Abstract : Bucket wheel stacker-reclaimers controlled by PLC have advantages such as low failure rate, good anti-interference, high reliability, and stable operation. Using PROFIBUS-DP bus control can save a lot of control cable and installation costs, and is simple to install and easy to maintain.
1. Introduction
Bucket wheel stacker-reclaimers are large-scale bulk material handling machines, widely used in ports, mines, steel mills, power plants, and other locations for the stacking and retrieval of bulk materials such as ore, coal, and sand in storage yards. They require continuous operation and frequent handling. Traditional sequential control of bucket wheel stacker-reclaimers uses logic control devices composed of relays and contactors. While this traditional control device can meet the needs of automatic control within a certain range, the large number of contact devices and cables makes the control circuit overly complex, unreliable, and difficult to maintain. PLCs, with their stable performance, low cost, powerful functions, and convenient programming, are widely used in industrial control.
This paper uses a Siemens TP270 touchscreen, a Siemens S7-300 programmable controller, and a Profibus-DP (distributed I/O) controller for each actuator. The control network is simple, the system is optimized, and it fully meets the control requirements of each actuator on the machine.
2. TP270 Touchscreen Configuration
The Simatic TP270 boasts advantages such as low price, robustness, compact structure, clear display, and simple and efficient configuration. The introduction of the TP270 can greatly reduce the number of instrument panels, indicator lights, and digital tubes in the driver's cab, thereby reducing wiring between devices and making operation more reliable. At the same time, it allows designers to flexibly change the display content and method according to actual conditions, greatly improving the overall control performance and level of the machine.
This article uses Protool V6.0 to configure the touch screen. Based on the needs and characteristics of the bucket wheel excavator, the screen design is generally divided into one main screen and six sub-screens, and their logical relationship is shown in Figure 1:
The sub-screens are divided into four main categories: First, the screen operation section (operating the equipment's operation, stopping, stacking, and picking up materials on the touch screen); second, the status display of each piece of equipment (displaying the position and operating status of each piece of equipment) and the data display section (displaying the operating current of each motor, the values of the traveling, slewing, and luffing positions on the machine); third, the fault display and fault alarm list, historical alarm list (displaying fault indicators and fault queries for each piece of equipment); and fourth, the system settings, which allow for convenient setting of various parameters of the touch screen itself without exiting the program.
Some of the PT screens are shown in Figures 2 and 3:
After the configuration program is written, when downloading the program to the TP270 using the programmer, pay attention to setting the TP270's transmission channel and transmission baud rate. Considering the download speed, the baud rate is set to 115200 in this example.
3. System Composition and Communication
3.1 System Hardware Configuration
Based on the control requirements of the bucket wheel stacker-reclaimer and utilizing mature bus technologies, the system adopts Profibus bus technology. The system consists of a Profibus-DP network and an MPI network. Profibus is an open asynchronous communication standard that enables data exchange between various automated devices. It comprises three compatible parts: Profibus-DP, Profibus-PA, and Profibus-FMS. Profibus-DP is an open fieldbus system that enables fast response and high-speed data communication, used for communication between device-level control systems and distributed I/O. The Profibus-DP master station periodically reads input information from slave stations and periodically sends output information to them. In addition to periodic user data transmission, it provides non-periodic communication required by intelligent devices for configuration, diagnostics, and alarm processing. RS485 transmission technology is used, with the physical transmission medium being twisted-pair cable, two-wire cable, or optical fiber, and the baud rate ranging from 9.6kbps to 12Mbit/s. Master-slave communication uses a token-based method, with master-slave transmission between master and slave stations, supporting single-master or multi-master systems.
MPI is a multi-point interface communication network that can connect up to 125 MPI nodes at a communication rate of 187.5 kbit/s. It can transmit data between different controllers and can also be used as a Profibus-DP interface. Operator control and monitoring devices (HMIs) and programming devices (PGs) can be connected via both interfaces (MPI and Profibus-DP).
Based on the characteristics of the bucket wheel stacker-reclaimer's electrical control system, Profibus-DP master and slave stations are equipped in the upper and lower electrical rooms respectively. A touchscreen is located in the driver's cab for convenient operation and fault diagnosis. The upper electrical room uses a Siemens S7-300 CPU315-2DP programmable controller as the master station. The CPU315-2DP has one MPI interface and one DP interface. The slave station is located in the lower electrical room, using an ET200M I/O module with an IM153-1 communication interface module. The trolley travel, cantilever luffing, and slewing mechanisms are controlled by frequency converters, which act as slave stations connected to the system bus via a CBP2 communication board (Profibus communication module). The system hardware configuration is shown in Figure 4.
