Most relevant to predicting the behavior of shape-memory-alloy (SMA)-based actuator-sensor applications activated by Joule heating are the electro-mechanical characteristics of the material under consideration. For a comprehensive characterization, a single setup that is able to provide all relevant data and information is desirable. This work covers the design, implementation and validation of such a high-end test bench for the holistic characterization of SMA micro-wires. In addition, the setup provides the possibility of application simulation experiments. Key elements of the design are the clamping mechanism guided on air bearings, a linear direct drive, a high-resolution load cell, a high-precision constant current source and a stress-controlled in-line wire sample installation. All measurements take place inside an isolated, temperature-controlled chamber. With the presented setup, the electro-mechanical and thermal characteristics of SMA wire samples with diameters from 20 µm to 100 µm can be determined. Via hardware-in-the-loop (HiL) implementation, the outputs with different biasing mechanisms and additional end-stops can be simulated even at high ambient temperatures. The generated results facilitate the prediction of the exact characteristics of SMA-driven actuator-sensor systems in a variety of applications and lead to a better general understanding of the alloy's properties. All functionalities and features of the setup are presented by discussing the results of exemplary experiments.

Motzki et al.: Artificial Muscles and Nerves in Industry 4.0 – Multifunctional Actuator-Sensor Systems with Shape Memory Alloys (SMAs) and Dielectric Elastomers (DEs). Industry 4.0 Science 1 2023, S. 8-15. Online: https://doi.org/10.30844/I4SE.23.114

This contribution provides an experimental analysis of relevant parameters to determine the air cooling potential of elastocaloric shape memory alloys (SMAs) in the form of wires. An essential step consists in developing a scientific test setup, which offers the ability to determine these parameters under varying conditions like wire geometry, material composition, and process parameters.With the help of this test setup the influence of different airflow rates on the heat transfer from SMA wire to air and the coherent heat transfer coefficients as well as the approximate air temperature ΔT values are determined and presented in this contribution. Furthermore, the latent heat of the SMA material is experimentally identified with a novel approach based on comparing dynamic temperature changes achieved by elastocaloric phase transformation with direct caloric heating. Analysing these parameters will greatly support the simulation, development and construction of elastocaloric air cooling devices to provide environmentally friendly alternatives to the prevalent vapour compression based cooling principles.

Thermal shape memory alloy (SMA) actuators are known for their superior energy density (force-volume ratio) compared to other actuation principles, allowing the construction of lightweight and compact systems. Furthermore, SMA actuators can be used as sensors, as their electrical resistance changes during activation. Using this multifunctionality, this work aims at presenting the development, fabrication, and validation of an SMA-driven robotic end-effector. The end-effector prototype is designed in a modular concept and consists of four independent arms with two degrees of freedom (DOF). Each arm can rotate in-plane and also tilt out-of-plane to allow gripping of various workpiece geometries. Both DOF actuator components consist of an SMA wire working against a tension spring. The tilting joint has an additional mechanism that creates two energy-free rest positions to improve energy efficiency. The end-effector is designed to carry a maximum load of 10 kg. In a test bench for the validation of the SMA-driven end-effector joints, hall sensors are used to measure the gripping arm displacement. In addition, the resistance of the SMA wires is monitored during activation. The dynamic system performance is analyzed using different activation current levels. Finally, a proportional integral (PI) control with Hall sensor feedback is implemented to position the first DOF at arbitrary angles within its 90 degrees rotation radius.

Open-cell metal foams are biomimetic open-porous materials mimicking the construction elements of bones. Based on their special porous microstructure, they are used as lightweight construction elements and for crash energy absorbers. Ni/Al hybrid foams are aluminium (Al) foams electrochemically coated with nanocrystalline nickel (Ni) leading to enhanced strength and energy absorption capacity. A robust understanding and knowledge of the deformation behaviour under different strain rates are essential to design crash absorbers made of foams. The present contribution is focused on the investigation of strain rate effects and is furthermore a pioneering work dealing with a full thermomechanical characterisation of the deformation behaviour in Al foams and Ni/Al hybrid foams by a combination of digital image correlation for measuring local strain fields and infrared thermography for measuring local temperature fields during the deformation process.

