Application of PLC in Electromagnetic Induction Heating Papermaking Dryer

0 Introduction
At present, in the papermaking industry, domestic and foreign manufacturers of all sizes still widely use traditional oil heating and steam heating papermaking drying cylinders. In the production process for many years, traditional papermaking drying cylinder equipment has exposed many shortcomings: low energy utilization, large equipment investment, environmental pollution, frequent production accidents, etc. With the intensification of international energy shortages and the enhancement of people's environmental awareness in recent years, various industrial sectors are constantly transforming old equipment and developing new energy-saving and efficient equipment. As a major energy consumer in the papermaking production line, the transformation of the heating drying cylinder requires a fundamental change in the heating method using hot oil and superheated steam as the heating medium, and finding a simple, convenient and pollution-free alternative.
"Induction heating" is a suitable heating method with the characteristics of high thermal efficiency, uniform heating and safety. It has been successfully applied in industries such as steel smelting and automobile manufacturing. This paper applies "induction heating" to the development of papermaking drying cylinder equipment, designs a small test paper machine medium-frequency induction drying cylinder, and uses SIEMENS S7-200 series PLC products to develop a control system with low cost, precise control and easy operation. After debugging and operation, the electromagnetic induction drying cylinder can fully meet the requirements of the original production process, has stable operation and obvious energy-saving effect. It can be used as a substitute for traditional drying cylinders and has broad market prospects.

1 Induction Heating Power Supply
1.1 Principle of Induction Heating Power Supply
The key to developing an induction heating electromagnetic drying cylinder is to develop a stable and reliable induction heating power supply system. The main structures of the induction heating power supplies currently used in various fields are roughly the same, and are composed of the following parts:
(1) Rectification conversion circuit (AC-DC);
(2) Inversion circuit (DC-AC);
(3) Resonant load circuit;
(4) Power protection and control circuit.
The schematic diagram of the induction heating power supply is shown in Figure 1. Among them, the rectifier conversion circuit converts 50 Hz industrial frequency mains into direct current; according to whether the DC voltage obtained by rectification is variable, the rectifier circuit can be divided into "controlled rectification" and "uncontrolled rectification", among which "uncontrolled rectification" only needs to use high-power diodes as rectification elements, the circuit structure is simple, and the equipment cost can be effectively reduced; the rectified DC power is then converted into alternating current whose size and direction change with time through the inverter circuit, and its frequency and amplitude can be controlled by the inverter circuit; the alternating current obtained as required is loaded on the resonant load circuit, and an alternating magnetic field is generated on the load coil, thereby generating eddy currents on the surface of the metal device placed in the alternating magnetic field, completing the energy transfer and achieving the purpose of heating the metal device; the "protection control circuit" plays a protective role and adjusts the size of the heating power.

1.2 Series resonant induction power supply
Series resonant induction power supply is also called "voltage type induction power supply", and its main circuit schematic is shown in Figure 2. The A area is a three-phase bridge uncontrolled bridge rectifier circuit, which rectifies the industrial frequency AC into pulsating DC ud; B is the DC intermediate circuit, which consists of a filter inductor and a filter energy storage capacitor, which separates the 50Hz industrial frequency network from the medium frequency network. The capacitor is very large, and the two ends of it are basically smooth DC voltage u'd. This voltage is the voltage established by the capacitor before the inverter circuit C works, so as to start the inverter circuit; C is a single-phase bridge series inverter circuit, which inverts the DC voltage u'd into a medium frequency square wave voltage ud and adds it to the load circuit; the load circuit D is a series oscillation circuit composed of an inductor and a capacitor, which performs induction heating on the workpiece. The intermediate frequency voltage u'd contains obvious fundamental and harmonic components: when the fundamental voltage close to the resonant frequency is added to the series oscillation circuit, the oscillation circuit presents high impedance, and when the harmonic voltage several times higher than the fundamental frequency is added to the series oscillation circuit, the oscillation circuit presents very small impedance. Therefore, when the square wave voltage is added to the series oscillation circuit, the inductor load current ia is actually close to a sine wave.
1.3 Inverter power regulation
Inverter power regulation is a method in which the rectifier part does not use phase-controlled rectification, but uses simple uncontrolled rectification instead. The rectifier output voltage is uncontrollable, and the power regulation is achieved by changing the frequency or phase angle during the inversion process. Common power regulation methods mainly include pulse frequency modulation (PFM), pulse density modulation (PDM), pulse width modulation (PWM), etc.
The PWM method achieves power regulation by adjusting the pulse width and output frequency of the output voltage. Since the pulse period changes when the pulse width is changed, the frequency changes accordingly. In order to distinguish it from the phase-shift PWM method with unchanged working frequency, this method is also called pulse width and frequency hybrid modulation (PWM & PFM) method.
In general inverters such as DC-DC conversion inverters, the operating frequency of the commonly used phase-shift PWM method is fixed, and there is no need to consider the characteristics of the load at different operating frequencies. However, the series resonant induction heating power supply requires that its operating frequency must track the resonant frequency of the load. When the series resonant induction heating power supply uses the phase-shift PWM method, the phase of the drive pulse signal of a certain bridge arm is usually kept consistent with the phase of the output current, and the phase difference between the drive pulse signal of the other bridge arm and the output current can be adjusted. By changing the phase difference between the drive signals of the two bridge arm switching devices, the output voltage effective value is changed to achieve the purpose of adjusting the power. Since the PWM & PFM power regulation method has the advantages of uncontrolled rectification, simple circuit, low cost, wide power regulation range, easy control circuit implementation, and small frequency change, this power regulation method will be adopted in the developed electromagnetic induction drying cylinder.

