WATER HYDRAULIC HIGH SPEED SOLENOID VALVE AND ITS APPLICATION

Transcription

1 WATER HYDRAULIC HIGH SPEED SOLENOID VALVE AND ITS APPLICATION Akihito MITSUHATA *, Canghai LIU *, Ato KITAGAWA * and Masato KAWASHIMA ** * Department of Mechanical and Control Engineering, Graduate school of science and engineering Tokyo Institute of Technology Ookayama, Meguro, Tokyo, Japan ( mitsuhata@cm.ctrl.titech.ac.jp) ** Tohoku Steel Co., Ltd., Japan 23, Nishigaoka, Ooazamurataaza, Murata, Shibata, Miyagi, Japan ABSTRACT This study aims at the optimization of an attenuator and the improvement of linearity of the main flow rate of water for Water Hydraulic High-speed Solenoid Valve. In this study, the parameters of optimum attenuator are defined, and verified experimentally. And the experiment of position control using a water hydraulic cylinder is performed to confirm whether the control performance is improved by the new HSSV. Though proportional poppet and attenuator improved the pressure oscillation of the control chamber, the linearity of main flow rate of water became worse than before changing the structure. In this study, the linearity of main flow rate of water is improved by adjusting a shape of control window which controls the pilot flow rate of water. Effect of new shape of window is verified experimentally. KEY WORDS Key words, Water hydraulics, Poppet valve, High speed solenoid valve, Attenuator NOMENCLATURE Q : Pilot flow rate [L/min] C : Flow coefficient of pilot valve [-] A : Cross sectional area of pipeline [mm 2 ] p s : Supply pressure [MPa] p a : Pressure in attenuator [MPa] : Density of water [kg/m 3 ] V : Volume of attenuator [cm 3 ] K e : Equivalent bulk modulus [GPa] t : INTRODUCTION Recently, there has been great interest in the water hydraulic systems because they are not harmful to the environment. As a flow control valve of the water hydraulic systems, which have as high dynamic performance as the oil hydraulic systems, a PWM controlled water hydraulic high-speed solenoid valve (HSSV hereafter) with a two-stage mechanism has been developed by Park et al [1], which is shown in Fig.1. The flow characteristic of the main valve corresponding to the PWM duty ratio is linear because the main poppet valve repeats an on-off motion according to PWM signals. The on-off motion of the main poppet valve

2 strikes at the poppet seat and at its stroke end once every cycle of the PWM signal. Resultantly, large noise and shock are generated. To reduce that, an attenuator and proportional poppet has been applied by Minematsu et al [2]. In this paper, the optimum attenuator is built. In addition, the experiment of position control is performed using HSSV having the optimum attenuator for the best control performance. Also, though proportional poppet and attenuator reduced noise and shock, the linearity of the flow rate characteristic is sacrificed to some degree. A new shape of the window of control orifice to improve the linearity of main flow rate and to reduce the noise simultaneously is introduced. Attenuator Orifice Control chamber Figure 2 HSSV with attenuator for experiment Under the conditions shown in Tabel 1, the displacement of the main valve is measured. Appropriate size of the attenuator and the control chamber which enable to reduce the pressure oscillation is verified in Measurement 1 to 5. The diameter of the orifice used in this experiment is 0.6mm. Table 1 Conditions of Measurements 1 to 7 Figure 1 Water hydraulic HSSV OPTIMIZATION OF ATTENUATOR In this section, the optimum attenuator is defined as the smallest chamber which makes the main poppet valve not to hit the valve seat in all duty ratios. The optimization is necessary because the rapid response of the main poppet valve and the reduction of pressure oscillation in control chamber are a trade-off. The larger the attenuator is, the worse the response of the main valve is. Figure 2 shows the experimental circuit. HSSV is divided into the pilot valve and the main valve. Attenuator, orifice, and control chamber are inserted between the pilot valve and the main valve. Displacement of the main valve is experimentally measured using the valve with attenuator and control chamber of various sizes. In this experiment, PWM signal whose duty ratio is less than 50% is given under the supply pressure of 10MPa. Volume of control chamber [ml] Volume of Attenuator [ml] Meas Meas Meas Meas Meas Meas Meas Figure 3 and 4 show the results of measurements 1 to 5. The volume of the attenuator influences the reduction of pressure oscillation in the control chamber. Meanwhile, the volume of the control chamber doesn t reduce that sufficiently. From these results, only the attenuator is larger, it is possible to reduce the pressure oscillation. Figure 5 shows the results of measurements 6 and 7. The pressure oscillation in measurement 7 using the larger volume is smaller than that of in 6. From this measurement, it is concluded that the larger the volume of the attenuator is, the smaller the pressure oscillation in the control chamber is. Meanwhile, the rising time of the main poppet valve of measurement 7 is 20ms longer than that of measurement 6 as shown in Fig.6. Therefore, the optimum attenuator is revealed by increasing the volume of the attenuator gradually. As shown in Fig.7, it was revealed that the volume of the

