Principle and application analysis of electro-hydraulic proportional valve for construction machinery?

Electro-hydraulic proportional valve of construction machinery is an element in which the proportional electromagnet in the valve generates corresponding action according to the input voltage signal, which makes the valve core of the working valve shift and changes the size of the valve port, thus completing the pressure and flow output proportional to the input voltage. The displacement of the valve core can also be fed back in the form of mechanical, hydraulic or electric.

The plug-in proportional valve and proportional multi-way valve developed and produced in recent years fully consider the characteristics of construction machinery and have the functions of pilot control, load sensing and pressure compensation. Its appearance is of great significance to the improvement of the overall technical level of mobile hydraulic machinery. Especially, it shows a good application prospect in electronic control pilot operation, wireless remote control and wired remote control operation.

1. Types and forms of electro-hydraulic proportional valves for construction machinery

Electro-hydraulic proportional valve includes proportional flow valve, proportional pressure valve and proportional reversing valve. According to the characteristics of hydraulic operation of construction machinery, there are two main types of electro-hydraulic proportional valves: one is spiral plug-in proportional valve, and the other is slide valve proportional valve.

Spiral cartridge valve is an element that fixes electromagnetic proportional cartridge valve on oil manifold block through thread. Spiral cartridge valve has the characteristics of flexible application, pipeline saving and low cost, and has been widely used in construction machinery in recent years. Commonly used spiral plug-in proportional valves have two-way, three-way, four-way and multi-way forms. Bi-directional proportional valve is mainly proportional throttle valve, which is often combined with other components to control flow and pressure. The three-way proportional valve is mainly a proportional reducing valve, which is also widely used in the hydraulic system of mobile machinery. It mainly operates the pilot oil circuit of the hydraulically operated multi-way valve. The three-way proportional pressure reducing valve can replace the traditional manual pressure reducing pilot valve, which has greater flexibility and higher control accuracy than the manual pilot valve. It can be made into a manual multi-way valve with proportional servo control as shown in figure 1. According to different input signals, the pressure reducing valve makes the output piston have different pressures or flows, thus realizing proportional control of spool displacement of multi-way valve. Four-way or multi-way spiral plug-in proportional valve can control the working device independently.

Slide valve proportional valve, also known as distribution valve, is one of the most basic components of mobile mechanical hydraulic system, and it is a compound valve that can realize direction and flow adjustment. Electro-hydraulic slide valve proportional multi-way valve is an ideal electro-hydraulic conversion control element, which not only retains the basic functions of manual multi-way valve, but also adds advanced control means such as position electric feedback proportional servo operation and load sensing. Therefore, it is an updated product of construction machinery distribution valve.

Considering the low requirements of manufacturing cost and construction machinery for control accuracy, the general proportional multi-way valve is not equipped with displacement sensor and has no electronic detection and error correction functions. Therefore, the displacement of the spool is easily affected by the pressure fluctuation caused by the load change, and the operation process depends on visual inspection to ensure the completion of the operation. In electronic and remote operation, more attention should be paid to the influence of external interference. In recent years, due to the development of electronic technology, more and more displacement sensors, such as built-in differential transformer (LDVT), are used to detect the position movement of the spool, thus realizing the closed-loop control of the spool displacement. This highly integrated proportional valve is composed of electromagnetic proportional valve, position feedback sensor, driving amplifier and other electronic circuits. It has a certain correction function, which can effectively overcome the shortcomings of general proportional valves and greatly improve the control accuracy.

2. Load sensing and pressure compensation technology of electro-hydraulic proportional multi-way valve.

In order to save energy, reduce oil temperature and improve control accuracy, at the same time, several synchronous actuators do not interfere with each other during the movement. At present, more advanced construction machinery adopts load sensing and pressure compensation technology. Load sensing and pressure compensation are very similar concepts, both of which use the pressure change caused by load change to adjust the pressure and flow of pump or valve to meet the working requirements of the system. For the quantitative pump system, load sensing leads the load pressure to the overflow valve with remote pressure adjustment through the load sensing oil circuit. When the load is small, the set pressure of the relief valve is also small. When the load is large, the setting pressure is also large, but there is always a certain overflow loss. For the variable pump system, the load-sensitive oil circuit is introduced into the variable mechanism of the pump, so that the output pressure of the pump increases with the increase of the load pressure (always a small fixed pressure difference), so that the output flow of the pump is equal to the actual demand flow of the system, and there is no overflow loss, thus realizing energy saving.

