Abstract: Currently, industrial control automation technology is developing towards intelligence, networking, and integration. This paper reflects the current development trend of automation technology through the design of a walking beam furnace system. It also introduces the software design concepts and the characteristics of pulse combustion control technology, and their application in this system. 1 Introduction Heating furnaces are essential heat treatment equipment in the steel rolling industry. With the continuous development of industrial automation technology, modern steel rolling mills should be equipped with large-scale, highly automated walking beam furnaces. Their production should meet the process requirements of high output, high quality, low consumption, energy saving, pollution-free operation, and automated production, in order to improve product quality and enhance market competitiveness. In China's steel rolling industry, there are two types of heating furnaces: pusher furnaces and walking beam furnaces. However, pusher furnaces have disadvantages such as short length, low output, high burn loss, and the risk of steel sticking and production problems if not operated properly, making it difficult to achieve automated management. Because pusher furnaces have inherent drawbacks, walking beam furnaces, on the other hand, use a dedicated walking mechanism to move steel pipes in a rectangular motion within the furnace. Gaps can be left between the pipes, eliminating friction between the pipes and the walking beams. The pipes are removed from the furnace via a lifting device, completely eliminating rail scratches. This results in smaller temperature differences across the heating cross-section of the pipes, more uniform heating, unrestricted furnace length, high output, and flexible operation. Its production meets the requirements of high output, high quality, low consumption, energy saving, pollution-free operation, and automated production. Fully continuous and fully automated walking beam furnaces have the following characteristics: ① Significantly reduced energy consumption. ② Significantly increased output. ③ Extremely high level of automation. While traditional furnace control systems were mostly single-loop instrumentation and relay logic control systems, and drive systems were mostly analog power supply devices, modern furnace control systems are PLC or DCS systems, and most also have secondary process control systems and tertiary production management systems. Drive systems are all fully digital DC or AC power supply devices. This project involves the walking beam furnace equipment in the heat treatment line of a newly built φ180 small-diameter seamless continuous rolling steel pipe production line of a steel group. 2. Process Description The process flow diagram of this system is shown in Figure 1. [align=center]Figure 1 Walking Beam Furnace Process Flow Diagram[/align] Both the quenching furnace and tempering furnace are walking beam furnaces. Loading and unloading method: side inlet, side outlet; furnace material distribution: single row. Both the movable and fixed beams are made of heat-resistant cast steel with toothed surfaces on the top. For steel pipes with a diameter less than 141.3mm, one steel pipe is placed in each toothed slot. For steel pipes with a diameter greater than 153.7mm, one steel pipe is placed every other tooth. The movable beam has a lift of 180mm, 90mm up and down, a tooth pitch of 190mm, and a step distance of 145mm. Therefore, each step allows the steel pipe to rotate at an angle, ensuring uniform heating and preventing bending and deformation within the furnace. The walking beam can perform various actions including positive circulation, continuous circulation, single-action, and inching. During lifting and lowering, it gently supports and lowers the steel pipe; during forward movement, it starts and stops slowly, without vibration, impact, or loss of control. It also has a stepping function, with a backward step distance of 45mm, allowing the steel pipe to rotate continuously within its original tooth groove. The fixed beam is supported by insulated pillars, with its top surface 520mm above the furnace bottom. This ensures good circulation of furnace gas around the steel pipe, guaranteeing uniform heating. The quenching furnace is divided into a charging section, a heating section, and a holding section along its length. The charging section has a low furnace top and does not have exhaust preheating steel pipes (three temperature measurement points) installed to prevent the steel pipes from bending and deforming due to sudden strong heat flow. The heating section is divided into four zones along the furnace width for proportional combustion and temperature control. The holding section is divided into four zones along the furnace width for pulse combustion and temperature control (using two sets of controllers) to ensure the furnace temperature in the holding zone is ±5℃ and the temperature uniformity of the entire steel pipe after holding is within 10℃. This design also allows for easier user control of the steel pipe end temperature, meeting quenching requirements. The quenching furnace has a maximum control temperature of 960℃ and a maximum allowable temperature of 1050℃. The tempering furnace is divided into a charging section, a heating section, a homogenizing section, and a holding section along its length. The charging section does not have burners. The heating section is divided into two temperature zones along its width: one zone has eight burners in the middle, and the other two zones have four burners on each side, controlled by a single pulse controller. The homogenizing and holding sections are each divided into four zones along their width (one