Application and Development of Real-time Control System for Induction Furnaces

Modern induction melting furnaces are a combination of high-power semiconductors and microelectronics, especially computer technology. The foundry industry requires high-power, high-power-density, continuous-batch, environmentally-friendly, and computer-managed melting equipment, which is proposed for manufacturers of induction melting equipment. With new challenges, various induction furnace manufacturers have done a lot of research and practice in this area. It is generally divided into three situations: an embedded real-time control system with a microprocessor as a control unit; a real-time control system represented with a tablet-type industrial integrated computer; and a real-time control system represented by an industrial computer. The following is an example of the real-time control system of Yingda's induction melting furnace.

I. Embedded real-time control system with microprocessor as control unit

Hardware components

The hardware of the embedded real-time control system for the microprocessor is composed of three parts (see Figure 1): 1 Microprocessor control board with CPU, memory, input and output interfaces. 2 keyboard. 3 character display.

Figure 1 Microprocessor embedded control system for the control of the real-time control system

Input signal

The input signal has the operating parameters of the intermediate frequency power supply; the weight of the electric furnace; the temperature signal of the oven.

Output signal

The output signal monitors and displays the parameters of the power supply operation; diagnoses and displays faults; receives control signals linked to other devices; and prints out.

4. Several main control methods for electric furnace

(1) KiloWatt (kWh) Power Consumption Control The kWh melting mode is used to determine when the required kWh power has been put into the smelting process. When the transmission of this energy value is completed, the intermediate frequency power system can automatically reduce the output power value to the holding power or turn off. The operator can directly input the kW·h value or input the kW·h/t data of the charge material. The system can automatically calculate the kW·h count value according to the weight and output the electric energy value needed from the melting to the pouring temperature.

(2) Automatic sintering lining The sintering mode is used for the operator to sinter the lining and set the temperature profile. Sintering mode can automatically control the on/off of the VIP power supply. With the sintering setting screen, the temperature profile can be entered.

The temperature curve generally has the following four areas:

The first area: starting heating. "Temperature rise rate 1" is used as °C/h, and "Temperature 1" is used as the initial target temperature. This stage should take into account the environment or starting furnace temperature. The time is calculated only in this initial heating phase (temperature increase rate 1/temperature 1 = time course).

The second zone: "Temperature rise rate 2" and "Time 2" are used to determine the next target temperature, temperature rise rate x time = next target temperature.

Third Zone: Use "Temperature Rise Rate 3" and "Time 3" to determine the next target temperature.

Fourth Zone: Use "Temperature Rise Rate 4" and "Time 4" to determine the next target temperature.

Not all areas need to be used. The following is an example of the use of sintering curves in three areas (see Figure 2). 1 The initial temperature is 650°C, (1200°F, target temperature 1), and the rate is about 95°C/h (200°F/h, temperature rise rate 1). 2 Keep the temperature at 650°C for 2 hours. 3 Raise the temperature at 150°C/h (300°F/h, temperature rise rate 3) for 4h (time 3) until 1300°C (2400°F). The total sintering time is 12h.

The sintering process uses a "K-type" thermocouple connected to the lining and the thermocouple signal is fed back to the regulator. The regulator output voltage is 0 ~ 5V, access to the microprocessor control board. If the thermocouple breaks or burns, the feedback is the thermocouple's full scale or 1360°C (2500°F). This allows the microprocessor control board to determine when the thermocouple was damaged or burned out and stop the sintering process.

Figure 2 Sintering curve using three regions

(3) Automatic furnace start mode is used to allow operators to preheat furnace charge and lining before starting their melting cycle. The automatic cold start can automatically start the VIP device according to the time specified by the operator and control the power value of the intermediate frequency power supply so that the charge is slowly heated.

