The 12th Conference on Selected Problems of Electrical Engineering and Electronics WZEE 2015

Transcription

1 The 12th Conference on Selected Problems of Electrical Engineering and Electronics WZEE 2015 Proceedings September 17-19, 2015 Kielce, Poland 1

2 Measurements of Shape Memory Alloy Actuator for a Self-Switching Electrical Fuse Grzegorz Kłapyta, Marek Kciuk Department of Mechatronics Faculty of Electrical Engineering, SUT Gliwice, Poland marek.kciuk@polsl.pl, grzegorz.klapyta@polsl.pl Kamil Świątek Leonid Szenwald Secondary School Dąbrowa Górnicza, Poland Abstract This paper describes chosen results of the measurements of various characteristics of Shape Memory Alloy wire in purpose of its usage as an actuator for selfswitching electrical fuse. The experiments were performed by talented pupil of secondary school as a research project cofinanced by Polish Ministry of Science and Higher Education. Measurement system consisting of power supply unit PSH- 3620a and PC computer with running LabVIEW application is used to control the measurement procedures. Program of experiments consists of static measurements (static switching current), dynamic measurements (switching times) and exploitation measurements (switch off time). The paper presents chosen resultant characteristics. Keywords Shape Memory Alloy; SMA wire; automatic fuse; self-switching electrical fuse, mechatronics I. INTRODUCTION This project is realized as the didactic project co-financed by Polish Ministry of Science and Higher Education, and is focused on the development of skills and talents of young pupil of Secondary School Kamil Świątek. The pupil is extraordinary and very talented young man who has won many scientific competitions (especially in math and chemistry) and is learning at secondary school within individual program. Within this project he is designing new device and conducting necessary experiments under supervision of scientists of Department of Mechatronics at the Faculty of Electrical Engineering of the Silesian University of Technology. The technical goal of the project is to design self-switching electrical fuse for automatic breaking and reconnecting of electrical circuits when the overcurrent appears. Novelty of the project is assured by one of the main assumptions - usage of the Shape Memory Alloy wire as an actuating component of the designed fuse. Shape Memory Alloy is one of modern and recently very popular material with quickly increasing number of applications. [1, 2, 3, 4]. Most often used kinds of SMAs consist mainly of nickel and titanium and has various trade names in our case it is Flexinol. Unique properties of the SMA wires, examined by authors during several years of research [5, 6, 7], resulted in new concept of creating the automatically working device for everyday use. General rule of operation is not new and was published in several patents [8]. II. CONCEPT AND PROTOTYPE A. Concept Classical electric fuses, used in every house, are switching off the overcurrent automatically, but must be manually restarted. Some over currents appearing in the electrical circuits are temporary and the circuits can be switched on again almost immediately. Usage of Shape Memory Alloy (SMA) actuator for breaking the circuit allows for automatic reconnection of the circuit when only the SMA wire cools down. Cooling process is depending on many factors but usually takes some seconds. This time can be sufficient for disappearance of short circuit cause. Automatic reconnection of broken circuit can save time and help to avoid loss connected with long lasting lack of power supply. B. Modelling the Prototype Initial task in the project it was to work out the concept and to apply the SMA behavior for rule of operation of the device. First ideas were discussed during many discussions and brainstorms when several hand drawn sketches were created. One of them is presented for example in Fig.1. Fig. 1. Exemplary hand drawn sketch of the concept. Finally the team decided to start with design of the prototype device allowing for quick confirmation of the The project is co-financed by the European Union under the Ministerial Programme Uniwersytet Młodych Wynalazców, carried out within the systemic project Support for the management system of scientific research and its results, implemented under the Innovative Economy Operational Programme (Sub-action 1.1.3) 99

