I. Introduction
With the increasing national investment in high-grade road construction, the demand and quality requirements for road construction materials are higher and higher, especially the quality of subgrade materials has a particularly obvious impact on the overall project quality. The subgrade material is a cement stabilized soil mixed with quicklime, hydrated lime (water content of 10-15%), soil, sand, fly ash and cement according to a certain proportion. According to the national quality standard, the qualified cement stabilized soil must meet the following requirements: ① plant mixing equipment must be used; ② Batching must be automatically measured in the production process; ③ Strict production process supervision and inspection. These three key points are reflected in the requirements for the equipment of cement stabilized soil mixing station, and the key is metering and batching.
Considering the metering and batching system used in cement stabilized soil mixing stations in China, there are mainly the following two modes: ① batching system controlled by PLC; ② Batching system controlled by upper computer software. Due to the convenient operation and powerful function, most of the mainstream products use the upper computer software control system. In the batching and metering system, the frequency converter is mainly used for regulation and control. According to the market requirements, Sifang company has developed a special frequency converter for subgrade construction, which has been widely used in plant mixing equipment with its excellent performance.
II. Equipment composition
1. Batching supply system: aggregate storage bin (which can be added or subtracted according to the situation), speed regulation (frequency conversion) metering belt scale and belt conveyor;
2. Cement material supply system: Cement bin (cement hopper), speed regulating (frequency conversion) screw conveyor, metering screw electronic scale.
3. Automatic water supply system: submersible pump, pipeline, regulating valve and sprinkler.
4. Mixing system: motor reducer, double horizontal shaft continuous mixer.
5. Finished product bin device system: double discharge storage bin, inclined belt conveyor and air compressor.
6. Electrical system: operation control cabinet and operation room.
7. Metering control system: microcomputer control, manual and automatic.
In the system, where the motor needs to be adjusted, the frequency converter is used to adjust the speed of each motor through the microcomputer command to achieve accurate control.
The equipment diagram is as follows:
III. equipment operation principle and requirements
Principle of automatic batching system: the upper computer accurately calculates the instantaneous flow and cumulative measurement of materials through the material weight signal and speed pulse signal sent by the batching scale (belt or screw) and processed by a pre-designed program. The upper computer compares the instantaneous flow with the set flow, and its deviation is used as the frequency input signal of the frequency converter, so as to adjust the speed of the motor, so that the actual flow per unit time is equal to the set flow, so as to realize automatic proportional quantitative batching. At the same time, the upper computer collects the water flow signal through the special water meter (or rotameter) and compares it with the given value of water flow, and adjusts the speed of the water pump to stabilize the water flow at the given value. In addition, the system has the proportional automatic tracking function of feed water. When the output increases, the water automatically increases in proportion, and when the output decreases, the water automatically decreases in proportion, so as to keep the proportion of feed and water unchanged, It also has delay function.
In the raw material proportioning control, the upper computer software is mainly used to collect operation data, send instructions to control the operation frequency of the frequency converter, adjust the motor speed everywhere, so that the discharge amount of various raw materials can reach the appropriate proportioning, and become qualified cement stabilized soil after mixing. Data is transmitted between the frequency converter and the host computer through 485 / 232 signal conversion device. The frequency converter receives the 4-20mA current signal from the electronic scale on the weighing belt conveyor. After collecting this signal, the upper computer gives the frequency signal of the frequency converter after calculation combined with the software. The communication between the frequency converter and the host computer is completed through Modbus communication protocol.
According to the above principle: since the frequency converter plays the role of connection and control, it is required that the frequency converter should first have strong data communication ability and be matched with the functions of special control software. Secondly, in operation, it should achieve the performance of large torque, stable and accurate speed, strong overload capacity, short acceleration and deceleration time and strong error correction ability. In addition, since the frequency converter connects strong current and weak current to realize communication and control, it is required that the frequency converter has strong anti-interference ability, weak electromagnetic radiation, simple control, convenient wiring and simple operation.
Performance characteristics of quad frequency converter
According to the equipment requirements, Sifang e380 series universal frequency converter can fully meet the needs. On this basis, the company has improved the function of e380 and developed a special model suitable for stabilized soil plant mixing equipment. Thus, with its powerful functional characteristics, it has been widely practiced in the electrical application of stabilized soil plant mixing equipment:
1. The standard 485 communication interface can easily realize the communication with the upper computer;
2. Modbus communication protocol is simple and easy to use, which greatly facilitates the preparation of upper computer programs by customers;
3. Unique low-frequency torque setting, large torque during startup and stable torque during operation, which effectively solves the problem of material blockage;
4. Accurate speed control to make the motor speed consistent with the requirements, ensure uniform discharge and accurate discharge quantity, and ensure the quality of products;
5. A special data acquisition and transmission channel collects the current value of the sensor, defines the value as the feedback value and transmits it to the upper computer;
6. Proprietary pulse signal output channel, which can realize special functions;
V. electrical principle and structure
In this equipment, the frequency converter, as an important equipment connecting the microcomputer control system and power device, plays an important role in motor operation, signal acquisition and transmission, microcomputer command realization and other important functions. The accurate proportion of raw materials is realized by adjusting the motor speed through the frequency converter. The electrical wiring is shown in the figure (frequency converter can be added or reduced according to the actual batching situation):
The wiring diagram of frequency converter control line is as follows:
Vi. equipment operation and maintenance
After the parameters of the frequency converter have been set, in the process of equipment installation and commissioning, the production requirements can be met only through the console and microcomputer according to the procedures, and the operation is simple and convenient.
Sifang frequency converter has simple maintenance and can operate stably for a long time under correct wiring and operation requirements. If the machine is not used for a long time, it needs to be powered on for about 2 hours every week. As the equipment is open-air and there are many metal objects, it is necessary to pay attention to lightning protection in thunderstorm weather to prevent lightning damage to the equipment, and pay attention to waterproof to prevent the machine from water and moisture.
VII. Conclusion
According to the on-site process requirements, Sifang engineers, through further in-depth research, developed a method without upper computer. By changing the mode, all the on-site data are directly collected into the frequency converter, and the automatic metering and blending production can be realized through the calculation and processing of the special software of the frequency converter.