As you can see from the drawing above, this is a emulsion pump station with the double-pumps in parallel. The pump station depends on electro-hydraulic-mechanical unloading valves (3) to control the pressure. When the system pressure is higher than the setting valve, the automatic unloading valve (d) will be opened and redundant hydraulic oil be sent back to the emulsion box(10). If the system pressure valve is lower than the setting valve, the automatic unloading valve (d) will be closed and not do any action

In order to improve the stability of the pressure, electro-hydraulic proportional relief valve group were set to replace electro-hydraulic-mechanical unload valve group, than through the electro-hydraulic proportional control way to control the system pressure, electro-hydraulic proportional relief valve group as figure 2.

Applied Mechanics and Materials Vols. 190-191 861

Fig.2 Electro-hydraulic proportional relief valve group a-non-return valve  b-throttling valve  c-pilot operated check valve d-mechanical unload valve  e-pilot electro-hydraulic proportional relief valve

In figure 2, this valve set added a pilot electro-hydraulic proportional relief valve (e) in the original foundation, PLC control the size of the relief valve mouth by proportional electromagnetic current. When the system program or relief valve failure, the system can’t be unloaded in time, the system pressure rose sharply, when to secure value, the hydraulic controlled pilot valve (c) action, by mechanical unload valve (d) to uninstall, guarantee the system pressure is not too large[2]. This system adopts the electro-hydro proportion relief valve instead of electro-hydro  machinery relief valve. Electro hydraulic proportional relief valve was as the execution element of the system, the system uses the electro-hydraulic proportional electromagnetic valve to control the system pressure. System control diagram in figure 3 is as follows:  

Fig.3 The whole system of control chart

In the full pressure control system, the pressure sensor as the signal collector of control system, testing the pressure of the emulsion pump infusion mouth, the signal directly be input into PLC analog module, than the signal be transferred from the simulation module of the signal to PID controller of PLC processor, comparing PID operation between the acquisition signal and the set valve in PID controller, by the simulation output module the result signal be transferred to the electro-hydraulic proportional relief valve electromagnet, through the current size of the input electromagnet, controlling the size of the relief valve mouth. The pressure sensors collect the pressure signal, the signal value with the set value subtracting. when calculating value is positive value, electromagnet pick up current, the valve open mouth, the mouth open degree and the calculated value of proportional to unload system, the system pressure drop; When calculating value is negative, the electromagnet without action, the mouth closed, the system pressure rise or remains the same [3]. According to the overall control diagram of the system, all aspects formed the system can be obtained: proportional amplifier, proportional control components, and detection feedback element.  

862 Digital Manufacturing & Automation III

PID controller：PID controller is a software controller combined with PID algorithm in PLC.

Formula 1 shows the transfer function. p k

is the scale factor; I TDT is the derivative time constant.is the integration time constant;

Proportional amplifier: Its band is wider than other, so it can be as the proportional element. Its

transfer function is m K

. Electro-hydraulic proportional relief valve: this system chooses indirect measure pressure type pilot relief valve[4], and according to the main structure, transfer function can be simplified as

Fig.4 The transfer function diagram of electro-hydraulic proportional  relief valve of the transfer function diagram