This paper introduces the implementation of monitoring software and PLC communication in a high-power solid-state laser processing system under two different scenarios, and compares the two methods. One method involves serial communication between monitoring software developed in VB6.0 and the PLC, with specific examples provided. The other method involves communication between monitoring software developed based on a touchscreen and the PLC.
In high-power solid-state laser monitoring systems, the control PLC of the laser processing system can collect and store real-time status data of the laser processing system. The monitoring software monitors the laser by periodically querying this data. This paper takes two examples: serial communication between an OMRON PLC and monitoring software developed in VB6.0 via an RS232 interface, and communication between a Delta PLC and Delta touchscreen configuration software, to study the communication between the PLC and the upper-level software under different conditions.
1. Communication between the host computer and PLC based on VB
The monitoring function is mainly reflected in the timely and accurate display and recording of real-time data of the processing system. It is mainly divided into a control management module, a data communication module, a fault handling module, a data processing module, and an information processing module. The data communication module is responsible for data communication with the PLC control system, converting the operator's operation instructions into a unified data format and sending them to the PLC control system through the serial communication port. The data communication module is also responsible for reading the data transmitted from the PLC control system and calling the data processing module to process it.
The design of the VB communication program in the monitoring software is as follows :
(1) Initialize the communication port
In high-power solid-state laser processing systems, the monitoring system uses an OMRON CQM1H series controllable programmer as the lower-level machine and a regular PC as the upper-level machine. The communication port is COM1, with communication parameters set to a baud rate of 9600 , even parity , 7 data bits, and 2 stop bits. These parameters must match the PLC settings; otherwise, communication will fail. The specific program is as follows :
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(2) After the serial port is opened, data is sent.
The data transmission format must be standardized, including a start character, node number, header code, data , FCS (Frame Sequence Code), and end character. If a string of data is being transmitted, the host computer and PLC must use the same data format for easy identification. After calculating the FCS, the frames can be transmitted. The maximum data block for a single frame is 131 characters. If it exceeds 131 characters, it must be split into more than one frame before transmission. When a set of transmitted data needs to be broken down, the end of the first frame and each subsequent frame is replaced with a delimiter instead of an end character. (The following example uses a communication program from the host computer monitoring software to the PLC as an example .)
(3) The PC receives data from the PLC.
The process of sending data from the PLC and receiving it through monitoring software is similar. This communication method simplifies data transmission and overcomes the data loss issues that occur when using custom data formats, ensuring accurate data transmission and reception.
2. Communication between touchscreen software and PLC
Industrial-grade human-machine interfaces (HMIs) can function as small workstations for PLCs, enabling them to connect and monitor PLCs, and simultaneously display internal contact status and data using text, numbers, or graphics. Touchscreen manufacturers provide configuration software for their products, offering various general-purpose modules. Users don't need extensive programming language skills to effectively complete almost all the functions required for a project. Delta's touchscreen software is one such example, using intuitive graphics and relatively simple macro instructions to edit the system interface. Macro instructions are similar to PLC instructions; the basic functions achievable with PLC instructions can also be achieved with macro instructions, and the programming method is relatively simple.
The system hardware uses Delta series PLCs and touch screens, while the software consists of two parts: a configuration environment and a runtime environment. The configuration environment is used to design and construct the user's application system; the runtime environment runs the configuration project constructed in the configuration environment to achieve the user's configuration design goals and functions. A system monitoring configuration interface is developed on the computer, and communication parameters are set. In the "Options" menu, the "Module Parameter Setting" option is selected, which will pop up a window for setting communication parameters (as shown in Figure 1). After the parameters are set, the prepared configuration project is downloaded from the computer to the touch screen via USB for use. As shown in Figure 1, it is simple and easy to use; no special programming is required. In the touch screen software, the instruction execution process is the same as the PLC, using a polling method to continuously scan. A certain thread in the process periodically queries the serial port's receive buffer. If there is data in the buffer, it reads the data; if there is no data in the buffer, the thread continues execution. Under normal circumstances, the polling method is the most direct way to read the serial port, but because it must continuously poll the serial port, it consumes a lot of CPU time.
3. Conclusion
The advantages of manually developing industrial control software are high flexibility and strong specificity. Modules can be implemented according to the system's needs, and the software cost is low. However, the disadvantages are a large workload, long development time, poor reliability, and low versatility. Different applications require redesigning the program, and even minor system modifications may necessitate software redesign. Delta touchscreen software is a small-scale configuration software, powerful in function, easy to use, and requires less specialized programming skills, saving time and manpower. However, it also has disadvantages such as limited functionality, poor flexibility, and inability to meet certain special requirements. Therefore, in practical applications, different development methods should be selected based on the characteristics and requirements of different systems to achieve the best results.
