Current transmitter problem solving
one What is a two-wire current transmitter?
The two-wire system means that the field transmitter and the control room instrument are connected with only two wires, which are both power cables and signal lines. Compared with the three-wire system (one positive power cord, two signal wires, one of which is common GND) and the four-wire system (two positive and negative power cords, two signal wires, one of which is GND), the advantages of the two-wire system are:
1, not easily affected by parasitic thermocouples and resistance voltage drop and temperature drift along the wire, can be very cheap thinner wires; Can save a lot of cable and installation costs;
2, when the output resistance of the current source is large enough, the voltage in the wire loop induced by the magnetic field coupling will not have a significant impact, because the current caused by the interference source is very small, generally the use of twisted pair can reduce the interference; Two wire system and three wire system must use shielding wire, shielding layer of shielding wire should be properly grounded.
3, capacitive interference will lead to receiver resistance related error, for 4 ~ 20mA two-wire loop, receiver resistance is usually 250Ω (sampling Uout=1 ~ 5V) This resistance is not small enough to produce significant error, so the wire length can be allowed longer and farther than the voltage telemetry system;
4. Each single reading device or recording device can be exchanged between different channels with different wire lengths, without difference in accuracy caused by different wire lengths, so as to achieve decentralized collection. The advantages of decentralized collection are: decentralized collection and centralized control….
5, the 4mA is used for zero level, so that it is very convenient to judge open and short circuit or sensor damage (0mA state).
6, it is very easy to add one or two lightning and surge protection devices in the two-line output, which is conducive to safety lightning and explosion protection.
Three-wire and four-wire transmitters do not have the above advantages will be replaced by two-wire transmitters, from the foreign industry dynamics and the supply and demand of the transmitter core can be a little know, the current transmitter in use to be installed on the power line of the field equipment, and the single-chip microcomputer as the core of the monitoring system is located far away from the equipment field monitoring room, The two are generally separated by tens to hundreds of meters or even more. The environment of the equipment site is relatively harsh, the strong electric signal will produce a variety of electromagnetic interference, and the lightning induction will produce strong surge pulse, in this case, a tricky problem encountered in the single-chip microcomputer application system is how to reliably transmit small signals in a long distance in a harsh environment.
The appearance of the two-wire transmitter makes this problem better solved. We design a small and cheap perforated two-wire current transmitter based on DH4-20 transmitter module. It features low offset voltage (< 30 μV), low voltage drift (< 0.7μV/C°), and ultra-low nonlinearity (< 0.01%). It converts the current isolation of the power line of the field equipment into a standard current signal output of 4 ~ 20 mA in linear proportion, and then sends it to the input interface of the monitoring system through a pair of twisted pairs, and the twisted pair also sends the 24V working power supply located in the monitoring system to the current transmitter. The measurement signal and power supply are transmitted simultaneously on the twisted pair, which eliminates the need for expensive transmission cables, and the signal is transmitted in the form of current, which greatly enhances the anti-interference ability. The principle of the two-wire current transmitter is shown in Figure 1.
2. How to convert the 4-20mA output of the current transmitter?
The two-wire current transmitter has an output of 4 to 20 mA, which is converted into an analog voltage signal of 1 to 5 V or 2 to 10V by a precision resistance of 250 Ω. There are many ways to convert to digital signals, if the system is used for a long time in a harsh industrial site, so the safety and reliability of the hardware system must be considered. The input module of the system uses the voltage frequency conversion device LM231 to convert the analog voltage signal into the frequency signal, and the photoelectric coupler TL117 is used to isolate the analog and digital quantities.
At the same time, the analog signal processing circuit and the digital signal processing circuit respectively use two sets of independent power supply, analog and digital are separated from each other, which can improve the safety of the system. The use of voltage frequency conversion device LM231 also has a certain anti-high-frequency interference effect.
3. What are the advantages and disadvantages of current output type and voltage output type?
In many applications controlled by the single chip microcomputer, transmitters are used to convert signals that can not be directly measured by the single chip microcomputer into electrical analog signals that can be processed by the single chip microcomputer, such as current transmitters, pressure transmitters, temperature transmitters, flow transmitters, etc.
Most of the early transmitters are voltage output type, that is, the measurement signal is converted to 0-5V voltage output, which is the direct output of the op amp, the signal power is <0.05W, and the digital signal is converted by the analog/digital conversion circuit for the single-chip microcomputer to read and control. However, in the case that the signal needs to be transmitted over a long distance or the grid interference is large in the use of the environment, the use of voltage output sensors is greatly limited, exposing the poor anti-interference ability, the line loss destroys the accuracy and so on, and the two-wire current output transmitter has been widely used with its very high anti-interference ability.
Voltage output transmitter anti-interference ability is very poor, the damage of line loss, not to mention how high the accuracy is, sometimes the output DC voltage is also superimposed on the AC component, so that the single chip computer produces misjudgments, control errors, serious will also damage the equipment, the output 0-5V can not be transmitted far, the line voltage drop after far transmission is large, the accuracy is greatly reduced. At present, many ADC,PLC,DCS input signal ports are made into two-wire current output transmitter 4-20mA, which proves the inevitable trend of voltage output transmitter being eliminated.