controller for every two zones), all using pulse combustion temperature control. This ensures furnace temperature control within ±5℃ and a 10℃ temperature uniformity along the entire length of the steel pipe after holding. The tempering furnace has a maximum control temperature of 750℃ and a maximum allowable temperature of 800℃ (as a reserve for the production of high-pressure boiler tubes). The flue gas from the quenching and tempering furnaces is drawn out through eight branch flue pipes below the charging section, enters the collecting flue pipe, and is then led from the middle of the collecting flue pipe to the main flue pipe. Waste heat is recovered from the flue gas through an air preheater before it is discharged into the atmosphere through the chimney. This method effectively prevents the flue gas from flowing out of the furnace. Both the quenching and tempering furnaces use a side-in, side-out method for loading and unloading. The cantilever rollers for loading and unloading the quenching furnace are installed at an 80-degree angle, causing the steel pipes to rotate and align with the loading/unloading end wall during loading and unloading. This prevents the steel pipes from bending during loading and facilitates positioning during loading and unloading. 3. Key Control Technologies in Walking Beam Furnace Production 3.1 Production Rhythm Control Controlling the production rhythm in walking beam furnace production is crucial. During fully automated and continuous operation of the pipe production line, the operating rhythm of the mechanical equipment in the furnace area, such as the feeding roller conveyor, walking beam, discharging roller conveyor, hydraulic station, and other auxiliary facilities, must be highly unified to achieve accurate positioning of the pipe material throughout the entire process, thus enabling fully automated and continuous operation. When loading is required according to the production schedule, the pipe is conveyed to the loading roller conveyor via the loading platform, and automatically transported to the external roller conveyor via photoelectric switches and metal detectors. When the loading end of the furnace becomes available, the furnace door automatically opens, and the pipe is lifted into the furnace and placed on the fixed beam by the internal roller conveyor, thus initiating the tracking of the pipe material flow within the furnace. The pipe is moved step by step from the loading end to the discharging end of the furnace via the furnace walking beam. The photoelectric switch installed at the discharge end detects the edge of the tube and pauses the walking beam's movement after it completes its step distance. Simultaneously, the PLC calculates the position of the tube awaiting discharge. After the heating furnace receives the tapping signal, the discharge furnace door automatically opens, and the tube is transported by the discharge roller conveyor to the outer discharge roller conveyor. When the metal detector detects the tube, it is then transported by the discharge roller conveyor to other equipment for the next process. Tube conveying, measurement, loading and unloading, logistics tracking, and data exchange are all controlled sequentially, timed, interlocked, and logically by the PLC and a secondary computer system, achieving automated operation and computer management. 3.2 Heating Furnace Combustion Control The combustion control level of an industrial furnace directly affects various production indicators, such as product quality and energy consumption. Currently, most industrial furnaces in China use continuous combustion control, which controls the flow rate of fuel and combustion air to achieve the required furnace temperature and combustion atmosphere. Because this continuous combustion control method is often constrained by fuel flow rate adjustment and measurement, the control effect of most industrial furnaces is currently unsatisfactory. With the rapid development of the industrial furnace industry, pulse combustion control technology has emerged and has been applied to a certain extent both domestically and internationally, achieving good results. Currently, high-end industrial products have high requirements for the uniformity of the temperature field within the furnace and the stability and controllability of the combustion atmosphere, which cannot be achieved using traditional continuous combustion control. With the advent of wide-section, large-capacity industrial furnaces, pulse combustion control technology is essential to control the uniformity of the temperature field within the furnace. This system primarily employs a pulse combustion system. It is an intermittent combustion method that uses pulse width modulation technology to control the furnace temperature by adjusting the duty cycle (on/off ratio) of the combustion time. The fuel flow rate during combustion can be adjusted online through the main fuel control valve. Once the burner is igniting, it is in its designed optimal combustion state, ensuring a constant gas outlet velocity during combustion. The control system causes the burners within the furnace to burn alternately, and through continuous stirring of the gas within the furnace, a uniform temperature field distribution is achieved. When heating is required, the burner combustion time is extended and the intermittent time is reduced; when cooling is required, the burner combustion time is reduced and the intermittent time is extended. The system controls the fuel flow rate during combustion based on the set temperature inside the furnace. When the set temperature is low, the main fuel control valve is closed slightly; when the set temperature is high, the main fuel control valve is opened wider. This prevents excessive temperature difference between the fuel gas and the furnace interior when the furnace is at a low temperature, thus avoiding direct thermal shock to the products inside the furnace. The main advantages of the pulse combustion system are: 1) Simple, reliable, and low cost. 2) Improves the uniformity of the temperature field inside the furnace. 