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Abstract— Pneumatic driving systems are mainly used in industrial applications where the moving parts are usually fixed by the mechanical stops. For flexible and precise positioning tasks of pneumatic drives relatively expensive proportional valves have been implemented. In order to develop cheaper pneumatic servo systems, the employment of low-cost on/off solenoid valves have received considerable attention. But, the traditional technologies used in on/off solenoid valves manifest different problems caused by the electric and thermal effects, inertial forces of their mechanical parts and friction phenomenon. However, it seems that innovative technology used in fast switching solenoid valves gives better possibilities in control of pneumatic systems due to their negligible internal friction and modular architecture. In this paper a control method based on pulse-width-modulation algorithm for position control of a pneumatic actuator with high-speed solenoid valve is considered and experimentally verified.
an electric signal from control device. In order to develop an inexpensive pneumatic actuator system a considerable research has been addressed to implement inexpensive on/off solenoid valves rather then proportional or servo valves. Binary-position solenoid valves put their open or closed position in accordance with an electric on/off control signal. The use of pulse-width modulation (PWM) technique in pneumatic systems makes possible the use of inexpensive, fast switching on/off valves in closed-loop control applications, which are implemented to digital systems by using real-time programming. Such systems can provide actuation characteristics at a significantly lower cost compared to the electromechanical actuation systems. However, precise control is difficult to achieve because of significant delay time of the solenoid valves and their discrete on/off characteristic [7]. Also, it is not easy to derive and to use an analytical dynamic model of pneumatic system, which uses a PWM control signal for switching solenoid valves. From the viewpoint of engineering practice, the standard solenoid valves realized by traditional technologies have several imperfections due to the inertial forces of their moving parts and thermal effects of electromagnet caused by the electric current. In this paper instead of traditional solenoid valves we have been implemented a high-speed solenoid valve, which uses some new construction conceptions [8]. The mass reduction of the moving parts and the using of material with energetic high efficiency have resulted with a negligible internal friction during opening and closing the valve. This innovative solution enables fast valve switching, which is crucial in the implementation of PWM based control method. The construction of the valves is based on the modular architecture, allowing the assembly of several outlets in a single body. In this study the implementation of a pulse-width modulated, computer controlled, closed-loop electro-pneumatic drive, which consists of a rodless cylinder and fast switching on/off valve is investigated and experimentally verified. II. DESCRIPTION OF THE E XPERIMENTAL DEVICES
I. INTRODUCTION Technological improvements and innovations in pneumatic components and digital signal processing have made possible some new modalities in traditional pneumatic systems applications. Over the years, pneumatic actuators are extensively used in industrial automation only in the open-loop control mode i.e. for ‘pick-and-place’ positioning problems [1], with simple on/off solenoid valves to control their motion. This sequence control technique is widely used in pneumatic driving systems for relatively simple tasks. Namely, in manufacturing processes there exist a great number of pneumatic systems where the process operation is based on the use of such simple valves of the on/off type, which are operated electronically or even in some cases adjusted manually. For less demanding applications this form of control is fairly satisfactory and their employment will be continued in the future. Increasing requirements for precise positioning when the pneumatic systems are used for robotic manipulators and mechatronics applications make demands on more effectively control techniques and closed-loop control strategies. Accurate position control of pneumatic driving systems is generally a difficult problem because of the fundamental problems associated with the air compressibility and significant friction effects of moving parts. During the past two decades a notable amount of research has been realized to development of position-control systems for pneumatic drives. In most of them relatively expensive proportional directional control valves are used [2]–[6]. Proportional solenoid valves place a certain position between full open or closed in terms of
Željko Šitum, Tihomir Žilić and Mario Essert
Faculty of Mechanical Engineering and Naval Architecture Department of Robotics and Automation of Production Systems, Zagreb, Croatia
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High Speed Solenoid Valves in Pneumatic Servo Applications