3 proof of concept. The prototype is much bigger then the final planned device and is just a kind of measurement stand for easy and efficient examination of the mechanism. Final concept of the design of the prototype measuring stand was modelled using Autodesk Inventor environment. At this stage the idea of mechanics of the stand was still optimized and there was several versions of the chassis shape and components forms. Exemplary 3D model of one of the working versions of the prototype is presented in Fig.2. The 12th Conference on Selected Problems of Electrical Engineering and Electronics WZEE 2015 Fig. 2. 3D model of the early prototype of the measuring stand. C. Producing the Prototype Before producing the prototype unit it was necessary to choose all the additional components of the system (like micro-switch, return spring, mountings, etc.). All the choices also influenced the final shape of the chassis and the moving parts. Prepared 3D model of the measuring stand was finally optimized once again for the modern technology of producing 3D printing. Most of the walls were slimmed and some of the sharp angles were rounded. All plastic parts for final version of prototype were printed with ABS filament using polish commercial printer - 3D Kreator Motion. Mounted prototype measuring stand, ready for tests is presented in Fig.3. D. Rule of Operation The rule of operation of the prototype measuring stand can be explained using picture presented in Fig.3. The SMA wire actuator, heated by flowing current, is used to pull down the left side of rotary lever while a mechanical steel spring is used to move it back. Right arm of the lever presses the micro-switch causing closure of the electric circuit. The steel spring is holding the micro-switch contact normally closed. The SMA wire and micro-switch circuit are connected in serial so the same current flows through both components. The flowing current heats the SMA actuator and then it contracts. For some specific value of current (called the switching current) the SMA actuator is thermally activated and pulls the lever opening the electrical circuit. Fig. 3. Prototype measuring stand for laboratory experiments. When the circuit is broken the SMA wire cools down and its force decreases. As a result the mechanical spring pulls the lever back turning on the circuit again. Length of the SMA wire as well as the tension of the mechanical spring can be adjusted by the mounting screws. The adjustment allows to change the switching current of the micro-switch in quite wide range, depending on the kind of SMA material and the wire diameter. III. MEASURING METHODOLOGY AND EXEMPLARY RESULTS A. Measuring System Measuring and power supply system consists of several components connected to PC computer. Electrical circuit (consisting of micro-switch and SMA wire connected in serial) is supplied by power supply unit PSH-3620a. The power supply unit can work both in the voltage or the current control mode. In the current mode circuit current is regulated but when the circuit is broken the power unit switch to the voltage mode and the no-load voltage is set. The power supply unit settings are controlled in real time by specially designed LabVIEW application running on the PC computer. The PC computer uses the General Purpose Interface Bus (GPIB) protocol to communicate with the power supply unit. Measurements are realized by a data acquisition card (DAQ) - NI USB-6009, connected to the PC computer through USB port. Scheme of the measuring system is presented in Fig

4 Fig. 5. Static measurement waveforms. Fig. 4. Scheme of the measuring system. All the measurements were done for SMA wire purchased under trademark Flexinol. The wire used in experiments had 0,51mm diameter. The switching current of used SMA actuator can be adjusted by the mounting screws in quite wide range of 0,9-1,5A. For all the described experiments the switching current was set to have the constant value equal to 1,06A. Methodology of the measuring experiments was planned to examine the properties of SMA actuator bearing in mind its usage as the self-switching fuse. The experiments were divided for three main groups: static measurement, dynamic measurements and the exploitation measurements. B. Static Measurement First experiment, called the static measurement, was prepared to determine the static switching current. Initially the current is set to the starting, small value and then it is increased by equal small step each two minutes. Two minutes is experimentally determined time which is enough to achieve the thermally static state of the system. For each step the SMA voltage waveform is registered and SMA wire resistance is calculated according to the Ohm s Law. When the SMA actuator is heated enough to pull the lever it opens the micro-switch, breaking circuit which ends the measurement. The current set just before the break is called the static switching current. It is strongly dependent on the initial tensions of the SMA wire and mechanical spring but also ambient temperature has influence on the switching current value by changing the heating and cooling conditions. Exemplary static measurement waveforms are presented in Fig. 5. In the given example Iset is the set current forced by the power supply unit. It starts from 0,44A and rises by 0,02A every two minutes. For the current equal to 1,06A the circuit was broken by the SMA wire which opened the microswitch. The current sufficient to break the circuit is called the static switching current (Istatic). The static switching current will differ for various SMA wires and for other components (the spring and the micro-switch). C. Dynamic Measurements Dynamic measurements procedures are focused on the determination of SMA activating times. The time is counted from the appearance of overcurrent till the automatic switch off the electric circuit. In the first experiment constant current (bigger then the static switching current) is forced in the circuit and time for SMA activation is measured. Exemplary dynamic characteristic presenting the experiment is presented in Fig.6. Fig. 6. Dynamic characteristic for Iset=1,46A. For the examined SMA wire with the static switching current Istatic=1,06A the set current was forced to Iset= 1,46A. Measured average switching time (for 20 trials) is equal to 2,75s. The second dynamic measurement experiment checks the switching times for several repeating switching sequences. First turn on of the circuit current is done by the power supply unit and the SMA actuator starts to heat from the ambient temperature. Hence the first break of the circuit is noticed after longer period of time then all the others in the sequence, when the SMA wire is already hot. Despite the time of the first circuit break all other switching times are similar. Resultant measurement characteristic showing the experiment is presented in Fig

