Alexander Czechowicz

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.

Computer-Aided Development and Simulation Tools for Shape-Memory Actuators

This article discusses the dynamic properties of shape-memory alloy (SMA) actuators, which are characterized by their rate of heating and cooling procedures and can be described only insufficiently for different boundary conditions. Based on an analysis of energy fluxes into and out of the actuator, a numerical model implemented in MATLAB/SIMULINK (MathWorks, Natick, MA) is presented. Besides the fluxes, the time-variable parameters like the latent heat of transformation or the influence of stress on the transformation temperatures are also included in the model. These parameters, depending on actuator geometry and temperature, the fraction of martensite, and the environmental conditions, are considered in the simulation in real time. In addition, this publication sums up the needed empirical data (e.g., fatigue behavior) to create a general-purpose engineering tool. The SMA wire-based actuation system can be configured by drag-and-drop tools and finally simulated and graphically displayed for different actuator systems. The development and verification of such a tool (called CASMADA) from theoretical equations to the verification on real elements is the main topic of this publication.

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

An Investigation of Service-Oriented Shape Memory Actuator Systems for Resource Efficiency

Nowadays companies working in the field of industrial applications realize that traditional electromagnets reach their technical limits. Additionally, there is an increasing awareness for resource-efficiency and safety, due to shortages in resources and rise in legal requirements. Furthermore customers demand more and more system solution, because of increased complexity in technical applications. Service-oriented shape memory actuator systems are an innovative approach that can help companies meeting these challenges and thus not only keep but enhance their competitiveness. Therefore, this paper is focusing on the presentation of a service-oriented smart memory actuator system based on condition monitoring or reconfiguration by heat treatment. Hence an experimental actuator system is designed. This system is used to evaluate feasibility of condition monitoring to predict lifetime and the ability to reconfigure properties in use by heat treatment. Finally, an appropriate business model for service-oriented shape memory actuator systems is discussed. First results of the project are presented within the paper as well as a business model draft for a service-oriented shape memory actuator system. Further studies should be done investigating the feasibility of remote heat treatment as well as developing applicable business models for shape memory actuator systems.

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.

On the functional characteristics of pseudoelastic adaptive resetting of shape memory actuators in the field of automotive applications

Shape memory alloys 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, and so on), these elements are seldom integrated by engineers into automotive systems. One reason for this phenomenon among others is 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. This article presents different solutions and longtime experiments compared to conventional shape memory alloy actuators at automotive conditions. It concentrates on the possibility of utilization of a pseudoelastic resetting element working in combination with a shape memory alloy actuator.

Cooling Strategies for a SMA Wire Actuator in a Feed Axis

Shape memory alloys (SMA) are smart materials which can be activated thermally. They are suitable for the use as actuators due to their ability to remember an imprinted shape through thermal activation. In addition, actuators based on shape memory alloys offer a higher work output in relation to their volume compared to other actuator concepts. Other advantages of using SMA in actuation applications include the ability to design lightweight systems and the comparatively low material costs.On the other hand, designing an SMA actuator poses a challenge in case a specific rate of feed has to be achieved. These difficulties become especially apparent if the actuator is used to create a defined displacement not only in its activation direction, but in the returning (deactivation) direction as well. This might occur, for example, while devising an SMA-driven feed axis.During the activation of the SMA, the speed of the actuator and therefore the speed of the axis can be influenced by choosing a specific thermal energy transfer method. For instance, when using the intrinsic resistance for heating purposes, the speed can be controlled by changing the electrical current running through the SMA. However, after the deactivation (end of the heating phase) of the shape memory alloy, the transformation needs a considerably longer time. For an exemplary SMA wire actuator, the transformation time in room temperature can be five times higher than the activation time. For usage in a feed axis, the actuator should produce similar speeds in both the activation and deactivation direction.To achieve this, different strategies for cooling the SMA after cutting off the current are investigated. These strategies include an active air cooling system with different flow characteristics and the operation of the actuator in a cooling fluid.In a nutshell, the paper compares different ways of cooling an SMA wire actuator to increase the transformation speed after deactivation. The aim is to make the deactivation speed as manageable as the activation speed.Copyright