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Application of programmable controller in the control system of nitrogen generator

[Key words] PLC, control system, input/output points nitrogen generator

[Abstract] this paper introduces the working mechanism and simple process flow of nitrogen generator, PLC control system design scheme, input/output wiring diagram, software flow chart and debugging results, The skills and methods of saving input/output points used in the design process and the communication between the display setting unit and the host are introduced. The practical application shows that the control system has stable and reliable performance, simple operation, economy, practicality and strong flexibility

1 introduction

nitrogen generator is a new nitrogen production technology that uses air as raw material, carbon molecular sieve as adsorbent, and uses the principle of pressure swing adsorption to make the carbon molecular sieve full of micropores selectively adsorb gas molecules to obtain nitrogen with purity of 98 ~ 99.99%

carbon molecular sieves are obtained by grinding hard coal and sintering through a series of processing. Numerous micropores are distributed in the grain body of activated carbon molecular sieve. Carbon molecular sieve is a speed separation adsorbent. Because oxygen molecule O2 with small molecular diameter is ten times faster than nitrogen molecule N2 with slightly larger molecular diameter in motion, from the perspective of molecular dynamics, when air enters the carbon molecular sieve bed, oxygen with small molecular diameter enters the carbon molecular crystal micropores at a faster speed, and oxygen is enriched in the carbon molecular solid phase, while nitrogen molecules are enriched in the gas phase, This separates oxygen and nitrogen from the air. The equilibrium adsorption capacity of carbon molecular sieve for oxygen increases with the increase of adsorption pressure and decreases with the decrease of pressure, which is called pressure swing adsorption. When the air pressure into the carbon molecular sieve bed is reduced to 0.1MPa under normal pressure, the oxygen molecules adsorbed in the so-called sieve micropores are released, that is, the so-called atmospheric desorption. In order to continuously produce qualified nitrogen, each process in the production process must be automatically controlled

psa nitrogen generator control system originally used relay contact control method, with many devices, fragile contacts, single control, large volume, and unable to detect the quality of Y1 ~ Y8 solenoid valves at any time. Through the analysis of the process flow and control requirements, in order to make the control system more flexible, reliable, convenient and universal, we selected sm-16t PLC of Wuxi Huaguang company for control. PLC control method is flexible, without time relay, it can display the working process and monitor the working time in real time, and has a simple, practical, beautiful and convenient man-machine interface

2 analysis of control requirements of nitrogen generation system

the air compressor is used to provide sufficient gas volume and relatively constant feed gas with input pressure of 0.75 ~ 0.8MPa. After purification treatment such as water removal, oil removal and solid particle removal by the cold dryer, in order to continuously output constant nitrogen, the system sets up two adsorption towers a and B to work alternately. The purified compressed air from the gas source system enters the tower body from the lower part of the adsorption tower a through the electromagnetic pneumatic control valves Y1 and Y2, is adsorbed by the carbon molecular sieve bed in the adsorption tower, and gradually advances upward. In this process, the oxygen molecules in the air are adsorbed in the carbon molecular sieve micropores, while the nitrogen is concentrated in the gas phase, flows out from the upper part of the tower, and enters the nitrogen storage tank through the electromagnetic pneumatic control valves y6 and Y8. This process is the adsorption and nitrogen production of tower a. At the same time, the oxygen molecules adsorbed in Tower B are evacuated through electromagnetic pneumatic control valve Y5, that is, Tower B is desorbed to atmospheric pressure. A. B the two towers supply nitrogen continuously alternately. When the adsorption capacity of carbon molecular sieve for oxygen in tower a will reach equilibrium, the tower will immediately stop adsorption. At this time, Y1, Y4, Y5 and Y8 are all in the closed state, while Y2, Y3, y6 and Y7 are in the open state at the same time. Implement the pressure equalization of two adsorption towers a and B, and switch to the adsorption of Tower B and desorption of tower a after pressure equalization. At this time, the compressed air enters the lower part of the B adsorption tower through the electric control valves Y1 and Y3, and is adsorbed by the carbon molecular sieve bed in the B tower. The separated nitrogen enters the nitrogen storage tank through Y7 and Y8, that is, Tower B adsorbs to produce nitrogen. In this way, tower a and Tower B alternately adsorb and desorb, that is, form a continuous transmission of nitrogen to the nitrogen storage tank. The action sequence and switching time of the above Y1 ~ Y8 electric control valves are all controlled by PLC, so that the second tower can continuously supply qualified nitrogen

during normal operation, the automatic circulation process is as follows:

press the program start key → start the cold dryer → delay x seconds → start the air compressor → delay x seconds → enter adsorption a → delay x seconds → equalize pressure a = B → delay x seconds → adsorb B → delay x seconds → equalize pressure B = a → delay x seconds → enter adsorption a again, so the automatic circulation. Press the stop key, and the system will stop working

control requires manual and automatic working modes, and Y1 ~ Y8 valves can be checked in manual mode

the air compressor and the cold dryer can be started and stopped independently during manual operation; It can display the opening of Y1 ~ Y8 in four stages: adsorption a, pressure equalization a = B, adsorption a, pressure equalization B = a; All operations shall be indicated by indicator lights

when working automatically, it should be able to work according to the requirements of automatic process flow, and the corresponding working state can be displayed in the simulated process flow diagram. At the same time, each delay time in the automatic operation process should be able to be adjusted arbitrarily, and the current delay setting value can be displayed and queried in real time

