Application of PLC Programmable Controller in Experimental Device of Process Control System

1 Introduction

With the rapid development of modern science and technology, not only the automation of the production process, but also put forward higher requirements for production management. The combination of automatic control and computer management systems through computer network technology, integrating management and process control as a whole is the trend of industrial automatic control development today. Complex process control systems often use two-level network topology. The bottom layer uses fieldbus to make the control device as close as possible to the controlled production process site. The upper layer uses industrial Ethernet, and the monitoring level is relatively concentrated in the main control room, thereby realizing the production process. Centralized management and decentralized control. The control system constructed in this way has the advantages of good real-time performance, high reliability, and strong anti-interference ability, and is more economical and more reliable than the traditional DCS system. In order to adapt to this form of development requirements and improve the quality of experimental teaching, engineering students can receive good engineering practice during their studies. Therefore, a process control system experimental device based on industrial Ethernet and fieldbus has been developed.

2 system configuration and network structure

The experimental device control system consists of two parts, the upper computer monitoring system and the lower computer PLC control system. The entire network adopts a two-tier network topology. The upper layer is Industrial Ethernet. It is used for communication between host PCs and PCs and lower-bit PLCs. The bottom layer is PROFIBUS-DP field bus for the lower-level PLC master station ( Communication between DPM1) and four slave stations (DPS1-DPS4), where the PLC master station and slave station control the process control experimental devices such as liquid level, pressure, and temperature flow. The system uses SIMATIC STEP 7 software to carry on the network configuration, the hardware configuration and the PLC control program preparation, and uses the configuration software SIMATIC WinCC to realize the host computer and the PLC dynamic connection. The overall system composition is shown in Figure 1:

Fig.1 Structure of experimental device of process control system

2.1 The site part of the site is the experimental device for the liquid level, temperature flow and pressure required to be controlled. The transmitter converts the sampled data into a current signal of 4 to 20 mA and directly connects to the SM334 module (analog input/output module). The analog/digital conversion becomes a digital quantity of 0 to 27648. The input and output of the switch are connected to the SM323 module (digital input/output module).

2.2 Control Unit The control unit uses Siemens PLC. The S7-300 series PLC is powerful and modular in design. It has a central processing unit (CPU), various signal modules (SM), a communication module (CP), and a function module (FM). Power modules (PS), interface modules (IM), etc. are available in a variety of CPU sizes. The AS-Interface, PROFIBUS and Industrial Ethernet systems can be connected via the CPU with integrated PROFIBUS-DP interface, MPI interface or communication module.
The main station of this system adopts Siemens S7-300 series PLC, its CPU is 315-2DP. It has a short instruction execution time and does not require 10ms to scan 1000 instructions, which is sufficient to meet the control time requirement. The master station also has 2 signal processing modules (DI 16/DO 16, AI 4/AO 2) and a communication module CP 343-1 (for communication between the host and the PLC via Industrial Ethernet). Slave station selects PROFIBUS-DP distributed I/O ET 200M with two signal processing modules (DI 16/DO 16 and AI 4/AO 2). The slaves do not have a central processor unit and each slave station passes through the IM 153 interface. The module is connected via the DP bus. After configuration, the added distributed I/O and the local I/O in the PLC station have a uniform addressing.

2.3 Host computer The upper computer is four industrial computers. The host interface design adopts Siemens WinCC configuration software to ensure full compatibility with the industrial computer. The software integrates advanced technologies such as configuration, scripting languages, and OPC, and provides functions for using various general-purpose software in the Windows operating system environment. The software has functional modules for graphic display, control and alarm screens, real-time and historical trend curves, filing and report printing for industrial production processes. In addition, WinCC also has the option of system diagnostics for SIMATIC PLCs, which provides convenience for hardware maintenance.
The system application development and operation software is STEP7 V5.2, which is a standard software package for programming and configuration of S7-300/400 PLC series. With STEP 7 V5.2 users can complete the following tasks:
(1) Network configuration, setting up connections and interfaces;
(2) Configuration hardware;
(3) Write and debug user programs.

3 Network System Principle

PROFIBUS-DP is an international, open fieldbus standard that is mainly used in the industrial process control field. With reference to the ISO/OSI reference model, there is no Layer 3 to Layer 7 in PROFIBUS-DP. Direct Data Link Mapping (DDLM) provides easy access to Layer 2 user interfaces. The user interface specifies that users and systems and different devices can call. Application features. It is designed for communication between industrial control systems and equipment-level distributed I/O, and is used for high-speed data transmission in distributed control systems. Its modules can replace expensive 24V or 4-20mA parallel signal lines. The central controller communicates with decentralized field devices through high-speed serial lines. Most data exchange processes are cyclic. The master station periodically reads the input information of the slave station and sends the output information to the slave station. In addition to periodic user data, PROFIBUS-DP also provides the aperiodic communication required for intelligent devices for configuration, diagnostics and alarms.
SIMATIC Industrial Ethernet is a network based on international standards. It is optimized for industrial applications and supports ISO and TCP/IP protocols. Through it, you can quickly establish communication between PLC and PC/PG. The development of the product follows the distributed “open control structure”, which has the advantages of simple network configuration (plug and play), reliable communication, and short network recovery time (less than 0.3 seconds). Since the full-duplex sharing load mode is used for industrial networks with high performance requirements, a very large network structure can be achieved through switching technologies.