3) High heat transfer efficiency, significantly reducing energy consumption. 4) Large burner load regulation ratio. 5) Precise control of the air-fuel ratio can be achieved without online adjustment. Compared with traditional proportional combustion control, the number of instruments involved in the pulse combustion control system is greatly reduced, with only temperature sensors, controllers, and actuators, omitting a large number of expensive flow and pressure detection and control mechanisms. Furthermore, since only two-position switching control is required, the actuator has been changed from the original pneumatic (electric) control valve to a solenoid valve, increasing the system's reliability and significantly reducing the system cost. The air-fuel ratio of a conventional burner is typically around 1:4. When the burner operates at full load, the gas flow rate, flame shape, and thermal efficiency all reach their optimal states. However, when the burner flow rate approaches its minimum, the heat load is at its lowest, the gas flow rate decreases significantly, the flame shape fails to meet requirements, and the thermal efficiency drops sharply. When a high-speed burner operates below 50% of its full load flow rate, these indicators fall significantly short of design requirements. Pulse combustion, on the other hand, operates in only two states regardless of the conditions: full load operation and inactivity. Temperature regulation is achieved by adjusting the time ratio between these two states. Therefore, pulse combustion can compensate for the low adjustability of burners, ensuring optimal combustion even when low-temperature control is required. When using a high-speed burner, the rapid gas ejection creates a negative pressure, drawing a large amount of flue gas into the main combustion mixture for thorough stirring and mixing. This prolongs the residence time of the flue gas within the furnace, increasing the contact time between the flue gas and the product, thereby improving convective heat transfer efficiency. 4 System Introduction 4.1 System Composition The system topology diagram is shown in Figure 2. [align=center] Figure 2 System Topology Diagram[/align] The automatic control system of this heating furnace consists of a basic automation system (L1) and a process computer control system (L2). The primary basic automation control system consists of four control stations: the electrical drive section of the quenching furnace, the electrical drive section of the tempering furnace, the instrumentation and control section of the quenching furnace, and the instrumentation and control section of the tempering furnace. The PLC for the electrical drive section of the quenching furnace is a Siemens MM440 series frequency converter with an S7 315-2DP, four ET200M slave stations, and three PROFIBUS-DP cards. It realizes the functions of sequential control of the roller conveyor, positioning control of the steel pipe on the furnace entry roller conveyor, and stepper beam control. The quenching furnace drive system adopts frequency conversion control, and the frequency converter is an MM440 series provided by Siemens. Three MM440 series frequency converters with PROFIBUS-DP cards control three sets of roller conveyors. The PLC for the electrical drive system of the tempering furnace is an S7 315-2DP with three ET200M slave stations and two Siemens MM440 series frequency converters with PROFIBUS-DP cards. It controls the sequential operation of the roller conveyor, the positioning of the steel pipes on the furnace feed roller conveyor, and the stepper beam control. The tempering furnace drive system uses frequency conversion control, with the frequency converters being Siemens MM440 series. Two MM440 series frequency converters with PROFIBUS-DP cards control two sets of roller conveyors. The PLC for the instrumentation and control system of the quenching furnace mainly consists of an S7 315-2DP and a function module FM355C closed-loop control module. Each PLC and its corresponding PID module are responsible for controlling all instruments in the quenching furnace, used to complete the data acquisition and process control of the heating furnace process parameters. The Siemens FM355C closed-loop control module is used to control the heating furnace process parameters to achieve optimal combustion control. The structure and function of the PLC for the instrumentation and control system of the tempering furnace are basically the same as those of the quenching furnace. In the process computer control system, one computer is set up for each of the quenching furnace and tempering furnace. These computers configure and set parameters for the hardware of their respective heating furnace systems, define communication protocols, write and debug user programs, and monitor and record data, ultimately achieving centralized monitoring and operation. Both computers are SIEMENS industrial PCs, configured with a P4 2.0G processor, 256M bandwidth, and equipped with an industrial Ethernet processor CP1613. The computers communicate with the PLC via an industrial Ethernet fiber optic switch (OSM) using the CP1613. The OSM has two fiber optic interfaces and six electrical interfaces. Multimode fiber optic cables are used for connection, suitable for environments with strong electromagnetic interference. Redundant 10M/100M industrial Ethernet significantly improves network performance, and network configuration and expansion are very simple. Only two fiber optic cables are needed, and the configuration is redundant, making control cabinet wiring straightforward. 