Sven Langbein

Smart Control Systems for Smart Materials

Shape memory alloys (SMAs) are thermally activated smart materials. Due to their ability to change into a previously imprinted shape by the means of thermal activation, they are suitable as actuators for microsystems and, within certain limitations for macroscopic systems. Most commonly used SMAs for actuators are binary nickel-titanium alloys (NiTi). The shape memory effect relies on the martensitic phase transformation. On heating the material from the low temperature phase (martensite) the material starts to transform into the high temperature phase (austenite) at the austenite start temperature (As). The reverse transformation starts at the martensite start temperature after passing a hysteresis cycle. To apply these materials to a wide range of industrial applications, a simple method for controlling the actuator effect is required. Today’s control concepts for shape memory actuators, in applications as well as in test stands, are time-based. This often leads to overheating after transformation into the high temperature phase which results in early fatigue. Besides, the dynamic behavior of such systems is influenced by unnecessary heating, resulting in a poor time performance. To minimize these effects, a controller system with resistance feedback is required to hold the energy input on specific keypoints. These two key points are directly before transformation (As) and shortly before retransformation (Ms). This allows triggering of fast and energy-efficient transformation cycles. Both experimental results and a mechatronical demonstrator system, exhibit the advantages of systems concerning efficiency, dynamics, and reliability.

Adaptive resetting of SMA actuators

Shape memory alloys (SMAs) have essential advantages compared with conventional actuators, in particular their high-power density and their silent mode of operation. However, this material has not yet gained acceptance in technical applications. The main reasons are the missing simulation tools and a lack of knowledge of materials as well as the companies’ uncertainty as to how to handle SMA. The resetting of the SMA element to generate a repeatable movement is often a defined problem. In this context, reset springs made of steel are conventional solutions, although their characteristics are a disadvantage. To reach a high level of power output and hence a high degree of efficiency, a reduction of the preload is necessary. A solution for this problem is an adaptive resetting. One main possibility to generate an adaptive resetting is given by the agonist–antagonist principle where two SMA elements work against each other. Here, the reset force can be applied if necessary. The advantage of this type of design is that a conventional return spring or a mechanical brake for clamping the position (electrically operated) is not necessary. Another possibility for adaptive resetting is to change the spring characteristics of a pseudoelastic SMA element by heating. The aim of this publication is to sum up the different possibilities of adaptive resetting of shape memory actuators. It also provides methods and the knowledge to support the development process of such resetting principles. The development of these methods is based on the analysis of different designs and requirements. Based on the experimental results, a conclusion of the possibilities is given.

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.

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.

Service Systems for Shape Memory Technology

Shape memory alloys (SMA) are well-known for their ability to transform into an imprinted shape by means of thermal activation (pseudoplasticity) or after a mechanical deformation (pseudoelasticity). The thermal effects can be used in a wide range of industrial applications like valves, unlocking devices or comfort applications in the field of automotive mechatronics. While there are many ideas concerning shape memory actuators, only few thoughts have been spent on service applications around these unique actuators. At present, product-related services are usually considered as an add-on to the actual product. But in future, industrialized countries are subject to a structural change toward service societies. For this reason, new concepts and methods which enable the companies to design the potential services in an optimal way are necessary already during the development of a product. This is a paradigm shift from the separated consideration of products and services to a new product understanding consisting of integrated products and services. In the case of shape memory technology, recycling processes present an interesting field for such integrated services. Starting with general ideas towards recycling concepts for and with shape memory components, this paper focuses on refresh-annealing as an example of an interesting recycling process. Finally, the paper is summed up by an outlook on future works on development methods for generic shape memory actuators and their service systems. The aim of this study is to show the possibilities and the importance of services in the field of shape memory technology. As a result, new applications and markets for SMA can be developed.Copyright

Concepts for Standardized Shape Memory Actuators for Positioning Applications

Actuators based on shape memory alloys (SMAs) have been developed to be used only in special applications. Therefore solutions based on SMAs generally cannot be transferred to other tasks. Focusing on the development for special applications has two important disadvantages. Firstly, the effort and costs reach a high level due to the individual development and secondly, for many companies the development of complex SMA-actuators turns out to be an insuperable barrier. Reasons for this are the complex characteristics and the missing simulation and design tools. In order to make statements about the functions and durability of the SMA-component, extensive tests need to be conducted. As a result there is a significant interest in providing SMA-actuator systems with complex and also variable functions. Modular systems allow a transfer to different areas of applications and they also lead to a reduction of variants. Using standardized components is an interesting opportunity to reduce the risk of individual development and the effort for single applications effectively. However, the increased system complexity of conventional modular systems is a problem (additional functions are required, e.g. the mechanical and electrical coupling of the modules). Apart from the conventional form of a modular system there is the possibility of a variable SMA-actuator system generated by standardized SMA-components which can be assembled to a stack system. The existing and unique potential of SMAs for function integration and therefore standardization can be used to its full extent. The aim of this paper is to show an application of such an SMA-actuator in stack design. Besides, the study presents the development process and the control concept of this actuator. An actuator system like this can be used in positioning solutions, for example. The task of this variable SMA-actuator is the conversion of a controller output variable into an exact displacement. The evidence of the realization of simple SMA-based actuators in modular design is provided with the development of this actuator system.Copyright

