulate the rotation angle, output torque, and the actuation performance degradation of the SMA motor, Lagoudas’s constitutive model was modified to consider the cyclic actuation influence. After design and simulation, a prototype of the SMA motor was fabricated and experimentally tested to verify the feasibility and performance of the proposed design decision. The test results indicate that the SMA motor can realize continuous rotation. The rotation speed is 0.28 r/min, and the torque is 1008 N mm.     

Torsional Properties of TiNi Shape Memory Alloy Tape for Rotary Actuator

In order to develop novel shape memory actuators, the torsional deformation of a shape memory alloy (SMA) tape and the actuator models driven by the tape were investigated. The results obtained can be summarized as follows. In the SMA tape subjected to torsion, the martensitic transformation appears along both edges of the tape due to elongation of these elements and grows to the central part. The fatigue life in both the pulsating torsion and alternating torsion is expressed by the unified relationship of the dissipated work in each cycle. Based on an opening and closing door model and a solar-powered active blind model, the two-way rotary driving actuator with a small and simple mechanism can be developed by using torsion of the SMA tape.     

形状记忆合金微型阀的研制

本文阐述了形状合金的特性,形状记忆合金作为驱动元件,具有结构紧凑、功率重量比大、易控制等优点,大大推动了微型机器人的发展。本课题利用形状记忆合金作为驱动元件,开发了微型气动阀,实现了与微机器人主体的集成。     

Application of shape memory alloy wire actuator for precision position control of a composite beam

In this paper are presented the design and experimental results of using a shape memory alloy (SMA) wire as an actuator for position control of a composite beam. The composite beam is honeycomb structured, having wires of SMA embedded in one of its face sheets for the purposes of active actuation. Nickel-titanium SMA wires were chosen as actuating elements due to their high recovery stress (>700 MPa) and tolerance to high strain (up to 8%). A simple proportional and derivative controller plus a feed-forward current is designed and implemented for controlling the tip position of the composite beam. Experiments have demonstrated the effectiveness of the SMA wire as an actuator for active position control of a composite beam.     

NiTi Alloy Negator Springs for Long-Stroke Constant-Force Shape Memory Actuators: Modeling,Simulation and Testing

This work aims at the experimental characterization and modeling validation of shape memory alloy (SMA) Negator springs. According to the classic engineering books on springs, a Negator spring is a spiral spring made of strip of metal wound on the flat with an inherent curvature such that, in repose, each coil wraps tightly on its inner neighbor. The main feature of a Negator springs is the nearly constant force displacement behavior in the unwinding of the strip. Moreover the stroke is very long, theoretically infinite, as it depends only on the length of the initial strip. A Negator spring made in SMA is built and experimentally tested to demonstrate the feasibility of this actuator. The shape memory Negator spring behavior can be modeled with an analytical procedure, which is in good agreement with the experimental test and can be used for design purposes. In both cases, the material is modeled as elastic in austenitic range, while an exponential continuum law is used to describe the martensitic behavior. The experimental results confirms the applicability of this kind of geometry to the shape memory alloy actuators, and the analytical model is confirmed to be a powerful design tool to dimension and predict the spring behavior both in martensitic and austenitic range.     

Design and control strategies for SMA actuators in a feed axis for precision machine tools

In a current project, the Chair of Production Systems (LPS) is examining the use of standardized shape memory alloy (SMA) actuators in the feed axis of a small machine tool. These machines are used to manufacture small work pieces with high precision. Therefore, the actuators must be qualified to handle movements in the micrometer range. To achieve this goal, different design elements for the actuators are considered, as well as different measures to control the actuator. In the scope of this paper, binary actuators with just two possible positions are examined next to actuators that can reach every possible intermediate position. For the latter, different control strategies are proposed. The displacement of the actuators can either be measured by external sensors or by using the change in resistance in the SMAs during transformation as feedback.     

Effects of Loading and Constraining Conditions on the Thermomechanical Fatigue Life of NiTi Shape Memory Wires

The availability of engineering strength data on shape memory alloys (SMAs) under cyclic thermal activation (thermomechanical fatigue) is central to the rational design of smart actuators based on these materials. Test results on SMAs under thermomechanical fatigue are scarce in the technical literature, and even the few data that are available are mainly limited to constant-stress loading. Since the SMA elements used within actuators are normally biased by elastic springs or by antagonist SMA elements, their stress states are far from being constant in operation. The mismatch between actual working conditions and laboratory settings leads to suboptimal designs and undermines the prediction of the actuator lifetime. This paper aims at bridging the gap between experiment and reality by completing an experimental campaign involving four fatigue test conditions, which cover most of the typical situations occurring in practice: constant stress, constant-strain, constant stress with limited maximum strain, and linear stress-strain variation with limited maximum strain. The results from the first three test settings, recovered from the previously published works, are critically reviewed and compared with the outcome of the newly performed tests under the fourth arrangement (linear stress-strain variation). General design recommendations emerging from the experimental data are put forward for engineering use.     