If a set parameter is entered and the automatic cold start mode is enabled, the intermediate frequency power supply will pause and wait until the specified start time arrives. When the specified time has come, this mode will issue a command to open the pump system of the intermediate frequency power supply, reset the alarm of the intermediate frequency power circuit monitor (eliminate the low water pressure alarm of the water system), and then start the intermediate frequency power supply. Once started, the IF power will increase to the level set by the operator and operate within the set time. After this time arrives, the intermediate frequency power supply device will be automatically shut down, and the sound and light alarm will be activated to inform the automatic cold furnace that the start has been completed.

(4) Control of the trigger pulse Some manufacturers integrate the pulse control of the intermediate frequency power supply with the microprocessor control system, while others do not integrate together.

Second, the real-time control system represented by the flat-panel industrial integrated computer

Its basic function and form are the same as microprocessor-controlled embedded real-time control systems. The biggest difference is that the output display screen is changed from character display to computer image display (see Figure 3).

Fig. 3 Schematic representation of a real-time control system represented by a tablet-type industrial integrated computer

Hardware components

(1) A microprocessor control board with CPU, memory, input and output interfaces is required on the IF power supply.

(2) Configuration of tablet-type industrial integrated computer: CPU, memory, video display card, hard disk, USB interface, touch-type or non-touch-type display, and input and output interfaces, etc. One-piece computer for data communication.

2. Input and output signals

The input and output contents of the real-time control system of the microprocessor are the same.

The above two kinds of induction melting furnace real-time control system content, focusing on the control and management of the intermediate frequency power supply and the electric furnace own equipment, with the continuous development of the casting industry control technology, the induction melting furnace real-time control system also needs to closely related to the electric furnace equipment Manage it. As a modern foundry, it is necessary to conduct real-time monitoring and management of the main equipment in the factory, such as the batch feeding system, electric furnace and molding line, which also requires the real-time control system of the induction melting furnace in addition to its own management. The data is transmitted to the computer at the next level so that the computer of the foundry can receive the data of the real-time control system of the induction melting furnace.

For this purpose, the real-time control system of the induction melting furnace needs to develop a control system with a wider coverage, requires an additional configuration of an industrial computer, handles the control and management of the aforementioned intermediate-frequency power supply and the equipment of the electric furnace itself, generates various melting furnaces and each The report of the time period; plus the calculation and management of the added material; receive the spectral analysis data of the secondary iron liquid composition of each furnace, and record it in the corresponding report. At the same time, a communication port is reserved for the need of the foundry management computer to read data from the induction melting furnace real-time control system.

Third, the real-time control system represented by industrial computers

The real-time control system represented by an industrial computer is shown in Figure 4.

Hardware components

(1) The microprocessor control board with CPU, memory, input and output interfaces is required on the intermediate frequency power supply.

(2) Configurations of industrial computers include: CPU, memory, video display card, hard disk, DVD-ROM, keyboard, mouse, USB interface, and monitor, as well as multi-serial input and output interface cards and other computer configurations, industrial computers and micro-processing Control board, weighing equipment, and temperature measuring equipment for data communication.

(3) In general, an enclosed computer control cabinet is additionally provided, which contains data transfer interfaces for the computer and microprocessor control boards, weighing equipment, and temperature measurement equipment.

Input signal

(1) Operating parameters of the intermediate frequency power supply.

(2) The weight of the electric furnace.

(3) Temperature signal. Including: 1 oven thermocouple signal. 2 iron transient contact temperature thermocouple signal. 3 electric furnace cooling water temperature thermocouple signal.

(4) The signal of the iron component spectrometer.

Output signal

The output signal mainly has the parameters for monitoring and displaying the power running; diagnosis and display failure; receiving control signals linked with other devices; printout.

Fig. 4 Schematic representation of a real-time control system represented by an external industrial computer

4. Several main control methods for electric furnace

(1) kWh power control.

(2) Automatic oven. Compared with the microprocessor real-time control system, the electric furnace sintering is more intuitive, and the temperature and time curve of the electric furnace sintering can be edited and compared with the actual temperature curve. At the end of the furnace sintering process, the control system updates the refractories statistics, including date of relining.

