QUAN Long, XU Xiaoqing, YAN Zheng, and ZHANG Xiaojun

Institute of Mechatronics Engineering, Taiyuan University of Technology, Taiyuan 030024, China

1 Introduction

Pilot operated proportional direction valve is the key element in electro-hydraulic control system of mass flow.Its function is to control the operating speed, direction,position, and the strength of output force from the hydraulic actuator. Considering the principle, pilot operated proportional direction valve includes three types of open loop control valve, internal mechanical feedback valve, and electronic closed loop control valve. The open loop control type employs the proportional pressure reducing valve, or the proportional relief valve of less flow to act as pilot valve, and to control the opening of the valve by controlling the pressure at the terminals side of the main valve spool indirectly[1]. Because of the flow force influence, its static characteristic and dynamic characteristic is poor, and this type of valve is normally used in open loop control system. The internal mechanical feedback type employs the principle of displacement internal mechanical feedback. This type includes following forms like displacement force feedback, displacement follow-up feedback, and displacement pressure difference feedback[2–4]. Comparing with the first type, static characteristic and dynamic characteristic of this type are greatly improved. Owing to the mechanical coupling between the pilot valve and the main valve, it is difficult to manufacture, and is also mainly used in open loop system.The third type is electronic closed loop controlled valve,employing the external sensor to detect the spool displacement. It can replace the traditional servo valve in many industry fields and be used both in open and closed loop system. This proportional direction valve is generally named servo proportional valve. This valve can either employ a servo valve as pilot valve, while the main spool position is electronic closed loop controlled[5], or employ 4 high-speed on-off valves as pilot valve in pulse width modulation, and the main spool position is electronic closed loop controlled[6–7]. In order to improve its stability and to promote the dynamic response speed, this valve of top characteristic is controlled by pilot spool position and main spool position dual electronic closed loop[8–9], and this will perfect its control accuracy and the dynamic characteristic just like that of servo valve[10–11]. Based on this, intelligent proportional direction valve technology is employed to perform information exchange, state monitoring, fault diagnosis, and forecast[12–13]. However,because two sets of position sensor is to be employed, and related electronic amplifying and control unit is required,this will make the valve complex and expensive. Moreover,any failure in detection and monitoring section in pilot valve or main valve will make the valve invalid, and cause the system shutdown. Since the electronic closed loop valve is not compatible with that of internal feedback in structure, and is complicated in production, the element cost will be very high.

In order to overcome the difficulties that limit the development and application of the electro-hydraulic control technique, and to perfect the existing technology,this paper raise a combination control principle that will combine the internal feedback and the electronic closed loop technique in one body. By means of feed forward and decoupling technique, interference between the two types of feedback channels may be canceled. This paper also raise an innovative theory which employ the displacement pilot flow feedback principle in the 3-position 4-way valve structure, and combine the internal feedback valve with the electronic closed loop valve in one structure. This will conclude proportional direction valve of faulty tolerance acceptable in electronic closed loop type that may operate under open loop condition and in closed loop system; and will simplify the design to the proportional direction valve of top characteristic, and simplify the manufacturing skill and reduce the cost. Moreover, redundant function under failure of electronic detecting and control circuit can secure the system to operate smoothly.

2 Working Principle

2.1 Theory of displacement pilot flow feedback

Theory of displacement pilot flow feedback was originally raised by a Sweden scholar ANDENSSON[14],and this theory had become the highlight of proportional throttle valve product of American Vickers Corporation.Owing to the simply structure and the excellent dynamic and static characteristic, it found a wide application[15]. Its working principle is shown in Fig. 1.

Fig. 1. Principle of displacement pilot flow feedback

The notable character is that there is a slot on the main valve core. Throttle slot c connects the inlet of main valve at one side, and the another side of it through the throttle edge to the upper chamber of the main valve. Since the spool is still at the stable state, feedback relations between spool displacement and flow in pilot may be established.Flow in the throttle slot c must be equal to that in the pilot valve. Relationship can be described by Eq. (1):

where ωy—Area gain of pilot spool, mm;

xi—Preopening of throttle slot c, mm;

ωc—Area gain of throttle slot c, mm;

Cdc, Cdy—Flow coefficient of throttle slot c and pilot valve.

To simplify the analysis, based on Eq. (1), and when the flow force is not considered, the equations of main spool displacement and the flow through main spool may be described by Eqs. (2) and (3):

where Cdx—Flow coefficient in main valve, mm;

ωx—Area gain in main spool, mm.

Obviously, main spool displacement is proportional to the pilot spool displacement. Opening of the main spool can be dominated by displacement of pilot spool.Proportional coefficient can be regulated easily by size ωcof throttle slot c, as well as the area gain ωyof pilot valve.This process is just like that of electronic transistor, so it is normally called hydraulic transistor. Nevertheless, an area difference between upper and bottom side in main spool is required in this structure, otherwise it can’t operate. That is why it is only applied in the 2-way cartridge valve. When we want to apply this principle to the proportional direction valve of 3-position 4-way, the structure must be innovated.

