Application of Industrial Computer in Arc Furnace Control System

With the popularization and application of computer technology, digital technology has penetrated into various industries, especially in the transformation and innovation of some traditional industries, playing an important role. The industrial computer introduced in this article participates in the automatic control system to control the arc furnace, which greatly improves the technical content of the control system. The production practice has proved that the system is much better than the traditional analog control method in terms of safety, reliability, sensitivity, etc.

1 System composition

The system consists of an Advantech 5X86/133 industrial computer, input and output circuit boards (D/A board, A/D board and trigger pulse board), WB series current and voltage sensors, Fuji G11 inverters (four units, one of which is a spare) and AC asynchronous motors. The system block diagram is shown in Figure 1.

This system uses electrical transmission to control the up and down movement of the electrode. After calculation, an asynchronous motor with a power of 7.5kW is selected, and the inverter is selected based on this. This system uses Fuji G11 inverter, which has the following characteristics: it does not require positive and negative conversion signals, and can control the speed and direction of the motor by only controlling the size and polarity of the output voltage according to the given frequency.

Since the arc current of the electrode is too large (about 25kA), the sensor samples from the current transformer. Because the input signal of the A/D conversion card is voltage type, we use the electromagnetic isolation, inductive WBI412as1Q current-voltage sensor. For the arc voltage (generally around 250V), there is no need for an intermediate link, and the electromagnetic isolation, inductive WBV414asQ voltage-voltage sensor can be directly used.

In order to facilitate control and maintenance, each phase of the electrode is an independent device (including sensors, inverters, motors and printed circuit boards). Therefore, the principle of independent control by phase separation is adopted in the design of the control software. In addition, the setting of the rated current and the 0-12V power supply required for the power supply of the power module can be achieved through a circuit composed of a small transformer and WY1-7812, WY2-7912, etc., which will not be described in detail here.

2 Hardware Design

The control principle of the system is shown in Figure 2.

This system uses the PCL-730 A/D conversion card developed by Advantech. This card provides 32 DIO (Digital Input Output) channels, including 16 inputs and 16 outputs. Each channel has optocoupler isolation to improve the system's anti-interference ability. The PCL-728 D/A conversion card with a resolution of 12 bits is selected as the output card. This card has 2 channels, and its output voltage range can be set to between -10V and +10V using the jumper of the card, so that the inverter can be directly controlled. In the above system, the PCL-813 A/D conversion card with a resolution of 12 bits is selected. It can provide 32 analog input quantities, so only one card is needed to realize the analog-to-digital conversion of arc current, arc voltage and current setting value.

Figure 1 System composition block diagram

Figure 2 Control system block diagram

Figure 3 Switching between analog system and digital system

Figure 4 Frequency converter and related circuits

Figure 5 Inverter switch

This system is divided into two working states: manual and automatic. The automatic working state is further divided into analog state and digital state. In the automatic working state, the relay contact can select the analog path or the digital path (the digital path is generally connected to the normally closed contact) through the conversion switch. The switching structure is shown in Figure 3. In the manual working state, the circuit is relatively simple to implement. It can directly drive the inverter without going through the intermediate circuit and signal processing process. Therefore, it has a higher priority and is suitable for emergency operations in emergency situations. Its switching with the automatic state is shown in Figure 4.

Terminal 12 of Fuji inverter is the set voltage input terminal, terminal 11 is the common terminal of analog signal, FWD is the run/stop command terminal, and CM is the contact input common terminal. Because it is not possible to operate the inverter by directly connecting and disconnecting the main circuit power supply. Using the circuit shown in Figure 5, while keeping the main circuit connected, just press the button switch SB to energize the AC contactor coil, attract KM-1 to light the indicator light, and KM-2 to supply power to the inverter (see Figure 4).

This type of inverter is set, monitored and queried through the keyboard and panel operation system (screen conversion hierarchy). The operator sets the parameters he needs to determine through the LCD monitor and operation keys, such as the maximum frequency, rated voltage, acceleration time, torque characteristics, etc. During the operation, select the data he is concerned about (such as output voltage) as the real-time monitoring object. The designer can also design appropriate braking resistors, process alarm output signals and anti-interference measures according to the actual requirements of the system.