3 optimum attenuator is 200ml. In this case, the main poppet valve doesn t hit the valve seat even in low duty ratio. Meas6 Meas7 Displacement [m] Meas1 Meas2 Displacement [m] Figure 6 Comparison of displacement response Figure 3 Results of measurements 1 and 2 Best Attenuator Displacement [m] Meas3 Meas4 Meas5 Displacement [m] Figure 4 Results of measurements 3, 4 and 5 Figure 7 Displacement of main poppet with optimum attenuator Displacement [m] Meas6 Meas7 Figure 5 Results of measurements 6 and 7 Although the volume of the attenuator is optimized, this volume is too large. If the volume of water in the attenuator increased, the rising pressure can be determined from the volume of the attenuator V and the equivalent bulk modulus of the attenuator K e. In this study, is reduced by making the volume of the attenuator larger. Therefore, such a large volume is necessary. This is explained through the following equations and an experiment. The flow rate passing through the pilot valve is given by 2( p s pa ) Q CA (1) The flow rate considering the equivalent bulk modulus of the attenuator K e is expressed as

4 Pressure[MPa] V dpa Q (2) K dt e Using Eq. (1) and Eq. (2), the pressure response in the attenuator p a can be expressed as in Eq. (3). p a p s 1 CAK 4 V e 2 t 2 2 p s The pressure response in the attenuator p a was acquired in the experiment. Figure 8 shows the experimental setup. The experiment was performed under 10MPa. Figure 9 shows the result. By calculating from the equations and the result of the experiment, the equivalent bulk modulus of the attenuator K e is 1.15GPa. The large volume of the attenuator is necessary to reduce the pressure oscillation in the control chamber because of its large bulk modulus. The volume of the attenuator is able to be reduced by reducing the bulk modulus of the attenuator. (3) POSITION CONTROL OF WATER HYDRAULIC CYLINDER USING THE NEW VALVE Position control experiment of water hydraulic cylinder under supply pressure of 10MPa has been performed to measure control performance and to estimate the effect of the improvement of HSSV. Figure 10 shows the experimental setup. The cylinder has a single rod and its pressurized area ratio is 1:2. An accumulator is connected to the rod-side chamber of the cylinder. HSSV is connected to the inlet of head-side chamber of the cylinder, and the orifice whose diameter is 1.1mm is also connected to it. The cylinder moves according to the flow rate supplied from HSSV. In this experiment, a step signal or stationary signal is given as a reference, and the position of cylinder is controlled by PI control method. Figure 10 Experimental setup Figure 8 Experimental setup 4 2 Ps Pa Figure 9 Pressure response Figure 11 shows stationary responses of the cylinder. Comparing new HSSV and original HSSV, the response of cylinder is steady with the first one, since there is a difference in pulsation amplitude of the main flow in low duty ratio between valves. In original HSSV (without attenuator), the main valve hits the valve seat in low duty ratio. Therefore, the pulsation amplitude of main flow is larger and the position of cylinder becomes unstable. On the other hand, in new HSSV (with attenuator), the pulsation amplitude of the main flow is small because main valve does not hit the valve seat. As a result, the stationary response of cylinder is stable. Figure 12 and 13 show step responses of the cylinder. By original HSSV, response of the cylinder shows vibration because of the large pulsation, and because of the long periodical oscillation too. On the other hand, by new HSSV, the response is steady but it is a little lower than the reference, since a certain amount of water constantly flows out from the orifice. Therefore, its position is a little lower than reference.

