Two wire and four wire signal transmission modes o

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Two wire and four wire signal transmission modes of the transmitter

in the two-wire transmission mode, the power supply, load resistance and transmitter are connected in series, that is, two wires simultaneously transmit the power supply and output current signals required by the transmitter. At present, most transmitters are two-wire transmitters with sports drinks in their bags; In the four wire system, the power supply and load resistance are connected to the transmitter respectively, that is, the power supply and the transmitter output signal are transmitted by two wires respectively

I. what is a two-wire current transmitter

what is the two-wire system? What are the advantages of the two-wire system

two wire system means that only two wires are used to connect the field transmitter with the instruments in the control room. These two wires are both power lines and signal lines. Compared with the three wire system (one positive power line, two signal lines, one of which shares GND) and the four wire system (two positive and negative power lines, two signal lines, one of which has GND), the two-wire system has the following advantages:

1. It is not easy to be affected by parasitic thermocouples and resistance voltage drop and temperature drift along the wire, and can use very cheap and thinner wires; It can save a lot of cables and installation costs

2. When the output resistance of the current source is large enough, the voltage in the conductor loop induced by the magnetic field coupling will not have a significant impact, because the current caused by the interference source is very small, and the interference can be reduced by using twisted pair generally; Two wire system and three wire system must use shielded wire, and the shielding layer of shielded wire shall be properly grounded

3. Capacitive interference will cause errors related to the receiver resistance. For Ma two-wire loop, the receiver resistance is usually 250 (sampling uout=v). This resistance is too small to produce significant errors. Therefore, the allowable wire length is longer and farther than the voltage telemetry system

4. Each single reading device or recording device can be connected between different channels with different wire lengths, so as not to cause accuracy differences due to different wire lengths, and realize decentralized acquisition. The advantages of decentralized acquisition are: decentralized acquisition, centralized control

5. 4mA is used for zero level, which makes it very convenient to judge open circuit, short circuit or sensor damage (0ma status)

6, it is very easy to add oneortwo lightning protection and surge protection devices at the two-wire output port, which is conducive to safe lightning protection and explosion-proof

three wire and four wire transmitters do not have the above advantages and will be replaced by two-wire transmitters. It can be seen from the foreign industry trends and the supply and demand of transmitter chips. The current transmitter should be installed on the power line of field equipment when in use, while the monitoring system with single chip microcomputer as the core is located in the monitoring room far away from the equipment site, and the two are generally tens to hundreds of meters or even farther away. The environment of the equipment site is relatively bad, and strong electric signals will produce various electromagnetic interferences. ② closing the oil delivery valve will produce strong surge pulses. In this case, a thorny problem encountered in the SCM application system is how to reliably transmit small signals from a long distance in a bad environment. The appearance of two-wire transmission device has solved this problem. Taking DH transmission module as the core, we designed a small and inexpensive perforated two-wire current transmitter. It has the characteristics of low offset voltage (<30 V), low voltage drift (<0.7 v/c) and ultra-low nonlinearity (<0.01%). It converts the current isolation of the power line of the field equipment into a 4 ~ 20mA standard current signal output varying in linear proportion, and then sends it to the input interface of the monitoring system through a pair of twisted pairs. At the same time, the twisted pair also sends the 24V working power supply located in the monitoring system to the current transformer. The measurement signal and power supply are transmitted on the twisted pair at the same time, which not only saves the expensive transmission cable, but also the signal is transmitted in the form of current, and the anti-interference ability is greatly enhanced. The principle of two-wire current transmitter is shown in Figure 1

II How is the Ma output of the current transmitter converted

the output of two-wire current transmitter is 4 ~ 20mA, which is converted into 1 ~ 5V or V analog voltage signal through 250 precision resistance There are many ways to convert to digital signals. If the system is used for a long time in an industrial site with a harsh environment, it is necessary to consider the safety and reliability of the hardware system. The input module of the system uses the voltage frequency converter LM231 to convert the analog voltage signal into the frequency signal, and uses the optocoupler tl117 to isolate the analog quantity from the digital quantity

at the same time, the analog signal processing circuit and the digital signal processing circuit use two groups of independent power supplies respectively, and the analog ground and the digital ground are separated from each other, which can improve the safety of the system. The voltage frequency converter LM231 can also resist high-frequency interference

III What are the advantages and disadvantages of current output type and voltage output type

in many applications controlled by single chip microcomputer, transmitters are used to convert signals that cannot be directly measured by single chip microcomputer into electrical analog signals that can be processed by single chip microcomputer, such as current transmitter, pressure transmitter, temperature transmitter, flow transmitter, etc

most of the early transmitters were voltage output type, i.e. the measured signal was converted to V voltage output, which was directly output by the operational amplifier. The signal power was 0.05W, and the digital signal was converted through the analog/digital conversion circuit for single chip microcomputer to read and control. However, when the signal needs long-distance transmission or the electrical interference is large in the use environment, the use of the voltage output sensor is greatly limited, exposing the disadvantages of poor anti-interference ability, line loss damaging the accuracy and so on. The two-wire current output transmitter has been widely used because of its high anti-interference ability

the anti-interference ability of the voltage output transmitter is extremely poor, and the damage of the line loss can not be said how high the accuracy is. Sometimes the output DC voltage is also superimposed with AC components, which causes the single chip microcomputer to make wrong judgments, control errors, and even damage the equipment in serious cases. The output 0-5V must not be transmitted remotely. After the remote transmission, the line voltage drop is large, and the accuracy is greatly reduced. At present, many input signal ports of ADC, PLC and DCS are made into two-wire current output transmitter Ma, which proves the inevitable trend that voltage output transmitter will be eliminated