Thermal shape memory alloy (SMA) wires exhibit a mechanical hysteresis of which the shape depends on both temperature and loading rate. Commercially available actuator wires typically exhibit polycrystalline behavior, which also depends on training effects. Polycrystallinity may lead to complex hysteresis loops, differing substantially from standard box shapes often employed in modeling attempts. In addition, actuation often results in loading trajectories leading through the interior of the hysteresis, making accurate modeling and control of SMA systems a highly challenging task. In this paper, we present a novel dynamic model for polycrystalline SMA actuator wires based on a modified version of the Miiller-Achenbach-Seelecke model. The model permits to predict time evolution of stress and resistance of a one-dimensional SMA wire under arbitrary input strain and Joule heating profiles. The constitutive equations are developed by properly exploiting the concept of a representative single-crystal, resulting in an optimal trade-off between physical interpretation and computational efficiency. After developing constitutive model equations, experimental validation is performed by means of two case studies, given by a superelastic NiTi wire and a quasi-plastic NiTi wire, respectively. The experiments are intended to illustrate the model capabilities in predicting internal hysteresis loops, loading rate effects, as well as actuation and sensing behavior at the same time. A remarkable accuracy is observed in all of the investigated experimental scenarios, making the model particularly suitable for high-precision control and self-sensing applications.

The following contribution provides analyses of the air cooling potential of elastocaloric shape memory alloys (SMAs) in form of NiTiCo wires. An essential step consists in developing a scientific test setup, which offers the ability to determine the air-cooling potential under varying conditions like wire geometry, material composition, and process parameters. With the help of this test setup the influence of different airflow rates on the heat transfer from SMA wire to air and the coherent heat transfer coefficients as well as the expectable air temperature Delta T values are determined and presented in this contribution. Analysing these parameters will greatly support the simulation, development and construction of elastocaloric air cooling devices to provide environmentally friendly alternatives to the prevalent vapour compression based cooling principles.

Elastocaloric cooling based on NiTi exhibits an excellent cooling capability, due to the high specific latent heats activated by mechanical loading/unloading and the small required work input. The current funding period of the DFG Priority Program SPP 1599 focuses on the development and realization of a continuously operating elastocaloric cooling device. A novel mechanical loading concept and a novel bundle concept of thin SMA wires enable a thermodynamically optimized cooling process by utilizing a minimal amount of SMA material in a compact design space and high cooling capability. The versatile realization of the device allows the independent variation of process parameters, thermal boundary conditions, and material parameters. This work presents the design process from the development of the novel loading and heat exchange concept, the fluid management and the wire arrangement up to the fabrication and validation of an efficient continuously operating cooling device based on SMA.

Innerhalb der Konzepte von Industrie 4.0 steht der Begriff Smart Factory für die Schaffung von effektiven Produktionsumgebungen durch Digitalisierung und Cyber-Physische Systeme. Fertigungsanlagen sollen stärker automatisiert, flexibel und adaptiv werden. Im Zuge dieser Bestrebungen geraten auch intelligente Materialien immer mehr in den Fokus der Industrie. Kombinierte aktorische und sensorische Eigenschaften ermöglichen den Aufbau leichter und kompakter multifunktionaler Aktor-Sensor-Systeme, welche zudem energieeffizient, geräusch- und emissionslos betrieben werden. Dadurch bieten sie sich insbesondere für den Aufbau vernetzter Systeme an. Speziell Formgedächtnislegierungen (FGL) und dielektrische Elastomere (DE) eignen sich für den Aufbau intelligenter Aktoren und werden anhand von einigen Anwendungsbeispielen in diesem Beitrag vorgestellt.

