A DEVELOPMENT OF WATER HYDRAULIC HIGH SPEED SOLENOID VALVE

Transcription

1 1B22 A DEVELOPMENT OF WATER HYDRAULIC HIGH SPEED SOLENOID VALVE Sung-Hwan PARK*, Ato KITAGAWA*, Masato KAWASHIMA**, Jin-Kul LEE***, Pindong WU* * Department of Mechanical and Control Engineering Tokyo Institute of Technology O-okayama, Mekuro-ku, Tokyo, , Japan ( kitagawa(a)ctrl.titech.ac.jp) **Department of Engineering, Technical Managing Team Tohoku Steel Co.,Ltd 23 Daijimuratajinisigaoka, Muratamati, Sibatagun, Miyagiken, , Japan ***Department of Mechanical Engineering, Faculty of Engineering Pusan National University 30 Changjeon-dong, Kumjeong-ku, Pusan, , Koera ****Electro. Mechanics Centre Beijing Institute of Technology 7 Baishigiao Road P.O.Box327, Beijing, , China ABSTRACT Oil Hydraulic systems are employed in a wide range of engineering fields because of their rapid response and high power density. However, increasing public awareness of environmental issues call for environmentally friendly pressure medium like water to prevent environmental damage caused by potentially harmful material leakage from oil hydraulic system. In this study, a water hydraulic high speed solenoid valve with two stage mechanism and no inner leakage is developed. The particular feature of this valve is the utilization of the leakage from the clearance between main poppet and sleeve as inner pilot flow. The experimental results using the prototype valve show excellent response characteristics and that proposed valve is valid for the control of water hydraulic systems. It is also discussed to improve the performance of this valve by the alteration of the design parameters. KEY WORDS Water hydraulics, High speed solenoid valve, Quick drive circuit NOMENCLATURE A: area of orifice Aa: clearance area Acm,Acp: gap area between poppet and seat Ahm, Ahp: cross section area of orifice Bm,Bp: viscous damping coefficient of poppet Cd: discharge coefficient of flow etm, etp : difference of lag time f: PWM carrier wave frequency Fkp: spring force of pilot valve Fluid Power. Fifth JFPS International Symposium(c)2002 JFPS. ISBN

