The quality and efficiency of concrete production are crucial for the control system of concrete mixing plants. This also reflects the level of automation of the entire equipment. This article mainly introduces the composition and characteristics of four common automatic control methods combined with software and hardware used in the current concrete mixing plant control system, providing multiple choices for selecting mixing plant control methods.
The control system of the concrete mixing plant was initially controlled manually by relays in sequence. With the application of microcontroller technology in the 1970s and 1980s, it gradually entered the era of intelligent automatic control. After the 1990s, with the rapid development of computer technology, the control system mostly adopts fully automatic control by computers, which has the following advantages: it can achieve automation of the production process and improve productivity; It can store a large amount of ratios for users to call up actual production plans, and it is very convenient to call and modify the ratios; Can dynamically adjust the mixing ratio based on the moisture content of aggregates, automatically reduce water content and ensure consistency in concrete consistency; Can automatically compensate for the material in the lower batch based on the ingredient error in the upper batch; It can intuitively simulate and display the entire process status of the concrete mixing plant, achieve real-time monitoring, automatically record and store various data, and print tables to achieve optimal control. The specific control methods of the automatic control system for concrete mixing plants can be divided into the following types according to the different control methods:
1. Control mode of industrial computer+batching weighing instrument:
In the control mode of industrial computer+batching weighing instrument, the industrial computer is mainly responsible for coordinating the automatic operation; By using the serial port according to the communication protocol of the instrument, the target value, point momentum, stable weighing time and other control parameters of the controlled material are set, and a batching signal is issued. Various threshold positions are directly interlocked in the output circuit of the batching instrument. After disconnecting from the industrial computer, the system remains an independent instrument control system (which may lack certain functions). This control system requires the instrument to have certain intelligent requirements. In addition to basic weight collection and display functions, it also has automatic material batching functions (accumulation or subtraction). The batching process of each material is completed by its own batching instrument. The industrial computer only needs to determine which batching stage the material is in, whether the batching is complete, and whether it exceeds the tolerance signal. The batching algorithm is solidified in the instrument, and only the corresponding batching parameters need to be adjusted.
2. Industrial computer+board control mode:
The industrial computer+board control method can be further divided into two types based on different weight acquisition methods:
1) Industrial computer+board+weighing module. In this control method, the weighing sensors of each material weighing hopper are connected to the weighing module, and the output signal of the weighing module is connected to the analog board (A/D) of the industrial control computer. The material weight is calculated by the industrial control computer software and displayed on the upper computer software interface; By using digital input/output boards (I/O) to monitor the start and stop status of each batching mechanism, the input terminals are connected to various buttons and limit signals on the control panel, and the output is connected to intermediate relays to control the batching mechanism. Due to the fact that the entire control core in this control method is an industrial computer, the design of control software requires high resistance to interference and other factors. Once the industrial computer system malfunctions, the weight information cannot be displayed, resulting in the entire system being paralyzed and unable to produce.
2) Industrial computer+board+weighing instrument. In this control method, the weighing sensors of each material weighing hopper are connected to the weighing instrument, and the corresponding weighing instrument displays the weight. The control software reads the material weight of each instrument through the serial port for further calculation; By using digital input/output boards (I/O) to monitor the start and stop status of each batching mechanism, the input terminals are connected to various buttons and limit signals on the control panel, and the output is connected to intermediate relays to control the batching mechanism. This control method can be switched to fully manual production when the industrial computer malfunctions, but the disadvantage is that the input and output boards (I/O) limit the types of materials and are not suitable for expansion.
Control mode of industrial computer+PLC+weighing instrument:
The control method of industrial computer+PLC+weighing instrument can be further divided into soft and PLC biased solutions based on whether the control center is in the industrial computer or PLC:
3.1. Software oriented solutions.
The control logic is completed by the upper computer software of the industrial control computer. The PLC input terminal is connected to various limits and buttons, while maintaining communication with the industrial control computer, telling the software what material and its status are. The industrial control computer directly reads the real-time values of each material scale through the serial port, sends measurement signals, starts, jogs, stops... Compared with the first control method, the software needs to simulate instruments to achieve many functions such as reading scales, batching (not only addition and subtraction), communication, data statistical analysis, archiving and printing. What algorithm is used to implement the ingredient process and how is it implemented? Is it an efficient and reliable implementation? All of these have high requirements for software writers. However, the advantages of this control are obvious. Based on the powerful hardware advantages of industrial control computers, weighing instruments and PLCs can be quickly defeated. No matter how much demand there is, the ultimate problem may be to achieve algorithm optimization in the most reliable way.
3.2. Partial PLC solution.
PLC is born for industrial control and has high reliability. The weighing instrument is connected to a sensor to collect the scale value and is also connected to a PLC for automatic control. The PLC controls the actions of various executing mechanisms, and the upper computer collects operational information from the PLC to complete production monitoring and data management functions. There are many ways to implement the upper computer here, which can be based on Windows programming language, or can be developed based on the selected PLC and configuration software. However, most PLCs do not have display functions and cannot monitor the operation process without industrial computers.
Control method of 4 industrial computers and remote I/O modules:
Compared to the previous control method, this control method delegates "control" to various I/O modules located in the "field". The I/O modules are programmable and the programs inside also determine which situations the I/O modules can decide how to handle, which problems need to be "reported" to the main industrial control computer for "above" opinions to handle. The I/O modules are directly placed in each control site, collecting information accurately, and receiving less interference. In addition to having highly reliable I/O hardware, communication technology is also one of the key factors directly affecting system performance. Ethernet (EtherNet) mode can be chosen, and each on-site collection station only needs to connect to a hub (HUB) with twisted pair cables to communicate with the remote master through networking. This method is suitable for control systems within a distance of 180 meters. Electrical signals can also be converted into optical signals through a photoelectric converter and transmitted outward through optical fibers. A photoelectric converter can also be installed at the other end to analyze optical signals into electrical signals, with a communication distance of up to 40 kilometers. This control method is particularly suitable in production areas with large spatial spans, as communication between various contact points only requires a shielded twisted pair (fiber optic) cable, greatly reducing wiring strength and cost.
The above four control forms vary in terms of cost and implementation advantages, and the best solution can be determined based on different emphasis needs. The author believes that the control method of industrial computer+PLC+weighing instrument structure is most suitable for concrete mixing plants. Due to the harsh industrial environment, industrial computer has good seismic resistance, dust resistance, and pressure resistance, while PLC control has the advantages of high reliability, complete functions, and simple programming. PLC manufacturers are also paying more attention to the implementation of network interfaces, which are easy to operate and manage in coordination with industrial computer. Once the industrial computer malfunctions, PLC+weighing instruments can also be used for semi manual production temporarily.