3 PLC control system

3.1 determination of input/output points and selection of PLC

according to the analysis of process flow and control requirements, there are 19 input signals of PSA nitrogen generator, including manual operation button, automatic operation button, automatic start button, stop 6, vibration table (including impact table and collision test table); Stop button, air compressor start button, 5 Always keep the machine clean and tidy. There are 8 cold dryer start buttons, adsorption a button, pressure equalizing a = B button, adsorption B button, pressure equalizing B = a button, valve detection button, Y1 ~ Y8 valve detection button; The valve detection button only works in the manual mode, while the Y1 ~ Y8 valve detection button only works after the valve detection button is pressed and there is a valve detection indication

there are also 19 output signals, including 8 Y1 ~ Y8 valve outputs, manual status indication, automatic status indication, automatic start indication, stop indication, air compressor work indication, cold dryer work indication, adsorption a indication, pressure equalizing a = B indication, adsorption B indication, pressure equalizing B = a indication, valve detection indication. That is, there are corresponding indications next to each input signal. The working voltage of solenoid valves Y1 ~ Y8 is DC 24V and the power is 8W

according to the general design method, at least select the PLC with 19 input points and 19 output points, or adopt the mode of host plus expansion. In order to make the control system reliable, practical and economical, we choose sm-16t of Wuxi Huaguang Co., Ltd. as the basic unit, plus l-16t as the expansion unit, and cl-02ds as the parameter setting and monitoring unit

sm-16t has 10 input points and 6 output points. It adopts DC 24V sink input mode and DC 24V Transistor NPN output mode. The working voltage of sm-16t is AC 220V, and it is equipped with RS-485 and RS-232 communication interfaces, which can realize the well-known CCM protocol and protocol free communication. RS232 interface is also used as programming port. The program is stored in flashrom without backup battery; The l-16t has 16 output points and adopts the NPN output mode of DC 24V transistor. The l-16t and sm-16t communicate through RS-485; Parameters can be set and monitored through cl-02ds LCD setting unit. It communicates with sm-16t through RS-232

3.2 number assignment of input/output points and input/output wiring

in the design at 15:00 on October 25, due to the relatively large number of input signals, we used the design skills of hardware and software at the same time. 19 input signals only occupied 8 input points of sm-16t, and the output points used 3 points on sm-16t and 16 points on l-16t, a total of 19 points. The specific input/output numbers are assigned as follows:

i2-- cold dryer button, manual operation button, Y1 valve detection button

i3-- air compressor button, automatic operation button, Y2 valve detection button

i4-- adsorption a button, automatic start button, Y3 valve detection button

i5-- pressure equalizing a = B button, stop button, Y4 valve detection button

i6-- adsorption B button, Y5 valve detection button

i7-- pressure equalizing B = a button, y6 valve detection button

i10-- valve detection button, Y7 valve detection button

i11--y8 valve detection button

q0 -- intermediate relay and indication of cold dryer

q1-- intermediate relay and indication of air compressor

q2-- valve detection indication

q20-- manual operation indication

q21-- automatic operation indication

q22-- program start indication

q23-- program stop indication

q24-- adsorption a work instruction

q025-- voltage equalizing a = B work indication

q26-- adsorption B work instruction

q27-- voltage equalizing B = a work instruction

q30 ~ Q37 -- control the output and indication of Y1 ~ Y8 valves respectively

among the input points, there are four points for one point three use and three points for one point two use. In order to distinguish which input signal works at a certain time, we use the three output signal points Q3, Q4 and Q5 of sm-16t as the common end of each input signal for one point three use. At the same time, we use software control to make Q3, Q4 and Q5 cycle on. Through the cooperation of software and hardware, the input signal is reduced by more than half

3.3 software design and skills

for one point three use or two use of input points, Q3, Q4 and Q5 are used as their different common ends in hardware, but at a certain time, which signal works? At this time, the method of software can be used to determine. See the ladder diagram shown in Figure 3 for the specific method. In Figure 3, a counter C0 is used to connect Q3, Q4 and Q5 at different times. The "and" of the normally open contacts carried by Q3, Q4 and Q5 and the direct input contact of PLC are used to determine which signal works, and the internal coil is used to represent different input signals

the software programming adopts the programming software directsoft2.3 of Huaguang company to design the ladder diagram and determine the communication parameters. Directsoft2.3 is a PLC programming software under Windows environment, which is easy to use, simple and powerful. It can be used for program design, programming implementation, writing notes and instructions, syntax check and communication with PLC

after ladder diagram design debugging and syntax check with directsoft2.3 software, the following transmission can be written into sm-16t. At this time, PLC can carry out simulation operation. Before operation, the communication parameters of RS-232 interface and RS-485 interface must be correctly set. We all choose 9600bps, CCM communication protocol and 8-bit data bit odd check method

in addition, the setting of the delay time parameters in each stage of the workflow is completed by the cl-02ds human-machine interface unit. First, the cl-02ds communicates with the computer through the RS-232 interface, and uses the cl-02ds software programming to determine the register number used to set the delay time parameters and the register number used to monitor the time parameters. In addition, the alarm can also be set

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