Application of PLC in the Net Milk Process of Dairy Factory

1 Overview

With the development of science and technology and the continuous progress of human society, more and more dairy products are emerging in the Chinese market and continue to meet people's needs. However, the quality of these dairy products varies greatly from their intrinsic quality; in addition to other factors, the quality of the clean milk process plays a very important role, directly affecting the quality of the product. The purifier control system designed and developed by us is compared with the products of the same type used abroad, and has the advantages of intuitive operation, convenience, accurate control, high performance and low price, and the like. And it has already gone through the field and started to move toward productization.

2 system process

Net milk is a very important production process in modern dairy companies. It can remove the mechanical impurities in the product, at the same time it can also remove dust, cell debris, etc., and effectively control the cleanliness of the raw milk. The process settings of the net milk machine are mainly composed of five parts: starting, preheating, circulation, products, CIP, etc. The actions consist of large slag discharge, small slag discharge, and overflow. In each part, the action is not the same; start-up, warm-up: For the purifier, receiving signals from the starter system and completing the star-angle conversion of the motor, etc. In the initial state, the system performs a large slag discharge; : The cleansing machine only finishes its own regular small slag removal action. Under the product status: In addition to its own regular small slag discharge, the cleansing machine also periodically receives the large slag discharge and overflow commands issued by the remote controller; CIP: In this state, the purifier regularly receives and completes the large slag and overflow commands of the remote controller.

2.1 Turn off bowl water and open bowl of water

Close the bowl of water and open the bowl of water action: Open the bowl of water is mainly to open the cavity, clean the cavity of the purifier, close the bowl of water is through the pressure of water, hold the cavity as shown in Figure 2: During the operation of the system In a fixed period of time, a pulse can be sent to shut off the water to meet the system's requirement for timely water supply.

2.2 Large slag, small slagging and overflow

As shown in Table 1, the action of large slag discharge and small slag discharge is a combination of two solenoid valves that open the bowl of water and close the bowl of water. The main task of small slagging: clean the equipment in a relatively short period of time. The action sequence, as shown in Fig. 1, the action of closing the bowl of water is composed of a front flush and a rear flush. Partial slag discharge is a bowl-opening action; the main task of large slag discharge is to send the controller to the controller at a fixed time to clean the equipment. For relatively small slag discharge, after several small slag discharges, the large slag discharge is a relatively thorough cleaning of the purifier cavity. Overflow is the cleaning of the upper part of the chamber and the pipe, and is achieved by timing the closing (pressure) of the backup valve. The three action times can be set on the panel. In the automatic state, this command is issued by the remote controller. Small slag discharge and large slag discharge also have certain differences. When the small slag discharge is performed, the replenishing machine still feeds the material. When the slag discharge is large, due to the large impact on the rotation speed of the purifier, it is necessary to close the feed valve and wait for the net. After the number of revolutions of the milk machine has increased to a certain level, the feed valve is opened again. This time can be set.

3 system structure composition

In general, a high-quality net milk system requires a complete and reliable control function structure in addition to an accurate temperature treatment and a reasonable process structure. At present, the control of net purifiers generally adopts distributed local control methods, which are difficult to control centrally and are not conducive to the unified production management. This system is designed to use upper and lower machine control to form a multi-controller centralized control network system. The status of the purifier can be set on the upper computer screen, and the working status of the purifier can also be displayed. Historical trend charts of important parameters and the preservation of relevant data can be easily operated, and the operation process of net milk can be controlled and monitored in real time. .

As shown in Figure 2, the centralized controller selects the CONTROLLOGIX5550 series, which is easy to integrate with existing PLC systems. It not only has advanced communication and input and output capabilities, but also provides sequential, process and motion control functions at the same time. The modular system makes Users can easily design, assemble and modify. The on-site net purifier controller system adopts SIEMENS's more mature SIMATIC S7-200 series CPU224 processor. The processor integrates 14INPUT/10OUTPUT digital I/O points, and expands EM222, EM231 modules, adds extended functions, selects MPI communication protocol, and forms a communication network with other devices. The operation panel adopts the touch screen OP7 of SIEMENS Company. The relevant parameters can be set and modified on the OP7. These function configurations meet the design requirements.

