1 Introduction
Domestically designed 6m mobile coke oven machinery has achieved mechanized operation, but its electrical control system uses relay-contactor control, resulting in a large number of relays, complex wiring, frequent malfunctions after commissioning, and difficulty in ensuring the full realization of the machinery's functions. Based on extensive learning from domestic and international experience, we adopted PLC control for the coke pusher and quencher cars of Wuhan Iron and Steel's No. 7 and No. 8 coke ovens, independently developing and designing both hardware and software to achieve automated operation of individual units.
The coke pushing car production process is divided into 12 operating units according to the technological flow, and the coke quenching car has 10 operating units, all employing a "5-2 sequential coke pushing" and "single alignment" process. After the coke pushing rod or coke guide grid is aligned with the furnace position, both the furnace door can be controlled and the furnace frame can be cleaned. Compared with the original "9-2 sequential" process, this process reduces the number of vehicle movements, shortening the time per coke outlet by 3-4 minutes. Although it increases the complexity of the control system, it improves operational efficiency. To meet the complex operation of multiple tasks occurring simultaneously, the control system design adopts both "automatic unit operation" and "manual operation" modes.
2. Composition of the coke oven PLC control system
The electrical control systems for the coke pusher and coke quencher both use the 984-680 PLC controller from Modicon, USA, and employ 800 series modules, intelligent I/O modules, S908 remote I/O drivers, J890 remote I/O interface processors, and printers, among other equipment.
The coke pusher control unit consists of one local station, equipped with four I/O racks and 31 I/O modules, as shown in Figure 1. The coke quencher control unit consists of one local station and one remote station, each equipped with two I/O racks and 17 and 11 I/O modules respectively. The power supply system design utilizes an isolation transformer and a UPS power supply.
3. Functions of the control system for the coke pusher and coke catcher
3.1 Basic Control Functions
Due to the limitations imposed by high temperatures and dust, the detection elements of the mobile machinery in coke ovens primarily utilize limit switches and travel switches. Furthermore, the solenoid valves used in the hydraulic system lack position signal feedback, making it impossible to directly determine the execution status of the output signal. Therefore, design techniques such as rework, skipping steps, and time checks are employed in the software programming. This objective is achieved indirectly based on travel or time principles. The program consists of eight blocks: equipment status detection, interlock check, control mode selection, unit automatic control, single-step manual control, signal display, fault location and process data detection, and data printing. The hierarchy is clear, the structure is simple, and it facilitates debugging and maintenance. This explanation uses the coke pusher car control as an example; the coke quencher car control function is essentially the same and will not be repeated. Figure 2 shows the flowchart of the automatic control system for the coke pusher car.
(1) Automatic control of units
The system is divided into units based on the nature of the work, enabling automated operations such as door retrieval, coke pushing, coal leveling, and furnace frame cleaning. Units operate independently of each other, but multiple units can also be operated simultaneously through reliable interlocking control.
(2) Manual control
Each work unit is further divided into several control steps, and each control step can be manually operated independently through a separate operation button.
(3) Redo function
During automatic operation of the unit, a switch representing the completion of an action may fail to operate within the specified stroke or time for some reason. To correctly determine whether these switches have an irreversible fault, a redo function will be automatically executed based on manual confirmation after an automatic redo request. If the redo is successful, the program continues; otherwise, a fault alarm will be triggered.
(4) Skip function
During operation, if some control actions fail to be completed within the specified time or schedule, but do not significantly affect the overall functionality, then skipping this part of the program and continuing execution downwards will be used.
(5) Time verification function
The time principle is used to determine whether an action command has been completed. A time setpoint for verification is calculated based on the equipment's operating speed and distance traveled. If the action command is completed within this time, it is considered normal; otherwise, it is considered a equipment malfunction. The time verification function is further divided into single-time and dual-time verification functions due to changes in control factors within the same control process.
(6) Deceleration verification
When the coke pusher reaches the forward or backward end, the proper engagement of the deceleration mechanism is crucial to the safety of both the pusher and the coke oven body. Deceleration verification involves determining whether eddy current braking is engaged based on the operating status of the deceleration verification switch. If not engaged, the control circuit is immediately cut off.
(7) Coal leveling frequency selection function
Based on process requirements, coal leveling is divided into two methods: long-stroke and short-stroke, with several different options available for each method. The program design employs 0 to 9 different long and short stroke methods, which can be combined arbitrarily. As long as the switch position is selected in advance, the program can automatically perform coal leveling according to the prescribed method and number of times.
3.2 Monitoring Function
(1) Equipment status monitoring function
The program uses the "stat" function block to detect the operating status information of various devices in the controller. This information includes CPU operating status, memory protection status, backup battery working status, AC power status, etc. When these devices are in normal status, the program allows the execution of various control functions. If there is an abnormality, an alarm is immediately generated, the control loop is cut off, and data is printed.
(2) Interlocking inspection
The interlocking check includes checks on the unit's automatic initial conditions, automatic operating conditions, and single-step operating conditions. Through these interlocking checks, reliable operation is ensured when multiple units are operating simultaneously, preventing equipment accidents.
(3) Data acquisition, processing and recording functions for push-coke current.
(4) Status display and alarm functions
During the production process, the equipment operating status can be displayed by lights and sounds according to normal, major fault, and minor fault status, and corresponding control measures can be taken according to the nature of the fault when a fault occurs.
(5) Process data detection and printing function
Providing accurate operational information and maintenance data to operators and maintenance personnel is crucial for stable operation and reliable system operation. During each operation, when faults such as "redo," "timeout," "skipped step," or "interlock failure" occur, various information about the faulty component is detected in real time and transmitted to the intelligent I/O module via the internal bus. After data processing, the relevant data is automatically printed out. This data includes the operating mode, unit number, data type, and the equipment code of the specific faulty component.
4. Performance
This system was successfully commissioned on the first attempt at production at Wuhan Iron and Steel's No. 7 and No. 8 coke ovens, realizing all software design functions. Within just two months of commissioning, it achieved the comprehensive design target of a 10-minute coke pushing interval, setting a new record for smooth start-up and production capacity of similar coke ovens in China. Compared with domestically designed relay-controlled 6m coke ovens, it has the following advantages:
(1) The coke pusher car operates for 405 seconds per furnace position, which improves its operating efficiency by 10%.
(2) The cleaning of doors, frames, furnace platforms, and the handling of coke at the head and tail have all been automated, which greatly reduces the labor intensity of workers.
(3) The system has high reliability and has been operating normally since it was put into operation. Compared with the 4.3m coke oven in this plant, its monthly average electrical failure time ratio is 1:10, and the failure rate is greatly reduced.
5. Conclusion
Practice has proven that the PLC control system has significant economic and social benefits in the application of the mobile machinery system of Wuhan Iron and Steel's No. 7 and No. 8 coke ovens, and has high promotion value, creating conditions for the comprehensive adoption of PLC control in the future coke oven mobile machinery.
