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DOI: 10.1007/s00170-003-1973-7ORIGINAL ARTICLEAdvanced Manufacturing Technology (2004)J.-C. Renn C. TsaiDevelopment of an unconventional electro-hydraulic proportional valve withfuzzy-logic controller for hydraulic pressesReceived: 23 May 2003 / Accepted: 22 September 2003 / Published online: 20 February 2004Springer-Verlag London Limited 2004Abstract In this study, an unconventional electro-hydraulic pro-portional flow control valve based on a switching solenoid anda fuzzy-logic controller is proposed for application to hydraulicpresses. The main purpose of this study is the attempt to de-velop an electro-hydraulic proportional flow control valve withlowest cost. Since the switching solenoid possesses quite non-linear force/stroke characteristics, it is basically not suitable forthe development of hydraulic proportional valves. Therefore, thefuzzy-logic controller is employed to linearize the force/strokecharacteristics. The basic idea is the utilization of the numer-ically estimated pseudo-force as the feedback signal. Finally,this newly developed electro-hydraulic proportional flow controlvalve is installed in a hydraulic press. Experimental results showthat the control of the ram velocity of the press cylinder usingthe proposed electro-hydraulic proportional flow control valve isquite successful.Keywords Fuzzy-logic controller Hydraulic press Linearization Proportional valve Solenoid actuator1 IntroductionPresses are perhaps the most commonly used machine toolsfor metal forming 1. According to different types of actua-tion, they can be classified as mechanical and hydraulic presses.Some distinct advantages of hydraulic presses over mechani-cal ones are the simpler design and construction, the linearlyvariable output press force and ram velocity of the press cylin-der, the ready adjustment of the ram position and the more ef-fective protection from overload. Figure 1 shows two typicalcircuits of hydraulic presses 1. The first (Fig. 1a) representsa pump-controlled system, in which a complex variable-dis-placement pump is used to control the output volumetric flowJ.-C. Renn C. Tsai (a117)Department of Mechanical Engineering, National Yunlin University of Sci-ence and Technology,123, University Road, Sec. 3, Touliu, Yunlin, Taiwan 640, R.O. ChinaFax: +886-5-5312062rate in a manner that is proportional to an applied electrical sig-nal. Consequently, the ram velocity of the press cylinder canalso be linearly controlled. Besides, a pressure-relief valve isemployed in the circuit to control the output press force of thepress cylinder. In the second hydraulic circuit (Fig. 1b), how-ever, a simple fixed-displacement pump is utilized. The controlof the volumetric flow rate and the ram velocity of the presscylinder is achieved by an electro-hydraulic proportional flowcontrol valve. Therefore, this hydraulic circuit basically repre-sents a valve-controlled system. In addition, it may also be foundthat a pressure-relief valve and an accumulator are installed inthis circuit. The former is employed to control the output pressforce of the press cylinder, and the latter is charged with high-pressure oil at the beginning and then serves as an auxiliarypower source. Thus, the hydraulic pressure required to producea corresponding output press force is maintained constant and,consequently, a smooth control of the ram velocity of the presscylinder can be obtained.Generally speaking, the price of a variable-displacementpump is much higher than that of a conventional fixed-displace-ment pump. Besides, the response speed of a pump-controlledsystem is much lower than that of a valve-controlled sys-tem 2. Nowadays, therefore, the latter design is still pre-ferred though the former possesses the advantage of energysaving. On the other hand, the valve-controlled hydraulic cir-cuit shown in Fig. 1b requires an electro-hydraulic proportionalflow control valve, which is the most important but expen-sive component in the circuit. Figure 2 shows the schematicstructure of the valve. It can be observed that such a valveconsists of two parts. One is the proportional valve body,in which the proportional solenoid is utilized to regulate theopening area of the valve orifice. The other is the pressurecompensator, which maintains a constant pressure drop acrossthe valve orifice in case of possible variations of externalloads.Figure 3 shows the structures and characteristics of two com-monly used solenoid actuators 2. The first is the switchingsolenoid, which is widely applied to the design of traditionalhydraulic directional control valves. The second is the propor-Fig.1. Two typical circuits of hy-draulic pressestional solenoid, which is the standard component for the de-velopment of electro-hydraulic proportional valves. As shownin Fig. 3, the switching solenoid and the proportional solenoidhave quite different force/stroke characteristics, though theypossess almost the same structure. In detail, the former pos-sesses a highly nonlinear behavior regarding the force/strokecharacteristic； the static force/stroke characteristic of the lat-ter, however, is quite linear. Such a linear