The Design of PLC temperature control system of Vacuum heat treatment equipment
Vacuum heat treatment equipment is more and more widely used, generally need to buy temperature control meter control temperature. In this paper, the design of the PLC temperature control system, the use of the equipment in the PLC PID module design temperature control system, no need to buy temperature control meter, greatly reduce the cost of equipment and reduce the failure. At the same time, the temperature control program and vacuum equipment original program in a software system programming, communication lines can be reduced, the system is stable, rapid response.
1. Vacuum heat treatment technology
Vacuum heat treatment is a new heat treatment technology combining vacuum technology and heat treatment technology. The vacuum environment of vacuum heat treatment refers to the atmospheric environment under one atmosphere pressure, including low vacuum, medium vacuum, high vacuum and ultra-high vacuum. Vacuum heat treatment actually belongs to atmosphere controlled heat treatment. Vacuum heat treatment refers to all and part of the heat treatment process in the vacuum state, vacuum heat treatment can achieve almost all conventional heat treatment can be involved in the heat treatment process, but the quality of heat treatment greatly improved. Compared with conventional heat treatment, vacuum heat treatment can achieve no oxidation, no decarburization, no carburization, can remove the phosphorus chip on the workpiece surface, and have degreasing and degassing, so as to achieve the effect of bright surface purification.
1.1 vacuum metallurgy
Metallurgical operation under standard atmospheric pressure. It can realize the metallurgical process that cannot be carried out in the atmosphere, prevent metal oxidation, separate substances with different boiling points, remove the gases or impurities in the metal, enhance the deoxygenation ability of the carbon in the metal, and improve the quality of the metal and alloy. Vacuum metallurgy is generally used in the smelting, refining, casting and heat treatment of metals. With the rapid development of cutting-edge science and technology, vacuum metallurgy is increasingly widely used in the smelting of rare metals, steel and special alloys.
1.2 temperature control
Temperature control has become a very important link in industrial production and scientific research activities. Whether the temperature can be successfully controlled within the required range relates to the success or failure of the whole activity. Due to the diversity and complexity of control objects, the temperature control means adopted are diverse. In the temperature control system of vacuum heat treatment equipment, will first need to control the temperature measured parameters, the sensor is converted into a signal, and then compared with preset value, compare with the differential signal, after a certain regularity of calculation, get the corresponding control values, at this time will control quantity to control system to control, and continuously carry on the work, so as to achieve the aim of automatic adjustment.
1.3 PID control principle
In process control, PID controller (also known as PID regulator) controlled by deviation proportion (P), integral (I) and differential (D) is the most widely used automatic controller. It has the advantages of simple principle, easy implementation, wide application, independent control parameters and simple parameter selection. In addition, it can be proved theoretically that the PID controller is an optimal control for the typical process control objects -- "first-order lag + pure lag" and "second-order lag + pure lag". PID regulation is an effective method for dynamic quality correction of continuous system.
The CPU adopts Siemens s7-300 series 314c-2 PLC, which has Ethernet and d-p communication, analog input and output functions, and can meet the requirements of conventional heat treatment equipment.
In addition, thermocouple temperature transmission can measure the actual temperature value. Man-machine interface TP1900 delicate panel, and 300 series PLC compatibility is good, more convenient connection. The whole temperature control system structure is shown in figure 1.
FIG. 1 system block diagram
3. PLC program design
PLC program is written in SCL language in botu V 13, including heating PV startup program, SP calculation program in heating process, segment insert/delete/skip program, PID module program. The PID program is written in the loop interrupt block O B1 whose cycle is 100m s to ensure the accuracy of PID function calculation cycle. The PID module adopts the FB58 temperature control PID module, which has the functions of control and self-setting and is convenient to use.
3.1 variable definition
All important parameters are stored in D B data block to prevent loss.
3.2 the startup value follows PV
When heating starts, make SP=PV. Even in high temperature failure heating interruption, reheating, can be directly from PV start, saving time. See figure 2.
Figure 2 PV startup diagram
3.3 SP calculation in the heating process
During heating, as time T increases, SP increases with time.
3.4 operation of deleting, inserting and jumping segments
When performing a segment deletion operation, delete the Del segment and move all subsequent segments forward; When the Ins segment is inserted, the Ins segment and all subsequent segments are moved back and the inserted segment is placed in the Ins segment. When performing a skip operation (when heated), jump directly to the next section of the program. Segment insert deletions are shown in figure 3.
Figure 3 segment insertion and deletion
3.5 PID function debugging
(1)0Initial PID parameter setting: according to practical experience, set gain P=5, integral time I=120s, differential time D=30s. The screenshot of PID loop setting interface is shown in figure 5.
(2) PID self-tuning: start the program, 800 ℃, suspend the program, open PID self-tuning function, after a period of time the setting of the obtained after setting the PID value of P = 6.0, respectively, 23, I = D = 5.78, as shown in figure 5. Starting from the low temperature heating process of the test again, as shown in figure 6 temperature curve, the temperature control accuracy is higher, there is a small amount of overshoot at low temperature, 400 ℃ temperature control accuracy is + / - 1 ℃, meets the requirement.
3.6 automatic protection program during heating
When heating, sometimes a large number of materials deflated, resulting in a lot of vacuum quickly reduced, generally using manual maintenance, until the vacuum recovery and then continue to heat up. This operation can be written into an automatic program. When the vacuum degree is lower than the set value, it will be maintained automatically. After recovery, it will automatically heat up to reduce the operation amount.
Figure 4 PID parameter setting
Figure 5 PID self-tuning
Figure 6 man-machine interface
4. Man-machine interface design
Figure 6 shows the touch screen temperature control screen, which is composed of process editing, operation button, data display, bar graph and temperature curve. The process curve can be set up to 20 sections in total. The process parameters can be set directly in the process editing section or in the formula. Then the formula data can be exported to variables. The process name can be modified in the formula. Can be inserted segment, delete segment operation, in the run time can be a jump operation. Display running time, running section number and other parameters to facilitate the observation of the process.
Different operation permissions can be set for each parameter modification button and module to prevent accidents caused by operational errors. All operating parameters are displayed in the man-machine interface to observe the running state of the equipment at any time. Add formula function to facilitate process storage and reading.
This paper designs PLC temperature control system, including hardware system design, PLC software system design, touch screen interface design. The system is simple, in line with the conventional heat treatment equipment operation habits, easy to operate.
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