With the expansion and reconstruction of the electrolysis flue gas purification system, the purifying material conveying system is designed as a dense phase conveying system. The original air supply volume cannot meet the process requirements of the dense phase conveying system. To ensure the normal operation of the system, an air compression station is established. The air supply system provides a stable and clean source of air for the dense phase conveying system and the flue gas purification system. According to the actual production needs, four sets of LU315~500 W 81 m 3 /min series screw air compressors, three dryers and one set of circulating water cooling system are equipped. A communication control system based on Profibus fieldbus is proposed. Using the Modbus communication protocol, S7-300 programmable controller is selected to design a set of aluminum electrolysis flue gas purification automatic air supply control system. The system can be started as needed or The air compressor is loaded to keep the system pressure stable, the automatic monitoring interface is added, and the level of automation control is improved.

Design background of air supply control system

1. The control principle of air compressor

After the equipment is turned on, the system first checks the operating conditions of the air compressor. When the cooling water pressure and the oil pressure in the air compressor separation tank meet the requirements, the air compressor starts, and the motor starting mode adopts the Y-â–³ starting method. , Y-â–³ start delay 5 s.

After starting, the gas storage tank begins to inflate. When the pressure of the gas storage tank reaches the set value of 0.8 MPa, the air compressor relief valve is closed and the air compressor is idling. When the pressure of the gas storage tank drops to 0.7 MPa, the relief valve opens. The air compressor is loaded and operated and inflated again.

2. System control mode

The air compressor comes with Schneider's small PLC Twido series TWDLCAA24DRF controller, which adopts two analog modules of TWDALM3LT and TWDAMM3HT models. One TWDDMM8DRT expansion unit can connect 0~10 V or 4~20 mA signals. Schneider's small controller uses a standard analog signal TWDAMM3HT to collect temperature and pressure signals without using a transmitter. The control method provides a 4 mA constant current source to the PT100 via an analog output. The algorithm linearizes the measured temperature signal to obtain the desired temperature value. With this method, the accuracy of 1 °C can be achieved. The man-machine interface adopts MODICON NEIA, can be connected to Twido PLC through Modbus, and has RS485 interface to communicate with the host computer, monitor the parameters to be controlled on the host computer control interface, and perform remote start, stop and fault alarm. During the operation of the air compressor, the protection function has phase loss protection, motor thermal overload protection and so on.

Control system overall structure design

The system SIMATIC S7 uses the host computer software WinCC as the man-machine interface for operation and monitoring, with the air compression station as the main station, and the air compression station PLC control system adopts S7-300 PLC. Field equipment 4 sets of 6.3 kV 450 kW screw air compression Machine, 3 micro-heat dryers as slave stations. Each air compressor is equipped with Schneider Twido PLC. Through the operation panel of the Twido PLC controller, the operator can control the start and stop of a single air compressor, check the operating status and set operating parameters. The air compressor Twido PLC controller and dryer all have an RS485 communication interface and support the Modbus RTU protocol, which provides conditions for centralized monitoring of the air compressor unit. The circulating water of the air compression station acts as the cooling system of the air compressor, and communicates with the PLC control system of the air compression station through Profibus-DP. The transmission rate is 1.5 Mbit/s, and communicates with the PLC of the main station through the Modbus RTU protocol. It is 187.5 Kbit/s. In the master D slave system, the master station and the slave station use the master D to communicate with the program. The master D slave system allows the master station to currently have the right to send and access the slave device assigned to it, the master station. The information can be sent to or from the slave station, and the PLC program mainly performs organization, monitoring and coordination.

Wind supply control system hardware design

The air compressor station PLC master station control system selects Siemens S7-300 series CPU315-2DP, 1 PS307 power module, 2 SM321 digital input modules, 3 SM331 analog input modules, 1 SM322 digital output module and 1 block. CP341 communication module. The CP341 communication module communicates with 4 air compressors and 3 dryers via RS485 interface. The operator station performs Profibus-DP communication with the CPU of the main station PLC and the CPU of the circulating water control system through the CP5611 communication card. On-site signals are centrally monitored and controlled at the operator station.