5 Displacement[mm] Displacement[mm] Displacement[mm] Target Old Valve New Valve Figure 11 Comparison of displacement response main poppet valve which generate main flow rate corresponds to the pilot flow rate. As shown in Fig.14, pilot flow is composed of the leakage which passes through the clearance between the sleeve and the main poppet, and the control flow provided from the control window. Leakage through the clearance is constant. Therefore, the displacement of the main poppet valve corresponds to the opening area of the control orifice which supplies the control flow. So, it is possible to adjust the displacement of main poppet by adjusting the shape of control window. Concept of displacement and control window of main poppet is shown in Fig.15. The smaller the width of control window is, the higher the displacement of main poppet is Pilot Flow Control Chamber Control Flow 56 Target 52 Measured Figure 12 Step Response by HSSV without attenuator Leakage Control Orifice Sleeve p s Figure 14 Water flow around main poppet Target Measured Figure 15 Relationship between displacement of main poppet and width of control window Figure 13 Step Response by HSSV with attenuator CONSIDERATION OF MUTUALITY BETWEEN DISPLACEMENT OF MAIN POPPET VALVE AND PILOT FLOW RATE As shown in Fig.1, the HSSV is composed of a main valve and a pilot valve. The pilot valve is controlled by a PWM signal, which supplies a pilot flow rate according to the duty ratio of PWM signal. Displacement of the MEASUREMENT OF THE MAIN FLOW RATE USING THE MAIN POPPET VALVE WITH THE NEW CONTROL WINDOW As shown in Fig.16(b), a new built main poppet valve is manufactured, which improves the linearity of the main flow rate. The main flow rate has been measured using new HSSV and original HSSV under supply pressure of 10MPa (Fig.17). Comparing the main flow rate by original HSSV and by new HSSV, the latter is a little larger than

6 the former because of accuracy of fabrication. However, the linearity of the main flow rate by new HSSV is improved in whole duty ratio. It was revealed that the linearity of the main flow rate is able to be improved by adjusting the shape of the control window and the displacement of the main poppet valve. CONCLUSION The optimum attenuator is investigated, and the smallest possible attenuator with which the poppet does not hit the valve seat has been developed. From the experiment of position control of a water hydraulic cylinder, it is revealed that the position of actuator follows the reference accurately by the improvement of HSSV discussed in this study. Method of adjusting the shape of control window of main poppet has been proposed to improve the linearity of main flow rate of HSSV. The main flow rate of HSSV with new shape of control window was measured. As a result, the improvement of linearity of main flow rate by adjusting the shape of control window has been verified. REFERENCES (a) Original window (b) New window Figure 16 Control window of main poppet 1. Park, S.H., Kitagawa, A. and Kawashima, M., Water Hydraulic High Speed Solenoid Valve (Part1: Development and Static Behavior), Proceedings of the Institution Mechanical Engineers, Part1, Journal of systems and Control Engineering, 2004, 218, pp Park, S.H., Kitagawa, A., A Study on Noise and Flow Fluctuation Reduction of Water Hydraulic Two-Stage High Speed Solenoid Valve Using Leakage Flow around the Main Poppet as Pilot Flow, The Japan Society of Mechanical Engineers, Part C, 2005, , pp (in Japanese) 3. Minematsu, S., Kitagawa, A., Liu, C., Fuchigami, K. and Kawashima, M., Study on Proportional Poppet Type 2-stage Water Hydraulic High Speed Solenoid Valve with Attenuator, The Proceedings on Autumn Conference of Japan Fluid Power System Society, 2009, pp (in Japanese) Figure 17 Average main flow rate by duty ratio