Design of intelligent ignition control system based on single chip microcomputer

    Intelligent ignition control devices are widely used in the metallurgical industry, mainly in steel rolling annealing furnaces, ring furnaces, bell furnaces, etc. However, there are certain defects in the ignition control devices used in the metallurgical industry. The ignition control time is fixedly designed according to the on-site production environment, but different production processes have different requirements for the control of ignition time. The
    utility model intelligent ignition control device can control the ignition time to ensure that the ignition process is realized one by one according to a certain procedure during ignition. At the same time, the control parameters can be modified at any time to facilitate the use of different production processes to ensure that the ignition process is completed smoothly and safely. During the use of the ignition device, an ultraviolet (UV) sensor is used to detect the flame in time, and when there is no fire in the heating furnace, the software will alarm. If the software alarm fails, the hardware circuit will alarm after a certain delay. Such double protection ensures that the entire ignition control device is safer and more reliable when used.

1 System Design
1.1 System Overall Block Diagram
    The overall block diagram of the system is shown in Figure 1. In the figure, all the input and output of the CPU are completed through optical coupling, which enhances the anti-interference ability of signal transmission. Remote reset puts the device into working state, mode control controls the ignition time and the main valve opening time, the flame sensor transmits the signal to the CPU through the flame detection circuit, the CPU outputs the signal to the hardware control logic, the hardware control logic performs ignition, opens the ignition valve, the main air valve and the air valve, and alarms. Finally, the alarm signal is input to the CPU to let the CPU know the operating status of the device, and then issue the correct instructions to make the entire ignition control process proceed in an orderly manner.

1.2 Main functions of the system
    Under normal circumstances, the CPU controls the hardware control logic according to the input signal to perform ignition, fire detection, opening and closing valves or alarms. When the CPU runs away or stops working due to other reasons and enters an abnormal working state, the hardware control logic will automatically start the alarm after a certain delay. Such multi-layer protection makes the entire device safer and more reliable during operation.
    The main functions of the system are as follows:
    1) Press the start signal to enter the working state. First, detect whether there is a false flame signal. If there is, output a fault signal. If normal, turn on the ignition transformer and ignition valve to enter the ignition state. If there is fire, the ignition is successful. Open the main gas valve. If there is no fire, the ignition fails. After 2 seconds, all control outputs are closed, the alarm is locked, and a failure signal is output.
    2) After the ignition is successful, the opening time (safety time) of the main gas valve can be set by the switch (3S, 5S or 10S).
    3) The action of the air valve can be selected to follow the opening of the ignition valve, or follow the opening of the main gas valve, or be controlled by external manual opening (for furnace purging).
    4) After normal operation, if there is a flame signal, keep normal operation until the working signal is cancelled and return to standby state. If there is no flame signal, the CPU will control the processing method. There are 3 methods as follows:
    ① Immediately close all control outputs, alarm lock, and output failure signal.
    ② After a delay of a period of time (the time is adjustable), if there is still no flame signal, close all control outputs, alarm lock, and output failure signal.
    ③ Close all control outputs and restart. If ignition is successful, enter the working state. If ignition fails, close all control outputs, alarm lock, and output failure signal.
    When the CPU runs away or stops working due to other reasons, the hardware control logic will automatically start the alarm after a certain delay and output a fault signal.
    5) The controller still detects the flame signal in the standby state. If a flame signal appears for 10 seconds continuously, the control output power supply will be disconnected, the alarm will be locked, and a fault signal will be output.
    6) After the fault/failure state occurs, it will remain locked and will not be affected by the working signal or power failure. The fault/failure state can only be released after manual intervention and reset.
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2 Hardware Design
    This device is mainly composed of CPU circuit, ignition circuit, fire detection circuit, hardware control logic and relay protection circuit. The CPU uses MEG16 chip to control the entire ignition process; the ignition circuit ignites the heating furnace; the fire detection circuit detects the flame in the furnace in real time; the hardware control logic controls the output of the entire ignition device, and can automatically start the alarm when the CPU fails; the relay protection circuit is to control the valve opening sequence and protect the circuit. The following is a detailed introduction to several circuits.
2.1 Hardware Control Circuit
    The hardware control circuit is shown in Figure 2. J is a double-coil relay. The upper coil can only control the switch to move downward, and the lower coil can only control the switch to move upward, and it has a memory function when the power is off. It can be seen from the figure that the alarm is controlled by 555 and CPU at the same time. When there is no alarm, the control chip ULN2003A is enabled, and the CPU controls the ignition transformer, ignition valve, main gas valve and air valve through ULN2003A, and the device works normally. When working normally without fire, the flame sensor transmits the signal to the CPU, and the CPU gives an alarm signal to turn on the transistor Q, and the relay J works. ULN2003A is not enabled, and all CPU outputs are locked. The device enters a locked state without any output.