4.2 Programming and Configuration and Functional Modules FM355C (1) The PLC programming software uses STEP7, which runs under Windows 2000/XP. The STEP7 programming language provides a very rich instruction set, which makes programming complex functions simple and fast. STEP7 provides a structured programming method, which manages user-written programs and data in blocks. They can be combined into structured user programs by calling statements, which increases the readability and maintainability of the program. The system provides users with a large number of pre-compiled function blocks, which users can use directly, thus greatly shortening the programming time. Functions of the Standard Software Package The standard software supports all stages of the automatic task creation process, such as: • Creating and managing projects • Configuring and assigning parameters to hardware and communication • Managing symbols • Creating programs, such as creating programs for S7 programmable controllers • Downloading programs to programmable controllers • Testing automation systems • Diagnosing equipment faults (2) The SCADA software installed on the computer is WinCC, and the operating system is Windows 2000. WinCC boasts wide applications and high compatibility, offering mature and reliable operation and efficient configuration performance. WinCC can be used for all operator control and monitoring tasks in the automation field. WinCC can clearly express the status of the production process in the form of images, text, bar graphs, curves, or alarms. It can also record occurring events and process data for historical data retrieval. It can easily be configured to generate required report formats and print based on time or event triggers. In the Windows environment, WinCC can easily integrate other controls into application software via OLE and ODBC. It can also communicate with other applications via DDE. A standard C language is embedded within WinCC, allowing for flexible task completion within projects. Furthermore, WinCC's API programming interface can be accessed to achieve certain special functions. WinCC has an open communication protocol and supports various PLC systems. (3) The FM355C is a 4-channel closed-loop control module for closed-loop control tasks. It has the following functions: • Can be used for temperature, pressure and flow control • Convenient online self-optimizing temperature control for users • Pre-programmed controller structure • FM 355C as a continuous action controller • 4 analog output terminals for controlling actuators • Can still connect and run after CPU shutdown or failure The FM 355C controller has the following performance: • Factory pre-built controller structure for fixed setpoint control, series control, proportional control, and 3-component control; depending on the selected controller structure, several controllers can be combined into one structure. • Different operating modes: automatic, manual, safety mode, follow mode, backup mode • 2 control algorithms; self-optimizing temperature control algorithm, PID algorithm This system applies the basic functions of the module and achieves good results. 5 Design of Control Software The control modes of this system are as follows: 1) Manual mode: used for equipment debugging and maintenance. 2) Semi-automatic mode: used for automatic control of individual equipment in manual mode. 3) Automatic mode: used for automatic control of all equipment. The control software design mainly consists of the following parts: ● Roller conveyor control ● Walking beam control ● Furnace temperature control ● Furnace pressure control ● Gas main pipe pressure control ● Air main pipe pressure control ● Hot blast temperature control ● Emergency shutdown protection. The most important are the walking beam control, furnace temperature control, and emergency shutdown protection. The following mainly discusses the control principles of walking beam control, furnace temperature control, and emergency shutdown protection. 5.1 Walking Beam Control The walking beam has two movement modes: periodic and stepping. The periodic mode is used to transport the steel pipe forward, while the stepping mode is used to wait for steel to be tapped. The periodic mode of the walking beam: The moving beam rises 180mm, advances 145mm, descends 180mm, and retreats 145mm. The steel pipe advances one tooth pitch. Its running trajectory is as follows: [align=center] Figure 3 Walking beam movement trajectory diagram[/align] Its running speed is shown in the following figure: [align=center] Figure 4 Walking beam running speed diagram[/align] When the walking beam approaches the fixed beam surface, the rising speed of the walking beam is slowed down so that the walking beam lightly contacts the steel pipe on the fixed beam. The same applies when descending. The control system of the walking beam is shown in Figure [align=center] Figure 5 Walking beam control principle diagram[/align] Stepping method of the walking beam: The moving beam rises 180mm, retreats 45mm, descends 180mm, and advances 45mm. The steel pipe rotates in the original tooth groove of the fixed beam. The running trajectory is shown in the following figure: [align=center] Figure 6 Walking beam stepping trajectory diagram[/align] 5.2 Furnace temperature control ● The furnace temperature zone is divided into 8 temperature control zones. The heating section is divided into four zones along the width of the furnace, namely heating 1, heating 2, heating 3, and heating 4. The heat preservation section is also divided into four zones along the furnace width: Heat Preservation 