Abstract: This paper introduces a control system for a coal feeder designed based on the coal feeder's technological process. The paper explains how the system uses controllers, frequency converters, and PLCs to achieve precise control of the coal feed rate. Application results demonstrate that the system has excellent controllability and reliability.
Keywords: coal feeder; controller; frequency converter; PLC
Abstract: This article introduced the control system of coal feeder which is based on the technics process of coal feeder. It explained how can the system control the coal quantity exactly by using controller, transducer and PLC. The practical application proves that the system is very controllable and reliable.
Key words: Coal feeder; Controller; Transducer; PLC
1. Introduction
The coal feeder is a crucial power piece of equipment in an ironmaking plant. Its main task is to uniformly, continuously, and quantitatively feed coal into the coal mill. It can also continuously weigh the coal during transport and adjust the feed rate in real time based on the coal quantity signals required by the combustion system. The electronic weighing coal feeder control cabinet is the supporting control equipment for the first-phase bituminous coal feeder at the Wuhan Iron and Steel Plant, playing a vital role in the entire operation and maintenance of the feeder. The coal feeder control system can accurately measure the coal during operation and control the coal feeding rate based on manually set values and feedback information from the combustion control system, automatically adjusting the conveyor belt speed.
Figure 1 Process diagram of coal feeder control system
The coal feed rate is adjusted to match the coal supplied to the boiler with the required fuel, thereby achieving ideal economic benefits and management results. Figure 1 shows the process diagram of the coal feeder control system. Coal from the raw coal hopper in the coal bunker falls onto the conveyor belt through the raw coal hopper valve, is transported by the belt to the coal drop hopper, and then falls into the coal mill under the control of the coal drop hopper valve.
2. Design of the coal feeder control system
2.1 Working principle of the coal feeder control system
The working principle of the coal feeder control system is to measure the weight of coal per unit length on the conveyor belt, q (kg/m), and the conveyor belt speed, V (m/s), and multiply these two values to obtain the actual coal feeding rate. The actual coal feeding rate is then integrated over time t (s) to obtain the actual coal feed quantity. The controller continuously compares the actual coal feeding rate with the manually entered or remotely controlled 4-20mA setpoint, and then uses PID calculations to automatically adjust the conveyor belt speed to achieve a precise coal feeding rate.
As shown in Figure 2, the coal feed rate is as follows: where t0 and t1 are the time it takes for the coal to pass through the conveyor belt from X0 to X1; q·v is the coal feed rate.
Figure 2 Weighing principle diagram
2.2 Composition of the Control System
The hardware of this control system mainly consists of a Siemens S7-300 PLC, a Fuji FRN3.7G11S-43.7KW inverter , an MT2105 digital feeder controller , a weight detection mechanism, a coal feeder motor speed detection mechanism, and various actuators . The interfaces and signal transmission between the main components of the control system are shown in Figure 3.
The core of this control system is the Ramsey 2105 coal feeder controller, which is specifically designed for weighing belt feeders. It has a built-in PID controller that can adjust the feed rate based on a set value. As shown in Figure 3, the controller sends a belt speed command signal to ports 11, 12, and 13 of the frequency converter to adjust the speed of the belt drive motor. It also measures, statistically analyzes, and displays relevant data from the coal feeding control system (such as coal feed rate and feed quantity) for easy use.
Frequency converters are mainly used in various variable frequency speed control drive devices, especially suitable for drive devices of water pumps, fans, and conveyor belt systems. They feature a modular design; the operation panel and communication module can be easily replaced without any tools. After receiving a 4-20mA speed command signal from the coal feeder controller, the frequency converter adjusts the running speed of the belt drive motor accordingly to regulate the coal feeding rate. Simultaneously, based on the status signal of the universal transfer switch and the belt start/stop signal from the PLC, it sends its working/fault signal to the PLC.
This system uses a Siemens S7-300 series PLC . Figure 3 only shows the signal transmission between the PLC, the frequency converter, and the controller. It has 30 input points and 24 output points. The PLC program makes corresponding outputs to the controller, frequency converter, relays, and other devices based on the input control signals. The specific program design concept can be found in section 3.2 of this document.
2.3 System Operation (in conjunction with WZK1)
This system has five operating modes, corresponding to the five states of the universal transfer switch WZK1: Standby (local standby manual operation), Manual operation (remote manual operation), Running (local running), Remote control (remote remote control), and Stop (local). The following is a detailed description of some of these operating modes:
(1) On-site standby manual exercises and remote manual exercises
When the PLC or counter in the coal feeder control system fails, the belt speed can be directly adjusted using the potentiometer on the control cabinet panel (with the universal changeover switch WZK1 set to the "standby" position) or the 4-20mA current signal provided by the central control room (with the universal changeover switch WZK1 set to the "manual" position). At this time, the control system of the control cabinet is out of control.
(2) Local operation
When the central control room sends a start signal for the coal feeder and the emergency stop button on the control cabinet is not pressed, and there is no protection shutdown signal, the coal feeder will be started, and then it will be determined whether to enter the local mode or the remote mode to enter different operating modes.
(3) Remote control method
Remote control is the primary operating mode of the coal feeder control system. "Remote control" refers to the control system relative to the central control room. In this mode, the coal feeder receives control signals from the central control room to control the coal flow. These control signals mainly include start/stop signals and requested feed rate signals. When the operator selects "remote control mode," the coal feeder achieves constant flow control according to the user-requested feed rate.
(4) Stopping method
There are two ways to stop the coal feeder: one is normal shutdown, where the central control room sends a stop signal to gradually stop the conveyor belt, the coal feeding rate gradually drops to zero, the cleaning chain stops, and the machine enters a shutdown state; the other is emergency stop, where the "emergency stop" button on the control cabinet is pressed when the system alarms or an emergency occurs, and the gate on the coal feeder is closed.
Figure 3. Interfaces and signal transmission between the main components of the control system.
3. Control flow of the control system
The basic idea of the program design is shown in Figure 4:
Figure 4. Control system program flowchart
4. Conclusion
The electronic weighing coal feeder control cabinet is a crucial component of the first-phase bituminous coal feeder at Wuhan Iron and Steel Plant, playing a vital role in the operation and maintenance of the entire feeder system. This system enables precise metering of coal during operation, ensuring that the coal fed into the boiler matches the required fuel, thereby achieving ideal economic benefits and management effectiveness. Utilizing the latest and most advanced control equipment, the system boasts high equipment availability and utilization rates, thus guaranteeing the normal and safe operation of the production line.
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