2 Drying cylinder design and PLC hardware
2.1 Physical structure of electromagnetic drying cylinder
During the production process of the system, the drying cylinder equipment needs to rotate all the time. How to combine the inductive coil with the rotating drying cylinder equipment to meet the needs of energy transfer between the coil and the drying cylinder equipment without affecting the rotation of the drying cylinder and the induction and other operations in the production process? The physical realization must also be simple and practical. This combination of coil and drying cylinder is called the physical structure of electromagnetic drying cylinder.
The papermaking drying cylinder is mainly composed of a drying cylinder body, a drying cylinder cover, a variable frequency motor and a gear box. Unlike traditional oil-heated drying cylinders and steam drying cylinders, the electromagnetic drying cylinder body does not need to be filled with heating medium or closed. It only needs three spokes to support the surface of the inner cylinder body. The drying cylinder cover is located at the top of the drying cylinder body and can move up and down. When paper feeding is required, the drying cylinder cover can be lifted by the button on the control box, and the electromagnetic drying cylinder stops heating at this time; when the paper feeding operation is completed and the normal operation is performed, the drying cylinder cover is lowered, and the induction circuit heats the drying cylinder. The lifting and lowering of the drying cylinder hood is done manually with a small air compressor. The upper and lower travel switches are used to ensure that the drying cylinder hood has a suitable distance from the cylinder surface when it is lowered.
2.2 Papermaking production process
In the papermaking production process, after the pulp is filtered and dehydrated, a thin layer is formed on the conveyor belt. The wet pulp is first conveyed to several ovens and some moisture is removed by preliminary baking. Hot air above 100°C is blown into the oven from top to bottom, and the temperature of the hot air can be adjusted. Part of the moisture of the wet pulp thin layer evaporates in the hot air, and the water vapor is blown away with the hot air to achieve the purpose of pre-drying. After the wet paper comes out of the oven, it is baked on the drying cylinder with a surface temperature of about 1400°C to remove most of the remaining moisture. The temperature of the drying cylinder surface needs to be adjusted up and down according to the different paper thickness and paper material, as well as the speed of paper feeding.
In the papermaking system developed in this project, after the wet pulp thin layer is formed in the entire process, it is attached to the conveyor belt and passes through three ovens. The three ovens have basically the same structure and use electric ovens. Each oven is equipped with an electric heating plate on top. When current is supplied, the electric heating plate heats up, and the cold air blown in from the top is heated into hot air and blown onto the conveyor belt to pre-dry the thin layer of pulp. A standard 4-20 mA current signal is used to control the heating current flowing through the electric heating plate, thereby realizing the control of the heating power and adjusting the temperature in the oven.
The heat dissipation, dehumidification and air circulation system is a cold air inlet main pipe, into which air is blown by a blower. The main pipe is divided into three branch pipes, which pass through several layers of electric heating plates and enter the three ovens. At the entrance of each branch pipe, there is a plug-in baffle. Changing the insertion depth of the baffle can change the air intake. After
the cold air is heated by the electric heating plate, the hot air blows against the wet paper to dry and dehumidify, and mixes with the volatile water vapor, the humidity of the hot air increases, and the temperature decreases; the wet air in the three ovens is discharged through the dehumidification port at the bottom of the oven. Each dehumidification entrance is also equipped with a plug-in baffle, which can adjust the flow rate of the discharged wet air and the humidity in the drying oven. In addition, an exhaust fan is installed at the outlet of the dehumidification main pipe to suck out the humid air in the three drying ovens and the drying cylinder hood. Although the temperature of the humid air after passing through the drying oven is lowered to a certain extent, it still contains a considerable amount of heat. It is a bit wasteful to exhaust all of it and not use it from the perspective of energy saving. Therefore, the reflux of humid hot air is designed, and a part of it is separated from the dehumidification main pipe and re-enters the air inlet main pipe, thereby effectively improving the energy utilization rate. The amount of reflux air is also adjusted by a plug-in baffle.
The electromagnetic drying cylinder is the last papermaking heating process. After pre-drying, the paper contacts the high-temperature drying cylinder surface to remove the remaining moisture and generate a large amount of water vapor. Since the density of water vapor is less than that of air, it will rise and be collected by the drying cylinder hood. A dehumidification outlet is also left on the top of the drying cylinder hood and is connected to the dehumidification main pipe. Under the suction of the exhaust fan at the dehumidification outlet, it is smoothly discharged to the outside.
2.3 PLC Hardware
2.3.1 S7-200 Series Small PLC System
SIMATIC S7-200 series PLC is a PLC system for micro and small applications launched by SIEMENS. It has been applied to the automation of detection, monitoring and control in various industries and occasions. The S7-200 series inherits the characteristics of SIEMENS PLC products, which are comprehensive functions, reliable quality and flexible application. Its powerful functions enable it to realize complex control functions whether in independent operation or when connected to a network. It has a small appearance, easy installation, a variety of expansion modules, and can expand the memory capacity of the CPU. It can meet the control requirements of most small systems and has a very high cost performance compared with similar PLC products. The S7-200 series excels in the following aspects: a. High reliability; b. Very rich instruction set; c. Easy to master; d. Convenient operation; e. Rich built-in integrated functions; f. Real-time characteristics; g. Strong communication capabilities; h. Rich expansion modules.
The S7-200 series of products can provide 5 CPU modules with different performances for customers to choose from, so as to control the system cost. It includes a series of expansion modules, such as digital input (DI), digital output (DO), analog input (AI), analog output (AO), thermal resistance module (RTD), Ethernet, modem module, Prohibits-DP expansion module and other powerful modules, which can be easily used with various field instruments to collect various signals on site and control the operation of various equipment.
The S7-200 series of products also provides a small human-machine interface (HMI) device - TD200 text display (Text Display) device, which can easily set various parameters of the system and display the key variable values ​​in real time. It is simple to configure, convenient and easy to use, and cheap, eliminating the need for special IPC equipment and host computer monitoring software, saving system costs. Using TD200 as the human-machine interaction interface of this system can fully meet the requirements of parameter changes in the production process. In addition, SIEMENS provides Micro/Win step7 software specifically for S7-200 series PLC programming, which supports three PLC programming methods such as ladder diagrams, is easy to learn and use; and integrates common functions such as PID algorithm module and digital filter algorithm module for programmers to call, which greatly improves development efficiency, shortens development time, and saves system development costs.
2.3.2 PLC peripheral expansion module
In the PLC program, the main purpose is to realize the control of five sets of loops: three sets of power control loops for electric ovens, and power control of two electromagnetic induction drying cylinder coils. The temperature detection of the three ovens uses PT100 platinum thermal resistors, which are directly connected to the temperature detection (RTD) expansion module using a three-wire connection method; the temperature detection of the electromagnetic drying cylinder cannot use platinum thermal resistors, but uses infrared thermometers. The infrared temperature probe directly converts the detected temperature into a 0-5V standard voltage signal, which is passed into the analog input module (AI) and occupies one AI channel. The two electromagnetic induction drying cylinder coil loops can use the same drying cylinder temperature signal. Five inner loop current detections use mutual inductance coils to convert 0-50A current signals into 0-5V voltage signals, which are input into the AI ​​module and occupy 5 AI channels.
Another thing that needs to be controlled is the rotation speed of the drying cylinder. The drying cylinder is driven by a variable frequency motor, which is decelerated by a gear box so that the drying cylinder rotates at different speeds. By outputting a 4-20mA control current to the inverter in the control cabinet, the paper feed speed of the entire system is adjusted. The output of the inverter occupies one AO ​​point. The above analog quantity requirements are statistically required, and the total number of points required is:
(1) RTD input points: 3
(2) Analog input points: 6
(3) Analog output points: 6
In addition, digital signals such as the start and stop of the blowing and dehumidification motors, and the lifting and lowering of the drying cylinder hood need to be controlled. The CPU226XM has 24-bit digital input and 16-bit digital output channels, which can already meet the digital input and output of the system, and there is no need to select a digital expansion module.
To sum up, the S7-200 series PLC hardware modules finally selected are as follows:
(1) CPU controller: CPU226XM;
(2) Temperature detection module: EM231, 2 pieces;
(3) Analog input module: EM231, 2 pieces;
(4) Analog output module: EM232, 2 pieces;
(5) Power supply (24V DC regulated source, 5A), 1 piece.