1, Heat Preservation 2, Heat Preservation 3, and Heat Preservation 4. The tempering furnace has a total of 10 temperature control zones. The heating section is divided into two zones along the furnace width: Heating 1 is the middle section, and Heating 2 consists of two sections on the left and right. The temperature equalization section is divided into four zones along the furnace width: Temperature Equalization 1, Temperature Equalization 2, Temperature Equalization 3, and Temperature Equalization 4. The heat preservation section is also divided into four zones along the furnace width: Heat Preservation 1, Heat Preservation 2, Heat Preservation 3, and Heat Preservation 4. Each zone has independent temperature control. ● Temperature Setting Methods for Each Zone There are two temperature setting methods for each zone: Manual Setting Method: The furnace temperature is set manually by changing the temperature setting value of each zone on the industrial control computer. Program Setting Method: For steel pipes of different specifications and materials, different set temperatures are required according to process requirements. Operators can first save the furnace temperature set values ​​for different specifications and materials in a database within the PLC. Then, on the steel pipe selection interface of the industrial control computer CRT, the furnace temperature for each zone can be set in batches using a one-touch soft button as needed. ● The temperature adjustment method for each zone adopts PID control. The process involves transmitting the actual furnace temperature detected by the thermocouple to the FM355 PID module, comparing it with the set value for that zone, and then the module performs PID calculations and outputs a 4-20mA signal. This signal is transmitted to the input of the Krom continuous control or pulse controller to control the combustion system and achieve temperature control. The specific control process is as follows: For the four temperature zones of the quenching furnace (heating zones 1, 2, 3, and 4), a proportional combustion continuous control system from the German company Krom is used. The PID output of the temperature module (4-20mA signal) controls the air solenoid butterfly valve. The opening degree of the air solenoid butterfly valve changes, which in turn changes the gas pressure before the burner through the air/fuel proportional regulating valve, thereby changing the heating supply and achieving automatic control of the furnace temperature. See Figure 7. Figure 7 Continuous Combustion Control Principle Diagram 1 Gas Solenoid Valve 2 Burner Controller 3 Ignition Transformer 4 Air-Fuel Proportional Valve 5 Flow Regulating Valve 6 Manual Butterfly Valve 7 Bellows 8 Burner For the four heat preservation zones 1, 2, 3, and 4 of the quenching furnace and the zones of the tempering furnace, a pulse combustion control system from Krom (Germany) is used. The pulse controller MPT-700 receives the PID output signal from the PID module and converts it into a pulse width modulation timing signal to control the switching sequence and switching time ratio of the pulse burner, thereby regulating the flow of air and gas to control the furnace temperature. To ensure the furnace temperature uniformity of ±5℃ and the temperature uniformity along the entire length of the steel pipe of less than 10℃, the heat preservation section of the quenching furnace and the tempering furnace are both controlled by pulse combustion. In pulse combustion control, the burner only works in two states: on or off. According to the requirements of the burner power, mixing ratio, and spray speed, the burner is adjusted to the optimal working state in one go. We use Krom temperature-adjustable burners, which have a significant effect on improving combustion efficiency and reducing the degree of pollution from emissions. The schematic diagram of its control system is shown in Figure 8. [align=center] Figure 8 Schematic diagram of pulse combustion control[/align] 5.3 Emergency shutdown protection and interlock (1) Automatic shutdown The furnace should be automatically shut down when the following situations occur: ● The pressure of the main gas pipe in the workshop is below the limit; ● The pressure of the hot air is below the limit; ● The pressure of the cooling water is below the limit or the water supply is cut off; ● The combustion fan stops due to failure; ● Power outage. Automatic shutdown process: Emergency cut-off of main gas pipe → nitrogen purging of pipeline → gas release in pipeline. (2) Emergency Manual Shutdown: This is used in special circumstances such as control system failure. This system is an interlocking system independent of PLC control; the operator presses the emergency stop button to shut down the furnace. The shutdown process still has the following interlocking functions: Emergency cut-off of main gas supply → Nitrogen purging of the pipeline → Gas release from the pipeline. 6 Conclusion This system design adopts advanced automation technology, achieving the process requirements of high output, high quality, low consumption, energy saving, pollution-free operation, and automated production in the heating furnace. Since its commissioning, the system has been operating stably and reliably, and is easy to operate, which has been well received by on-site staff, achieving good expected results. References [1] Chen Nanyue. Modern heating furnace process control technology and its mathematical model. Metallurgical Automation, 1995.5 [2] Hu Ren'an. New technologies in steel industry automation. Metallurgical Automation, 2001.2 [3] Siemens STEP 7 Programming Manual. Siemens (China) Co., Ltd., 2003.3 [4] Siemens Industrial Communication Network and Fieldbus Manual. Siemens (China) Co., Ltd., 2002.2