PLC Control Manufacturing System Based on Fieldbus Monitoring System

1 Introduction

Vacuum systems for electroplating metals, such as zinc and aluminum films, are very common in the capacitor industry. Its main goal is to increase the film transfer speed as much as possible while maintaining the film tension. The film tension and speed should be kept as a reference value. The main problem is the coupling between film speed and tension. There are many sources of disturbance between their speed (roller non-circularity, film slippage). If the transfer speed fluctuates, it will cause uneven processing of the die; on the other hand, improper tension may cause wrinkles or film ruptures. Once the film ruptures, the operator needs to open the winding chamber again, so that the pressure of the vacuum winding chamber returns to standard atmospheric pressure. Then, it takes about 20-30 minutes for the winding chamber to reach the required pressure (about 1.3 × 10^1--2.67×10-2Pa). Therefore, the output will be significantly reduced; therefore, a monitoring system for detecting tension fluctuations to avoid film rupture is very necessary.

CC-Link is a local network system that can provide high-speed process control and information data processing, providing customers with efficient and comprehensive industrial and process automation. Users who use CC-Link can reduce the number of control variables and the power lines required for complex production lines. Therefore, users can choose the appropriate device from the remaining 354 fields of equipment that support CC-LINK, and the expansion of multi-vendor environments will become easy.

As for high-speed communication, it enables the input of communication sensors to meet the requirements of intelligent devices and high-speed responses that require large amounts of data communication. As a CC-link function, the RAS function is: standby master function, detached slave function, which enables automatic recovery during communication failure and system debugging. As for the test and monitoring function, hardware testing and circuit testing are available to confirm the data connection status. The CC-Link network diagram is shown in Figure 1, and its monitoring system consists of master station> slave station> "personal computer"> inverter> servo motor> remote I/O unit, etc.

In order to meet the requirements of automation and flexibility, many complex manufacturing systems are controlled by programmable logic controllers ( PLC ). This is because PLC is highly adaptable, modular, easy to use and low in purchase cost. Vacuum winding coating is such a complex manufacturing system.

This paper introduces an intelligent monitoring system based on CC-LINK fieldbus, which includes a host personal computer and a lower programmable logic controller for control and information management. This monitoring system uses the new technology of CC-Link fieldbus.

2 Structure of the monitoring system

The monitoring system based on fieldbus has two functions, including monitoring and information management unit in the monitoring area and control unit in the control room. All intelligent monitoring units located near the vacuum cleaner have microprocessors with the following functions: respective signal sampling, A/D conversion, data calculation, etc. The fieldbus is the most important connection between discrete detection units and the master station in the unit slave station. Analog signals are replaced by digital signals to form two-way communication, which is convenient for operators to control, check and set parameters in the control room. This structure, on the one hand, improves the accuracy and anti-interference ability of the monitoring system, and on the other hand, saves investment costs.

The digital intelligent monitoring unit should be determined according to actual needs. The system has five monitoring units: temperature, vacuum degree, roller diameter, film tension and transmission speed.

Whether the monitoring system can work properly depends on whether the performance of the sensors is above the required level. Their accuracy and stability have a direct impact on the monitoring system. The data obtained from these sensors are processed by the PLC and displayed on the HMI human-machine interface, as shown in Figure 2.

The intelligent monitoring module has two functions: on the one hand, it filters, amplifies and adjusts the output signal of the sensor so that it can be processed by the PLC (FX series) to obtain appropriate values; on the other hand, it relies on the programmable controller of the communication interface to perform data acquisition, A/D conversion, data processing, software anti-interference, calculate parameters and transmit data to the central control unit using the communication interface on the PLC.

The central control unit (CCU) is the center of the monitoring system, which contains a personal computer and RS232. Its working state is to control each unit of the intelligent monitoring, process the obtained data and field information, and save them. In addition, the controller also supports computer access by upper users, and its data analysis is feasible, and the data is diagnosed using data management and fault diagnosis software.