2.2 Principle of the new proportional direction valve

In order to apply the pilot flow feedback principle on spool displacement to the proportional direction throttle valve, newly innovated structure is shown in Fig. 2. New structure of this includes the added valve sleeve at both ends of the spool. Throttle slot c then is designed on the sleeve, and the throttle edge then is designed to the main valve body. Passage in the main spool can conduct the compressed oil into the throttle slot c on sleeve at both ends of it. Shoulder on the main valve body can limit the displacement of the sleeve. When the main spool intends to move leftward, right side of it will suffer a inlet pressure,and left side will suffer a pressure after throttling.Meanwhile, the right sleeve is limited by the valve body and can’t move leftward. And now, a spool with area difference is constructed by the main spool and the left sleeve, and now it drives the spool moving leftward according to the amplifying principle. Oil port P–A is connected, so does the B–T. Area difference can be easily regulated by sleeve diameter variation. Provided the spool is to be driven rightward, the pilot valve can be applied to control the oil in the right chamber of main spool, and then area difference is formed between left ends of main spool and end of right sleeve. In this way, control to 3-position 4-way valve is realized by means of the pilot flow feedback principle. Obviously, there is no rigid connection between the main spool and the pilot valve, and arrangement to them is easy. As to control to pilot valve, opening of it is enough. Applicable device includes one bidirectional electric-mechanics transformer, or two 1-way electromechanical transformers, such as moving-coil type force motor, linear force motor, step motor, and proportional electromagnet, etc. Then a direction and flow control element with simply structure, and suitable for many purposes in electro-hydraulic control technique is applied.

Fig. 2. Principle of new proportional direction valve based on displacement pilot flow feedback

3 Experimental Study of New Valve

3.1 Introduction to the experiment system

Fig. 3 shows the experiment system. This system is controlled by one PC and one real-time control card dS1003 of dSPACE from Germany. In this system, flow is controlled by a high quality servo pump A10VDFEE—71,and load is changed by a proportional throttle valve,back-pressure is regulated by a proportional relief valve.Parameters to be tested include outlet pressure, pressure in chamber A and B of the tested valve, displacement of the tested valve, pilot valve, and main valve, as well as the flow in them. Flow measurement is tested by means of the gear type dynamic flow meter at high accuracy from Germany. Its model is VC-SF-1PS. Signals are fed into the computer and processed by specially designed software.Computer can display the control signals fed into the tested valve, as well as the output flow signals from the pump.

Fig. 3. Experiment system of proportional direction valve

3.2 Experiment on steady state characteristic

Fig. 4 shows the control characteristic under experiment condition of the new proportional direction valve. In this experiment, system pressure of 5 MPa is applied. By means of the proportional amplifier, set value to pilot valve is given from minimum to maximum, and then decreased from maximum to minimum. As the curve shows, prototype valve is excellent in linearity, and is poor in hysteresis. The poor hysteresis may be caused by poor processing and poor heat treatment to the spool, or by friction between the sleeve and spool, and that between the valve body and the spool because of their bad coaxial condition. Sticky of the main valve also may be caused by the bad design to that spring in the valve (used the old one in the switch valve).

Fig. 4. Characteristic curve of steady control to open loop valve

For fine control, two segments area of main throttle orifice is used. During first stage, the flow increases slowly with the input value, and then after that point, the flow increases proportionally at great gain with the set value.

Fig. 5 shows the characteristic curve of the valve under steady load condition. In this experiment, 1.6 mm, 3.5 mm,and 5.7 mm in three different opening positions of main spool are applied to the tested valve. When the outlet pressure of the load throttle valve is regulated, pressure difference at the outlet of the valve may be raised from a reasonable minimum value continuously. Because of the pressure limit of the test table, maximum value is 16 MPa.This experiment shows that the valve is in an excellent equal displacement condition even under open loop control condition. Especially under the small displacement condition, opening of the spool is almost free from the pressure difference. Even under a pressure of 0.5 MPa, the displacement of spool can be controlled. This experiment also shows that, along with the growth of opening value,required pressure difference shall be increased. Provided the difference is less than the minimum acceptable value,the spool displacement will not reach the required value.And along with the growth of pressure difference,displacement will be increased gradually until to a set value.Influenced by the bad sticky loop, error of spool displacement at lower pressure is a little severe when the pressure difference is in rising stage as curve 2 shown in Fig. 5. However, error of spool displacement at higher pressure is less as curves 1 and 3 shown in Fig. 5 when the pressure difference is in descending stage.