(3) automatic cold furnace start.

(4) Automatic smelting. Its main screen is shown in Figure 5.

The automatic smelting we are talking about now is not a true “automatic smelting”. The diagnosis is because there is no continuous contact type iron temperature measuring device so far. Ability. Therefore, the melting process is now decomposed into several stages. In each stage, the temperature rise parameters are set. According to the weight and the required temperature, the electrical energy needs to be calculated. After reaching the set temperature, the operator will be prompted with information. Reduce the power supply to the insulation power to avoid overheating of hot metal in the furnace. The melting operation of the normal automatic melting control method is shown in FIG. 6 .

Figure 5 Automatic Smelting Home Screen

Input parameters in the system: charge warning temperature setpoint; setpoint to the holding power temperature; sampling temperature setpoint; pouring temperature setpoint; minimum charge for the first charge of the electric furnace; full furnace charge The minimum weight and so on.

Figure 6 Operation decomposition of the melting process

After charging the electric furnace to the minimum weight of the first charge, as the power is added to the furnace, the temperature in the bath rises. If the temperature exceeds the charge warning temperature setpoint, the melt management system will give the operator a warning indicating that the operator should quickly add metal to the weld pool or it will reach the next set point.

The melting management system keeps track of the holding power temperature set point. If the bath temperature rises above this set point, the intermediate frequency power will be automatically reduced to the holding power and will remain at that level until there is enough metal. Add the furnace so that the calculated bath temperature is below this set point. When the bath temperature falls below the set point, the intermediate frequency power will return to full power. Each time the furnace operator continues to add charge to the furnace, the melt management system will repeat this process until the pool weight exceeds the minimum full weight.

When the bath weight exceeds the minimum full weight, the system will ignore the setting of the charging warning temperature and the holding power temperature, and run at full power until the sampling temperature set point is reached. When the bath temperature is higher than the sampling temperature set point, the control system again reduces the intermediate frequency power supply to the holding power level to maintain the bath temperature.

During the holding power, the operator can perform various operations on the molten bath, such as slag removal, temperature measurement, chemical analysis of the sample, and direct temperature readings of molten iron. The melting management system will replace the calculated bath temperature on the screen with the measured actual temperature of the sample. The temperature information will be immediately transmitted to the computer, and the computer program will update the temperature calculation based on the sampling temperature. If the sample is the first sample of the heat, when the difference between the calculated temperature and the actually obtained temperature is very large, the system will automatically adjust and correct the error. Therefore, from this temperature point, the calculated temperature of the molten pool should be more accurate than before. .

At this point, the operator can give the intermediate frequency power supply a warming signal and the system raises the bath temperature to the pouring temperature set point. When the bath temperature reaches the pouring temperature set point, the system will again reduce the intermediate frequency power supply to the holding power level. The furnace operator then pours out the required molten metal liquid, which is the end of the auto-melting process of the furnace.

5. Various statistical reports

(1) Calculation of Charges and Elemental Additives Furnace operators are provided with a means to calculate the cost-effective combination of charge and elemental additives to meet the alloy composition required for a single heat.

(2) Statistical record report The operator can generate reports according to the order of day and heat, mainly including daily heat record, cooling water temperature discharge report, detailed furnace electrical data (see Figure 7) and sintering report.

Figure 7 Electrical parameter curve report

IV. Conclusion

The modern induction melting furnace real-time control system not only focuses on the control and management of the medium-frequency power supply and the electric furnace's own equipment, but also requires the management of equipment closely related to the electric furnace, such as real-time monitoring of some equipment such as batching feeding system, electric furnace and molding line. And management to generate reports for various melting furnaces and various time periods; plus calculations for the addition of added materials; receive spectral analysis data for the composition of the secondary molten iron per furnace, and record them in the corresponding reports, etc. At the same time, there is a communication port to prepare the needs of the foundry management computer to read data from the induction melting furnace real-time control system so that the computer can realize real-time control of the entire foundry.

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