Reconstruction Design of Lime Kiln Electric Control System Based on PLC and Frequency Converter
“With the advent of PLC in the electrical control system, the PLC internal program (virtual relay) has been used to replace the actual relay control, which has been widely used. With the continuous development of PLC, the control function is becoming more and more powerful. One PLC can control dozens of frequency converters at the same time, but the increase of control signals requires a large number of control cables and Display devices. We are looking for a control system with higher speed, lower cost, low power consumption and high reliability. Using PROFIBUS-DP bus technology is the best method at present.
With the advent of PLC in the electrical control system, the PLC internal program (virtual relay) has been used to replace the actual relay control, which has been widely used. With the continuous development of PLC, the control function is becoming more and more powerful. One PLC can control dozens of frequency converters at the same time, but the increase of control signals requires a large number of control cables and display devices. We are looking for a control system with higher speed, lower cost, low power consumption and high reliability. Using PROFIBUS-DP bus technology is the best method at present.
1. Introduction to the application of PROFIBUS-DP technology
PROFIBUS is an international, open, and manufacturer-independent field bus standard that is very suitable for electrical control systems.
PROFIBUS communication protocol has been widely used in many fields, but the development of PROFIBUS-DP slave station must meet the requirements of users, so the development of PROFIBUS-DP intelligent slave station is more and more people’s attention, but due to the complexity and complexity of PROFIBUS protocol Cumbersome, the PROFI used by most domestic manufacturers at present. BUS-DP intelligent slave station products are all imported.
The embedded PRO-FIBUS bus bridge developed by Beijing Dingshi Co., Ltd. in 2006 provides a simple interface and is widely used in frequency converters, motor starting protection devices, intelligent high and low voltage electrical appliances, electrical measuring devices, and various transmitters. , Intelligent field measurement equipment and instruments, etc.
2. The application of PROFIBUS-DP technology in the transformation of lime kiln electrical control system
2.1 The use of PROFIBUS-DP in the electrical control system of the lime kiln
The need for technology
In the lime kiln electrical control system, the traditional PLC control method is shown in Figure 1, and the power lines and control lines are directly connected to the main control room cabinet. Due to the huge electrical control system of the lime kiln (for example, in the lime kiln PLC control system used in a steel plant, about 1,500 points are used for digital, about 600 points for analog, and 20 frequency converters), and the electrical equipment is scattered and controllable. The amount of wires is very large, and the cost of a set of 300 t lime kiln control signal cables accounts for about 40% of the total cable amount.
By adding intelligent instruments and using PROFIBUS-DP bus technology (see Figure 2), on the one hand, since the field devices use a unified communication protocol, the controller only needs to use a communication cable to connect the scattered field devices, and complete the communication of all field devices. The monitoring, thereby removing the transmission wire in the control, can save 300,000 funds. On the other hand, in the traditional electrical control system, large bundles of transmission wires need to be dug cable trenches or cable trays separately, which brings a lot of inconvenience to the construction, and also brings great difficulties to maintenance and repair. The establishment of the slave station ensures the reliability of control and the convenience of maintenance and transformation. In particular, it brings great convenience to the on-site control. People can issue commands to the PLC and inverter through intelligent instruments to control the start, stop, forward rotation, reverse rotation, and speed regulation of the motor according to the conditions of the operating equipment.
2.2 Slave hardware composition
The core of the slave station is a self-developed intelligent instrument, and its structural block diagram is shown in Figure 3.
It consists of two parts, the first part is the embedded PRFI-BUS bus bridge; the second part is the user product template: including the user microcontroller, data acquisition, data output, keyboard control, output display and address setting 6 parts. One side of the RS485 bus driver is connected to a 9-pin D-type socket, and the other side is connected to SPC3 through an optocoupler.
2.2.1 Embedded PROFIBUS bus bridge
The PRFIBUS interface part adopts the embedded PRFIBUS bus bridge PB-OEM2-SE interface board developed by Beijing Dingshi Co., Ltd. The CPU uses AT89S52 single-chip microcomputer. Since Siemens PLC (s7-300) is used as the master station, Siemens slave The station protocol chip SPC3 is used to connect the PROFIBUS-DP master station and the slave station. For AT89S52, SPC3 is equivalent to an external RAM extended by it, and it is a DP bus bridge chip integrated with PROFIBUS-DP protocol.
