Application of the hottest PLC control system in t

2022-08-23
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Application of PLC control system in the transformation of plastic extrusion granulation equipment

1 introduction

in the production process of polyethylene plastics, extrusion granulation equipment is a kind of mechanical equipment with complex process, and its control system is the key to ensure the normal operation of equipment and product quality. In the past, most of the equipment of each production line used imported supporting DCS control system, which is difficult to maintain and improve, and the cost is very high. In the recent construction of domestic extrusion granulation equipment, we have adopted PLC control system, which has stable performance and low cost

in the production and operation before the transformation, due to the instability of the feed and the frequent change of the product brand, it is not only difficult to make the product reach the ideal level by manual operation, but also there are often blocking, damage of the explosion-proof membrane due to high pressure and other mechanical failures, resulting in low production operation rate and failure to reach the nominal output of the equipment. After using PLC control system, the reliability and continuous stability of equipment operation are greatly improved, and the product quality reaches a high level

2 equipment process characteristics and control requirements

2.1 equipment process characteristics

Figure 1 is the process flow diagram of the plastic production line, which mainly includes mixer, feeder, melt pump and pelletizer. The function of the mixer is to mix different raw material components and plasticize them into a thick fluid - melt plastic; The feeder is used to absorb the melt plastic flowing out of the mixer and transfer it evenly to the inlet of the melt pump for rapid pressure boosting and extrusion. The melt plastic after plastic refining is extruded through the template into a filament under high pressure; It is evenly cut in the pelletizer, and finally forms standard granular products, which are cooled by granular water and transported to the post system process

the automatic control process of the extruder is shown in Figure 2: in the figure: ft-2425 is the feed flow of the mixer, and the range is 0 ~ 15t/we are further strengthening the material research and development h

pt-2426 is the outlet pressure of the feeder, and the range is 0 ~ 10MPa

pt-2427 is the outlet pressure of the melt pump, with a range of 0 ~ 12. The deformation measurement accuracy: 1%; 50MPa;

2.2 control requirements

2.2.1 main circuit control

according to the equipment process requirements, the Extruder mainly needs three control links: feeder control circuit, melt pump control circuit and pelletizer control circuit. In addition, there are temperature control circuits of the lubricating oil storage tank of the melt pump and the outlet temperature control circuit of the lubricating oil of the melt pump

① feeder control circuit - an electronic scale is set at the inlet of the mixer to detect the dynamic flow of feed, and its change is used to control the feeding speed of the feeder

② melt pump control circuit - this circuit adopts cascade control, that is, the working speed of the melt pump is jointly controlled by the rotating speed of the feeder and the outlet pressure of the feeder. Among them, the rotating speed of the feeder is used as the auxiliary control parameter, and the PID output of the outlet pressure of the feeder is used as the main control parameter, that is, through the adjustment of the rotating speed of the melt pump, it has the professional ability to professionally detect the mechanical properties, thermal properties, combustion properties, electrical properties, optical properties, weather aging resistance and reliability of high-molecular materials and products, and control the outlet pressure of the feeder to be stable at 5MP

③ pelletizer control circuit - this circuit also adopts cascade control, that is, the working speed of the pelletizer is jointly controlled by the speed of the melt pump and the outlet pressure of the melt pump. Among them, the rotating speed of the melting pump is used as the auxiliary control parameter, and the PID output of the outlet pressure of the melting pump is used as the main control parameter, that is, through the control and adjustment of the rotating speed of the granulator, to ensure that the particle size of the product is uniform and the appearance is qualified

2.2.2 interlock control

some alarm interlock controls are also required in the equipment control. The main alarm interlock controls are:

① feeder lubricating oil pressure alarm interlock

② melt pump lubricating oil pressure alarm interlock

③ feeder, melt pump and pelletizer motor bearing temperature alarm interlock

④ melt pump front Rear bursting disc state interlock

⑤ pelletizer pelletizing room window switch interlock

⑥ pelletizer particle water flow low interlock

⑦ feeder, melt pump and pelletizer low speed interlock

2.2.3 manual/automatic switching control

because the equipment is in startup, the operating state and working parameters, but the main role is the complex forming and shrinkage, which generally requires manual control by the operator, When the equipment system operates stably for a period of time, it can be switched to the automatic control system. Therefore, undisturbed and stable switching must be realized in the control circuit of feeder, melt pump and pelletizer

3 system composition

the automatic control system is shown in Figure 2, which adopts the architecture of upper and lower computers. In the control room, two industrial computers are used as the man-machine interface operation station, and the lower PLC adopts the SYSMAC CS1 system of OMRON company; CS1 system is a PLC system with high speed, high efficiency, high reliability and compact CPU, and its CPU is cs1-cpu65h model

the controller link network is used between the computer and PLC. The twisted pair communication cable is used, with the maximum communication rate of 2M and the maximum distance of 1km. 3.1 hardware configuration

3.1.1 operation station

two industrial control computers are set in the control room as the man-machine interface operation station, one of which can be used as the system program administrator station. The industrial control computer is mainly configured with P Ⅲ 1.3 CPU, 256MB memory, 40g hard disk and 19 "flat CRT. A controller Link card 3g8f5-clk21 is inserted into the industrial control computer for communication with PLC. The printer is used for regular printing of main process parameter reports

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