Control system software design

1. Application of CP341 communication module

This system selects the CP341 communication module and implements the Modbus Master/Slave RTU protocol using the ASCII Driver communication protocol integrated in the CP341, which can realize point-to-point communication. It is a serial communication module in the Siemens S7-300 series PLC. 1 serial communication interface RS422/485, rate 9 600 bit / s. The system selects RS485 interface communication template, RS485 transmission message content is in bytes, the transmission order in the channel is: low byte first, The high byte is sent later. The low bit in the byte is sent first, and the high bit is sent later.

RS485 Modbus RTU communication mode is required to include information such as station number, data area and read/write instructions in the transmitted data packet, for which the slave device connected to the CP341 module authenticates the data packet to which station, and the data packet It is the function of reading or writing which data area. The CP341 receives data with function block FB7 (P-RCV-RK) and transmits data with FB8 (P-SND-RK). In the user program, FB7/FB8 respectively have an instance data block, and FB7/FB8 is unconditionally called. The transmission or reception of data can be cyclic or time-driven.

2. Implementation of Modbus communication protocol

The communication protocol adopts the Modbus protocol of single master multi-slave structure, and the CP341 selects the RTU format communication protocol of Modbus to realize the serial communication of CP341 and 1#~4# air compressor computer controller and 1#~3# dryer PLC. Data exchange of equipment. To implement Modbus communication, you need to insert the corresponding protocol hardware dog Dongle (6ES7 870-1AA01-0YA0) on the CP341 module, which can be used as the master station. The CP341 template can support the Modbus protocol. The CP341 module can simultaneously work with multiple air compressors. The dryer serial communication device communicates.

3. Setting of communication parameters

First set the communication parameters, double-click the CP341 template in the hardware configuration window, open the property window of the CP341 template, and record the hardware address of the template 256. This address parameter is required when writing the communication program. Then click the Basic Parameters button on the Properties window, select the communication protocol you want to use, and select the Modbus Master protocol. Double-click the envelope icon to pop up the Modbus Master protocol communication parameter setting window. In the Interface option, set 9 600 bit/s, 8 data bit, 1 stop bit, even parit. Select RS485 wiring, voltage R(A) is 0 V, R ( B) is 5 V, after determining the download driver. Compile and compile the hardware configuration, download the hardware configuration. If the SF light is on at this time, connect the communication cable to another communication partner, the SF light goes out, indicating that the hardware configuration is correct.

4. Control program design

The design idea of ​​the program is: CP341 module is set as the master communication module of the bus, 4 Twido PLCs and 3 S7-200 PLCs are set as slaves, each slave station is assigned a unique address, the communication speed of the master station and the slave station The unified setting is 1.5 Mbit/s. The command/response communication mode is used during operation. Each command frame corresponds to a response frame. The Modbus protocol defines many function codes for the command frame. Different function codes require slave stations. Make a different response. The CP341 module issues a function code command frame, and the PLC matching the address responds with the air compressor operation information (pressure, differential pressure, temperature, voltage, current, load status, running time, fault information, etc.) stored in the register. The composition response frame is sent to the CP341 module, and the above process is repeated, and the CP341 module can realize the polling operation information of the collected air compressor unit. The master station PLC uses the polling method to read and write the slave data, and calls the Modbus block FB7 (P-RCV-RK) and FB8 (P-SND-RK) in OB1.

In the program, LADDR is the hardware address 256, first DB data block, one slave corresponds to one DB transmit data block and one DB receive data block. There are seven slave stations in the control system, such as: 1# slave station sends data block to DB1, receive data block to DB11; 2# slave station sends data block to DB2, receive data block to DB12, and so on to make seven corresponding data block. Apply a timer in OB1 to make a 600 ms time pulse with a pulse width of 70 ms, which is used to trigger the transmission of data block FB8, and then call a counter to count the transmitted pulses, ranging from 0 to 8, if the count value is greater than or equal to 8 It is classified as 0, and the value of the counter is used to correspond to the slave station. If the value is 1, the DB transmission data block block number 1 and the DB reception data block block number 11 corresponding to the 1# slave station are respectively transmitted to the word MW300 (send DB block). Block number) and word MW302 (receive DB block number).

Conclusion

The control system forms a monitoring interface with dynamic effects. Since use, the operation is stable, the monitoring data is reliable, and the stable air supply of the flue gas purification system is ensured.

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