2 Fsol: suction force of solenoid h: clearance between poppet and sleeve I2: retention current of quick drive circuit Km,Kp: spring constant Ksm,Ksp: steady-state flow force Ktm,Ktp: transient flow force Kv: bulk modulus of water Mm,Mp: mass of poppet PA,PD: pressure of port A, D pm: lower chamber pressure of main poppet pp: pilot pressure ps: supply pressure qa: leakage flow from main poppet qm,qp: flow rate of main valve pilot valve T1,T3: magnetization demagnetization time ton, toff: valve opening closing lag V1,V3: magnetization demagnetization voltage Vp: volume of upper chamber of main poppet xkm0, xkp0: initial compressed length of spring Subscripts xm, xp: displacement of poppet p: density of waterĖ : duty ratio m: main valve p: pilot valve INTRODUCTION Hydraulic systems are used in a wide range of engineering fields in the aircraft, marine, mobile and manufacturing sectors because of their rapid response and high power densities. Additional benefits of hydraulic systems include instantly reversible motion, automatic protection against overloads, and infinitely variable speed control. In spite of all these highly desirable features, it is not a panacea for all power transmission. Hydraulic systems also have some drawbacks. Hydraulic oils are messy, and leakage is impossible to eliminate completely. Also, most hydraulic oils can cause fires and environmental pollution if an oil leak occurs. Pneumatic systems can be one of the good solutions for clean and safety systems. However, these systems do not.offer sufficient force and controllability. Water hydraulic systems that use water as a pressure medium are good solutions for these problems. In recent years there has been growing interest in the use of tap water to the hydraulic systems, which are designed to improve safety and to protect environmental pollution. These systems consist of specially designed pump, actuators and several control valves, which can be obtained in the market area. However, the price level is much higher than that of oil hydraulic systems and these systems still have some limitations for the performance and efficiency. In many cases of water hydraulic applications, the water hydraulic servo valve has been used for the control, but the large amount of internal leakage and wasteful flow from the nozzle flapper should not be ignored for the control accuracy around the null and efficiency. Through the use of fast switchable poppet, it should be possible to prevent internal leakage and wasteful flow and to realize much lower price rather than servo valve, whilst achieving the fast response and good control performance. In respect to high speed solenoid valve which use poppet, there already exists that for oil hydraulic systems. However, when it is implemented direct to the water hydraulic systems, large amount of an internal leakage occurs at pressure above 2 [MPa] [1]. In addition, high speed solenoid valve for the water hydraulic systems is also developed by the Vilenius[2], but it is only able to use at low pressure and flow rate is too small. This study is concerned with the development of high speed solenoid valve for the water hydraulic systems. In order to improve above mentioned problems, it is proposed that simple two-stage mechanism which uses leakage flow from the clearance between the main poppet and sleeve as a pilot flow is necessary, which permits relatively large flow rate and use of small capacity solenoid. STRUCTURE OF THE PROPOSED VALVE The schematic diagram of structure for the proposed valve is shown in Fig.l. This valve is designed to be structure of two-stage mechanism. As shown in Fig.1, the pilot stage ball type poppet valve is located in the upper valve body and driven by its own built-in solenoid. The lower body contains main valve shaped like needle type poppet, which is connected to actuator. Since both stage take up poppet type, inner leakage does not occur at the time of valve closing. This particular structure permits no leakage from even each part of components through the use of 0 ring which is installed in several places. In addition, it is possible to get rid of wasteful flow by the fact that main flow from the port A and pilot flow from the port D are joined. Three ports of S1-S3 on the housing side are designed for measurement of transient pressure. This valve is normal closed type valve whose basic function is to control the flow rate from the main valve by the fast switching of pilot stage ball valve. In normal state, main valve is held seated on its seat by a light spring and slightly unbalanced pressure. When the pilot

3 stage ball valve is driven by energized solenoid, the restricted flow q, through the orifice result in an increase. This causes an unbalance in hydraulic force, which tends to raise the main valve off from its seat. When the pressure difference between the upper and lower chambers reaches larger than the spring force, the main valve lifts off from its seat to permit flow direct to port A. Conversely, if the pilot stage ball valve is closed according to the de-energized solenoid, the pressure of the upper and lower chambers goes up rapidly and becomes equal to the supply pressure. Eventually, main valve is closed and held seated on its seat by a light spring and slightly unbalanced pressure. Dynamic equation of the main valve can be written as Also the equation for the pilot valve, which is similar with the above equation, can be expressed as Eq.(4). It is affected by a pressure difference of the orifice area Ahp because the pressure pp acts upon the plunger. (2) (3) (4) The water hydraulic valve proposed by Vilenius[2]is general one-stage mechanism poppet type valve, which can only be applicable to low-pressure. The structure of pilot valve for this study is almost same with that, although this valve is applicable to high-pressure because of the two-stage mechanism. From this analysis, it is possible to explain the reason why the one-stage mechanism poppet type valve can only be applicable to low-pressure. It is necessary as for the control valve that the valve have to be opened and closed according to turning on and off of the solenoid regardless of the value of the downstream lateral pressure pp and pp. From the Eq. (4), the stationary closing condition for the spring can be given by (5) The condition of suction force of the solenoid for the valve opening is given by (6) Fig. 1 Water hydraulic high speed solenoid valve The equations, which are used in characteristic analysis of this valve is shown in below. The amount of pilot pressure change can be expressed as (1) The size of the solenoid is restricted by the size which is required to the valve body, also size of the suction force Fsal is decided attendant upon that. In addition, in order to obtain the process precision which does not have a leakage, there is a limit to make the hole area of the seat Ahp small. Consequently, from the Eq.(6), the system pressure is restricted. Furthermore, since the area A for the orifice of the pilot valve and maximum value of the pressure difference Ap is Ahp, Ppmax-PDmin respectively, It can be noted that there is the upper limit in flow rate from Eq. (2). The flow rate from the each orifice is given by In order to expand flow rate, the valve which is

