1. Introduction The Lian Steel hot-rolled plate leveling machine is a supporting facility for the hot-rolled thin plate plant. Its main function is to level or cut some of the hot-rolled steel coils into smaller coils, improving the straightness and mechanical properties of the steel plates. The leveling machine is designed to produce 102,100 tons per year. The leveling thickness range is 0.8–6.5 mm, the slitting thickness range is 0.8–12.7 mm, the coil width range is 900–1600 mm, and the coil outer diameter range is 900–1950 mm. The leveling machine is driven by five variable frequency motors from uncoiling to coiling. The frequency converters are Siemens 6ES7 series products, with two frequency converters (main and auxiliary) for both the rolling mill and the coiler, used for speed and tension control respectively. There are also two hydraulic stations: a high-pressure station for AGC (Automatic Roll Gap Control) during leveling, and a low-pressure station for all other hydraulic systems. According to the system control requirements, the Italian company EDM uses Siemens' S7-400 series PLC for sequential control of the production process, and then uses Siemens' TDC to control the mill's AGC and the overall line speed and torque. 2. System Structure The leveling unit consists of an engineering workstation, an operator workstation (PC containing three HMIs), an S7-400 PLC, and a TDC. All of these communicate with the outside world via industrial Ethernet. The S7-400 PLC uses a PS40720A power supply module; the CPU uses a CPU414-2DP, which has integrated MPI and PROFIBUS-DP interfaces. The MPI interface connects to the TDC and the engineering workstation PC, and the PROFIBUS-DP interface connects to the TDC. It also includes field ET200M modules, an ET200M module in the MCC cabinet, and an ET200M module in the main control room. This PLC also includes a counter module, analog and digital input/output modules. The analog input/output modules process analog signals from field sensors, while the digital input/output modules acquire signals from field proximity switches and other switching signals. It also includes communication and interface modules. The IM467 interface module connects to the field I/O station via a PROFIBUS line. Additionally, the TDC communicates with the PLC master station via a PROFIBUS line. The TDC has two CPUs, using a CPU551, one for AGC control of the four-roll mill and the other for calculating and controlling the speed and torque of the entire line. There are also two communication modules: one connected to the industrial Ethernet, and the other communicating with the frequency converter via a PROFIBUS line. The hardware configuration diagram is shown in Figure 1, and the communication network diagram is shown in Figure 2. 3. Software Design The leveling machine system uses over 1000 digital and analog input/output points, involving not only sequential logic control but also analog signal control, data processing, and fault handling. The entire system requires a significant amount of programming. Here, the S7 Graph language, used by S7-300/400 for programming sequential control programs, was employed. The language primarily uses ladder diagrams, statement lists, and function block diagrams. The program consists of over 10 OB blocks, over 120 FC blocks, over 60 background DB and common DB blocks, and 5 system blocks (SFC). The main program includes the automatic flow of the steel coil being wound from the walking beam or trolley #1 to the unloading trolley at the exit, completing the task-assigned process. In S7 Graph, the control process is divided into many clearly defined steps (STEPs) with defined functional ranges. The execution of the entire process is clearly illustrated graphically. The actions to be performed in each step can be specified, and the transition from one step to the next is controlled by conversion conditions. Ladder diagrams and function block diagrams are used for programming transitions, interlocks, and monitoring. Figure 3 shows the program structure diagram of the leveling machine, and Figure 4 shows the HMI screen block diagram. 4. Human-Machine Interface (HMI) The HMI consists of: a main screen, a loading car screen, and an unloading car screen displaying various real-time data and equipment statuses of the strip steel during its forward movement, such as the actual diameter of the coil on the uncoiler and coiler spindles, the actual torque of the straightener, the actual speed of the inlet and outlet guide rollers (measured by the encoder), the actual tension of the uncoiler and coiler, and the overall speed of the line; the loading car and unloading car screens display the transport status of the inlet and outlet coils; the alarm screen displays various alarm information, such as: emergency stop, fast stop fault, encoder fault alarm, MCC cabinet not ready alarm, frequency converter fault alarm, etc. All alarms connected to the HMI can be displayed on the HMI. The hydraulic screen is used to monitor the operation of various auxiliary equipment, hydraulic pumps, and the hydraulic valve oil supply status of the hydraulic station. 