Application of PLC and touch screen control system

0 Preface

With the rapid development of technology, more and more machines and field operations tend to use man-machine interfaces, and the powerful functions and complex data processing of PLC controllers also require a man-machine with matching functions and easy operation. interface. The emergence of touch screen is undoubtedly a huge innovation in the field of automation in the 21st century. The combination of touch screen and PLC has become the dominant form.

PLC is a new type of control element with simple structure, good versatility and complete functions. Its main advantage is its strong anti-interference ability, which can improve the reliability and stability of the system and production efficiency. It is especially suitable for industrial control.

The touch screen is a human-machine interface that connects people and machines. It replaces the original console and display, and can be used for data display and parameter setting. It can also describe the control process of the system in the form of dynamic curves; expands the functions of PLC, Reduced the use of buttons, switches, instruments and other instruments.

1 System components

This system adopts a system composed of Siemens S7-200 and Deven touch screen to control the actuators such as electric valve, solenoid valve, motor and temperature controller on site. S7-200 collects on-site temperature and pressure signals through analog input modules and temperature and pressure sensors. The signals are transmitted to the touch screen through A / D conversion and numerical conversion on the PLC. The touch screen displays real-time temperature values, pressure values, and temperature curves , Pressure curve and PID curve; and PID parameters can be set through the touch screen, the touch screen sends instructions to the PLC to control the on-site actuators. Since the PLC interface is RS-485 and the touch screen interface is RS-232, an RS-485 / RS-232 conversion line needs to be added. The composition of the control system is shown in Figure 1.

Figure 1 Block diagram of the control system

2 PLC and touch screen communication

The data transmission between computer and computer or computer and terminal can adopt serial communication and parallel communication. The communication of S7-200 series PLC is divided into three working modes: PPI communication mode, free port communication mode and Profibus-DP communication mode. This system uses the free port communication mode.

2.1 PLC free port communication

When S7? ? When the 200 series PLC uses free port communication, the data transmission protocol is completely determined by the user program, and all communication tasks must be completed by the user programming. Through the free port mode, the S7-200 can communicate with serial printers, bar code readers, and touch screens, and its baud rate range is 1 200 ~ 115 200 bit / s (adjustable). The core of free port communication is the two commands XMT (send) and RCV (receive) and the corresponding special register control. The free port communication of this system uses free port 0, and the S7-200 CPU uses SMB30 to define the working mode of free port 0. Through the special register SMB30, you can set the selection of the check, the data bit of each character, the baud rate of the free port and the protocol selection. The free port interface on the S7-200 CPU is RS-485, and the touch screen interface is RS? ? 232, a RS-485 / RS-232 communication line needs to be made during design. The RS-485 communication port of the S7-200 CPU is a half-duplex communication port. Sending and receiving commands cannot be activated at the same time. At this time, you can control the RCV (receive) by controlling the special register SMB87. When the PLC is within the specified time When the message is not received, the RCV command will stop receiving.

2. 2 Data transmission method of touch screen

The serial data frame structure of the Devon touch screen is composed of four data blocks: frame header, command, data and end of frame character. The frame header is fixed at 0XAA, and the instruction refers to the Devin instruction set. The data is at most 249 B, and the frame end character is fixed at 0XCC, 0X33, 0XC3, 0X3C. Among them, 0X represents a hexadecimal number. All instructions or data of the Devon touch screen are in hexadecimal (HEX) format. For font (2 byte) data, the byte transfer order adopts the high byte first transfer (MSB) method. When the transmission direction is down (Tx), the PLC sends data to the touch screen, and the data is input from the "Din pin" of the serial interface of the touch screen; when the transmission direction is up (Rx), the touch screen sends the data to the PLC, and the data is from the serial interface of the touch screen The "Dout pin" output. The process of data exchange and exchange between the touch screen and PLC serial port is shown in Figure 2.

Figure 2 Schematic diagram of the data exchange process

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