Typical applications of compact pressure transmitters in PLC and DCS

With the advancement of science and technology today, compact pressure transmitters play a very important role in industrial control systems, not only realizing the pressure of the liquid crystal display system on the control screen, but also participating in important logic calculations and parameter alarm functions set up. The design and use of compact pressure transmitters in industrial control are nothing more than two ways: an internal power supply mode, that is, the control system provides 24V DC power to the transmitter, and also accepts the 4 ~20mA DC signal;. The other is the external power supply mode, that is, the control system does not need the transmitter to provide 24V DC power to it, and the control system only accepts the 4~20mA DC signal from the transmitter to it. The first internal power supply mode is usually designed as a two-wire system, which is the most familiar and the most frequently used mode. We use the second method relatively little, most of the comrades are not familiar with it, and the magic effect of the pressure transmitter mentioned in this article is based on the second method and the author's personal test and successful use of the experience. , I hope to bring benefits to my peers and friends, and also play a role in attracting others. Principle Transmission PLC system provides 24VDC power supply, and accepts the 4-20mA DC signal supplied by the transmitter as the signal to control the operation of the variable frequency water pump. At the same time, the whole situation has achieved the expected goal. The pump running at power frequency is changed from power frequency to variable frequency operation, so that the pressure of the outlet main pipe is stable at left and right. It can be seen from the above description that the outlet pressure of the main pipe plays a key role in the safe and stable operation of the entire system. But our system is set up in an unattended manner, and the on-duty personnel in the control room have no idea about the operation of the local equipment. How to use a concise method to let the on-duty staff know the operation status of the local equipment at a glance is the problem we face. After repeated deliberation and bold experiments, the design system finally achieved the above difficulties with the least equipment and the most optimized method. The bus pressure is the core of the inverter control system, and its dominant position must be ensured. Therefore, the small pressure transmitter of the outlet bus is designed in the PLC control system. The power supply is supplied by the transmitter and the signal supplied to it by the transmitter is also supplied with two wires. working mode. At the same time, the 4-20mA DC signal of the transmitter is introduced into the DCS system as the pressure monitoring point of the water supply system to realize the monitoring function on the DCS screen. The 4-20mA DC signal of the loop is also sent to the DCS as a pressure source to monitor the actual operation of the pump. Condition. For the DCS system, accepting 4~20mA DC signal is equivalent to its own external power supply mode. Taken together, the above system is equivalent to the PLC as the power supply, the transmitter and the DCS are equivalent to the series connection of two resistors, the entire loop resistance is equivalent to an increase, and the current of the loop is reduced. But after my on-site test and calculation, the power supply voltage of the transmitter fully meets its requirements. After the whole loop current is reduced, the current through the PLC and DCS is reduced (but both can meet the requirements of PLC and DCS), and the card is reduced. The heat generation of the piece is more conducive to the normal operation of the card piece. The current signal terminal of DCS has four terminals. If internal power supply mode is required, connect A n and Bn; if only accept 4～20mA DC signal, connect Cn and Dn ends, and we only need Cn and transformer here. Connect the 24V + of the transmitter, and connect Dn to the 24V — of the PLC (ie, connect the Cn and Dn of the DCS in series in the loop).

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