force/stroke rela-tion of the proportional solenoid is the key requirement for thedesign of electro-hydraulic proportional valves. For example,the valve spool, which is subjected to a constant force in thelinear working range, reaches a definite position in the valvebody according to Hookes law. This definite position of thespool signifies a definite opening area of the valve orifice. Fur-thermore, it is also observed from Fig. 3 that the relation be-tween the output force and the input current is quite linear.Consequently, the opening area of the valve orifice is contin-uously variable and is proportional to the input current. Thisis exactly the basic function of electro-hydraulic proportionalvalves.The prerequisite to utilize the low-cost switching solenoidinstead of the high-cost proportional solenoid is the lineariza-tion of its force/stroke characteristics. Previous reports 35concerning the linearization of the force/stroke curves ofswitching solenoids can be found in the 1990s. However,only the linear proportionalintegral controller was employedin these papers. The results of the force linearization werealso not fully satisfactory. In this paper, therefore, a nonlinearfuzzy-logic controller together with a closed-loop force-controlscheme are proposed to linearize the force/stroke character-istics. Moreover, instead of the installation of an expensiveforce sensor, this closed-loop force-control scheme utilizesthe numerically estimated pseudo-force as the feedback sig-nal to minimize the cost of implementation. In the following,the experimental test device for solenoid actuators is firstlyoutlined.Fig.2. The schematic struc-ture of the proportional flowcontrol valveFig.3. The structure andcharacteristics of the switch-ing solenoid and the propor-tional solenoid2 Experimental test deviceThe static test device for solenoid actuators is shown in Fig. 4.An open-loop-controlled micro-stepping motor (American Pre-cision Industries, CMD-260) is utilized to control the plungerposition of the tested solenoid. In detail, the angular displace-ment of the micro-stepping motor is proportional to the numberof pulses of the square-wave signal sent to the driver. The direc-tion of rotation can be easily controlled by sending a Hi (5 V)or Lo (0 V) signal to one input port of the driver. In addition,a ball screw is attached to the rotor, which transforms the angularmotion into a rectilinear movement. Besides, this test device pro-vides a position sensor (RDP-LVDT-D2/200) as well as a loadcell (BAB-10M) for the measurement of the position and the out-put force of the plunger. In the practical application, however, theload cell is not necessary and a low-cost potentiometer is used tomeasure the plunger position.To measure the coil excitation current, a series resistor of10 is used in the electric circuit. To produce the linearly con-trollable input current as well, a voltage-to-current transducer(i.e. amplifier) is employed. Finally, the control of the unit aswell as the acquisition and processing of the measured data areall integrated in a Pentium-III-based software controller. Fig-ure 5 shows the measured nonlinear force/stroke characteristicsof the tested switching solenoid, which is originally developedfor NG02 hydraulic directional valves.In the following sections, two procedures involved in thelinearization of the force/stroke characteristics of the switch-ing solenoid are described. The first is the estimation of thepseudo-force signal and the second is the introduction of a fuzzy-logic controller for the closed-loop force control of the switchingsolenoid.Fig.4. The static test device forsolenoidsFig.5. The measured nonlinear force/stroke characteristics of the testedswitching solenoid3 Estimation of the pseudo-force feedback signalLet the pseudo-force, Fp, be a function of the plunger position,xm, and the coil current signal, im； thus,Fp(xm,im) = A(xm)im+ B(xm). (1)From the experimental results shown in Fig. 6, Table 1 can beestablished, in which three parameters are involved, the plungerposition, the output force of the plunger and the coil current.By a curve-fitting technique, the best-fit polynomials A(xm) andFig.6. Experimental results of the graphical relation between the three pa-rametersB(xm) with minimum order are found to beA(xm) =1.8211x4m54.3739x3m+247.4310x2m395.0201xm+287.5556 (2)andB(xm) = 26.7396x4m106.8007x3m+143.9303x2m67.1442xm10.9445. (3)The family curves of the measured actual force and the simu-lation pseudo-force by curve fitting are shown in Fig. 7. Themaximal deviation is only about 2 N. Since the position of theplunger, xm, and the coil current, im, are easily measured andusually known, the output pseudo-force, Fp, can be numericallyderived from Eqs. 13.Fig.7. Comparisons between the experimentally measured actual force andthe estimated pseudo-force by curve fittingTable 1. Functional relation between the polynomials and the plunger strokexm0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8A 220 160 130 106 82 70 60 60 40B 20 19.3333 19.6667 17.5 14 12.6667 12 14 74 Design of the fuzzy-logic controllerUnlike the conventional controller, there are three proceduresinvolved in the implementation of a fuzzy-logic controller, fuzzi-fication of the input, fuzzy inference based on knowledge anddefuzzification of the rule-based control signal.