The monitoring system uses CC-LINK fieldbus modules and twisted pair cables as communication media. Up to 64 stations can be connected to the fieldbus, and the communication module connects the monitoring unit and the substations through the CC-Link fieldbus. When the communication rate of the fieldbus is 156kps, the transmission distance can reach 1.2 kilometers.

3 PLC Programming

In this winding system, we use Mitsubishi A series PLC as the master station because of its fast response and powerful information processing capabilities. It is used to control the behavior of the total winding system and the FX series PLC winding system. The system's operating actions and action sequences have been pre-programmed into the control program by the designers. The control program sets a series of operating actions for the winding system to guide the PLC to control the entire system. The current status of all sensors or actuators is stored in the PLC memory as input, output or flag signals. Therefore, the PLC program is the basis for monitoring a PLC-controlled manufacturing system.

The main method used in PLC programming is the ladder diagram method, which provides a design environment in which software tools run on the host terminal to facilitate the construction, verification, testing and diagnosis of ladder diagrams. First, write the high-level program in the ladder diagram; second, the ladder diagram should be converted into binary code so that they can be stored in the random access memory (RAM) or the erasable programmable read-only memory (EPROM) that stores the code. Each consecutive instruction is decoded and executed by the CPU. The function of the CPU is to control the memory and I/O interface to process data according to the program. Each input and output node on the PLC can be used to distinguish the I/O address. The direct representation method of data is related to input, output and memory. Based on this fact, the memory of the PLC is divided into three parts: input image storage, output image storage and internal memory to directly represent the relevant data of input, output and memory.

The PLC program uses a main program loop scanning method, such as periodic checks of input variables. The loop program is started by scanning the input system and storing it in a fixed location in memory (input image memory). The ladder program then executes the response, scanning the program, solving the output state determined by the logical ladder, and storing the updated output state in a fixed memory location (output image memory). At the end of the program scan, the output values ​​saved in memory are used to set and reset the physical outputs of the PLC.

We all know that logic control is the distinctive feature of PLC, which can be used to process analog data efficiently.

1) Analog data acquisition and conversion The input and output of analog quantities, such as pressure and temperature, need to be measured in real time. For example, the temperature is first obtained by a platinum resistor, and then the signal conversion module converts it into a 1-5V voltage signal. The output of this conversion module is finally collected and transmitted to the above-mentioned PLC.

2) PLC control algorithm simulation variables can be any variable, such as temperature and pressure in our winding system. In fact, there are two control modes: automatic and manual. In manual mode, the operator modifies the output value according to the required level; while in automatic mode, the output value is given according to the pre-designed control algorithm. It is worth mentioning that the output of the PLC is always an incremental value. Although the self-tuning PID controller can meet the requirements, manual tuning is always used to initialize routine production and switch to automatic mode through the system; we emphasize that the user's experience-based fuzzy logic control is also applicable to improve production conditions. The film tension control system uses a fuzzy logic controller (PLC) of the PLC to overcome the uncertainty of the winding system. The fuzzy logic algorithm has been implemented by the basic PLC controller.

The parameter processing for fixed mode and conventional PID has reached a good stage, but stability is relative, the parameters of the winding system fluctuate within a certain range, so the controller must be robust to obtain higher performance. For this purpose, fuzzy reasoning is used to adapt PID control, in this way, the PID factors can be adjusted according to the system status and power plant parameters.

3) Servo motor control: The film is initially transmitted at a low speed, and then the servo motor is accelerated to a set high-speed winding speed of about 8 meters per second. The sensor detects the film tension and controls the servo motor speed to maintain a constant tension. The control target is not only to keep the film tension constant, but also to achieve the set winding speed in the shortest time. Therefore, the main issue is how to control the servo motor accurately.

4 Conclusion

This paper introduces a PLC monitoring and control manufacturing system for thin film electroplating production, which has been put into production and operation since 2003 and has achieved great success. The expected results have shown its great advantages in practice and the objective profit has also been greatly improved.