3 PLC software and control algorithm
3.1 PLC programming software and communication
For SIEMENS S7-200 series PLC, Siemens provides programming software Micro/Win step7 under Windows platform, which includes complete internationalization support and can be operated with a complete Chinese and English interface, reducing the difficulty of use. Micro/Win step7 software has the following features: a. Step7-Micro/WIN is a SIMATIC S7-200PLC programming software running on Windows platform, which is simple, easy to learn, and can solve complex automation tasks; b. Suitable for SIMATIC S7-200PLC model software programming; c. Supports three programming languages: IL, LAD, and FBD, and can switch between the three; d. The program block has a password protection function; e. Provides software to help you debug and test your program, which includes monitoring the status of the program being executed by the S7-200 user, specifying the number of scans for the program for the S7-200, forcing variable values, etc.; f. Command wizard function: PID self-tuning interface, PLC built-in pulse output (PTO), pulse width modulation (PWM), command wizard, data logging wizard, recipe wizard, etc.; g. Support TD200 and TD200C text display interface. Step7-Micro/Win programming software communicates, uploads and downloads programs through programming cables that support the protocol. It is the simplest and most economical communication method.
3.2 Cascade PID control
In paper production, the most important parameter of the dryer is the surface working temperature T of the dryer, which directly affects the quality of the finished paper. Because the temperature T has the characteristics of large delay, slow heating and large inertia, it is difficult to control. Therefore, cascade control is used in the design, which has the following main characteristics:
(1) When the interference to the sub-loop has not yet affected the controlled quantity, the sub-loop can be effectively controlled;
(2) The parameter changes in the sub-loop are controlled by the sub-loop, and the influence on the controlled quantity is greatly reduced;
(3) The inertia of the sub-loop is adjusted by the sub-loop, thereby improving the response speed of the entire system.
The control block diagram of the cascade control is shown in Figure 3. The control object of the outer loop is the temperature T, and the control object of the inner loop is the output current IO of the load circuit.