Conceptual Design and Simulation of a Compact Shape Memory Actuator for Rotary Motion

This work describes the conceptual design, the modelling, the optimization, the detail design and the virtual testing of a shape memory actuator purposely conceived to maximize torque and angular stroke while limiting overall size and electric consumption. The chosen design, achieved by means of a Quality Function Deployment approach, features a fully modular concept in which an arbitrary number of identical modules are assembled to produce the desired angular stroke and output torque. The basic module contains shape memory springs that actuate the device and also a conventional spring that reduces the torque ripple. Following the concept generation stage, a thermo-electromechanical model is developed and a numerical optimization performed, aimed at minimizing the electrical consumption of the actuator. Finally, the device is designed in detail and the actuator is tested virtually. Thanks to the proposed modular construction and the use of a conventional balancing spring, the device shows better performances than known rotary shape memory actuators in terms of rotation, torque and customization.     

Actuators and Drivers Based on CuAlNi Shape Memory Single Crystals

Our research reported here is concerned with the development of physical aspects of shape memory linear actuators and drivers made from CuAlNi single crystals. Our study is focused on reactive stress generation and reversible deformation in the clamped CuAlNi single crystals due to martensitic transformations during thermal cycling. The crystals can reproduce force generation in heating to 560 K for many times, and one time in heating to 700 K with the maximal stress 350 MPa. The theoretical model and calculations of linear actuators that use shape memory single crystals is discussed and we also demonstrate real model of an actuator based on these crystals.     

SHADE: A Shape-Memory-Activated Device Promoting Ankle Dorsiflexion

Acute post-stroke rehabilitation protocols include passive mobilization as a means to prevent contractures. A device (SHADE) that provides repetitive passive motion to a flaccid ankle by using shape memory alloy actuators could be of great help in providing this treatment. A suitable actuator was designed as a cartridge of approximately 150 × 20 × 15 mm, containing 2.5 m of 0.25 mm diameter NiTi wire. This actuator was activated by Joule’s effect employing a 7 s current input at 0.7 A, which provided 10 N through 76 mm displacement. Cooling and reset by natural convection took 30 s. A prototype of SHADE was assembled with two thermoplastic shells hinged together at the ankle and strapped on the shin and foot. Two actuators were fixed on the upper shell while an inextensible thread connected each NiTi wire to the foot shell. The passive ankle motion (passive range of motion, PROM) generated by SHADE was evaluated optoelectronically on three flaccid patients (58 ± 5 years old); acceptability was assessed by a questionnaire presented to further three flaccid patients (44 ± 11.5 years old) who used SHADE for 5 days, 30 min a day. SHADE was well accepted by all patients, produced good PROM, and caused no pain. The results prove that suitable limb mobilization can be produced by SMA actuators.     

Effect of Current Pulses on Fatigue of Thin NiTi Wires for Shape Memory Actuators

Shape memory alloys (SMA) are used in many technological applications, thanks to their unique properties: superelasticity and shape memory (SM) effect. Many efforts have been made to improve performance of SMA wires to utilize them as thermal actuators also for many thousands of cycles. Near-equiatomic nickel-titanium compound is the most used materials for SM actuators because of its high recoverable values of strain and good cycling stability, if compared to the other known SMA. In this study, the functional properties of NiTi thin wires (80 μm) thermally cycled under a constant load (200 MPa) were investigated. In particular, the effect of two heating conditions, carried out by a step and a ramp current pulses, on functional fatigue of SMA has been studied. By means of an experimental apparatus, thermomechanical cycling, thermal loop under constant load and actuation time (AT) tests were carried out to investigate the alteration and the trend of recovered strain, irreversible strain, characteristic temperatures, and ATs of the thin SM wires.     

Design of an electro-hydraulic CNC press brake

Design principles of computer controlled hydraulic press brake system is presented. A press brake is retrofitted with a pair of electrohydraulic actuators. The electrohydraulic system consists of a variable displacement pump, a pressure accumulator, two servo valves and two asymmetrical piston-cylinder type actuators holding the press ram. Basic modelling and identification of the electrohydraulic drive system dynamics is given for each actuator. The design and implementation of a digital pole placement control algorithm for a CNC system is presented. The position accuracy of the press is maintained within the resolution of the encoder unit. From a series of sheet metal bending tests, the compliances of each actuator and their influence on the bending precision are predicted.     

Analysis of Wire Position and Operating Conditions on Functioning of NiTi Wires for Shape Memory Actuators

In this work an experimental-numerical approach was used to analyze the thermo-mechanical behavior of thin NiTi wires, electrically heated, finalized to defining the influence of both wire position and the operating conditions of the actuator functioning. Tests were carried out on wires having diameters of 80 and 150 ??m, loaded by constant stresses of 100 and 200 MPa and characterized by DSC and strain/temperature hysteresis measurements. Two wire positions (horizontal and vertical) were adopted in single cycle tests and designed to obtain different typologies of the heating and cooling transients. In general, the heating time was selected to reach a steady state condition while the cooling time always allowed decreasing the wire temperature to the ambient one. Data concerning strain, applied current and voltage were simultaneously acquired during the tests. Moreover, for the optimization and validation of a numerical model, for the 150 ??m wire in diameter was used, its temperature was recorded by IR thermographic system. On the basis of the collected experimental data, a simple model was tested to reproduce the experimental results and data regarding the heat exchange coefficient and wire electrical resistivity dependence on temperature were obtained. The influence of the experimental wire positioning and wire diameter on the free convection coefficient is reported and the results indicate that the heating transient is associated with different convection coefficients depending on the heating modalities.