The Control Of Segmented Level By Plc

    In many fields of production, it is often necessary to monitor the liquid level in tanks, tanks, pools and other containers. In the past, traditional relay contact control was often used. Hard-connected appliances were used more, reliability was poor and automation was not high. At present, many enterprises have adopted advanced controllers to reform the traditional contact control. It greatly improves the reliability and control degree of the control system, and provides more reliable production guarantee for enterprises.level control This paper introduces a method of monitoring liquid level by using programmable logic controller (PLC). Its circuit structure is simple, its investment is small (the original facilities can be retrofitted). The monitoring system not only has high automation, but also has on-line modification function. It is flexible and suitable for multi-stage liquid level monitoring occasions.level control
1. Control requirements
The control system can divide the liquid level into several sections according to the need of production, and display them in segments. When the liquid level is the lowest, the material pump is automatically started to add liquid. When the liquid level reaches the set value, the sound and light alarm is issued and the pump is stopped. The operator can remove the sound alarm signal by confirming the button, and the flashing light turns flat. Two control modes, dynamic and automatic, with test function subsection
2. Selection of PLC
At present, there are many series of PLCs imported from the United States, Germany, Japan and other countries in the domestic market, and dozens of kinds of PLCs are assembled and developed by many domestic manufacturers. Therefore, the standard of PLC series is different, the function is uneven, and the price is very different. Under this circumstance, the choice of PLC should focus on the performance-price ratio of PLC, and the choice of PLC has high reliability. This paper takes four-stage liquid level control object as an example. According to the analysis and comparison of various factors and the requirement of input and output points of monitoring system, the C20P PLC of OMRON Company is selected.
3. System hardware configuration
In order to realize the manual/automatic control of liquid level, 12 points of input port and 8 points of output port are needed, and the PLC of C20P 20-point I/O unit is selected. The input photoelectric isolation, output relay isolation and strong load capacity are adopted. Reed tube sensor is used for liquid level detection, and double-level knobs are used for manual/automatic conversion, operation/test conversion and liquid level setting. Start, stop, confirm and test the pump with the normal button; output with the electronic sound alarm and 24V DC indicator lamp, relay PLC refer to Fig. 1 system hardware configuration diagram PLC.

 
Figure I   System Hardware Configuration Diagram
 
In order to save the number of input ports and investment, the optimization of I/O module wiring is adopted in the conversion of the system's operation/test function, which makes one input node in the input module of PLC play the role of two input nodes, and completes the I/O function of PLC in two ways. See Figure I I I/O module wiring optimization.

Figure II   Optimizing the wiring of I/O module
 
When the system is in normal operation, the operation/test conversion knob S is connected to 1-3 contacts, and the test buttons do not work. The liquid level signals are transmitted to the PLC by the reed sensors. When the system is in the test state, S is connected to 1-2 contacts, and the input signals of the sensors do not work. At this time, the test buttons can be used to simulate the liquid level signals transmitted to the PL. C. Under the two control modes, two signals share one input node, which multiplies the utilization of I/O ports and saves I/O points.
4. System Software Design
4.1  Flow chart of control program

 
 
Figure III   System Flow Chart
 
4.2  Programming instructions
 = 1 \* GB3 ① 本系统为液位的双位控制系统。液位可分四段设定和显示，在最低液位时自动启泵，当液位到达设定值时自动停泵。
 = 2 \* GB3 ② 采用IL/ILC分支指令，通过0008旋钮实现手动/自动两种功能的选择，当0008旋钮闭合时，自动指示灯亮，系统执行IL/ILC分支内程序，完成自动监控；当0008旋钮打开时，手动指示灯亮，系统执行分支外程序，通过0010、0011旋钮实现手动启泵、停泵。
 = 3 \* GB3 ③ 液位由0004~0007旋钮分最低、较低、较高、最高四段设定，系统设置由低到高的优先权，即当多个设定旋钮同时闭合时，低液位设定优先。
 = 4 \* GB3 ④ 采用干簧管检测液位时，当液位到达检测点时其触点闭合，指示灯点亮；液位离开检测点时其触点打开，为保证相应测量段指示灯不立即熄灭及不受液位波动的影响，每段指示灯的控制均采用KEEP保持指令，只有当液位上升或下降到相邻段时指示灯才熄灭。
 = 5 \* GB3 ⑤ 当液位到达检测点时，液位指示灯闪烁，灯光闪烁因子采用内部闪烁内标1902，以1S为周期闪烁；若液位到达设定值时，自动停泵，并设置电子音响报警，报警声设计为响3S停2S，循环30S后自停，或在30S内按0009确认按钮停音响，指示灯传平光。电子音响报警和泵的启停同样考虑液位的波动影响，设计时采用KEEP保持指令和DIFU微分指令联合使用。
 = 6 \* GB3 ⑥ 首次开车时，液位低于或高于最低液位时，需先手动启泵，再切换成自动运行；或先进入试验方式，按最低液位试验按钮启动料泵，再进入自动运行方式。
4.3  PLC Ladder Diagram

 
 
Figure IV    PLC Ladder Diagram
 References: 1. OMRON Programmable Controller Manual
          2.扬唯实.用PLC实现水位监控.自动化仪表.2001(3)