5. System Functions The system offers two control modes: manual and automatic control, achieved through switches on each control panel. During stand-alone debugging, manual control is used on the existing control panel. With both the equipment and PLC functioning normally, automatic control is employed, using the PLC to control and monitor the equipment. During automatic control, the human-machine interface on the host computer allows setting the weight of the steel coil during the slitting process. The automatic system tracks and calculates this weight, and when the coil reaches the set weight, the entire line decelerates to zero, and the slitting shear cuts it in the middle, thus separating the large steel coil into smaller coils. This system includes one main control panel (centralized control) and seven local control panels (one for preparation station, one for straightening machine, one for roll changing system, one for unloading, one for packaging, one for weighing, and one for hydraulic station). Each local control panel has a local/remote switch for auxiliary equipment; the main control panel has a manual/automatic switch. The combined use of remote and manual/automatic controls makes equipment control more standardized and facilitates maintenance and operation by the operators. When all equipment in the system operates in automatic mode, sequential automatic control of the entire leveling line system can be achieved. During normal program operation, the coiling trolley is manually or automatically controlled according to specific circumstances. The uncoiler is responsible for opening the steel coil, and the inlet equipment receives the opened coil head and sends it to the straightener, causing the coil to crawl forward. The straightener is responsible for straightening the strip steel, making it flat and crawling forward in a straight line. Afterwards, the program will automatically control the equipment sequentially to roll or slit the steel coil, which then passes through a slitting shear and exit pinch rolls before entering the coiler. Before coiling, the steel coil is head-cut at the preparation station, and after coiling, the tail is cut at the slitting shear according to requirements. This action aims to improve the quality of the strip steel by removing the poor-quality head and tail portions. Then, the unloading trolley transports the steel coil from the coiler. During transportation, it is packaged by a specialized packaging machine and then transported to the saddle of the exit walking beam, where a gantry crane transports the coil into the warehouse. Meanwhile, the coiling machine prepares for the next coil, and the auxiliary equipment at the exit prepares for corresponding actions. The system also implements necessary electrical interlocks for all electrical equipment to ensure normal equipment operation and production safety. For example, the coiling trolley can only begin the coiling cycle when the walking beam is in the rearward position and the #1 trolley is not in the cross saddle position; the exit auxiliary equipment only prepares for the next coil of steel when the steel coil is transported out of the production line to avoid collisions; the opening or raising of auxiliary equipment can only be activated when the closing or lowering limit is detected; in the event of an emergency stop due to a fault on the entire line, all equipment stops operating, etc. The system control flow is shown in Figure 5. 6 System Fault Handling The production environment of the Lian Steel leveling machine is not ideal, with a lot of iron oxide scale. Faults in the PLC control system are inevitable. Based on my experience, 95% of faults originate from outside the PLC, and only 5% are caused by the PLC itself. When a fault occurs, first determine whether the fault is inside or outside the PLC (first level); whether it is in the I/O circuit or within the controller (second level); and whether it is a PLC hardware fault or a software fault (second level). a. Use PLC input/output indicator lights to diagnose first-level faults. Whether the indicator lights are on or off is an effective and intuitive way to check and discover faults. Peripheral faults generally occur in relays, contactors, changeover switches; junction boxes, terminals; sensors, instruments; noise in power supply, ground wires, and signal lines, etc., and are relatively easy to troubleshoot. PLC faults themselves generally include: input module failure; abnormal power supply voltage; other circuits within the controller; and the controller itself. b. Use the PLC monitoring system to diagnose second-level faults. Use the PLC monitoring unit to monitor the status online, using ladder diagrams. For example, different colors displayed by soft contacts indicate short-circuiting and discharging at the positive and negative terminals, thus clearing different states in the user program. Locate input element XO; if it is ON, it indicates that the input signal has been generated in the controller's registers. Then reconnect the lithium battery and use the programmer to send an input signal to the second-level monitor. Then check the output element YO; if its state is ON, it indicates that the output signal has been generated in the controller's registers. If the output external I/O control, scanning, communication, etc., and a port that controls the CPU is faulty, a redundant port can be used to perform a cold start on the program. If the start still fails, it indicates that the hardware of the main unit system, including the CPU, needs to be checked again to restore normal operation. c. Diagnosing the PLC through fault phenomenon analysis: When the cold start is normal, it indicates that the main unit system is not faulty. The internal circuit of the controller is actually a microcontroller or microcontroller system. If the application program is incorrect, a backup program can be re-entered. If it is still not normal, a simple experimental program can be written and inserted before the source program and run separately. If all branches are faulty, the fault may be in the encoding control unit. The relevant circuits and components should be carefully checked and replaced if necessary. To determine if the CPU in the PLC is faulty, remove the lithium battery from the CPU motherboard and use a jumper wire to connect the CPU and the battery. The user program can be re-downloaded, and then the hardware and software can be added little by little or in sections and partitions to find the fault point. 7. Conclusion Two years after the leveling production line went into operation, the Siemens S7-400 PLC has operated stably and reliably. This PLC system is worth promoting and using in leveling production lines and other production lines with complex control systems. [b][align=center]For details, please click: Application of S7-400 Controller in Leveling Production Line System[/align][/b]