summary
This paper takes the design of a petroleum loop control system as an example, detailing the operation of the lower-level system controlled by a PLC. It focuses on analyzing the advantages of ControlNet among various buses and its application in the loop control system. Based on Rockwell's RSLogix5000, a freely configurable independent control software was developed. This enabled free control of various valves, using periodic control to address the lag problem caused by extensive computation. Furthermore, this paper combines LabVIEW virtual instrument technology with a PLC, implementing communication and monitoring between the upper-level computer and the field PLC monitoring network via serial port.
Keywords: ControlNet fieldbus; free configuration; virtual instrument; PLC monitoring system
Abstract
Basedonsomepetroleumring - roadcontrolsystem ,thispaperlaunchesthedetailedintroducesbitaboutcontrolsystemworksbyPLC . EmphasizeandanalysetheadvantagesofControlNetineverybusinthering- roadcontrolsystemapplication . Use the periodic control method to solve the problem of computing lagging. This paper is based on the LabVIEW virtual instrument technology combined with PLC , through aserial port , realize PC with PLC of monitoring network communication and monitoring system .
Keywords：ControlNetfieldbus;Freeconfiguration;Virtualinstrument;PLCcontrolsystem
1 Introduction
For 40 years, as a general-purpose industrial control computer, the Programmable Logic Controller ( PLC) has achieved a leap in industrial control from wired logic to stored logic; its functions have evolved from weak to strong, realizing the progress from logic control to digital control; and its application areas have expanded from simple control of individual devices to being capable of various tasks such as motion control, process control, and distributed control. This paper uses an advanced computer control system to form a host PLC control system as the slave computer, combined with ControlNet fieldbus technology [1 , 2], which has network control functions and remote control interfaces. At the same time, in order to ensure the safety and reliability of the system, both the host and slave computers are planned to adopt a hardware redundancy scheme to complete the control and data acquisition of the oil pipeline test system.
① Establish a lower-level PLC control system centered on field equipment, and develop independent control programs for each control valve to provide flexible and accurate control information for operation and management personnel. This enables the management of field equipment such as electric ball valves, electric plate valves, and frequency converters.
② The lower-level computer (PLC) provides control channels for the upper-level monitoring system, specifically by providing control variables and entry programs for the program;
③ In conjunction with process control, remote monitoring can be achieved on the client side via a local area network, enabling the management and control of field equipment. This is suitable for low-cost, small-scale distributed control systems.
Table 1 PLC Configuration List Figure 2 ControlNet logical network architecture | Figure 3. Main Program Design Flowchart | Figure 4. Flowchart of Alarm and Display Subroutines |
2. PLC-based system hardware design
To ensure the accuracy and security of data acquisition, the system employs dual-machine hot standby to prevent data loss due to unforeseen circumstances. The actuators controlled by the system include: 16 electric ball valves, 7 electric gate valves, 1 frequency converter, and 1 agitator. The required data collection points are: pressure (10 points), differential pressure (2 points), temperature (26 points), ground temperature (30 points), liquid level (1 point), and flow rate (2 points).
The overall system hardware design block diagram is shown in Figure 1:
in:
1) The PLC adopts the advanced ControlLogix series products from Rockwell Automation (USA), ensuring high reliability, dual-machine hot standby capability, and stable performance. It also employs high-speed transmission and real-time I/O control network (ControlNet) communication technology, enabling process data acquisition, real-time transmission, and process control, enhancing the expandability of the lower-level machine and the real-time performance of data acquisition. Software programming facilitates data acquisition from remote instruments (equipment), sensors, and actuators.
Processing and control to realize overall design functions and system hot standby functions.
2) The lower-level machine expansion adopts analog input and output modules, digital input and output modules, and a dedicated Pt resistor (temperature measurement RTD) input module from Rockwell Automation, USA, to collect and control data from field instruments, sensors, Pt resistors (temperature measurement) and actuators.
The functional modules of the system hardware design are shown in Table 1.
Figure 1 System Hardware Overall Block Diagram | Figure 5. Flowchart of the raft adjustment procedure | Figure 6 LabVIEW serial communication program |
3ControlNet Fieldbus Network Structure
ControlNet is a new real-time fieldbus network oriented towards the control layer. It provides time-sensitive control information and I/O data transmission, as well as non-time-sensitive information transmission, over the same physical medium link. This includes support for program upload/download, configuration data, and point-to-point message transmission. With its high-speed (5Mb/s) communication, advanced network model, efficient network protocol, and flexible and convenient installation, ControlNet offers a solution to meet these needs. ControlNet is also well-suited for applications requiring communication between multiple PC-based controllers, between different PLCs, and between PLCs and DCS. ControlNet allows multiple controllers, each with independent or shared I/O, to communicate with each other or to flexibly organize interlocks.
ControlNet bus can not only be directly connected to various devices or subnets, but also connect to the lower-level DeviceNet[3] and FF fieldbus products through a scanner; at the same time, it can connect to the upper-level Ethernet through the communication interface module to realize the integration of control network and information network and share information resources.
4PLC Lower-Level Machine System Software Design
The control of the PLC[4] includes two parts: control management of the ControlNet network and process control. Among them, the network control management uses RSLinx, which is dedicated to the ROCKWELL ControlNet network, as the network configuration software, and RSNetWorx as the network management software. By using these two software programs for relevant configuration and management, the PLC unit connected to the ControlNet network can be used correctly.