4 developed in this study has made two-stage mechanism structure. In respect to the main valve, a driving force is not the suction force of solenoid, being to be the pressure difference due to the pressure drop of pilot stream, It should be possible to obtain large driving force, which can make it possible to enlarge flow rate. The method of expanding flow by applying the two-stage mechanism has been already used in the field of oil hydraulics. In case of oil hydraulics, since the viscosity of the oil is high and the leakage from the clearance between the main valve and the sleeve is too small, it is necessary to provide a passage and the orifice to produce the pilot flow, which can cause pressure difference between the top and bottom of the main valve poppet. Therefore, when this structure is used for the water hydraulic systems in the same way, it does not operate properly because of much leak from the oil passage. developed valve. Fixed supply pressure, which is adjusted by a relief valve, is supplied to the experimental system from the water hydraulic power unit whose maximum pressure is 7 [MPa] and maximum flow capacity is 40 [L/min]. The valve is operated by the PWM signal which is calculated from the computer and passed through a driving circuit named Quick-Drive Circuit(QD)[3]. In this study, it is proposed that the leakage from the clearance is utilized as pilot flow. This is particular feature suited for the water hydraulic valve. The appearance of the prototype valve of this study is shown in Fig.2. Almost every part of this valve is composed of stainless steel and especially, only the part which is related to a drive of the solenoid is composed of electromagnetic solenoid steel KM-38. This valve is designed to permit 5 [L/min] as main valve ow rate when the supply pressure is 14 [MPa] and fl80% load was imposed. Fig. 3 Experimental apparatus Fig.4 shows principle of operation for the QD. This circuit is able to drive the valve in high frequency in comparison with when the conventional Darlington circuit is used. Therefore, it is obvious to be able to improve control performance because dead-band of the valve is reduced by the use of QD. Input voltage Output voltage Fig. 2 Appearance of developed valve Output current EXPERIMENTAL APPARATUS The basic block diagram for the experimental circuit is shown in Fig.3. The whole inside the dotted line reveals Fig. 4 Quick drive circuit

5 During the section 0 for magnetization, just between the ascent of the input signal and the time T1, high voltage Vi is impressed in order to compensate the rising lag of electric current due to inductance. This eventually shortens valve opening lag time. During the section 0 for retention, valve opening state is maintained with the small electric current I2. During the section 03 for demagnetization, just between the descent of the input signal and the time T3, opposite polarity voltage V3 is impressed in order to demagnetize residual magnetism forcibly by the use of backward magnetic field of O. As result of this, valve closing lag time is shortened. The output current of the QD is measured with the non-contact type electric current probe. Moreover, in order to investigate the operation lag time for opening and closing of the pilot valve, the shock sensor is installed on top of the valve. The impact which occurs instantaneously when the ball and the seat, the plunger and the stator crash is measured. Flow rate is measured by the time that the water of fixed capacity flows out. (a) Main valve The whole experiments are carried out at the condition of supply pressure Ps =7.0[MPa], PWM carrier wave frequency F =50[Hz] and no load. FLOW CHARACTERISTICS PROTOTYPE VALVE OF In order to use high speed solenoid valve as flow control valve of the system, it is ideal that flow characteristic due to duty ratio z of PWM signal has linear characteristic at all over the duty ratio range. The static flow characteristics of prototype valve and improved characteristics through the investigation of experimental results are shown in Fig.5. In Fig.5, experimental results represented by symbols o and ^ are obtained through first manufactured prototype valve. The ideal characteristic can be represented by the line which ties flow at duty ratio z =1 (100 [%]) and origin. However, actual experimental results reveal different tendency. All of the measured points are a little bit shifted up. Moreover, flow is saturated at high duty ratio and large flow comes out even in the vicinity of t =0. The difference with the ideal flow, in the ratio for maximum flow, is approximately 35% and 20% for the main valve and the pilot valve respectively. This like characteristic seems to be related to operation lag of the valve. In order to improve this characteristic into ideal one, on-off characteristic of the valve is deeply investigated. (b) Pilot valve Fig. 5 Flow rate characteristics according to duty ratio The PWM signal, output current of the QD and the shock wave of opening and closing the pilot valve are shown in Fig.6. The opening lag time of the valve can be measured by the time difference between the PWM signal rising up and the time to occurrence of impact. Also the closing lag time of the valve is measured in the same manner. As shown in Fig.6, it is obvious that valve closing lag toff is large in comparison with the valve opening lag ton. This difference etp = toff - ton becomes a cause of difference with ideal flow characteristic. Namely, the valve keeps opening and flow is saturated in the high duty range where length of off time of the PWM signal becomes short. In all over the range where the duty is lower than that equal to amount of the etp, flow increases according to the increase of z. As for the main valve, flow becomes larger because it is affected by etp and etm as well. Therefore, it is necessary to reduce valve closing lag in order to improve tendency of flow characteristic. In addition, when the supply pressure is 7.0[MPa] and these is no load, the theoretical maximum

