Abstract
In current field control, situations sometimes arise where the data source is located on a portable platform such as a programmable logic controller (PLC), heavy-duty crane, turntable, or other rotating instrument. These devices are constantly in motion or frequently moved, communication distances are variable, and hardware wiring is inconvenient or impractical. Some data sources are located in areas with strong radiation, high temperatures, high corrosion, or other hazardous conditions that pose a potential threat to human health, or experience high levels of radio wave interference (e.g., in the steel and manufacturing industries). Other situations require cost savings in wiring, and so on. When encountering such situations, wireless communication is a viable solution. Several examples of wireless communication are provided below.
1. Hardware selection for the automatic control system
Because sensors play a crucial role in industrial wastewater treatment, their placement is extremely important. Industrial water treatment is a large-scale process, typically involving several stages. Broadly speaking, these can be categorized as: pretreatment, hydrolysis and acidification, SBR (Sequencing Batch Reactor) process, contact oxidation, and sludge dewatering. More specifically, they can be divided into: coarse screens and wastewater lift pumps, fine screens, aerated grit chambers, biological treatment tanks, distribution wells, final sedimentation tanks, filters, and disinfection channels. A medium-sized water treatment plant typically occupies tens to hundreds of acres of land for water storage. Within such a large area, long data cables are required to collect analog data from sensors placed in the wastewater. Furthermore, shielded cables are sometimes necessary to ensure signal stability. The costs associated with trenching, cabling, and using high-performance conductors constitute a significant portion of the project budget, severely hindering the development of water treatment plants. The combination of the Dingshi PB-B-MODBUS bus bridge with Banner industrial wireless serial communication devices or Data-Linc Group wireless serial communication devices provides an economical and convenient solution.
2. Overall control requirements and functions
In this wastewater treatment system, due to functional division, data from remote subsystem PLCs needs to be centralized on the main control PLC for processing. Since the physical distance between the two systems is relatively far and the downstream equipment is relatively scattered, the two PLCs communicate with each other through a Banner SureCross Data Radio wireless serial communication device.
3. System Composition and Process Introduction
3.1 Connection between Dingshi PB-B-MODBUS bus bridge and Banner wireless serial communication device
(1) System network architecture:
The system adopts PROFIBUS fieldbus technology. Based on functional and process flow division, the system consists of 2 master stations, several substations, 1 operator station, Banner SureCross Data Radio wireless serial communication equipment, and Dingshi PB-B-MODBUS bus bridge. Its automatic control system is shown in Figure 1 below.
Figure 1. Connection between Dingshi PB-B-MODBUS bus bridge and Banner wireless serial communication device
(2) On-site photos:
(3) Process Introduction:
As wastewater passes through various processes and enters the corresponding treatment tanks, various monitoring instruments (including sensors for temperature, pressure, liquid level, flow rate, pH value, conductivity, suspended solids, etc.) collect relevant values of the wastewater and transmit them to the subsystem PLC. Then, the data is transmitted back to the main control PLC for centralized processing via the Banner SureCross Data Radio wireless serial communication device.
(4) Implementation method:
Here, the PROFIBUS DP communication between two PLCs is converted to MODBUS communication using the Dingshi PB-B-MODBUS bus bridge. Therefore, the two PB-B-MODBUS bus bridges need to be set to master and slave modes respectively.
3.2 Connection between Dingshi PB-B-MODBUS bus bridge and Data-Linc Group wireless serial communication equipment
(1) System network architecture:
Figure 2 below shows the network architecture for wireless communication using the Dingshi PB-B-MODBUS bus bridge and Data-Linc Group wireless serial communication devices.
Figure 2. Connection between Dingshi PB-B-MODBUS bus bridge and Data-Linc Group wireless serial communication equipment
(2) On-site photos:
(3) Process Introduction:
The left-hand PLC acts as the signal transmitter, and the right-hand PLC acts as the signal receiver, achieving transparent transmission through the Data-Linc Group.
(4) Implementation method:
By using the Dingshi PB-B-MODBUS bus bridge, the PLC's PROFIBUS DP communication is converted to MODBUS signal communication. The PB-B-MODBUS bus bridge needs to be set to MODBUS master mode, while the PBM-G-MBS itself acts as a MODBUS slave. In addition, the PBM-G-MBS acts as a DP master to communicate with the S7-200 PLC.
4. System Software
1) The computer operating system uses the Chinese version of Microsoft Windows XP Professional. The host computer monitoring software is implemented using WinCC 6.0 configuration software.
2) The configuration software realizes the acquisition and processing of analog and digital quantities such as liquid level, flow rate, pH value, conductivity, temperature and pressure of the entire system, and displays them on the HMI of the workstation; some important physical quantities in the processing, such as the start and stop status, frequency, pressure and material level of each pump, are displayed in real time on the main interface of the workstation, so that the dispatcher can keep abreast of the system's operation.
3) The hardware configuration software is Step7 V5.4 SP2.
5. Conclusion
According to statistics from relevant organizations, the overall application cost of using wireless sensors is three times that of wired sensors, assuming the same application conditions and output results. This is because wired sensors inevitably involve conventional operations such as wiring and slotting, while wireless sensors offer greater freedom and convenience. It is believed that with the improvement of application concepts, wireless sensors, with their higher cost-effectiveness and practicality, will be increasingly accepted by domestic enterprises. This significantly reduces hardware costs while ensuring project quality. It is worth noting that the use of the PB-B-MODBUS bus bridge module allows for easy connection of field serial devices with free protocols to the PROFIBUS bus, enabling accurate and rapid monitoring and control of various parameters of these devices within the PLC.