“One-Module”-Actuators Based on Partial Activation of Shape Memory Components

An advantage of shape memory alloys (SMAs) is their potential to generate integrated actuator systems with a shape memory component. This can be accomplished for example by activating the thermal shape memory effect in selected regions of the SMA-component. We refer to this process as partial activation. The purpose of the present study is to find a way to create universal actuators with properties adjustable for various applications solely by partial activation. Thus, an object of investigation is the analysis of properties and capabilities of partial activation. Furthermore this study also implicates the survey of possibilities for partial power supply and electrical contacting. One possibility to use partial activation in integrated systems is given by the agonist-antagonist design. This type of design offers the advantage that a return spring or a mechanical brake for clamping the position without feeding electrical power is not necessary. On the other hand retention force is limited by the martensitic plateau and positioning accuracy by the elastic portion of mechanical stress. To solve these problems with constructive or control-oriented solutions is furthermore an aim of this study. Another approach is to use partial activation for influencing passive superelastic structures like hinges, dampers, or return elements by changing the austenitic plateau stress in integrated systems. To create a multifunctional integrated system, the NiTi-elements presented in this study offer various options since they apply partial activation both for thermal shape memory and for influencing super elasticity.

Adaptive Resetting of SMA-Actuators

Shape Memory Alloys (SMAs) have essential advantages compared to conventional actuators, in particular their high power density and their silent mode of operation. However, this material has not gained acceptance in technical applications yet. The main reasons are the missing simulation tools and a lack of knowledge of materials as well as the companies’ uncertainty how to handle SMAs. The resetting of the SMA element to generate a repeatable movement is often a defined problem. In this context reset springs made of steel are conventional solutions although their characteristics are a disadvantage. To reach a high level of power output and hence a high degree of efficiency, a reduction of the pre-load is necessary. A solution for this problem is an adaptive resetting. One main possibility to generate an adaptive resetting is given by the agonist-antagonist principle where two SMA-elements work against each other. Here the reset force can be applied if necessary. The advantage of this type of design is that a conventional return spring or a mechanical brake for clamping the position (electrically operated) is not necessary. Another possibility for adaptive resetting is to change the spring characteristics of a pseudoelastic SMA-element by heating. The aim of this study is to show the properties of the agonist-antagonist design and the other concepts. It also provides methods and the knowledge to support the development process of such resetting principles. The development of these methods is based on the analysis of different designs and requirements. Based on the results, instructions for the conception of adaptive resetting systems have been created.Copyright

On the Potentials of Shape Memory Alloy Valves

Abstract Shape memory alloys (SMA) can be utilized as thermal and electrical-activated drives for valve applications. By using the high actuation forces and medium strokes in combination with SMA intrinsic sensor functions, smart and versatile valve elements for multi-purpose applications can be designed. The sensoric functions, based on the change of the electrical characteristics of the SMA drive, allow to detect the system’s condition as well as the system’s fatigue. The paper systematizes the usability of the intrinsic sensor function with particular emphasis on service potentials. A methodical overview over the design-options of different applications is presented in the first part of the publications. This is followed by a methodical analysis of the potentials of SMA in service applications. Since the product development process is not only a mechanical engineering matter, the production and the service options according to such valves have to be regarded. Besides this publication presents an innovative production process based on a fused deposition production process (FDPP) of valves which contains the installation of SMA actuators during production. The publications present several demonstrator systems which have been produced with FDPP and analyzed in applications.

On the Functional Characteristics of Adaptive Resetting of Shape Memory Actuators in the Field of Automotive Applications

Shape memory alloys (SMA) are thermally activated smart materials. Due to their ability to change into a previously imprinted actual shape by means of thermal activation, they are suitable as actuators for mechatronical systems. Despite the advantages shape memory alloy actuators provide (lightweight-actuators, lower costs etc.), these elements are seldom integrated by engineers into automotive systems. One reason for this phenomenon is among others the varying dynamic behavior at different ambient temperatures. A methodical approach through the problem definition as well as the presentation of different solutions using adaptive resetting introduces experimental results on the behavior of these actuator systems. The publication presents different solutions as well as long-time experiments compared to conventional SMA actuators at automotive conditions.Copyright