Control System of Concrete Mixing Station Based on PLC

With the large-scale construction of large-scale infrastructure, the demand for concrete mixing equipment is increasing, and the requirements are higher and higher. It is necessary to improve the quality and output of concrete. Therefore, people pay more and more attention to the automatic control system of mixing equipment in the process of concrete production. PLC has the characteristics of high reliability, perfect function and simple programming, which can effectively make up for the defects of the conventional relay control system of the concrete mixing station. Combined with the industrial computer, the control system of the concrete mixing station adopts the upper computer for management, and the lower computer PLC for control and monitoring, so as to realize the automation of the whole process from batching measurement, mixing, to discharging production Intelligent. In order to improve the accuracy of ingredients, improve product quality and output, reduce costs.

Composition of control system for concrete mixing plant

The equipment of concrete mixing station is composed of aggregate feeding system, powder feeding system, water and additive feeding system, transmission, mixing and storage system, and electrical automatic control system. In order to achieve the purpose of accurate measurement, fast speed and reliable control, the control of concrete mixing station is realized by the combination of lower computer PLC and upper industrial control computer, To ensure its reliability and function scalability. The upper IPC is installed with Siemens WinCC monitoring configuration software to realize the visual monitoring of concrete mixing production process. The lower PLC receives the command of the upper IPC, automatically and continuously controls the measurement of each part of materials according to the set formula, and automatically controls the batching, feeding, mixing and discharging.

Process design of concrete mixing plant

The concrete mixing plant is composed of mixing host, batching system (aggregate system, powder system, water supply system and additive system), pneumatic system (executive system) and control system. The main mixer is used to mix all kinds of raw materials which have been mixed in a certain proportion. The batching system is used to weigh and transport all kinds of raw materials such as aggregate, powder, water and additives needed for concrete production. The pneumatic system is part of the executive unit of the control system, which switches the feeding valve, premixed bin door and discharge door.

When the concrete is produced in the mixing station, the feeding valves of stone and sand are opened first, and the aggregate is put into the scale bucket for weighing. The aggregate in the scale bucket increases continuously. Until the electronic scale indicates the weight required by the proportioning, the feeding valve is controlled to stop feeding, and the conveyor belt is started to unload the aggregate into the premix bin. Similar to the above method, the measurement of cement and fly ash is carried out in the respective weighing bucket at the same time of aggregate batching. After the measurement of all kinds of aggregate and powder required for the production of a large round of concrete, if the last round of mixing has been completed, control the opening of the premixed bin to load all kinds of materials into the mixer for mixing, open the premixed bin door and measure the water and additives at the same time, and add them directly to the mixer after the measurement. After all the ingredients are finished, start the mixing time. After the mixer runs for a specified time, open the door of the mixer for discharging, and complete a cycle of concrete production.

In order to improve the production efficiency of the mixing plant, when the concrete is mixed by the mixer, the weighing and measurement of the aggregate and powder in the next production cycle are carried out at the same time. Each aggregate and powder shall be stored in the mixing bin according to the required mixing proportion. After the last mixing, all the materials in the mixing bin shall be put into the mixing chamber for the next mixing and a new mixing. It realizes the rapid weighing of materials, one-time feeding to the mixing chamber in proportion, mixing and batching at the same time, and improves the actual work efficiency of concrete mixing.

PLC control program design of concrete mixing station

1. Automatic feeding algorithm

The automatic feeding adopts the cycle control algorithm. First, the total production task is divided into each big round of batching based on the volume of the mixing chamber, and then each big round of batching is divided into each small round of batching. Each small round of batching is batched according to the normal maximum range of the smallest measuring bucket in each measuring bucket, and the rest is batched synchronously with the smallest hopper according to the proportion of batching. Each round of batching and small round of batching are counted respectively, which is used to calculate the tasks that have been produced. The specific algorithm is as follows:

Initialization: the total amount of each ingredient = the total amount of required production × Percentage of each ingredient.

The big wheel count and the small wheel count are cleared respectively.

