1. Introduction
Vacuum systems made of electroplated metals, such as zinc and aluminum films, are very common in the capacitor industry. The main goal is to increase film transfer speed while maintaining film tension as much as possible. Film tension and speed should be maintained as a reference value, and the main problem is the coupling between film speed and tension. There are many sources of disturbance to their speed (roller non-circularity, film slippage). If the transfer speed fluctuates, it will lead to uneven die processing; on the other hand, inappropriate tension may cause wrinkles or film breakage. Once the film breaks, the operator needs to reopen the winding chamber, thus returning the pressure in the vacuum winding chamber to standard atmospheric pressure. Then, it takes about 20-30 minutes to bring the winding chamber back to the required pressure (approximately 1.3 × 10^1 - 2.67 × 10^-2 Pa). Therefore, production will decrease significantly; thus, a monitoring system to detect tension fluctuations and prevent film breakage is essential.
CC-Link is a local area network system that provides high-speed process control and information data processing, offering customers efficient and comprehensive industrial and process automation. Users of CC-Link can reduce the number of control variables and the power lines required for complex production lines. Therefore, users can choose suitable equipment from the remaining 354 fields that support CC-Link, making expansion into multi-vendor environments easy.
Regarding high-speed communication, it enables the input of communication sensors to meet the requirements of intelligent devices that need large amounts of data communication and high-speed response. As a CC-Link function, the RAS function includes: standby master function and disconnected slave function, which enable automatic recovery in the event of communication failure and system debugging. As for testing and monitoring functions, they are available for confirming data connection status, hardware testing, and circuit testing. The CC-Link network diagram is shown in Figure 1, and its monitoring system consists of master station > slave station > "PC" > inverter > servo motor > remote I/O unit, etc.
To meet the requirements of automation and flexibility, many complex manufacturing systems are controlled by programmable logic controllers ( PLCs ). This is because PLCs are highly adaptable, modular, easy to use, and inexpensive to purchase. Vacuum roll-to-roll coating is one such complex manufacturing system.
This article introduces an intelligent monitoring system based on the CC-Link fieldbus, which includes a host personal computer and a programmable logic controller (PLC) for control and information management. This monitoring system utilizes the new CC-Link fieldbus technology.
2. Structure of the monitoring system
The fieldbus-based monitoring system has two functions: a monitoring and information management unit for the monitored area, and a control unit in the control room. All intelligent monitoring units located near the vacuum cleaning machine have microprocessors with functions including signal sampling, A/D conversion, and data calculation. The fieldbus is the most important connection between the discrete monitoring units in the slave stations and the master station. Analog signals are replaced by digital signals to establish bidirectional communication, facilitating control, inspection, and parameter setting by operators in the control room. This structure improves the accuracy and anti-interference capability of the monitoring system while also saving on investment costs.
The digital intelligent monitoring unit should be determined according to actual needs. The system has five monitoring units: temperature, vacuum degree, roll diameter, film tension, and conveying speed.
The proper functioning of the monitoring system depends on whether the performance of the sensors meets the required level; their accuracy and stability directly affect the monitoring system. The data acquired from these sensors is 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, its filters, amplification, and adjustment of output signal sensors can be processed by the PLC (FX series) to obtain appropriate values; on the other hand, it uses the programmable controller with the communication interface to perform data acquisition, A/D conversion, data processing, software anti-interference, parameter calculation, and uses the communication interface on the PLC to transmit data to the central control unit.
The Central Control Unit (CCU) is the heart of the monitoring system, containing a personal computer and RS232. Its function is to control each unit under intelligent monitoring, process and store the acquired data and information. Furthermore, the controller supports access from a host computer, and its data analysis is practical, utilizing data management and fault diagnosis software for data diagnostics.
The monitoring system uses CC-LINK fieldbus modules and twisted-pair cables as the communication medium. Up to sixty-four stations can be connected to this fieldbus, and the communication modules connect the monitoring units and substations via the CC-LINK fieldbus. When the fieldbus communication rate is 156 kbps, the transmission distance can reach 1.2 kilometers.
3PLC Programming
In this winding system, we use a 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 coexisting winding system of the main winding system and the FX-series PLC winding system. The system's operating actions and sequence of actions have been pre-programmed into the control program by the designers. The control program sets a series of operating actions for the winding system, guiding 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 semaphore signals. Therefore, the PLC program is the foundation for monitoring a PLC-controlled manufacturing system.
The primary method used in PLC programming is the ladder diagram approach. This provides a design environment where software tools run on the host terminal, facilitating the construction, verification, testing, and diagnosis of ladder diagrams. First, the high-level program is written in the ladder diagram; second, the ladder diagram is converted into binary code so that it can be stored in Random Access Memory (RAM) or Erasable Programmable Read-Only Memory (EPROM). Each consecutive instruction is decoded and executed by the CPU. The CPU's function is to control memory and I/O interfaces to process data according to the program. Each input/output node on the PLC can be used to distinguish I/O addresses. The direct representation of data relates to input, output, and memory. Based on this fact, the PLC's memory is divided into three parts: input image storage, output image storage, and internal memory to directly represent the relevant input, output, and memory data.
The PLC program employs a main program cyclic scanning method, such as periodically checking input variables. The cycle begins by scanning the input system and memory at fixed locations (input image memory). Following the ladder diagram program, the program executes a response, scans the program, resolves the output states determined by the logic ladder, and stores the updated output states in fixed memory locations (output image memory). At the end of the program scan, the output values stored in memory are used to set and reset the PLC's physical outputs.
As we all know, logic control is a prominent feature of PLCs, which can be used to efficiently process analog data.
1) Analog data acquisition and conversion of analog input and output, such as pressure and temperature, which need to be measured in real time. For example, 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 acquired and transmitted to the PLC mentioned above.
2) The PLC control algorithm can simulate 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 a pre-designed control algorithm. It is worth mentioning that the PLC output is always an incremental value. Although a self-tuning PID controller can meet the requirements, manual tuning is always used to initialize routine production and then the system switches to automatic mode. We emphasize that the user's experience-based fuzzy logic control is also applicable to improving production conditions. The film tension control system uses a fuzzy logic controller (PLC) to overcome the uncertainties of the winding system. The fuzzy logic algorithm has been implemented by a basic PLC controller.
While a good level of performance has been achieved for fixed-mode and conventional PID parameter handling, stability is relative. The parameters of the winding system fluctuate within a certain range, therefore the controller must be robust to achieve high performance. For this purpose, fuzzy inference is used to adapt PID control, allowing the PID parameters to be adjusted based on the system state and power plant parameters.
3) The servo motor initially transmits the film at a low speed, then the servo motor is accelerated to a set high winding speed of approximately 8 meters per second. Sensors detect the film tension and control the servo motor speed to maintain a constant tension. The control objective is not only to maintain constant film tension but also to reach the set value at the shortest possible time. Therefore, the most crucial issue is how to precisely control the servo motor.
4 Conclusion
This article introduces a PLC monitoring and control manufacturing system for thin-film electroplating production, which has achieved great success since its implementation in 2003. The anticipated results have demonstrated its significant advantages in practice, and its profitability has also greatly improved.
