Taking the electric control system of differential pressure casting as the research object, based on the analysis of the PLC automatic control system, the control system objectives are formulated, the mathematical model of the control object of differential pressure casting process is established, and the PID controller is designed to realize the automatic control of the whole gas path system. The experimental results show that the differential pressure casting gas path control system has high degree of automation, safe and stable operation, and good performance for easy operation. Differential pressure casting (DPC), as a liquid forming process of cash, can produce many large and complex thin-walled castings with high dimensional accuracy and yield.
With the development and maturity of industrial control automation technology in recent years, higher requirements are put forward for the accuracy and sensitivity of differential pressure casting equipment [1]. In this paper, the gas path control system of differential pressure casting is studied, and the whole process control of the gas path system is realized by using PLC control technology. The pneumatic control system of differential pressure casting equipment includes upper and lower tank differential pressure casting equipment to control the pressure and track the pressure and performance of the equipment. Its quality and control technology are related to the degree of automation and the quality of castings. The air path control system of differential pressure casting equipment mainly includes subsystems such as air source, air intake, filtration and decompression. In the working state, after pouring the differential pressure casting equipment, the air source system is called in, the compressed air in the air source system is filtered and decompressed. After treatment, the pressure is reduced to the working pressure range. After the treatment, the pressure is lowered to the working pressure range.
The air intake system is entered into the closed container. According to the setting of the process parameters, the throttle valve adjusts the opening of the valve and changes the gas in the pipeline. Flow rate, and then the pressure in the sealed container is controlled within a reasonable range. After crystallization, the exhaust valve is opened to reduce the pressure in the sealed container, and then a pouring process is completed. Compressed air, as the power source of air source system, is an important guarantee to provide pressure for casting process. Therefore, it is necessary to ensure that the compressed air entering the gas source is clean and free of impurities, and the pressure of the gas source is relatively stable. Compressed air contains a lot of impurities, among which the amount of water and oil and gas is the most. It needs to be treated with emphasis. If directly into the equipment, it will not only reduce the life of the equipment, but also affect the quality of castings.
Therefore, the treatment of water in compressed air can be carried out by cooling adsorption dryer, and the oil and gas in the gas can be separated by high efficiency oil-water separator. The pouring time of differential pressure casting is about 15 minutes, and the stability of gas pressure is very important. Therefore, the volume of gas storage tank with large volume is larger than the sum of the upper and lower tank volumes, so that the gas pressure can remain stable. There are many pipelines and various solenoid valves in the intake system and pressure regulating system. The solenoid valve can control the flow rate into the air through the closure of the switch. The opening of the throttle valve controls the flow rate and flow rate of the compressed air, and then ensures that the compressed air enters the sealed container with a proper amount and pressure, thus realizing the process of pressure increase. Differential pressure casting has two upper and lower sealed tanks. The process is to feed the upper and lower tanks at the same time. When the pressure of the two upper and lower tanks are the same, the gas from the upper tank can be discharged or the gas from the lower tank can be increased to achieve differential pressure, and then the filling can be obtained. Pressure regulating system not only needs to ensure the simultaneous intake of upper and lower gas tanks, but also ensures stable filling and no great fluctuation of pressure. The industrial computer of differential pressure casting is responsible for collecting and processing data, and controlling the pressure change in the sealed vessel during the casting process.
At the beginning of pouring, the industrial computer can set an ideal pressure curve, and then convert the actual pressure collected by the pressure transmitter into digital input into the industrial computer. According to the deviation between the actual pressure and the ideal pressure, the industrial control computer takes the input signal as the input signal, gets the control quantity through the control algorithm, and then converts the analog output to the control valve, adjusts the flow rate of compressed air in the sealed container, and then makes the pressure in the sealed container increase according to the change of the ideal pressure curve. After casting, the industrial control opportunity will automatically open all the exhaust valves to reduce the pressure in the sealed container.
The control system of PLC includes operation console, air circuit control module, differential pressure casting main machine, pipeline and so on. The control panel of various buttons and various programmable controllers constitute the operation console. Differential pressure casting machine mainly consists of upper and lower tanks, thermostatic element crucibles, lifting pipes and so on. Pipeline and circuit are mainly controlled by electric control cabinet, which centralizes pressure instruments, combination valves, solenoid valves and other devices. It is convenient for centralized management, easy to operate, safe and convenient, not easy to be corroded by gas, and aging of components. The air supply module mainly consists of air compressor, air mixing device, electric box and some pipelines, which is convenient for centralized management, maintenance and debugging.
In the experiment, the upper tank is opened, alloy is injected into the crucible, heated to the melting state, then the middle partition is covered, and the casting gate and the riser are aligned and pressed on the partition to prevent gas leakage from the upper and lower tanks. There are bolts on the upper and lower tanks. The main function is to control the sealing of the upper and lower tanks. Compression bolts can ensure the tightness between the upper and lower tanks, and withstand the whole pressure during pouring. In the manual operation panel, the corresponding process buttons are controlled, and the output signals automatically control the valve and gas pressure, so as to realize the automatic pouring process. The PID controller is a control law for the controlled object. Its structure is simple and its participation is easy to adjust. It is a common control scheme in the automatic control system. The combination of PLC and PID control technology can better satisfy the control of differential pressure casting electrical system.
In order to meet the needs of different controlled objects, it is necessary to improve the algorithm of PID and improve the application scope of the PID controller. In the design of the PID controller, the control parameters determine the control effect.
Among them, the main tasks of control parameters kp, kI and kD are to achieve the goal of the control system. (kp proportional control can improve the dynamic response speed of the control system, reduce errors, and improve the accuracy of control. KI is integral control, which can eliminate the static error of the system, but it will affect the stability of the system. KD is a differential control, which can shorten the time of system adjustment, improve the performance of the system, and then improve the anti-jamming performance of the system.
The three parameters of PID control vary with the actual control parameters. Therefore, in the design of PID controller, three parameters should be set according to the actual parameters of differential pressure casting equipment.
In the production of differential pressure casting, the pressure in sealed container needs to be sampled continuously. If the sampling period is smaller, the accuracy will be higher, but if the sampling period is too frequent, it will cause the control valve and the control valve can not react, thus increasing the burden of the control system. The change of pressure is a continuous beam, so it is difficult to determine the maximum frequency.
Therefore, in order to improve the accuracy of the control, the sampling period is controlled at 100 ms. The electric control system of the differential pressure bearing equipment designed has been commissioned and put into use. Practice has proved that the design selection and control scheme have remarkable effect in application. The use of PLC controllable editor in differential pressure casting not only improves the efficiency of work, but also simplifies and facilitates the operation. The precision of pressure tracking in operation is high, and the pressure tracking curve and ideal pressure are high. The force curves are consistent.