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УДК 255:29.1 PROPOSAL OF AUTOMATIC RESISTANCE RHEOSTAT FOR VMLAB J. Molnár Technical University of Košice Letná 9, Košice, 042 00, Slovak Republic. Е-mail: [email protected] This article deals with proposal of automatic resistance rheostat designed for distant measurement laboratory (VMLab). The distant measurement laboraty is a complex measurement system which serves for student´s education. In order to obtain the real measurement conditions, the automated resistance rheostat had to be created, which will serve as a regulated resistance in VMLab, which is controlled via RS-232. Key words: VMLab, RS-232, automatic. INTRODUCTION. VMLab is an unique system developed on Department of Theoretical and Industrial Electrical Engineering of FEI TU Košice. The system enables the creation of electrical circuit from predefined pool of electrical components from any place in the world connected to Internet. The designed circuit is then physically connected in lab from real components. Some of these components can be controlled (it is possible to set their parameters). Such example can be the voltage source, where the user can specify the desired voltage as needed. The resistance elements which were the constant resistors needed to be modernised, therefore they had been replaced with with the adjustable rheostats. These rheostats are controlled by the main server which controls the other VMLab elements, too. Due to the power nature of the system together with measurements of direct and alternating quantities, the electronic load couldn´t be used. Therefore unusual solution consisting of mechanical resistance rheostat and step motor combination was used. The RS-232 interface had been chosen for step motor control. This modernisation resulted in improved flexibility of whole measurement system. Following figure presents the VMLab system. Figure 1 – VMLab measurement system EXPERIMENTAL PART AND OBTAINED RESULTS. The automatic rheostat consists of power resistance potentiometer and step motor. The step motor had been chosen because of linear regulation of resistance, therefore the resistance will be changed continuosly depending on the steps of the step motor. With this combination, it is possible to set the exact value which can be repeated. The repeatibility is very impoertant factor, which could be problematic to achieve when using different type of motor. Following picture shows the block scheme of automated resistance rheostat for VMLab. Figure 2 – Block scheme of automatic resistance rheostat connection. This block scheme can be divided into two basic parts – hardware and software part of the connection. The hardware part consists of power electronics responsible for step motor control, step motor and power reostat. The software part consists of RS-232 communication interface and VMLab control server, which sets the specified resistance values on the resistance rheostat. The control algorhythm of step motor control will be explained later. Hardware part of the solution. At first, the mechanical connection of step motor and power resistance rheostat had to be created. It was essential to design and create the mechanical anchor of both components, so that the rotation of motor´s shaft and resistance rheostat was smooth. The control requests of such connection had to be considered, therefore the switching element was added to the construction. This switch will serve as an ending switch, which will serve an important role when setting the rheostat´s value. The following figure shows the proposal of such connection. Figure 3 – Block scheme of automatic resistance rheostat connection In order to solve the power control of step motor, the universal connection scheme was proposed. The main part of this connection consists of adjustable registry HCF4094BE and power circuit UNL2803 and other auxiliary components. The following figure shows the swchematics of connection of electronics used for step motor control. D3 and resistors R1 to R3 which together with diodes stabilise the voltage on 4,7V value. The unipolar step motor is feeded directly by 12V DC. The D0, D1, D2, D3 from IO2 outputs are connected directly to IO3 (UNL2803) outputs. This creates the power integrated circuit which outputs are rated to current up to 0,5 A. Together with retaining diodes this load can be controlled. The IO2 outputs are controlled by D0 to D3 for one direction of rotation and by D3 to D0 for another direction of step motor rotation. The used unipolar motor in this connection has maximum feeding voltage of 24 V DC, number of steps for one rotation is 48, the angle of step is 7,5 degrees. Software part of the solution. Software and hardware parameters of VMLab server had to be considered in order to ease the implementation of new component to existing system. Therefore, RS-232 standard communication interface was used. The main advantage of this interface is the good availability of communicating drivers together with its versatibility. The next step was adding the new components to client´s application, so they could be fully utilised. During the phase of scheme creation, the user uses the client application. The first step is the connection of application