Pressure compensation is a guarantee measure to improve valve control performance. The load pressure behind the valve port is introduced into the pressure compensation valve, which adjusts the pressure in front of the valve port to make the pressure difference between the front and back of the valve port constant, so that the flow through the valve port is only related to the opening of the valve port according to the flow adjustment characteristics of the throttle port, and is not affected by the load pressure.

3. Pilot control and remote control of electro-hydraulic proportional valve of construction machinery

The technical progress of special devices for construction machinery, such as electro-hydraulic proportional valve, makes the electrical control of various systems such as gears, steering, braking and working devices of construction vehicles become a reality. For the mechanism that generally needs displacement output, the manual multi-way valve driver can be controlled by proportional servo similar to Figure 1. Electrical operation has the advantages of quick response, flexible wiring, integrated control and easy interface with computer. Therefore, the electro-hydraulic proportional valve (or electro-hydraulic on-off valve) controlled by electronic control has been more and more used in the hydraulic valve of modern construction machinery to replace the multi-way valve controlled by manual direct operation or hydraulic pilot. Another significant advantage of using electro-hydraulic proportional valve (or electro-hydraulic on-off valve) is that it can greatly reduce the number of operating handles in engineering vehicles, which not only makes the cab layout simple, but also effectively reduces the complexity of operation, which has important practical significance for improving the quality and efficiency of operation. Fig. 2 is the JMF control rocker of TECNORD Company. One rocker can effectively control multiple electro-hydraulic proportional valves and on-off valves shown in fig. 2. The rocker can realize the proportional control or on-off control in X axis and Y axis, which is very convenient to apply.

With the rapid development of digital wireless communication technology, a wireless remote control system with stable performance and reliable operation for construction machinery has emerged. The remote control receiving device installed on the mobile machinery can convert the received radio signal into a proportional signal for controlling the electro-hydraulic proportional valve and a switching signal for controlling the electro-hydraulic on-off valve, as well as corresponding signals for controlling other devices, so that all manually operated components can accept the instructions of the remote control electric signal and perform corresponding actions. At this time, the construction machinery has actually become a remote control construction machinery.

The wireless remote control transmitting and receiving system has been successfully applied to the remote control transformation of various construction machinery. From the security point of view, every digital data instruction it transmits has a special system address code, which is only used once by the manufacturer. Each receiver only responds to the transmitted signal with the same address code, and other wireless signals, even with the same frequency, will not affect the receiving equipment. Coupled with the adoption of other security measures, the reliability of the system has been fully guaranteed. It has achieved success in the remote control transformation of various mobile machinery such as loaders, rock drills, concrete pump trucks, aerial work vehicles and bridge maintenance vehicles. The industrial remote control device and the electro-hydraulic proportional valve complement each other, providing a feasible interface for the remote control of construction machinery, and the remote control device makes the electro-hydraulic proportional valve play a greater role.

4. Application example of electro-hydraulic proportional valve in construction machinery

Schematic diagram of hydraulic system of truck crane, in which only the part related to electro-hydraulic proportional valve is drawn. The machine adopts three TecnordtDV-4/3LM-LS/PC proportional multi-way valves, and the three shuttle valves in the load sensing oil circuit select the maximum pressure of three working loads and send them to the remote control port of the remote pressure regulating relief valve to adjust the overflow pressure of the relief valve, so that the output pressure of the hydraulic pump just meets the needs of the system load, thus achieving a certain energy-saving purpose. The pressure compensation oil circuit makes the flow through each valve only related to the opening of the valve, and has nothing to do with the load it bears, and has nothing to do with the load borne by other valves, thus achieving the purpose of controlling the load speed at will under any load.

An example of manual and electro-hydraulic proportional pilot control of bulldozer shovel. When the two-position three-way solenoid valve is not energized, the pilot pressure is communicated with the manual decompression pilot valve, and the shuttle valve selects the pressure control hydraulic reversing valve from the manual pilot valve; When the two-position three-way solenoid valve is energized, the pilot control pressure oil is led to the three-way proportional decompression pilot valve, and the hydraulic reversing valve is controlled through the shuttle valve.

5. Summary

The working principle, structural form and working characteristics of electro-hydraulic proportional valve of construction machinery are analyzed above, and the load sensing and pressure compensation principle of proportional valve are analyzed and studied. This paper discusses the different applications of electro-hydraulic proportional valve, especially in the pilot control and remote control of construction machinery. Electro-hydraulic proportional valve is of great significance for simplifying the operation of construction machinery, improving efficiency and accuracy and realizing intelligent operation. The further improvement of its performance and the widening of its application scope will certainly improve the technical level of construction machinery products to a great extent.

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