The hottest nonlinear dew point temperature curve

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Implementation of nonlinear dew point temperature curve in Hollysys LM series PLC

Abstract: This paper takes the dew point temperature data processing related to humidity as an example to introduce the implementation method of nonlinear dew point temperature curve in Hollysys LM series PLC. At present, the system composed of PLC, text screen and sensor transmitter has been widely used in the field of analysis and detection

key words: Hollysys LM series PLC; Dew point temperature; Nonlinear curve

1 overview

with the higher and higher requirements for humidity measurement in all walks of life, humidity measurement has gradually become a new technical field. Taking the data processing of dew point temperature related to humidity as an example, this paper introduces the implementation method of nonlinear dew point temperature curve in Hollysys LM series PLC

in 1986, China formally established the humidity and moisture professional committee, and carried out many academic exchanges, and some metrological verification regulations of humidity have been gradually established. According to relevant regulations, humidity is defined as the content of water vapor in gas, and its common unit is ppm. Conventionally, the measured gas is divided into high humidity gas and low humidity gas with the dew point of -20 ℃ as the boundary. Dew point temperature refers to the temperature when the air is cooled to saturation without changing the moisture content and pressure. Figuratively speaking, the temperature when the water vapor in the air turns into dew is called dew point temperature. Dew point temperature is originally a temperature value, but why use it to express humidity? This is because when the water vapor in the air has reached saturation, the temperature is the same as the dew point temperature. When the water vapor in the air is not saturated, the temperature must be higher than the dew point temperature. Therefore, the difference between dew point temperature and air temperature can indicate the degree of water vapor saturation in the air. Table 1 shows the discrete statistical relationship between ppm and dew point temperature

2 design of humidity detection algorithm

ppm value is sent to the lm3310b high-precision input module of Hollysys LM series PL [coherent standard]: C by a special transmitter. According to the corresponding relationship in Table 1, PLC calculates the dew point temperature value and displays it on the text display. At the same time, it takes the average value of multi-channel dew point temperature value as the key parameter to participate in the discrete control and process control of the large-scale system

according to table 1, the relationship between ppm and dew point temperature is 55 discrete points. If PLC characteristic curve command (charcurve) is adopted, the number of coordinate points cannot exceed 11. Therefore, the method of curve fitting discrete points is adopted to draw the discrete point diagram and fit the curve formula. Practice shows that due to the nonlinear characteristics of the curve, a curve is not enough to meet the accuracy, so it is divided into five intervals and expressed by five formulas. Input these five formulas into PLC, and PLC will calculate the dew point temperature value according to the formula

first arrange the coordinate points in the order of ppm from small to large, and plan five intersecting intervals: [0.184,4.6], [4.6,80], [80841], [8412346], [23466032]. Then five nonlinear curves are fitted according to the planned interval, and the corresponding formulas of the nonlinear curves are given respectively, as shown in Table 2

Table 2 nonlinear fitting curve between ppm and dew point temperature

3 design of humidity detection system based on PLC

dew point analysis system is composed of control cabinet with PLC as the core, actuator, audible and visual alarm and primary and secondary peripheral components. The system device is shown in Figure 1. The control cabinet is equipped with PLC and alarm relay. The PLC uses Hollysys LM series PLC. The CPU module selected in this system is the smallest lm3104 of LM series. The body integrates 8-point digital input and 6-point relay output. The system uses lm3310b to expand the 4-channel high-precision analog input. The display part in the system adopts Hollysys hd2400l text display, which communicates with the RS232 serial port of PLC through the serial port with ModbusRTU protocol. Through the text display, the operation of the system can be monitored and the parameters can be set. Hd2400l is a small man-machine interface, which is mainly used with all kinds of PLC or intelligent controller with communication port to monitor and modify the value and status of PLC internal register or relay in the form of text or indicator light, so that the operator can freely control the machine and equipment. The system plans 8 windows, including cover, menu window, control interface, parameter setting, curve display, about us, system help, background parameter correction, etc. The menu window completes the switching function of each screen of the touch screen. Parameters should not break the setting window to complete the setting of process parameters of the testing instrument. The alarm list window can display real-time alarm information

Figure 1 Schematic diagram of hardware structure of dew point analysis system

4 application characteristics

the gas analysis pretreatment system based on Hollysys LM series PLC has the following characteristics:

(1) stability and reliability

accuracy and reliability are the key to the system. Hollysys LM series PLC is equipped with high-performance industrial processor, with ultra fast processing speed and large capacity memory. The application of gas analysis is generally in bad conditions. According to the site conditions, the three prevention modules of damp heat, salt fog and mold can be used to ensure that the system is in the harsh environment of damp heat, damp, high temperature and various chemical erosion, It still has high

(2) powerful functions and the performance characteristics of the expandable electro-hydraulic servo hydraulic universal experimental machine are as follows: sex

because Hollysys LM series PLC has the characteristics of small volume, high degree of integration, fast operation speed, large logical control capacity, etc., the whole program has complex functions such as automatic pipeline switching, parameter setting, and manual error prevention in addition to the functions of state alarm and analog quantity processing. In terms of hardware, it can also be expanded to 7 modules to facilitate system expansion. In terms of software, through mature programming means, the control strategy and display interface can be modified according to the requirements of customers to the greatest extent

(3) high cost performance

the humidity detection system based on PLC can not only flexibly solve the problem of nonlinear data processing, but also conveniently control multiple logic points of the system. In addition, Hollysys LM series PLC and hd2400l Series text screens have ultra-high cost performance, saving the investment of the system


[1]lm Small PLC software manual

[2]hd2000 user manual

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