5 Fig. 7. Single automatic switching sequence of the circuit. Fig. 9. The idea of exploitation measurements. The third experiment was measurement of several series of above described switching sequences. After each sequence the power delivery is interrupted for 5 seconds to allow for SMA wire cooling. The 5 seconds is experimentally determined time necessary to cool down the SMA wire to ambient temperature during experiments on the prototype measuring stand. Exemplary three series of switching sequences are presented in Fig.8. Fig. 10. Statistics of the exploitation measurements switching times. Fig. 8. Series of switching sequences. D. Exploitation measurements The goal of this experiment is to check the performance of the fuse in conditions similar to the normal work. Usually electrical circuits work continuously for a long period of time before the short circuit appears. To simulate real conditions SMA wire was supplied with various Iset currents (smaller than the static switching current) for some time called the Preheating Time. After the preheating time the Iset current is suddenly increased to the short circuit current and since then the time to automatic break of the circuit is measured. The idea of exploitation measurements is presented in Fig.9. For the examined wire several measurements were done using various set currents (Iset = 0,25%, 0,5%, 0,75%, 0,85%, 0,93% of the static switching current Istatic=1,06A) for SMA wire preheating. For each value of Iset there was 5 measurements for each of four preheating times (20s, 90s, 180s, 220s). After preheating time the short circuit current equal to 1,5A was forced in the system and since then the switching time was measured. As a result for each Iset there was 20 switching times measured. Statistics of measured switching times for various set currents are presented in Fig.10. IV. SUMMARY Presented results were achieved by secondary school pupil within the didactic project. Conducted experiments show that the SMA wire has properties allowing for its usage as an automatic self-switching electrical fuse. Wider research is necessary to check more configurations of the system and to work out the valuable conclusions. REFERENCES [1] G. Song, N Ma, HN. Li, Applications of shape memory alloys in civil structures, Engineering Structures, 2006, Elsevier. [2] L.G. Machado, M.A. Savi, Medical applications of shape memory alloys, Brazilian Journal of Medical and Biological Research, 2003, Volume 36(6), [3] Shape Memory Applications, Inc. [4] G. Kłapyta, Shape Memory Alloy Modern Smart Material for Various Applications, Slovakian Journal Communications Nr 1, [5] M. Kciuk, G. Kłapyta: SMA gripper for industrial robot, Proceedings of 7th International Workshop on Research and Education in Mechatronics, REM 2006, Stockholm, June 15-16, 2006 [6] M. Kciuk, G. Kłapyta, Computer-Based Measurement System For Complex Investigation Of Shape Memory Alloy Actuators Behavior, Applied Measurement Systems, Chapter 9, INTECH, [7] M. Kciuk, G. Kłapyta G.: Experimental measurements of Shape Memory Alloys wires, Archives of Electrical Engineering, Volume 61, Number 2, 2012, p [8] Patents: US b1, US , US