IV What are the general processing methods for 4 ~ 20mA current output type to interface

the output range of current output type transmitter commonly includes 0 ~ 20mA and 4 ~ 20mA. When the current transmitter outputs the minimum current and the maximum current, they represent the minimum and maximum rated output values calibrated by the current transmitter respectively

the following describes the current transmitter with the measurement range of 0 ~ 100A as an example. For the transmitter that outputs 0 ~ 20mA, the 0ma current corresponds to the input 0A value, the transmitter that outputs 4 ~ 20mA corresponds to the input 0A value, and the 20mA current of both types of sensors corresponds to the 100A value

for the transmitter with output of 0 ~ 20mA, we only need to select the appropriate step-down resistor in the circuit design, and directly convert the 0-5V or 0-10V voltage on the resistor into a digital signal at the input interface of the a/d converter. The circuit debugging and data processing are relatively simple. However, the disadvantage is that the damage of the transmitter can not be identified, and the open circuit and short circuit of the transmitter output can not be identified

for the transmitter with output of 4 ~ 20mA, the circuit debugging and data processing are cumbersome. However, this transmitter can judge whether the circuit is faulty and whether the transmitter is damaged by detecting the current within the normal range (the minimum value is 4mA in normal conditions) when the transmitter line is disconnected, short circuited or damaged. Therefore, it is more widely used

when the 4 ~ 20mA transmitter outputs 4mA, the voltage on the sampling resistance is not equal to 0, and the digital quantity directly converted by the analog-to-digital conversion circuit is not 0, so the single-chip microcomputer cannot be directly used, and the formula calculation is too complex. Therefore, the general treatment method is to eliminate the voltage drop generated by 4mA on the sampling resistance through the hardware circuit, and then conduct a/d conversion

this circuit forms a current loop by the 4 ~ 20mA current generated by the two-wire current transmitter, 24V and the sampling resistance, so as to generate a V voltage drop on the sampling resistance, and input this voltage value to the 3 pin of the amplifier lm258. The resistance voltage divider circuit is used to generate a fixed voltage value at pin 2 of the integrated circuit lm258 to offset the voltage drop caused by 4mA current on the sampling resistance. Therefore, when the minimum value of the two-wire current transmitter is 4mA, the voltage difference between pin 3 and pin 2 of lm258 is basically 0V. The lm258 and its connected resistance form an adjustable voltage amplification circuit, which amplifies the voltage value of the current of the two-wire current transmitter on the sampling resistance and outputs it to the analog/digital conversion circuit through pin 1 of lm258 for the MCU CPU to read in. The ma current of the two-wire current transmitter is displayed in the form of a value on the lcd/led screen through the data processing method. (Figure 2)

v How to distinguish between true and false current and voltage transmitters

after the marketization of the means of production, fierce competition has intensified, and it is difficult to distinguish the true from the false. Because the transmitter is a marginal discipline, many engineering designers are unfamiliar with it. Some manufacturers' indicators of industrial grade and civil commercial grade are confused (the price of industrial grade is twice that of civil commercial grade). Some manufacturers' products can make a transmitter with LM324 and lm431 for a few cents. If you don't believe it, open it and have a look, Do you use the LM324 and lm431 that you bought for hundreds of yuan? Do you dare to use such a transmitter

I try to take the commonly used current and voltage transmitter with accuracy of 0.5 as an example, and start with the following methods to distinguish the true from the false

(1) the reference shall be stable. 4mA is the corresponding input zero reference. The reference is unstable. In terms of accuracy linearity, the change of 4mA zero drift within 3 minutes of cold start shall not exceed 4.000maa0.5%; (i.e. 3..02ma), the voltage drop on load 250 is 0 005v, the expensive energy gap benchmark is widely used in foreign IC chips, and the temperature drift coefficient changes by 10ppm per degree

(2) the total current consumed by the internal circuit is 4mA, which is equal to 4.000ma after setting. Moreover, the current consumed by the active rectifier filter amplification constant current circuit will not change due to the change of the primary side input, and the foreign IC chips adopt constant current power supply

(3) when the working voltage is 24.000v and the full scale is 20.000ma, the reading of full scale 20.000ma will not change due to load change; The change shall not exceed


(4) when the full scale is 20.000ma and the load is 250, the reading of full scale 20.000ma will not change due to the change of working voltage 15.000v-30.000v; The change shall not exceed 20.000ma0.5%

(5) when the primary side is overloaded, the output current shall not exceed 25.000ma+10%, otherwise the 24V working power supply and a/d input clamping circuit for the transmitter in plc/dcs will be damaged due to excessive power consumption. In addition, the emitter output in the transmitter will also be damaged due to excessive power consumption, and those without a/d input clamping circuit will suffer more

(6) when the working voltage 24V is reversed, the transmitter must not be damaged and must be provided with polarity protection

(7) when the induced lightning and induced surge voltage between the two lines exceeds 24V, clamp them and do not damage the transmitter; Generally, only 1.5ke TVs transient protection diode is connected in parallel between two lines, which can suppress 4% of the 20ms pulse width every 20 seconds The test method of screw grip force is positive and negative pulse impact, and the transient impact power is 1.5kw-5kw

(8) whether 0.5% of the linearity of the product mark is an absolute error or a relative error can be identified by the following methods: if the following indicators are met, the true linearity is 0.5%

when the primary input is zero, the output is 4mA plus or minus 0.5% (3..02ma), and the voltage drop on load 250 is 0 005v

output 5.6ma plus or minus 0.5% when the primary side input is 10%

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