4 Programming and Programming

The entire system software is divided into two parts, the upper and lower machines. The upper computer is under Windows 2000 operating system and software configuration is performed using Intouch7.0. As a man-machine interface, the upper computer receives the data transmitted by the lower computer in real time and displays the status of various alarm data and valves. The lower computer software CONTROLLOGIX mainly completes the control of the feed inlet valve, discharge valve, and large slag discharge. Here focuses on the controller lower computer software.

Compile in V3.1 STEP 7-Micro/Win32 software environment. The abundant function instructions and symbolic addressing methods of the S7-200 series PLC greatly facilitate programming. The functions of digital filter, function block, data block, and communication interface of A/D conversion are applied in the program, fully embodying the features of PLC function control, making the program simple and intuitive. Program structure modular design, ladder diagram mode programming. The program is divided into the following sections: Main program, initialization program, input program, product program, CIP program, analog processing program, alarm program, output program, feed pump control program, inlet control program, and communication program. . Figure 3 shows the block diagram of the "product" processing module. In the program, V55.1 is the “Product” button on the controller panel, and V105.5 is the remote controller's setting bit. "Product" status, when V101.1 is 1, C1, C3 start timing, SM0.4 sub-pulse timing, when the full VW0, allowing small row action. C1 recounts, V55.1 is 1, starts C2, SM0.5 counts by the pulse of second, it is the front rinsing state, V60.2 is 1, closes the bowl water electromagnetic valve movement. When V55.2 is 1, V60.0 is 1, open bowl water solenoid valve action, and continue VW4 time, the same, when V55.4 is 1, set for the post-rinse state, keep off the bowl water action. In this way, a small slag discharge of one cycle of the program ends, and the program continues to execute cyclically.

Due to the use of a centralized control system, this requires program control in addition to guaranteeing the timing of the process. It must also coordinate the system's synchronization. That is, the CONTROLLOGIX processor and the SIEMENS processor must behave in a consistent manner.

In the program design, the software communication handshake bit is set to communicate with the CONTROLLOGIX processor in real time, and if necessary, corresponding adjustments are made to ensure the normal operation of the system. As shown in Table 1, when going from the “circular” state to the “product” state, the processor must not only complete its own small slag removal action, but also receive the large slag discharge action issued by CONTROLLOGIX. The normal operation sequence is large row. The slag interval is 1 hour, and the small slag discharge interval is 16 minutes. If the two are too close together, it will cause unnecessary waste and affect the effect of net milk. The above functions are implemented by the following programming methods. As shown in Figure 4, where C6, C7 are small slag discharge counters and reset counters respectively, m16.2 is a large slag discharge action, and v60.3 is a “circulation” status bit. When the "circulation", small slag always perform, C6 does not reset; "product" state, between the two large slag (1 hour), three small slag, C7 reset C6, recount.

The program has a complete fault detection function, real-time monitoring of the status of each detection point, when the motor temperature is too hot, emergency stop, running speed is too low, the compressed air working pressure is low, the operating water pressure is low, the system will produce Sound and light alarms, emergency stop, and after the fault is removed, the system operation can be resumed by releasing the alarm bell and light flashing.

5 Concluding remarks

After more than one year of operation, the control system is stable in operation, accurate in operation, simple in operation, easy to maintain, and fully meets process design requirements. After the clean milk, the technical indicators (grain size, sensory, odor) completely meet or exceed the requirements of this process, improve product quality, greatly enhance the company's production capacity, and achieve a good economy. benefit. The entire system is well received by users and has broad applications and development prospects.

references:

[1] SIEMENS SIMATIC STEP.7. Programming [S]. Programming Manual

[2] SIEMENS SIMATIC S7-200 application example [S]

[3] WESTFALIA Cleaner Manual [S]