Because other batching weighing procedures are similar to aggregate, aggregate and powder are batched at the same time, which can reduce batching time and improve production efficiency. Because water and additives are directly added to the mixing machine, and the amount of water and additives is not particularly large, and the batching is relatively fast, you can choose to open the premixed bin and carry out batching at the same time.

2. PLC control program design

Siemens S7 200 modular PLC system is selected as the field control device. In the first scanning cycle of PLC power on, the program is initialized. In the initialization program, the total amount of production, the capacity of the premixed bin, the percentage of each ingredient, the standard moisture content, and each counter are all initialized. At the same time, the total demand of each ingredient and the maximum feeding amount of each raw material in a large round of batching are calculated, and saved to the corresponding storage area to prepare for automatic production. In the process of proportioning, PLC collects the weighing sensor signals of aggregate measuring hopper, cement measuring hopper and fly ash measuring hopper, and stores them in the data register of PLC. After comparing and calculating the set values given by PLC and upper computer, PLC outputs the control signals of solenoid valve, motor and other actuators according to the obtained results and various conditions, Send out the corresponding feeding and discharging control action. Take the aggregate batching as an example, when the weight of each aggregate hopper reaches 85% of the set value, it will be fed quickly before it reaches 85%, and then it will be fed slowly after it reaches 85%. When it reaches the set value, it will stop feeding. In order to improve the measurement accuracy, two feeding doors are set in the aggregate hopper. At the beginning of measurement, the two feeding doors are opened at the same time. When the weight reaches 85% of the set value, the large opening feeding door is closed, and only the small door is used for feeding until the weight reaches the set value.

System features

1. Adaptability

According to the user's production requirements, the proportioning ratio can be arbitrarily selected. The system uses the method of calculating each proportioning separately for production. There is no necessary connection between each proportioning, and the production scheme can be easily changed at any time. A pre mixing bin with the same capacity as the mixing chamber is added to the main mixer to store the proportioned ingredients while mixing. After mixing, all ingredients are directly put into the mixing chamber for mixing.

2. Automatic control

It can automatically control the whole working process of the mixing plant. In the production process, each intelligent sensor collects its own measurement data and sends it to PLC. PLC carries out on-site control according to the corresponding conditions, and returns the production results to the upper computer for convenient management.

3. Manual / automatic selection

Considering that part of the production process needs to be operated manually at the same time of automatic production, the system sets manual / automatic selection, and the conversion between manual and automatic is interlocked. At the same time of manual operation, the automatic calculation of the system is not affected.

4. Automatic alarm

Each storage bin is equipped with a lower limit alarm value. When the batching is lower than the lower limit setting value, the system generates an audible and visual alarm, which can remind the staff to add ingredients to the storage bin in time during the raw material preparation process, so as to ensure the continuity of the automatic production process and improve the production efficiency. In the process of powder preparation, if there is agglomeration or foreign matter blockage, the production will also be affected. Set the time after the broken arch of powder. If the time exceeds the predetermined time, the system will send out a fault alarm to remind the staff to deal with it in time.

5. Improve the weighing accuracy

Variable speed feeding is used to reduce the impact of impulse. When weighing, first fast feeding, open the two feeding doors. When it reaches 85% of the set value, it will automatically change to low-speed feeding, so as to improve the weighing accuracy of aggregate.

6. Improve the accuracy of proportioning

The change of the moisture content of sand and gravel will change the water cement ratio of concrete and affect the quality and construction performance of concrete. Therefore, it is necessary to accurately measure the moisture content of sand and gravel and adjust the proportion according to the measurement results. The sensor of capacitance method is used to measure the over standard moisture content in aggregate, calculate the total moisture content, and then deduct the amount of water when adding water, so as to improve the proportion accuracy of concrete and improve the quality of concrete.

The control system of mixing station based on PLC makes use of the characteristics of PLC, such as strong anti-interference ability, stability, reliability and applicability to the industrial site, realizes the formula set according to the production needs, automatically and continuously controls the measurement of various parts of materials, and automatically controls the batching, feeding, mixing and discharging, so as to fully automate the concrete mixing station, The product quality and production efficiency are improved.

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