Fig. 5. Characteristic curve of steady load to open loop valve

3.3 Experiment on dynamic characteristic

Fig. 6 shows the actual characteristic curve of step response of spool displacement under pressure set value of 6 MPa and 15 MPa. In this experiment, displacement at 1 mm, 3 mm, and 5 mm are set to each pressure condition,output of the main spool is recorded.

Fig. 6. Dynamic step response curve of the new valve

From the above two curves, we can found that when the pressure is lower, time required to reach the set value is prolonged when it is increased, and there is one round corner near the set value. The nearer to the set value, the larger the corner is. However, there is no response delay in the closing stage. The main spool displacement can follow the pilot displacement without any delay under both closing and opening stage. When the pressure is raised to 15 MPa,response speed is promoted actively, even at large displacement and near the steady state, round corner is very small. That is to say, response of valve is very stable in the whole dynamic process, and free from any over-regulation and oscillation. Response in closing stage is better than that in opening stage.

4 Electronic Closed Loop Control

4.1 Working principle of electronic closed loop valve

According to the discussion in section 2 of this paper, the pilot flow feedback can control the displacement of main spool effectively, but comparison point for feedback is outside the pilot valve, and the performance is affected by the electro-mechanical transformer and design of the pilot valve. Although the performance of valve can be promoted by further improvement to control precision and electromagnet property, it still can’t meet the requirement for closed loop control at top precision demand. Superior to the existing technique, design of dual electronic closed loop control to pilot and main valve can result in a similar function as that of servo valve. That is, based on the new proportional direction valve of pilot flow feedback type,electronic closed loop control is applied, and the design combines the internal feedback principle with electronic closed loop control in the mechanical structure. Meanwhile,because control to original open loop is acceptable,redundant function of keeping operation near that original operation state under internal closed loop control can be utilized upon the electronic closed loop control was failure.Principle of electronic closed loop control is shown in Fig. 7. Controller of the valve is the PID and the feed forward controller. Role of the later is to control the vale in open loop condition, and minimum the error in the closed loop. Upon the feedback loop was failure, the electronic switch will automatically shut off the circuit, and the valve will operate in open loop condition along with alarming.

Fig. 7. Working principle of new proportional direction valve under electronic closed loop control

In Fig. 7, Kxqis the gain of feed forward controller, and Kxfis the gain of feedback.

4.2 Experiment research of electronic closed loop valve

System and process of this experiment is similar to that in open loop valve condition, refer to Fig. 3. Controller design and parameter setting shall be defined through the digital simulation. Fig. 8 shows the tested control characteristic of the new proportional direction valve.Fig. 9 shows the tested characteristic curve of steady load of the new valve at 1.5 mm, 3 mm and 4.5 mm three different opening values.

Fig. 8. Characteristic curve of steady control to electronic closed loop valve

Fig. 9. Characteristic curve of steady load to electronic closed loop valve

Fig. 10 shows the tested characteristic curve of flow control to the new valve. Obviously, control characteristic of the electronic closed loop valve is greatly improved to a tiny hysteresis and an excellent linearity. As manufacture error, there exists some asymmetry in fine control range that should be improved further. According to the load characteristic curve, if only the system pressure is greater than the minimum pressure difference, pressure variation of it will not affect the displacement. That minimum value can grow along with the growth of the spool displacement.However, the reason for the acceptable minimum pressure in system pressure decreasing stage is less than that in increasing stage shall be discussed and improved. Fig. 11 shows the step response curve of spool displacement at two directions under system pressure of 15 MPa. Since curve 2 is of integral regulation, overshoot of it is a littler higher.Curve 1 and curve 3 is of anti-saturation integral in which the extra regulation is eliminated, and response performance is greatly improved to a time of 50 ms at the maximum spool displacement condition. Characteristic difference at two directions can be improved by structure and parameter design.

Fig. 10. Characteristic curve of flow control to electronic closed loop valve

Fig. 11. Dynamic step response curve of electronic closed loop valve

5 Conclusions

(1) According to the internal feedback principle to displacement pilot flow valve, and supported by new design, normal electro-hydraulic direction valve can be changed to new proportional direction valve. Its opening area can be controlled in continuous way, while the position sensor and the complicated electronic control unit can be set free. Because of the cancel to tiny damp hollow and mechanical feedback, valve structure is simplified, and oil pollutant endurance is improved, pressure loss is reduced.However, the control performance needs to be improved further.

(2) According to the displacement pilot flow feedback principle, when the valve structure is remained and only added by one position sensor, it can be constructed to a pilot proportional direction valve in electronic closed loop control mode. Experiment has shown that the new valve is equipped with control accuracy and dynamic response characteristic just like that of the existing techniques of pilot valve and main valve displacement in dual electronic closed loop control mode. Mechanical structure and control circuit is greatly simplified.

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