This paper presents the design and the realization of an innovative SMA actuated bistable vacuum suction cup. The sealed, compact and fully integrated design enables the positioning and transport of inherent stable components in mobile and stationary applications. The bistable actuator mechanism based on SMA wires combined with a bistable spring represent an energy-efficient, noiseless gripping system without the need for compressed air. Additionally, the self-sensing effect of the SMA enables a sensorless condition-monitoring and energy-efficient control. The mechanics consists of antagonistic SMA wires, which are laterally arranged and connected to the bistable spring via levers. The membrane is directly connected to the bistable spring. The actuation of the wires leads to a rotational movement of the levers thus changes the state of the bistable spring, which directly deforms the membrane. When the membrane is sealed connected to the workpiece, the deformation of the membrane generates a vacuum. The integrated microcontroller electronics manages the joule heating of the wires by measuring the transmitted electrical energy. By applying an electrical energy to the pre-strained SMA wire, the wire heats up and contracts due to the phase transformation from martensite to austenite. The contraction of the wire is accompanied by a significant change in electrical resistance, which enables a resistance based strain feedback. The integrated electronics is able to correlate this resistance change to the actual state of the bistable spring, which leads to a position feedback of the membrane. This allows an adequate electrical energy deposition in the SMA wire by turning-off the heating directly after the position toggle of the membrane. Thereby, a successful position toggle is ensured independent from the ambient temperature and the real supply voltage. The new position of the membrane is then held by the bistable spring without the use of additional energy. This concept leads to a reliable gripping system with fast actuation times.

Elastocaloric cooling is a novel environment-friendly alternative to vapor compression-based cooling systems. This solid-state cooling technology uses NiTi shape memory alloys (SMAs) as cooling medium. SMAs are well known for lightweight actuator systems and biomedical applications, but in addition, these alloys exhibit excellent cooling properties. Due to the high latent heats activated by mechanical loading/unloading, large temperature changes can be generated in the material. Accompanied by a small required work input, this also leads to a high coefficient of performance superior to vapor compression-based systems. In order to access the potential of these alloys, the development of suitable thermodynamic cooling cycles and an efficient system design are required. This paper presents a model-based design process of an elastocaloric air-cooling device. The device is divided into two parts, a mechanical system for continuously loading and unloading of multiple SMA wire bundles by a rotary motor and a heat transfer system. The heat transfer system enables an efficient heat exchange between the heat source and the SMA wires as well as between the SMA wires and the environment. The device operates without any additional heat transfer medium and cools the heat source directly, which is an advantage in comparison to conventional cooling systems. The design of this complex device in an efficient manner requires a model approach, capable of predicting the system parameters cooling power, mechanical work and coefficient of performance under various operating conditions. The developed model consists of a computationally efficient, thermo-mechanically coupled and energy based SMA model, a model of the system kinematics and a heat transfer model. With this approach, the complete cooling system can be simulated, and the required number of SMA wires as well as the mechanical power can be predicted in order to meet the system requirements. Based on the simulation results a first prototype of the elastocaloric cooling system is realized.

When standard voltage levels commonly adopted in industry are used to activate thermal shape memory alloy (SMA) wire actuators, they often result in very high electrical currents which may eventually damage or destroy the actuators. To improve performance of SMA wire actuators operating in industrial environments, in this paper we investigate a novel, fast and energyefficient actuation strategy based on short pulses in the millisecond range. The use of higher voltages leads to a highly dynamic activation process, in contrast to commonly used quasi-static activation based on low-voltage. A test setup is designed to examine the effects of the control parameters (i.e., supply voltage, activation pulse duration, SMA wire pre-tension and wire diameter) on the measured displacement and force output of the SMA wire. It is shown that actuation times in the range of 20 ms and strokes of more than 10% of the SMA wire length can be reached. Additionally, energy savings of up to 80% with respect to conventional quasi-static actuation are achieved. Possible applications for this activation method are release mechanisms, switches or safety applications.

Solid-state cooling is an environmentally friendly, no global warming potential alternative to vapor compression-based systems. Elastocaloric cooling based on NiTi shape memory alloys exhibits excellent cooling capabilities. Due to the high specific latent heats activated by mechanical loading/unloading, large temperature changes can be generated in the material. The small required work input enables a high coefficient of performance. An overview of elastocaloric cooling from basic principles, such as elastocaloric cooling cycles, material characterization, modeling, and optimization, to the design of elastocaloric cooling devices is presented. Current work performed within the DFG (Deutsche Forschungsgemeinschaft) Priority Program SPP1599 Ferroic Cooling, which is focused on the development and realization of a continuously operating elastocaloric cooling device, is highlighted. The cooling device operates in a rotatory mode with wires under tensile loading. The design allows maximization of cooling power by suitable wire diameter scaling as well as efficiency optimization by implementing a novel drive concept. Finally, computer-aided design (CAD) models of the discussed solid-state air cooling device are presented.