An infrared temperature probe is installed on the drying cylinder frame, and the temperature signal is converted into an electrical signal and collected into the PLC module. In the program, the average temperature TO of the drying cylinder surface is obtained through smoothing and filtering steps. TO is compared with the set temperature Tsp, and the difference is used as the input of the PID module. After the PID algorithm described above, the output signal is used to adjust the set value Isp of the heating current IO of the load circuit. The actual heating current is collected by the PLC analog module through the mutual inductance coil, and compared with Isp as the input of the inner loop PID module. After calculation, the PLC outputs a 4-20 mA control current to adjust the power output. The various changes in the entire system, such as temperature T, heating current size IO, control signal milliampere number, drying cylinder speed and other data are displayed on the TD200 screen. The operator can clearly see the real-time changes of these variables, and can automatically or semi-automatically adjust the heating power by setting the given value of temperature T and the given value of heating current.
3.3 TD200 interactive interface and control panel
During the production process, due to changes in production process parameters, it is necessary to change parameters such as temperature settings. At the same time, in order to better implement monitoring, it is necessary to understand the various related parameters. TD200 is connected to the CPU module serial port through its own cable and does not require an additional external power supply. It can display multiple languages. When the CPU is running, the configured TD200 screen can display the real-time value of numbers or variables in the CPU. The value of the variable on the TD200 can also be changed through programming configuration and direct methods.
3.4 Programming precautions
In the design of PLC programs, special attention should be paid to the following issues:
(1) Because the coil can only heat a certain part of the surface of the drying cylinder at a certain time, if the drying cylinder does not rotate, it will cause local overheating of the drying cylinder body, and in severe cases, deformation and damage of the cylinder body. Therefore, the heating start should be protected in the program, and the heating can only be started when the drying cylinder speed is greater than a certain speed and the conditions are met.

(2) The coil is located inside the dryer hood and rises and falls with the dryer. The distance between the coil and the cylinder body affects the value of the coil inductance L. In production, due to the need to roll paper, the dryer hood often needs to be lifted. Protection should be added in the program. When the dryer is detected to be lifted, the heating should be stopped immediately.

(3) When starting and stopping the heating, the power changes greatly, which has a great impact on the power grid. In the control program, the rate of change of the output power should be limited. During the start and stop process, the control program should slowly increase the set power or slowly reduce the power until it stops completely.

4 Conclusion

This article gives a comprehensive and detailed description of electromagnetic induction heating dryers for papermaking from several aspects, including the principle and composition of electromagnetic induction heating power supply, the structure of the dryer, the process of papermaking heating production line, the composition of PLC control system, PID control algorithm, etc. On the basis of the transformation of the original steam heating dryer, a set of electromagnetic dryer heating system with convenient operation, energy saving and high efficiency has been developed, which achieves better drying effect while meeting the original "paper dehumidification rate". Compared with the original system, the dehumidification rate is increased by 20~30%, energy saving is 40~50%, and there are obvious economic benefits; the plant area is reduced, there is basically no pollution, and it has the characteristics of fast heating speed and convenient operation.