3.2 System Hardware Configuration
The PLC programming software used is Siemens SIMATIC STEP7 V5.4. After starting the software, the hardware is configured. Select SIMATIC300 (1), double-click the "Hardware" icon in the right window, and configure its hardware. Insert the selected CPU and each input and output module into the SIMATIC S7 network, and define the I/O address of each module. Connect IM153-1 and frequency converter to the Profibus network in sequence, set the PLC as the master station with address (2), and ET200M as the slave station with address (3) (note that the set address must be the same as the DIP switch number on the ET200M hardware). Set the addresses of the walking frequency converter, slewing frequency converter, and amplitude frequency converter to (4) (5) (6) respectively. The slave address of the frequency converter must match the address configured on the PROFIBUS master station, and the address of each unit on the bus must be unique. User program access in the PLC master station is entirely handled by the main interface and IM153-1 interface in the PLC master station. The PLC master station transmits digital signals to the frequency converter via the PROFIBUS-DP bus and the PROFIBUS module. The PROFIBUS module is installed on the front of the frequency converter and communicates with it via an RS485 serial interface. Its configuration diagram is shown in Figure 5.
4. PLC control of bucket wheel stacker-reclaimer
4.1 Stockpiling Procedure Control
In the semi-automatic PLC control program for stacking, the trolley is first pre-positioned at the designated coal stacking location. Stacking control parameters are set via the programmable terminal screen, specifying the number of boom rotations (M1 times) and slewing cycles (M2 times) during stacking. Upon starting the stacking program, the boom conveyor belt and tail conveyor belt start sequentially under program control, sending a bucket wheel stacking operation signal to the system and interlocking with the system conveyor belts according to the stacking conditions. The stacker automatically stacks material at the designated location. As material accumulates, the pile gradually rises. When the material hits the material switch on the bucket wheel, the boom rises one height, completing the set rise (M1 times). The boom then slew one angle to the left, repeating the stacking process until the set slewing cycle (M2 times) is reached. The trolley then reverses a short distance, and the boom slew one M2-fold to the right, completing the stacking cycle (M1 times) and slewing cycle (M2 times). The trolley then reverses again, repeating the process, thus realizing the PLC control operation of the bucket wheel stacker-reclaimer. Its control flowchart is shown in Figure 6:
4.2 Material Handling Procedure Control
In the material handling PLC control program, the trolley is first driven to the predetermined material handling position. Upon receiving the system's material handling command and the system's conveyor belt material handling operation, the material handling program is started. The cantilever conveyor belt and bucket wheel start sequentially under program control. The initial and final material handling angles are sampled, transmitted, and stored using an absolute photoelectric encoder to determine the material handling range of the cantilever rotation. When the cantilever rotates left or right for material handling, it is compared with the initial and final material handling angles, respectively. Each time they are equal, the trolley continues to advance a certain distance, and the cantilever rotates left or right again for material handling, until N1 times. Then, the trolley retreats N1 times the distance to the initial position, the cantilever descends a certain distance, and the second layer of material handling begins, with the process being the same as the first layer; ... until the N2 layer is completed. N1 should be set to an odd number to achieve full layer cycle. Its control flowchart is shown in Figure 7:
4.3 Signal and Instrument Indication Monitoring
Signal and instrument indication monitoring is achieved through communication between the PLC and the voltage transformer programmable terminal, forming a programmable terminal-assisted monitoring operating system for the bucket wheel stacker-reclaimer. Using a programmable terminal, the status signals, alarm signals, and instrument indication signals of the bucket wheel stacker-reclaimer enter the voltage transformer programmable terminal system, enabling real-time monitoring of the bucket wheel stacker-reclaimer. This is convenient, accurate, and fast, eliminating the cumbersome wiring of panel indicator lights and instruments, while also enriching the display functions.
5. Conclusion
Bucket wheel stacker-reclaimers using PLC control offer advantages such as low failure rate, good anti-interference capabilities, high reliability, and stable operation. The introduction of the PROFIBUS-DP bus significantly reduces control cable and installation costs, simplifying installation and maintenance. During operation, if equipment malfunctions, the PLC can quickly locate and display the fault via the touchscreen, facilitating equipment repair and shortening troubleshooting time. Furthermore, the reduced use of control cables effectively minimizes cable-related failures, greatly improving the reliability of the bucket wheel stacker-reclaimer and achieving excellent operational results, ultimately bringing significant economic benefits to the enterprise.
References :
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[4] Fang Zhenfa, Li Chunlei, Application of PLC in Stacker-Reclaimer, Automation Instrument, 2003.11, 24(11), pp.66-68
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