In this paper we present a control-oriented model for an actuator system driven by a shape memory alloy (SMA) wire. SMAs appear as suitable for the realization of compact actuator solutions, due to the inherently high energy density of the material. However, the strongly hysteretic response of SMAs significantly complicates modeling and control of devices based on such technology. The main goal of this paper is the development of a control-oriented model of a SMA wire valve actuator. The overall system model highly relies on an accurate model of the SMA material itself, which permits to describe the temperature-dependent hysteresis in a physics-based fashion. In addition, the model explicitly accounts for the polycrystalline nature of the material, and permits to reproduce the SMA hysteresis with high accuracy and in a computationally efficient way. Development of model equations is presented first. Subsequently, experimental identification and validation are performed on a 76 micron SMA wire. Model ability of predicting both stress-strain and resistance-strain curves, measured at different input powers, is shown. Finally, preliminary results on simulation of the overall actuator are presented.

The exploitation of the elastocaloric effect in superelastic shape memory alloys (SMA) for cooling applications shows a promising energy efficiency potential but requires a better understanding of the non-homogeneous martensitic phase transformation. Temperature profiles on sputter-deposited superelastic \({\mathrm {Ti_{55.2}Ni_{29.3}Cu_{12.7}Co_{2.8}}}\) shape memory alloy thin films show localized release and absorption of heat during phase transformation induced by tensile deformation with a strong rate dependence. In this paper, a model for the simulation of the thermo-mechanically coupled transformation behavior of superelastic SMA is proposed and its capability to reproduce the mechanical and thermal responses observed during experiments is shown. The procedure for experiment and simulation is designed such that a significant temperature change from the initial temperature is obtained to allow potential cooling applications. The simulation of non-local effects is enabled by the use of a model based on the one-dimensional Müller–Achenbach–Seelecke model, extended by 3D mechanisms such as lateral contraction and by non-local interaction, leading to localization effects. It is implemented into the finite element software COMSOL Multiphysics, and comparisons of numerical and experimental results show that the model is capable of reproducing the localized transformation behavior with the same strain rate dependency. Additionally to the thermal and the mechanical behavior, the quantitative prediction of cooling performance with the presented model is shown.

The following contribution presents a new concept of an air conditioning device based on the elastocaloric cooling effect of shape memory alloys (SMA’s). This technology provides an energy efficient and environment friendly alternative to conventional vapor compression based cooling principles. Starting from the thermodynamic investigation of the elastocaloric cooling process, a continuous operating elastocaloric air cooling device is developed. The device enables an optimized thermodynamic process control under various operating conditions as well as large temperature spans. This work presents the design process of such a system starting from SMA based heat engines to a thermodynamically optimized design of an elastocaloric air conditioning device.

One dimensional (1D) nanostructures offer a promising path towards highly efficient heating and temperature control in integrated microsystems. The so called self-heating effect can be used to modulate the response of solid state gas sensor devices. In this work, efficient self-heating was found to occur at random networks of nanostructured systems with similar power requirements to highly ordered systems (e.g. individual nanowires, where their thermal efficiency was attributed to the small dimensions of the objects). Infrared thermography and Raman spectroscopy were used to map the temperature profiles of films based on random arrangements of carbon nanofibers during self-heating. Both the techniques demonstrate consistently that heating concentrates in small regions, the here-called "hot-spots". On correlating dynamic temperature mapping with electrical measurements, we also observed that these minute hot-spots rule the resistance values observed macroscopically. A physical model of a random network of 1D resistors helped us to explain this observation. The model shows that, for a given random arrangement of 1D nanowires, current spreading through the network ends up defining a set of spots that dominate both the electrical resistance and power dissipation. Such highly localized heating explains the high power savings observed in larger nanostructured systems. This understanding opens a path to design highly efficient self-heating systems, based on random or pseudo-random distributions of 1D nanostructures.