3. Software Design

The software of the arc furnace control system is developed according to the actual situation of each furnace. In this project, the system software consists of three parts: interface module, operation module and printing module. The interface program provides a human-computer dialogue window, where the operator can clearly understand the current parameters of the electric furnace and the size of the digital value of the analog quantity input from the outside (such as the gear voltage). In addition, the revision of external parameters is also realized by inputting from the keyboard using the human-computer interface. The interface of this system is compiled with BORLANDC and assembly language, and strives to achieve the purpose of beauty, intuitiveness and easy operation during the compilation process.

Input and output acquisition and operation are the core part of this system. Based on the work experience of predecessors, the input and output channels and forms suitable for the system are selected through jumpers and small conversion switches (PCL is a highly integrated board that provides a variety of functional options, such as output in voltage form or current form, etc.). In the control operation process, according to the different conditions of the arc furnace's arc ignition period, melting period, refining period, and arc flow and arc pressure before the molten steel is discharged from the furnace, the algorithm implementation of the control subroutine should be different. The program flow chart is shown in Figure 6. In the control process, the characteristics of the voltage rate of the G11 inverter are fully utilized (as shown in Figure 7), reducing the complexity of hardware design. The determination of the current setting value and the voltage setting value selects different conversion algorithms according to the power supply voltage that the external circuit can provide. It is not enough to only consider acquisition and conversion in the software. This system uses the PID control algorithm for data processing. In terms of control mode, the simulation system is implemented through hardware circuits, while the software design is completed through the PID algorithm. In order to ensure that the smelting temperature is constant and the movement of the electrode does not fluctuate too much, a given integral link is added to the control system. The parameters of the circuit can be set by referring to the parameters of the analog regulator, the attenuation curve method, etc., to obtain good proportional integral (PI) control parameters.

Figure 6 Flowchart of running program

Figure 7 Speed-voltage curve

Figure 8 Current curve and power curve

(a) Current curve (b) Power curve

Due to the involvement of PC, the control software should also consider the recording and preservation of data, especially some important parameters, power curves, current curves and voltage curves should be printed out to provide first-hand information for future fault diagnosis, maintenance and equipment improvement.

In addition, the parameters involved are estimated and calculated in advance, and their maximum values ​​are determined to ensure that the acquired or processed data is within its value range and will not overflow. This not only ensures the rationality of the results and the reliability of the execution, but also avoids data jumps that cause shortened component life and mechanical damage.

4 On-site anti-interference design and operation results

The working environment of the smelting furnace is generally bad and the interference is serious. To address the problems on site, the following measures have been taken:

(1) Due to the high voltage in the steel plant (sometimes the control voltage reaches 270V

Around, midnight is higher), for this reason, the power supply of the industrial computer must be equipped with a voltage stabilizer to ensure the normal operation of the industrial computer;

(2) The electric arc furnace generates a very strong electromagnetic field during the smelting process.

Interference seriously affects the accuracy of the control circuit, so the input and output lines of the inverter and industrial computer all use shielded wires, and the display is selected as LCD (the electron gun type display screen will be interfered in the magnetic field, causing the picture to be distorted and unable to work normally);

(3) The ground wire of the inverter and industrial computer power supply is short-circuited with the fixed neutral wire;

(4) The air on site contains a large amount of metal dust, which is easy to be contaminated.

Seal the contaminated electrical equipment and remove dust regularly with a dustpan.

By adopting the above measures, the interference is basically eliminated. Figure 8(a) is the current curve obtained during the smelting period, and Figure 8(b) is the power curve, where 0~t1 is the initial arc ignition period, t1~t2 is the melting period, and t2~t3 is the refining period.

5 Conclusion

After the trial operation during the debugging period, this system has been well received by various departments of the factory for its simple operation, smooth electrode rise and fall, stable current, energy saving, and easy operation, which are much superior to the analog system. This control system can be further promoted and used in this industry.