with server, from which it will obtain the information about connected electronic components. VMLab can change the pool of electrical components connected to VMLab table.The program will allow to use only the components, which are on the component list. After the new automatic reostat was implementd into main program structure, the following algorhythm needs to be loaded into main control cycle. The following picture shows the algorhythm of automatic rheostat setting. Figure 4 – Schematics of connection of step motor This connection is feeded by 12V DC voltage. This voltage is stabilised by IO1 to 5V. This voltage feeds the IO2 (HCF4094BE) integrated circuit. The connected RS-232 outputs are protected by stabilising diodes D1 to Figure 5 – Algorhythm of automatic rheostat setting. This algorhythm at the beginning checks the rheostat´s value. With this value it will determine whether the ending switch is switched on or off. If it is switched on, then the resistor´s value is set to 0. If it is switched off, then the 0 is set. The 0 is the process, when the step motor will carry the necessary steps in set direction until the ending switch is switched on. Next, the resistor´s value can be set. This setting can be done only when the ending switch is switched on. The control program will send the information about the number of steps needed to the step motor. With these steps, the specified resistance value by user will be set. Then the algorhythm ends. When the resistance value will be changed, the whole algorhythm will be repeated. The algorhythm which controls the sources of voltage and currents needs to be optimised in order to avoid the short-circuit when the resistance is set to zero, during the change of the value. This setting of value to zero needs to be done everytime, otherwise the VMLab would be unable to determine the value specified on rheostat. As mentione previously, during this proposal, the specific parameters and constraints of VMLab had to be taken into account, therefore the rheostat is designed in the easiest way from the implementation perspective. CONCLUSIONS. In this article the proposal of automatic resistance rheostat was presented and realised for the VMLab system. This system allows the users who are connected to Internet create, connect and measure any electrical circuit. So far , the system used only the constant values of resistors, therefore, the user was limited with the choice. The system needed to be modernised, therefore it was essential to propose a solution for continuos resistance regulation which would result in more comfort in measurement. Due to power nature of the connections, the solution had to be the resistance rheostat, which will allow the continuous connection of whole measurement system. Therefore the unusual combination of resistance rheostat and step motor controlled by RS-232 interface had been chosen. REFERENCES 1. Kováč, D., Vince, T. and Molnár, J. (2010), "Modern Internet Based Production Technology", New Trends in Technologies: Devices, Computer, Communication and Industrial Systems, SCIYO, pp. 145–164. 2. Vince, T. and Kováčová, I. (2007), "Distance control of mechatronic systems via Internet", Acta Electrotechnica et Informatica, Vol. 7, no. 3, pp. 63–68. 3. Vince, T. and Kováč, D. (2010), "Remote and regulation of electrotechnical systems via Internet", FEI TU, Košice, Slovak. 4. Kolla, I. and Kováč, D. (2013), "Analýza vlastností mechatronických systémov riadených cez Internet", Technická univerzita v Košiciach, Košice, Slovak. 5. Dziak, J. and Tomčíková, I. (2013), "Creating visual application for analysis of electric circuits using MATLAB GUIDE", Electromechanical and energy systems, modeling and optimization methods: Kremenchuk Mykhailo Ostrohradskyi National University, pp. 317–318. 6. Vince, T. (2013), "Remote DC motor speed regulation using artific neural network support", Electromechanical and energy systems, modeling and optimization methods: Kremenchuk: Mykhailo Ostrohradskyi National University, pp. 75–76. 7. Perduľak, J., Kováč, D., Kováčová, I., Ocilka, M., Gladyr, A., Mamchur, D., Zachepa, I., Vince, T. and Molnár, J. (2013), "Effective utilization of photovoltaic energy using multiphase boost converter in comparison with single phase boost converter", Communications, Žilinská univerzita, no. 3, pp. 32–38, 8. Ocilka, M. and Kováč, D. (2012), "Calculation of capacitance of two spiral plates placed on printed circuit board using COMSOL, Calculation of capacitance of two spiral plates placed on printed circuit board using COMSOL", Electromechanical and energy saving systems, Vol. 19, no. 3, pp. 502–504. ЗАПРОПОНУВАННЯ АВТОМАТИЧНОГО ОПОРУ РЕОСТАТА ДЛЯ VMLAB Ж. Молнар Технічний університет м. Кошице вул. Лєтна, 9, Кошице, 04200, Словацька Республіка. Е-mail: [email protected] Запропоновано автоматичний реостат опору, призначений для дистанційного вимірювання лабораторії (VMLab). Лабораторія вимірювання відстані є складовою системою вимірювання, яка служить для навчання студентів. Для того, щоб отримати реальні умови вимірювання, має бути створено автоматизований опір реостата, який буде виступати у функції регульованого опору у VMLab, що контролюється за допомогою RS-232. Ключові слова: VMLab, RS-232, автомат.