Shape Memory Alloys (SMA) using elastocaloric cooling processes have the potential to be an environmentally friendly alternative to the conventional vapor compression based cooling process. Nickel-Titanium (Ni-Ti) based alloy systems, especially, show large elastocaloric effects. Furthermore, exhibit large latent heats which is a necessary material property for the development of an efficient solid-state based cooling process. A scientific test rig has been designed to investigate these processes and the elastocaloric effects in SMAs. The realized test rig enables independent control of an SMA's mechanical loading and unloading cycles, as well as conductive heat transfer between SMA cooling elements and a heat source/sink. The test rig is equipped with a comprehensive monitoring system capable of synchronized measurements of mechanical and thermal parameters. In addition to determining the process-dependent mechanical work, the system also enables measurement of thermal caloric aspects of the elastocaloric cooling effect through use of a high-performance infrared camera. This combination is of particular interest, because it allows illustrations of localization and rate effects - both important for efficient heat transfer from the medium to be cooled. The work presented describes an experimental method to identify elastocaloric material properties in different materials and sample geometries. Furthermore, the test rig is used to investigate different cooling process variations. The introduced analysis methods enable a differentiated consideration of material, process and related boundary condition influences on the process efficiency. The comparison of the experimental data with the simulation results (of a thermomechanically coupled finite element model) allows for better understanding of the underlying physics of the elastocaloric effect. In addition, the experimental results, as well as the findings based on the simulation results, are used to improve the material properties.

Due to their large latent heats, pseudoelastic Ni–Ti-based shape memory alloys (SMAs) are attractive candidate materials for ferroic cooling, where elementary solid-state processes like martensitic transformations yield the required heat effects. The present work aims for a chemical and microstructural optimization of Ni–Ti for ferroic cooling. A large number of Ni–Ti-based alloy compositions were evaluated in terms of phase transformation temperatures, latent heats, mechanical hysteresis widths and functional stability. The aim was to identify material states with superior properties for ferroic cooling. Different material states were prepared by arc melting, various heat treatments and thermo-mechanical processing. The cooling performance of selected materials was assessed by differential scanning calorimetry, uniaxial tensile loading/unloading, and by using a specially designed ferroic cooling demonstrator setup. A Ni[Formula: see text]Ti[Formula: see text]Cu5V[Formula: see text] SMA was identified as a potential candidate material for ferroic cooling. This material combines extremely stable pseudoelasticity at room temperature and a very low hysteresis width. The ferroic cooling efficiency of this material is four times higher than in the case of binary Ni–Ti.

Solid state refrigeration processes, such as magnetocaloric and electrocaloric refrigeration, have recently shown to be a promising alternative to conventional compression refrigeration. A new solid state elastocaloric refrigeration process using the latent heats within Shape Memory Alloys (SMA) could also hold potential in this field. This work investigates the elastocaloric effects in Ni-Ti- based superelastic Shape Memory Alloy (SMA) systems for use in an elastocaloric cooling processes. Ni-Ti alloys exhibits large latent heats and a small mechanical hysteresis, which may potentially lead to the development of an efficient environmentally friendly solid-state cooling system, without the need for ozone-depleting refrigerants. A systematic investigation of the SMA is conducted using a novel custom-built scientific testing platform specifically designed to measure cooling process related phenomena. This testing system is capable of performing tensile tests at high rates as well as measuring and controlling the solid-state heat transfer between SMA and heat source/heat sink.Tests are conducted following a cooling process related training cycle where the material has achieved stabilized behavior. First, a characterization of the elastocaloric material properties is performed followed by an investigation of the material under cooling process conditions.A comprehensive monitoring of the mechanical and thermal parameters enables the observation of temperature changes during mechanical cycling of the SMA at high strain rates. These observations can be used to study the rate dependent efficiency of the elastocaloric material.The measurement of the temperature of both the heat source/heat sink and the SMA itself, as well as the required mechanical work during a running cooling process, reveals the influence of the operating conditions on the elastocaloric effect of the material.Furthermore investigations of the process efficiency at different thermal boundary conditions (temperature of heat source/heat sink), indicates that the process is dependent on the boundary conditions which have to be controlled in order to optimize the efficiency.