6 fl ow can be calculated as 8.0 [L/min] from the Eq.(2), but the measured maximum flow is only 4.7 [L/min]. Also the theoretical pilot flow can be calculated as 2.9 [L/min], but the measured pilot flow is only 1.2 [L/min]. These also have to be improved. enlarged. At the beginning of manufacturing the prototype valve, the clearance h is selected as 30 [Đm]. This value was changed to 50 [Đm]. The reason for the maximum flow becomes smaller than the design specification is discovered from several experiments. When the main poppet is removed from the valve, the supply pressure ps and pilot pressure pp is same theoretically. However, it is recognized that the supply pressure ps is always larger than the pilot pressure pp from the experiments. This fact means that P port of the manifold becomes resistance for the supply pressure. Eventually the maximum flow rate is increase after enlarging the P port of the manifold. Fig. 6 On-off characteristic of pilot valve In this chapter, flow characteristic of the developed valve is analyzed and it can be obtained satisfactory flow characteristic by modifying design parameters. In Fig.5, experimental results represented by symbols + and reveal characteristics of improved valve. As for the result, maximum flow rate becomes large and also the tendency of flow characteristic is improved substantially in comparison with first manufactured prototype valve. CONCLUSION CHARACTERISTIC IMPROVEMENT If the spring which push down the pilot valve toward seat is made strong, valve closing time becomes fast. Conversely, this means there is possibility for the valve opening time to be slow. However, in order to improve the tendency of flow characteristic, it is good for the difference et, to become small. The spring implemented to the prototype valve is selected as weak as possible through the calculation with Eq.(5). But, it is recognized that, in real applications, it moves without problem after changing the spring of F = 11 [N] to that of F = 24[N]. Measured valve opening and closing time are shown in Table 1. Table 1 Opening and closing time of the valve In the case that if response of the pilot pressure p, is made fast, also response of the main valve poppet which uses the change of pressure as driving force becomes fast. This can be achieved by the fact that clearance h is High-speed solenoid valve which can be applicable to the high-pressure water hydraulic systems of large flow was developed anew. Moreover, it is proposed that two-stage mechanism is necessary to prevent inner leakage and expand the flow rate. Flow characteristic of the prototype valve is analyzed and it is obtained satisfactory flow characteristic by modifying design parameters. REFERENCES 1. S. H. Park, A. Kitagawa, T.Chenvisuwat, P. D. Wu, M.Kawashima,"A Development of Water Hydraulic High Speed Solenoid Valve (First report)", Spring Conference of Fluid Power System, 2001, pp M. Linjama, J. Tammisto, K. T. Koskinen, M. Vilenius,"Two-way Solenoid Valves in Low-pressure Water Hydraulics", ASME Fluid Power System and Technology, 2000, Vol.7, pp Y. Sato, S. Sato, H. Tanaka, Y. Yanai,"Influence of Eddy Current on the Dynamic of High Speed Switching Valve", Journal of thejapan Hydraulics & PneumaticsSociety, 1993, Vol.24, No.4, pp