4.1 Loop Control Design Based on RSLogix5000
Through the independent control section of this system, managers can easily control any process. By controlling each valve, any control process can be achieved, such as oil inlet process, self-circulation process, bypass process, pipeline cleaning process, tank transfer process, high-pressure relief process, etc. Process control is implemented through PLC programming, using ROCKWELL's RSLogix5000 Enterprise Edition programming software. The main program flow is shown in Figure 3:
The flowchart of the alarm and display subroutine is shown in Figure 4.
The flow chart of the regulating valve control subroutine is shown in Figure 5.
The second lower-level software scheme for the loop control system aims to individually control each valve, achieving flexible control. Using the RSLogix5000 programming environment, the program for the independent control section of the lower-level PLC was written based on the on-site hardware. This program includes one main program and 28 subroutines, used to call or control corresponding functional modules. The main valves controlled by the program are V101, V103, V104, V201, V301, V302, V403, V404, V405, V407, V408, V501, V503, V511, V513, V601, V602, V611, V612, V621, V622, V623, and V624. Among these, valves V101, V302, and V405 are adjustable valves that work with the frequency converter, controlling the liquid level, temperature, and flow rate parameters respectively. Their programming primarily considers applying PID control to achieve better control performance. The processing method for V101 is the same as that for the on/off valve, because it facilitates the control of the oil inlet speed in the future, thus providing a guarantee for future use.
4.2 PLC-based Virtual Instrument Measurement and Control System
In process control, due to the highly dispersed industrial sites, numerous I/O points, and harsh working environments for various instruments, it is obviously not advisable to use data acquisition cards and LabVIEW development platforms to complete on-site data acquisition and control. Considering that the process parameters in process control do not change rapidly, and that PLCs can overcome the shortcomings of data acquisition cards in process control and have a high cost-performance ratio, a PLC is adopted as the lower-level machine and an industrial control computer with LabVIEW software [5] is adopted as the upper-level machine development platform. Through RS-232 and RS-485 serial ports, communication with the PLC is achieved to monitor and analyze the industrial site data.
The communication port on the lower-level CPU is a 9-pin D-type connector compatible with RS-485. The PLC also provides a PC/PPI cable for connecting the RS-485 to the RS-232 on a PC, facilitating hardware connection between the S7 series PLC and a PC. If multiple PLC modules or other devices with RS485 serial communication capabilities are used in the system, they can be easily networked or configured into a network monitoring and control system.
The basic steps for serial communication in LabVIEW are as follows:
1) Serial communication functions in LabVIEW
① ISA Configure Serial Port. This function is mainly used for serial port initialization. The main parameters are shown in Figure 2.
②ISA Read Function. This function is a serial port read subroutine that reads data from a serial device, providing the necessary conditions for subsequent data processing. The main parameters are shown in Figure 3.
③ISA Write Function. This function is a serial port write subroutine used for write operations on serial port devices.
④ Close the port. After completing the serial port operation, you need to close the serial port to release hardware resources.
2) Implementation of serial communication between LabVIEW and PLC
Following the above process, a monitoring system program was designed, and the program flowchart and front panel are shown in Figure 6.
The PC-PLC serial communication program structure is divided into three parts:
(1) Serial port initialization. According to the communication protocol, the communication port is set to COM1, baud rate to 9600bps, with 1 start bit and 8 data bits.
1 bit, 1 stop bit, no parity check, no software handshake protocol.
(2) Sending commands and reading responses: Send commands to the PLC via serial port COM1 and receive response information from the PLC. If the response is normal (status information is 01 or 02), the obtained data will be displayed, calculated, analyzed, stored and other subsequent processing for use in real-time control and online status monitoring of the equipment. If the response is abnormal, the program will exit (03 or 04), and the user will re-enter the command to start working.
(3) Close the serial port. Use the VISA close serial port function provided by LabVIEW to release the serial port resources. When the program runs, LabVIEW first sends a read request to the PLC, then checks the number of bytes in the input buffer; when the predetermined number of bytes is reached, LabVIEW uses the read serial port function to read all the bytes in the input buffer at once, and then sends another read request to the PLC. This cycle continues until the end.
5. Conclusion
This system leverages ControlNet's superior remote control capabilities in the field of fieldbus control to facilitate convenient centralized remote data processing; its 5Mb/s data rate ensures real-time transmission; and its unique solutions guarantee data integrity and security during use. These successful examples provide strong assurance for ControlNet's application in loop control systems. With increasing attention, ControlNet is poised for further development. This system utilizes the ControlNet bus architecture.
As one of the most promising buses today, PLC free-port communication offers ample room for future expansion and performance improvements. Its advantages include convenient and flexible communication with peripheral devices and ease of microcomputer control, leading to its increasing adoption in monitoring systems. By utilizing the serial port of a PC or industrial computer, and following a free-port communication protocol combined with address mapping technology, a serial communication module can be developed on the LabVIEW platform. This allows for easy serial communication between the host computer and the PLC, enabling read and write operations on the PLC's memory units and thus facilitating PLC monitoring. The design presented in this paper can be readily applied to PLC-based industrial monitoring, offering advantages such as convenient development and flexible expansion.
