Automation based on monitoring computers (PCs), programmable logic controllers (PLCs), human-machine interfaces (HMIs), intelligent instruments, industrial control communication networks (DCS, distributed control systems, and FCS, fieldbus control systems) has become a technological pillar of modern factories and is widely used in modern production lines both domestically and internationally. According to statistics from relevant departments, among PLCs shipped globally in recent years, categorized by end-user: automotive (23%), food processing ( 16.4 %), chemical industry ( 14.6 %), metal mining ( 11.5 %), and papermaking ( 23.1 %). This shows that the application of PLC systems in the food processing industry ranks second only to the automotive industry.
In China, industries such as petrochemicals, metallurgy, electrical engineering, and fertilizers have completed the automation upgrade of their original relay control systems. In contrast, the relatively backward food industry only began to understand and embrace automatic control and management systems centered on PLCs and PCs after importing complete flour processing production lines from abroad in the early 1990s. Rapid technological advancements have powerfully promoted the technological transformation of the food industry. Food processing enterprises are gradually developing towards large-scale and modern operations during restructuring and reform, and food processing is moving towards refined and deep processing. Particularly noteworthy is the widespread application of PLC programmable control technology in technological upgrades and new construction projects in the food industry. PLC control has replaced relay control; dynamic displays of process flow counters have replaced analog screen displays; and PCs and distributed control networks have placed the entire production line under the control and management of counters, completely changing the backward state of traditional food processing and becoming the industry's development trend and direction.
This project mainly studies the application of an automatic control system composed of PLC and human-machine interface in an automated egg yolk pie production line. Starting with the control requirements proposed by the user, a comprehensive analysis of the assembly line is conducted. Basic control requirements are implemented through PLC programming. Precise timing and positioning of each component are then achieved through on-site debugging, and process parameters are adjusted. A user-friendly and simplified human-machine interface is designed using a touchscreen to ultimately meet both control and process requirements. The main focus of this project is the application of PLC on this production line.
1. Production line control requirements
(1) Equipment process flow: mixing - forming - baking - cooling - sterilization - packaging
(2) The control requirements for the sorting section are as follows:
①After the egg yolk pie is baked, drained of oil, and cooled, it enters the finishing stage.
The sorting section includes milk filling and scraping. The egg yolk pie is transferred from the suction cup section to the sorting section via a suction cup device. That is, the egg yolk pie is carried by the suction cup to the milk filling conveyor belt. At the front end of the conveyor belt, there is a material detection and conveying signal X0. When X0 detects a signal, after a milk filling and conveying delay, the milk filling and conveying servo Y0 starts running and starts the conveyor belt.
② The milk filling device can fill the egg yolk pies on the left and right sides with milk respectively. When the milk filling signal X1 on the left side is detected, the milk filling conveyor stops, and the left milk filling solenoid valve starts operating. After the milk filling solenoid valve operates for a delay, the left milk filling switch solenoid valve opens and starts pumping material. After the left milk filling switch solenoid valve operates for a delay, the left milk filling switch closes, and the left milk filling extrusion solenoid valve starts extrusion. After the left milk filling extrusion delay, the extrusion ends, and the milk filling conveyor starts operating. After the subsequent material delay, the milk filling conveyor stops, and step 2 is repeated until the predetermined number of pies is completed. (The milk filling solenoid valve, the switch solenoid valve, and the extrusion solenoid valve must all have operating and closing delays).
③ When the right-side milk filling signal X2 is detected, the milk filling conveyor stops, and the right-side milk filling solenoid valve starts operating. After the milk filling solenoid valve operates for a delay, the right-side milk filling switch solenoid valve opens and begins to pump material. After the right-side milk filling switch, the solenoid valve opens for a delay, and then closes. At the same time, the right-side milk filling extrusion solenoid valve starts operating and begins to extrude material. After the right-side milk filling extrusion delay, the extrusion ends, and the milk filling conveyor resumes operation. After the subsequent material delay, the milk filling conveyor stops, and step 3 is repeated until the predetermined number of rows is completed.
(The solenoid valves for filling milk, opening and closing, and extrusion must all have running delay and closing delay.)
④ The scraping servo scrapes the material when the milk filling and conveying stops (stopping at the end of a cycle).
⑤ There are three sets of sorting signals: sorting signal one, sorting signal two, and sorting signal three, corresponding to X3, X4, and X5 respectively. Upon detecting a signal, the corresponding sorting solenoid valve actuates and resets after a certain delay. Two feeding signals and a feeding detection signal correspond to two feeding servos.
⑥ The system is equipped with emergency stop reset functions for the first servo of material handling and feeding, the second servo of material handling and feeding, the cake conveyor, the scraper, and the milk filling. A signal alarm system is also required.
⑦ There should be a manual/automatic switching function, and the system should be able to switch to automatic control after pressing the start button.