4 network system configuration

Before the configuration, create a project (eg Project1) and insert the SIMATIC 300 station in the project.

4.1 Hardware configuration Hardware is configured for the SIMATIC 300 station in HW Config and includes the rack, power supply (slot 1), CPU (slot 2), communication module (slot 4) and input and output modules. Set the parameters of the DP master interface integrated on the CPU and establish a PROFIBUS network to be connected to the DP master interface.

4.2 DP Slave Configuration Take the ET 200M station connected to the DP master as an example. First select the interface module IM153-l from the hardware, connect to the PROFIBUS network of the DP master interface, as shown in Figure 2, and set the PROFIBUS address of this DP slave. The address must match the address set by the address selection switch on the IM153 module.
The ET 200M slaves are equipped with 2 signal modules and the corresponding module types are found in the DI/DO of the ET 200M and added to the corresponding slots of the slave as shown in Fig. 3. When using the hardware catalog, make sure that you are in the correct folder. For example, selecting the module for ET 200M should be done in the ET 200M folder. The added distributed I/O and the local I/O in the PLC master have a uniform addressing, so the distributed I/O can be accessed in the program as easily as the local I/O, without any consideration during programming. How is an I/O address physically connected?

Fig. 2 Configuration of ET200M slaves and DP masters

Fig. 3 Signal module configuration of ET20M slave

4.3 Port Settings
(1) The PG/PC interface is the interface for communication between the PG/PC and the PLC. To realize the communication connection between the PG/PC and the PLC, the interface must be set correctly. Open "Set PG /PC Interface" in the control panel, check "S7 OnLine (STEP7)" and select the network card type. Then enter the hardware configuration HWConfig of STEP 7 to set the MAC address of the communication module. The address is the physical address given on the CP343-1 label. The format is a 12-bit hexadecimal number (eg 08-00-06- 00-44-AE). In addition, the PLC must be assigned a unique IP address (eg, 192.168.0.130) and a subnet mask (eg, 255.255.255.0).
(2) Set up the PROFIBUS network: Use the graphical configuration tool NetPro to set the transmission rate, the highest station address, the bus profile, and the bus parameters of the PROFIBUS bus.
After the system configuration is completed, it should be downloaded to the PLC and debugged so that the hardware is connected.

4.4 Programming and Debugging of Programs STEP 7 is a standard software for creating control programs for S7-300/400. The programming languages ​​include ladder diagrams, statement tables, and function block diagrams.
Usually user programs consist of organization blocks (OBs) and function blocks (FBs).
Function (FC) and data block (DB) composition. OB1 is the main program loop block and is required. Depending on the complexity of the control program, simple programs can be programmed in OB1 linear programming, if the program is more complex should be structured programming, the program is structured with different logic blocks, call these logic blocks through OB1 .
For an actual process control, the user program is written in accordance with the adopted control strategy, and after simulation debugging, it is downloaded to the PLC, and it is combined with the actual system to complete the corresponding control function.

5 WinCC monitoring communication configuration

WinCC provides SIMATIC S7 Protocol suite. CHN driver. This driver supports various types of network protocols. Through its channel unit, it can communicate with various SIMATIC S7-300/400 PLCs. The type of channel unit depends on WinCC. The type of connection to the automation system. The system selects the Industrial Ethernet channel unit. Industrial Ethernet is the most effective subnet in the industrial environment. It is suitable for communication with management and field level.
First add the SIMATIC S7 Protocol suite. CHN driver, then select the "Industrial Ethemet" channel unit under "SIMATIC S7 Protocol Sute", open the "Connection Properties" input connection name, enter the MAC address of the communication module CP343-1 of the PLC to be connected in the connection parameters, PLC The rack number and slot number where the CPU is located. The slot number here is the slot number where the CPU is located, not the slot number where the communication module is located.
Then, according to the specific process control task, the user creates a variable under the new connection, associates the variable with the address to be connected in the PLC, and establishes a connection with the PLC. Finally use WinCC to complete the configuration of various display screens and data.

6 Conclusion

The field bus control network established in this paper can also realize remote monitoring by accessing standard Ethernet. The experimental device is based on the characteristics of automation and related professional teaching, based on process control based on the set of PLC technology, network technology as one of the advanced experimental device, using a variety of commonly used control algorithms and theory, in addition to the common PID algorithm It also added advanced control strategies such as fuzzy control and artificial neural network control.

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