Application of the hottest PLC in automatic cold s

2022-08-12
  • Detail

The application of PLC in automatic cold storage

1 introduction

Yantai ice Wheel Co., Ltd. is a large industrial enterprise with complete sets of industrial refrigeration equipment, central air conditioning and other leading industries. The company is the largest manufacturer of supporting equipment for fresh-keeping processing of fruits and vegetables in China. The independent research and development of new high-efficiency screw compressors represents the highest level in China and occupies a dominant market position in this field. Lulin ice wheel fruit and vegetable fresh-keeping warehouse is the first fully automated fluorine working medium cold storage of ice Wheel Co., Ltd. Because the cold storage system is relatively complex, highly professional, with various functional modes and strong system relevance, the requirements for the automatic control system are very strict, so it is very important to choose a reliable and advanced system. We selected Schneider micro series PLC, an internationally famous electrical appliance brand, including a main PLC, which is used to control the PLC communication of the whole cold storage system and four condensing compressors

2 process control process

the control process is divided into two parts: refrigeration and defrosting. Refrigeration is to realize the refrigeration function of cold storage; Frost flushing is to increase the refrigeration efficiency, because after refrigeration for a period of time, water vapor in the air will form frost on the refrigeration pipeline, which may freeze in serious cases. Therefore, we should melt the frost in time to increase the heat exchange efficiency

2.1 refrigeration

when the system is in refrigeration state, you can select automatic or manual operation. During manual operation, first select the room number of the cold storage to be cooled, and then set the required upper and lower temperature values to start the machine. After the power on command is issued, the system runs. First, start the cooling water pump; Then, turn on the cooling tower fan; Next, turn on the compressor condensing unit to be cooled and the warehouse cooler fan motor; After startup confirmation, turn on the corresponding liquid supply solenoid valve of the first refrigeration warehouse; Finally, the compressor will conduct energy adjustment according to the evaporation pressure to conduct the normal operation cooling process. During automatic operation, just select the room number of the cold storage to be cooled, and then set the required upper and lower temperature values. The system will automatically start and stop according to the upper and lower temperature values

2.2 frost flushing

automatic frost flushing is carried out according to the cumulative running time of each warehouse. When the running time of this warehouse reaches the set value, a defrosting command is issued, and the system automatically defrosts. The process is as follows: first, the liquid supply solenoid valve of the warehouse is closed; Then, turn off the cooling fan motor; Then, turn on the defrosting water solenoid valve and the defrosting water pump of this room; After a delay of 20 minutes, close the defrosting water solenoid valve and the defrosting process is completed

3 system composition and configuration

3.1 control system description

the control system adopts the mode of centralized monitoring and decentralized control. According to this principle, the whole cold storage control system is divided into three levels, namely, monitoring level, control level and equipment level. The composition of the system is as follows:

(1) monitoring layer: through the use of RS-485 communication interface on the main PLC, it can be converted into RS-232 through programming cable, and then it can communicate directly with the upper computer. The main PLC realizes data acquisition, centrally monitors the operation of each controlled equipment in the warehouse, and dynamically displays the changes of each control monitoring point. The upper computer equipped with Kingview software automatically generates tables for the important data of the cold storage, outputs them to the printer regularly for printing, and can conduct remote control and control the data setting of the set point. In principle, the computer only participates in the monitoring and management of the system, and is controlled in parallel with the man-machine interface (communicating with the main PLC) that controls the whole cold storage, which overcomes the terrible consequences of the paralysis of the whole system caused by computer failure in the past. Hardware configuration: upper computer, upper computer network connection card, printer, PLC communication cable, etc. Software configuration: the operating system software is Windows XP, and the monitoring configuration software is kingview6.02

(2) control layer: the control layer is the core of the whole control system and plays a "connecting role" in the whole control system. PLC control is a very mature industrial control technology. This system adopts TSX series PLC and its network system to receive the data information (digital or analog input signals) sent from the site, and send the corresponding instructions after the calculation and processing of the control program stored in its own CPU, Control the field equipment. At the same time, through the PLC network, the data exchange between each PLC in the layer can be convenient, and the main PLC uploads all relevant data to the upper monitoring computer through the network. Main hardware equipment: PLC controller TSX, communication card tsxscp114, man-machine interface, interface module, communication cable, etc. Software: PLC programming software pl7, man-machine interface configuration software, Microsoft office application software, etc

Figure 1 control system architecture

(3) equipment layer: the equipment layer is at the bottom of the pyramid structure of the whole control system, and it is the key link of the whole control system, mainly including the temperature sensor between libraries, the current transmitter collecting the working current of each motor, various solenoid valves on the pipeline, contactors controlling each motor and fan, etc., which are directly connected with PLC (control layer)

