Override (high selection/low selection) control field application program in the process control system

Release time:2021-09-29 03:43:14  Number of views:178

Application of analog quantity comparison transmitter (high-selection/low-selection isolation transmitter) in the override control system
In modern petrochemical, power monitoring, food processing, rail transit and other process industries, more and more production devices require control systems that can not only play a control role under normal process conditions, but also can still play an automatic role under abnormal working conditions. The control function can make the production process return to normal working condition as soon as possible, at least it is helpful or waiting for the working condition to return to normal. The control system in this abnormal working condition is a safety protection measure
There are two main types of security measures: one is hard protection, and the other is soft protection.

  The hard protection measure is the interlock protection control system. When the working conditions of the production process exceed a certain control range, the interlocking protection system takes a series of corresponding measures, such as alarming, switching from automatic control to manual control, interlocking actions, etc., to make the production process in a relatively safe state. However, such hard protection measures often cause the equipment in operation to stop or slow down, causing greater economic losses. Therefore, people have explored many safer and more economical soft protection measures in practice to reduce the loss caused by parking.
  The so-called soft protection measures are that when the production working condition exceeds a certain control range, it is not passively input interlock protection or even stop, but automatically switches to a new control system, which replaces the original control The system controls the production process, and when the working condition returns to normal, it automatically switches to the original control system. Since it is necessary to judge whether the working condition is normal, it is necessary to choose between two control systems. Therefore, it is called a selective control system, sometimes also called a replacement control or an override control.


Solution:
   The biggest feature of the selective control system in terms of structure is to use an analog signal comparison transmitter as a selector. The selector is usually two input signals and one output signal.
   For the high selector, the output signal Y is equal to one input signal X1 and the other input signal X2, whichever is larger.
   For the low selector, the output signal Y is equal to the input signal X1 and the other input signal X2, whichever has the smaller value.
         For example: X1=5mA, X2=4mA;  Ymax=X1=5mA. As shown in Figure 1.
 

模拟信号比较变送器的高选器和低选器逻辑框图
                                       Figure 1. Logic block diagram of high selector and low selector of analog signal comparison transmitter

Typical application wiring diagram (as shown in Figure 2)
 
模拟信号比较变送器接线应用图
                                       Figure 2 The principle application wiring diagram of analog signal comparison transmitter
 
Application example of analog quantity comparison transmitter in overriding control system
Application 1: Two sensors generate two 4-20mA analog signals, which are selected and compared by the transmitter, isolated and amplified, and then output 4-20mA. (As shown in Figure 3)
两路模拟信号比较选择、隔离放大原理框图
                                 Figure 3. Two-way analog signal comparison selection, isolation and amplification principle block diagram (signal low selection amplifier)
    

      The DIN ISO C1-P1-O1-SL comparison transmitter selects and compares the two 4-20mA analog signals generated by the two sensors, isolates and amplifies the output 4-20mA, and selects the lower one of the output to achieve industrialization. Soft protection measures for process control.
        This kind of comparison transmitter only needs a single power supply (5V/9V/12V/24VDC) as an auxiliary power supply, and 3KV three isolations between signal input/signal output/auxiliary power supply. The anti-interference ability and load driving ability of the 4-20mA output signal after isolation and amplification are greatly enhanced (maximum load can reach 650Ω), so it can also be used for long-distance signal transmission without distortion.

 
Application 2:
The two sensors generate two 4-20mA analog signals, which are selected and compared by the transmitter and output 0-5V after isolation conversion. (As shown in Figure 4)
两路模拟信号比较选择、隔离变送原理框图
                                Figure 4. Two-way analog signal comparison selection, isolation transmission principle block diagram (signal high selection transmitter)
    
      The DIN ISO C1-P2-O6-SH comparison transmitter selects, compares and isolates the two 4-20mA analog signals generated by the two sensors, and then converts them into a 0-5V voltage signal output proportionally, which is similar to the industrial field The data acquisition (usually 5V, 3.3V) circuits of PLC, MCU and DCS match. By comparing and selecting the highest signal output, the soft protection measures of industrial process control are realized. This kind of comparison transmitter only needs a single power supply (5V/9V/12V/24VDC) as an auxiliary power supply, and 3KV three isolations between signal input/signal output/auxiliary power supply.

       In process control, the automatic control system must be able to work in normal working conditions as well as in abnormal working conditions. This requires first to judge whether the working condition is normal, and then to select the two control systems. DIN ISO CPOS series products can compare two analog signals directly input, and select the high signal or low signal (high selection or low selection) according to the setting requirements, and then isolate and amplify the selected signal to achieve Industrial control is intelligent, reducing losses due to parking and other reasons.
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