(a) FuzzificationIn this study, the two input signals to the fuzzy controller arethe force error signal, e(k), and its change signal,e(k):e(k) = Fd Fp, (4)e(k) = e(k)e(k 1). (5)Here Fd: desired force signal,Fp: estimated pseudo-force signal.Figure 8 shows the fuzzy membership function for the twoinput signals to determine the degree of input.(b) InferenceThe inference process consists of a set of rules driven bythe linguistic values of the error and the error change signal.Table 2 shows the definition of the rules.(c) DefuzzificationThe defuzzification is to transform the control signal into anexact control output. In the defuzzification, the method ofFig.8. Fuzzy membership function for two input signalsTable 2. Definition of the control rulese(k)NB NM NS ZE PS PM PBPB ZE PS PM PB NB NM NSPM NS ZE PS PM PB NB NMPS NM NS ZE PS PM PB NBe(k) ZE NB NM NS ZE PS PM PBZS PB NB NM NS ZE PS PMNM PM PB NB NM NS ZE PSNB PS PM PB NB NM NS ZEcenter of gravity is used:y =nsummationtexti=1WiBinsummationtexti=1Wi, (6)where y: the output of the fuzzy controller,Wi: the degree of firing of the ith rule,Bi: the centroid of the consequent fuzzy subsetof the ith rule.The output actuating signal of the fuzzy controller, u(k),isdefined asu(k) = u(k 1)+u(k). (7)Figure 9 shows the output membership function for the actu-ating signal change, u(k).Fig.9. Output membership function for the actuating signal changeFig.10. Block diagram of the force-control schemeusing pseudo-force as feedback signal5 Experimental results of force linearization bypseudo-force feedbackFigure 10 shows the block diagram of the force-control schemeusing the estimated pseudo-force as its feedback signal. Thecorresponding experimental results of the force linearizationusing the fuzzy-logic controller are shown in Fig. 11. In therange of plunger positions 0.15 mm xm 0.95 mm, the ex-perimentally measured force signals are almost straight andhorizontal lines, which signify the successful linearizationof the force/stroke characteristics. In the same range, more-over, the estimated pseudo-force also agrees very well withthe experimental one. This further proves the validity of themodel described by Eqs. 13. However, it is also observedthat the measured force signals in the range of plunger pos-itions 0 mm xm 0.15 mm are not properly linearized. Thisis chiefly because of the obvious difficulties of compensatingthe highly nonlinear and almost vertical force/stroke curvesin this range, as shown in Fig. 5. Table 3 shows the compar-isons of the technical data between the conventional propor-tional solenoid and the new proportional switching solenoid.Because the output flow rate through the valve orifice with a con-stant pressure drop is proportional to the plunger stroke 2and the linear working stroke of the new proportional switch-ing solenoid is shorter, the available flow rate through theFig.11. Experimental results of the force linearization using the fuzzy-logiccontrollerTable 3. Comparisons between the two proportional solenoid actuatorsConventional New proportionalproportional switchingsolenoid solenoidType (Magnet-Schultz (Seven OceanGRF045) NG02)Linear working stroke 3 mm 0.8mmLinear maximal output force 65 N 25 NMaximal excitation current 0.81 A 0.45 Anewly developed proportional flow control valve is expectedto be smaller. Nevertheless, a low-cost proportional flow con-trol valve using the new proportional switching solenoid insteadof the conventional proportional solenoid has been successfullydeveloped.6 Installation in a hydraulic press and experimentalresultsA valve-controlled hydraulic press is used in the laboratory toverify the performance of the proposed unconventional propor-tional flow control valve. The piston diameter and the maximalstroke of the ram are 350 mm and 250 mm, respectively. Themaximal ram velocity is 300 mm/min, corresponding to a vol-umetric flow rate of 30 l/min. The output press force reaches2000 kN if the supply hydraulic pressure is set to be 210 bar. Tomeasure the ram velocity, a linear potentiometer is firstly utilizedto acquire the signal of the ram position. Then, the ram velocityis numerically obtained byvm(k) =xm(k)xm(k 1)Ts, (8)where vm: measured ram velocity,Ts: sampling time.Figure 12 shows the experimental step responses of the ramvelocity to three different force inputs. Because the pressurecompensator maintains a constant pressure drop across the valveorifice, the output flow rate is proportional to the stroke of thesolenoids plunger. It is thus observed that the ram velocity iscontrollable in a manner that is proportional to the desired in-put force and the performances are satisfactory. This proves thefeasibility of the attempt to develop a low-cost unconventionalproportional flow control valve. On the other hand, however,the maximal ram velocity corresponding to the maximal desiredinput force of 25 N reaches only 96 mm/min, which is approxi-mately 1/3 of the original one.Fig.12. Experimental step responses of the ram velocity to three differentforce sett