⑧ All overall parameters should be displayable and configurable on the touchscreen. Factory default settings and factory reset functions should be available, along with parameter logging and the ability to restore the last recorded value.
⑨ There should be a complete monitoring screen for the entire production line, reflecting the status of each control.
⑩ Set a service time. The service time can be changed to control the machine's effective running time; after the time expires, all output is disabled.
2. Control System Analysis
The first step in control system analysis is to allocate the number of I/O points according to the control requirements. Then, select the PLC model based on the number of I/O points and the control requirements. Next, select the inverter, touch screen, and servo model. Finally, draw the program flow function block diagram according to the control requirements.
3. Model Selection
PLC Model Selection: Based on the input/output point allocation, select a 60-point PLC, plus one expansion module. Since pulse output is required, a transistor-type PLC is selected, therefore the PLC model is XC3-60T-E, plus one expansion module XC-E8X8YR . TP Model Selection: A 10.4 -inch large-screen 256-color TFT true color display should support RS-232/RS485/RS422 communication, therefore the TPA61-T is selected. Inverter Selection: EDS1000-4T0022G/B . Servo Selection: VEC-VB-R40H21B-MB . 4. Control System Design
(1) Communication between PLC and frequency converter
This system uses four frequency converters, meaning four frequency converters communicate with one PLC. Because the PLC is a Xinje XC series while the frequency converters are EDS1000 series from ENERGY, they cannot communicate via MODBUS and require a free-format communication method.
Free-format communication means that the communication format can be defined by the user according to their needs.
Start character (1 byte) Data block (maximum 128 bytes) End character (1 byte)
Communication is performed in data blocks, with each block capable of transmitting up to 128 bytes. A start/stop character can be assigned to each block depending on the application, or it can be omitted. Once set, the PLC automatically adds the start/stop character when sending data and removes it when receiving data.
Therefore, when a PLC adopts a free-format communication protocol to realize data exchange between the PLC and intelligent instruments, it has great flexibility and avoids the limitations of the PLC's internal proprietary protocols.
To communicate between a PLC and three frequency converters, the addresses of the three frequency converters need to be set to 01, 02, and 03, corresponding to ASCII values 31, 32, and 33 respectively. To provide frequency input to the frequency converters, the master commands and auxiliary indexes can be found in the frequency converter manuals. The command indices are 12, 00, and 01 respectively, and then converted to ASCII format. The main task is to convert the input decimal frequency into ASCII code values, write them into four registers, and then accumulate the preceding numbers. Specifically, the ASCII code values of all bytes from "slave address" to "running data" are accumulated and placed into the four registers, with the frame end set to 0D . This is achieved by adding the ASCII code values of the unchanging numbers, performing an ASCII conversion on the frequency using 00FFH and FF00H, performing a logical shift, and then placing the resulting values into the registers. When writing the inverter station number, a timer is used to accumulate the D400 value, and then the D400 value is decoded and sent to M50. M50 is used to set the station number and frequency of the three inverters.
(2) Signal detection
The main material detection and conveying signals in the signal detection system correspond to the milk filling and conveying process, the sorting and unloading first servo operation process, and the sorting and unloading second servo operation process, respectively.
(3) Milk delivery
The milk filling and conveying process is S0. S0 is initiated by the preceding milk filling and conveying detection signal. After a milk filling and conveying delay, pulses are sent to the conveying servo to initiate milk filling and conveying. D8170 represents the cumulative number of pulses in PULSE_1. When M8170 (the milk filling and conveying servo running flag) receives a falling edge, the scraping and milking detection process begins. The reason the milking detection process is initiated here, rather than during the signal detection process mentioned above, is to prevent erroneous signal detection from mistakenly initiating the milking process.
(4) Scraping
The servo operation of the scraping process is similar to that of the milk filling and conveying process, so it will not be described in detail here.
(5) Milking inspection process
When the left milking detection photoelectric switch detects a signal, the milking process on the left is initiated. (The process is similar on the right.)
(6) Milking process
When the milking process begins, the left-side milking solenoid valve opens and is set to position Y12. After a delay of T200, the left-side milking switch solenoid valve opens and is set to position Y200. After a delay of T201, the left-side extrusion solenoid valve opens and is set to position Y201. After a delay of T202, the switch solenoid valve closes. After a delay of T203, the left-side extrusion solenoid valve closes. After a delay of T204, the operating solenoid valve closes. After a delay, the left milking process ends. (The right-side process is similar).
(7) Organize
The process of tidying up simply involves setting the tidying solenoid valve with a tidying signal and then resetting it after a running delay.
8) Organize the unloading servo operation
The unloading servo has two segments, which require a descent delay. Therefore, a segment flag is needed.
4. Summary
The application of PLC programmable control technology in the food industry has played a positive role in ensuring stable product quality and yield, stable equipment efficiency and process effects, improving the safety and reliability of production operations, reducing the labor intensity of operators and managers, reducing the number of production personnel while increasing their labor value, and realizing the modernization of production management.