    When there is no fire, and the CPU does not give an alarm signal for some reason, if one of the ignition valve or the main gas valve is open, 555 will delay for a period of time to give a high level to turn on Q, ULN2003A will not be enabled, and all outputs will be locked. During the delay period of 555, if there is fire again or the two valves are closed at the same time, 555 will automatically delay again.
    Due to the dual protection of CPU and 555, the entire device is safer when in use.
2.2 Ignition circuit
    The ignition circuit is shown in Figure 3. In the figure, MOC3023 is a bidirectional thyristor optocoupler, SCR is a bidirectional thyristor, and RV1 is a varistor, which plays the role of protecting the circuit. When there is an ignition signal (low level), the optocoupler works, the 4th and 6th terminals are turned on, and there is a voltage difference between the G and K terminals of the SCR, which triggers the SCR to turn on, and the A and K terminals are turned on. The transformer works and outputs high voltage to the ion rod to generate sparks, thereby igniting. Since greater interference will be generated during ignition, C2 and R5 are connected to the live wire and the neutral wire to filter and reduce interference.

2.3 Flame Detection Circuit
    The emission spectrum of flame is composed of electromagnetic radiation bands of ultraviolet light, visible light and infrared light. Flame detection can use ultraviolet sensors and infrared sensors. This device uses ultraviolet sensors.

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    The working principle of the UV sensor is shown in Figure 4. After a voltage is applied between the cathode and anode of the UV sensor, when the ultraviolet rays in the flame shine through the quartz glass tube on the cathode of the photoelectric surface, the cathode will emit photoelectrons because the cathode is coated with electron-emitting substances. Under the action of a strong electric field, the photoelectrons are attracted to the anode. When the photoelectrons move at high speed, they collide with the gas molecules in the tube to ionize the gas molecules. The electrons generated by the ionized gas collide with the gas molecules again, and finally the cathode and the anode are filled with a large number of photoelectrons and ions, causing avalanche discharge and generating a large current in the circuit. When there is no ultraviolet irradiation, there is no flow of electrons and ions between the cathode and the anode, presenting a very high impedance.

    The peripheral detection circuit is shown in Figure 5. The input end in the figure is the output end of the UV detector. When there is a flame, there is a large current signal at the output end, and the current size can be measured. The current passes through the resistor to the base of Q1, and the collector of Q1 is connected to the base of Q2. Then there is a voltage difference between the emitter of Q2 and Q1, so that the flame indicator light is on, and the flame signal is transmitted to the CPU through the resistor R8. The circuit can also adjust the sensitivity of flame detection by adjusting the potentiometer RV1. This circuit is simple in composition, but it is very accurate and sensitive for flame detection.

3 Software Design
    The software design block diagram is shown in Figure 6.

    1) During initialization, enter the standby state State=0 and turn off all outputs.
    2) If there is an external alarm, State=1, then wait for the remote reset signal to arrive, and when there is a reset signal, State=2. If there is no external alarm, directly State=2.
    3) State=2 enters ignition. Before ignition, first determine whether there is fire in the furnace. If there is fire, do not ignite. If there is no fire, ignite to State=3.
    4) State=3 ignites, turns on the ignition transformer and ignition valve. After ignition, determine whether the ignition is successful. If the ignition is successful, State=4. If the ignition is unsuccessful, an alarm is output, and all outputs are turned off, State=0.
    5) State=4 opens the main gas valve, and the device works normally. Real-time monitoring of the flame current signal, once there is no flame current signal, State=5.
    6) State=5 is the flameout response, which is selected by the mode control switch. State=6 is an immediate alarm; State=7 is a three-second delay alarm; State=8 is re-ignition.

5 Conclusion
    At present, the intelligent ignition control system has been put into use in the steel plant and is running well. The intelligent ignition control system has low cost but safe and stable operation. It can adapt to different production processes by modifying the software, and has the characteristics of dual protection of software and hardware and real-time flame detection. Its application environment should be broader, but the intelligent ignition control system is currently only used in hood-type heating furnaces. I believe that in the near future, the intelligent ignition control system will gradually be used in other heating furnaces and become familiar to more steel mills.