The internal structure of SPC3 is shown in Figure 4. The SPC3 chip includes mode registers, status registers, interrupt registers and various buffer pointers and buffers; the interior contains 1.5kB of RAM and a parallel interface with 11-bit address lines. In addition, SPC3 also integrates a watchdog counter, which can work in 3 different states: baud rate monitoring, baud rate control and DP control. When the application processor fails, PROFIBUS-DP will be prohibited, so as not to endanger the peripheral equipment. As the heart of SPC3, the microsequencer controls the entire working process. In the UART, the parallel data stream and the serial data stream are exchanged.
The interface circuit between CPU and SPC3 is shown in Figure 5. SPC3 has an internal address decoding circuit, so AB8~AB10 are grounded. At this time, DB7~DB0 are the data address multiplexing bus, which can generate the lower 8-bit data and address, the upper 4-bit address is generated by AB0~AB3, and AB4~AB7 generates the chip select signal of SPC3, only when AB4~AB7 is 0 at the same time Only selected SPC3.
At present, the driver chips that can meet the baud rate of 12Mbps include SN65ALS176 SN65ALS1176 SN75ASL176, ADM1485, SN65HVD1176 and so on. Photoelectric isolation is mainly used to eliminate interference from the outside world. The optocouplers that can meet the baud rate of 12Mbps include HCPL7720, HCPL0720, HCPL7721, HCPL0721, HCPL7710, etc. In addition, the power supply is also required to take isolation measures, such as adding transformer isolation or using DC-DC modules.
2.2.2 User Product Template Section
Its core part is an AT89C51 single-chip microcomputer, and the interface distribution circuit of the single-chip microcomputer is shown in Figure 6.
1) Digital input part (see Figure 7)
The analog quantity of the equipment (motor) is filtered by RC circuit, isolated by optocoupler, and then shaped by Schmitt trigger to become digital input CPU. RC circuit filtering to reduce misoperation due to mechanical chattering of contacts. In order to reduce the volume of the circuit board and make the overall structure more beautiful and compact, software anti-vibration can also be used, which can also achieve satisfactory results, but the program running speed is slowed down.
2) Digital output part (see Figure 8)
The digital output part adopts the 74LS373 latch to drive the electric coupler and outputs it through the high-power triode 3DD15. The purpose of using the 74LS563 latch, one is that the 74LS373 has a large driving capacity and can directly push the electro-coupler; the other is that the 74LS373 has a latch function, which can be easily hung on the bus, and the CPU accidentally crashes and runs away, and the gatekeeper When the dog is reset, the output can be kept unchanged to prevent misoperation caused by jitter.
3) Keyboard (see Figure 9)
In addition to completing the basic digital input/output, the DP slave station also transmits part of the control parameters of the inverter to the PLC through DP for on-site debugging of the inverter, such as forward rotation, reverse rotation, start, stop and frequency. settings. In order to complete a variety of functions, the keyboard adopts a typical circuit structure, and the 6 keyboards are arranged in a matrix, and the software is used for anti-vibration.
4) Display section
In order to provide users and debugging personnel with a good interactive interface, the dot-matrix LCD full Chinese character operation interface is used, which can not only clearly understand the details of each input and output point, but also watch the operation of the equipment on site, debug the inverter, Realize the setting of forward rotation, reverse rotation, start, stop and frequency of the motor, without having to go to the operating room. This function of on-site debugging of the inverter brings great convenience to the debugging personnel.
3. Software design
The main part of the software is to read and write to the DP bus bridge, and to complete the data processing by reading and writing to the DP bus bridge. This interface board is specially designed to provide PROFIBUS interface in OEM way for manufacturers who develop PROFIBUS products independently. Users do not need to understand PROFIBUS development technology, do not need to purchase PROFIBUS development systems; do not need to write GSD files themselves, the CPU of the user product reads/writes, inputs/outputs data to the interface board through the serial interface, and the interface board automatically converts to PROFIBUS-DP The protocol communicates with the PROFIBUS master. That is, the DP bus bridge transmits all data to the instrument by means of response.
This project is specially designed for the transformation of the electrical control system of a lime kiln in a steel plant. The changes of some performance indicators of the electrical control system of the 300 t lime kiln before and after the improved design are shown in Table 1. According to the preliminary calculation in Table 1, the improved design of the lime kiln can save 380,000 yuan, and at the same time improve the reliability of the control system and the convenience of control, which is well received by users.