3.2 refrigeration system description

the whole refrigeration system is equipped with four semi sealed condensing compressor units. The condensing compressor unit is the most critical component of the whole system. The unit is controlled by PLC microcomputer, which is the most advanced in the field of automatic control. Through the input and output of analog and digital quantities, the operation of the unit is accurately controlled and reliably protected. Various standard control functions make the unit efficient and energy-saving during startup and the whole operation process. The pressure sensor can detect the suction and exhaust pressure of the unit, and ensure the stepless adjustment of the compressor through PLC control. The reliable and advanced slide valve mechanism can automatically adjust the load of the unit through the control system, reducing the energy consumption of the unit and working efficiently. Data acquisition through temperature sensor and pressure sensor can cooperate with the actual situation of heat load, carry out complex control calculation and PID adjustment function, automatically control the loading and unloading of the compressor, and keep the system automatically shut down for supply when the cooling capacity of the system exceeds 2% and 5% of the maximum experimental force, so as to ensure the stability of storage temperature and avoid frequent startup of the compressor. Microcomputer has advance control function, which can timely predict the occurrence of faults and take correct measures before faults occur. For example, when the exhaust pressure is close to the limit value, the control system will make the compressor unload automatically and send out fault alarm signals. Compared with the traditional simple position control with pressure on and off, these can not only self diagnose the quality of sensors, but also pre diagnose the operation of the whole unit in advance, The number of fault shutdown is greatly reduced and the reliability of unit operation is increased. Through man-machine interface dialogue, you can read all the following important parameters: Table 1 unit operation parameter table

microcomputer can record the last eight faults, including the time of occurrence and recorded data, which is helpful for maintenance personnel to eliminate faults in time. The unit has the following automatic protection functions: Table 2 automatic protection functions of the unit

(1) cooling fan control: considering the large area of the cold storage, large temperature fluctuations and uneven distribution, each cold storage is equipped with four temperature sensors, taking the average temperature of the storage to control the cooling fan, and using Japanese physicochemical or Danfoss digital display thermostat to control and alarm. The temperature value is transmitted to the system control main PLC for defrosting control and judgment

(2) cooling tower control: start the circulating water pump according to the unit operation signal, control the start and stop of the cooling fan according to the condensing pressure controller, and the pool will automatically replenish water. It also has the functions of water shortage protection and super-high condensation pressure alarm. The principle of balanced wear and standby automatic input shall be observed during operation

(3) defrosting control of air cooler: there are two options for defrosting control: manual and automatic timing defrosting. (monofilament suture monacryl reg; is such a product, which adopts real-time clock control or timing clock control, and the defrosting time and defrosting interval can be freely set according to the actual situation.) Due to the danger of hot ammonia defrosting and the great influence of defrosting on the storage temperature, each defrosting system adopts Danfoss Evra solenoid valve and pmlx main valve for accurate control. In order to ensure the reliability of the system, filters, check valves and relief valves are imported products from Danfoss, and reliable electrical interlock and storage temperature over protection are added in the electrical aspect to ensure the stability and reliability of the system operation. In addition, due to the different evaporation temperature of the system, a Danfoss CVP pressure control pilot valve is set at the pmlx main valve of the return pipe of the cooling fan in room He 8 to regulate the evaporation temperature of the system, so as to ensure the stability of the operation of the whole system

(4) heat recovery device control: control the start and stop of the water pump according to the water temperature of the pool, and control the input and removal of the auxiliary electric heater at the same time

4 software design

4.1 PLC software design

plc software is divided into two parts: one part is the software of the whole control system except the compressor control system; The other part is the program software of the four compressors

the software design takes into account the protection of equipment and important parameters after the system is restarted, and makes full use of the cold start%s0, hot start%s1 provided by the micro series PLC programming software. The system is initialized by scanning%s13 for the first time, that is, resetting all equipment states to prepare for the next boot. Protect the internal words that need to be protected, such as equipment operation time, parameters set before shutdown, etc

since TSX has two RS485 communication ports, we use them to connect the man-machine interface and the upper computer respectively. However, in view of the limitations of Modbus communication protocol, if we want to use two interfaces at the same time, we can only configure the unitelway protocol when programming

4.2 upper computer software design

according to the system flow chart, outline the whole refrigeration system diagram with simple lines and cooperate with the picture library in Kingview upper computer software to make the monitoring picture vivid, and greatly save the time of designing the picture, and use the precious time in programming and picture configuration. In the refrigeration flow chart, we can take a comprehensive view of the overall situation and comprehensively grasp the operation of the whole cold storage mm; situation. Then make separate pictures of each cold storage room and compressor flow, and realize the switching function through the corresponding buttons in the refrigeration flow chart. After the picture is finished, we will connect the PLC device. That is, connect COM1 to PLC under the equipment directory in Kingview software. The configuration completion screen is shown in Figure 2

finally, the variables are established in the Kingview, corresponding to the i/o points in the PLC, and then connected to the corresponding equipment in the screen to realize the monitoring function of the system

5 conclusion

the automatic control system embodies the advantages of PLC control technology, network communication technology and Kingview picture monitoring technology of Schneider company in France, which greatly shortens the design and debugging time of the control system of the whole automatic cold storage

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