Abstract: This paper describes the communication between the S7-300/400 series PLC and the 6SE7 series frequency converter in the transport trolley control system of the steelmaking plant of Baoshan Iron & Steel Co., Ltd. By mitigating the high electromagnetic interference in the frequency converter's operating environment, such as the electromagnetic interference emitted by the electric furnace area and the frequency converter itself, and by resolving wiring and grounding issues, smooth and stable communication is ensured.
Keywords: transport trolley; PLC; frequency converter; communication
All transport trolleys on the stainless steel production line of the steelmaking plant at Baoshan Iron & Steel Co., Ltd.'s Stainless Steel Branch were put into operation simultaneously with the production line on April 18, 2004. They perform different functions at different positions in the process: receiving molten steel at the furnace tapping position; performing temperature measurement and sampling at the temperature sampling position; implementing online baking of the ladle at the baking position; and transporting molten steel between upstream and downstream processes and ferrying empty ladles at the overhead crane lifting position. Because the ladles contain molten, high-temperature steel, the trolleys must operate stably with smooth starts and stops. The motor drives of the transport trolleys use Siemens 6SE7 series frequency converters, and the basic automation uses S7-300/400 PLCs. The frequency converters and PLCs communicate via a Profibus-DP bus.
1 System Configuration
The basic automation level (L1) consists of a dephosphorization control system, an EAF (electric arc furnace) control system, an AOD (argon-oxygen decarbonization converter) control system, and a VOD (vacuum oxygen decarbonization furnace) control system. The system configuration is shown in Figure 1.
Figure 1 System configuration
a—Dephosphorization PLC; b—Electric Arc Furnace PLC; c—Argon-Oxygen Decarburization Converter PLC; d—Vacuum Decarburization Furnace PLC; 1, 2—Dephosphorization 1#, 2# Ladle Transport Trolleys; 3, 4—Dephosphorization 1#, 2# Slag Bag Transport Trolleys; 5—Electric Arc Furnace Ladle Transport Trolley; 6—Electric Arc Furnace Empty Ladle Transport Trolley; 7—Argon-Oxygen Decarburization Converter Ladle Transport Trolley; 8—Argon-Oxygen Decarburization Converter Slag Bag Transport Trolley; 9—Vacuum Decarburization Furnace Ladle Transport Trolley. The system uses a 100Mb/s industrial Ethernet to achieve data exchange between each unit PLC and between the PLC and the operator station server. Downlink (LO) communication is achieved via the Profibus-DP bus to the frequency converter. Communication between the PLC and the frequency converter is implemented using the Profibus-DP interface built into the CPU416-2DP or the communication interface of the CP443-5Ext board. A CBP/CBP2 communication board is added in the adapter box to establish communication between the frequency converter and the PLC.
2. Data exchange between PLC and frequency converter
During hardware configuration, each PLC and frequency converter is assigned a corresponding Profibus-DP bus address. The frequency converter address number is set during frequency converter parameter settings. In the PLC application, parameter identifier value addresses and process data addresses are defined. Parameter identifier values include parameter identifier, index IND, and parameter value; process data includes control word, setpoint, status word, and actual value. The PLC sends control words and setpoints via the Profibus-DP bus to the frequency converter via the parameter identifier value address port; the frequency converter returns its status word and actual value via the Profibus-DP bus, which is collected by the PLC via the parameter identifier value address port.
Parameter identifier values and process data are collectively referred to as available data. The process data area is used to transmit control words and setpoints (task master station) or status words and actual values (response master station). Transmission of process data is only valid if the control word, setpoint, status word, and actual value follow the path specified by the process data connection.
The process data soft connection only functions when the control word, setpoint, status word, and actual value are logically connected to the dual-port RAM interface. Received process data is stored in a fixed, predefined dual-port RAM address. The process data soft connection is implemented using BICO parameters. BICO parameters are intervention points established by connectors or digital input/output connectors to enable functional connections and observe internal signals. The BICO parameters can be used to determine the function block input signal source.
3. Solutions to Interference Problems
Due to the harsh on-site environment, some problems were encountered during application. For example, the main PLC sometimes stopped when the transport trolley frequency converter was running. In severe cases, the PLC would immediately enter STOP state as soon as the transport trolley frequency converter started, making normal production impossible. In addition, L1 and L2 communication failures also occurred when the transport trolley frequency converter was running, and the ON/OFF commands sent by the PLC to the frequency converter were frequently interfered with, etc.
The reasons for this are twofold: firstly, the operating environment of the frequency converter exhibits high levels of electromagnetic interference, such as in the electric furnace area and from the electromagnetic interference emitted by the frequency converter itself; secondly, there are problems with the wiring and grounding. Therefore, strict grounding must be ensured during frequency converter installation, paying attention to the following points:
(1) Ensure that all equipment in the control cabinet is properly grounded. Short, thick grounding wires should be used (preferably flat conductors or metal mesh, as they have lower impedance at high frequencies) and connected to the common grounding wire. The grounding resistance should be less than 4Ω.
(2) When wiring, power lines and control cables should be separated. If control lines and power lines cross, they should be wired at 90°.
(3) When using shielded wires or twisted pairs to connect control circuits, ensure that the unshielded parts are as short as possible, and use cable conduits if possible;
(4) Ensure that the contactors in the control cabinet have arc extinguishing function. AC contactors can be controlled by either RC suppressors or varistors. This is especially important if the contactors are controlled by relays of the frequency converter.
(5) When using shielded and armored cables as motor wiring, the two ends of the shielding layer must be grounded;
(6) Pay special attention when replacing the CUVC central control board and communication board of the frequency converter. If the grounding wire is missing or not connected tightly, it will often cause interference and make the PLC unable to work properly.
Some interferences cannot always be eliminated immediately or completely. For example, the ON/OFF commands sent by the PLC to the frequency converter from some transport stations are often interfered with. To address this problem, we used a hard-wiring method to receive the PLC's POWER ON signal from the 7th input terminal of the frequency converter, which originally transmitted the signal through Profibus-DP. This method has a better effect.
4. Conclusion
The PLC and frequency converter of the transport trolley at Baosteel Stainless Steel Branch use Profibus-DP communication, which greatly reduces equipment wiring and facilitates functional expansion and modification. After a period of application, the system is reliable and runs stably, meeting the needs of steelmaking production.
For details, please click: Communication between PLC and frequency converter of transport trolley in Baosteel steelmaking plant
