“You may be very familiar with RS232, RS485, CAN and other commonly used industrial buses, which are all ways of transmitting digital signals. So, how do we transmit analog signals? The industry generally needs to measure various non-electrical physical quantities, such as temperature, pressure, speed, angle, etc., which all need to be converted into analog electrical signals before they can be transmitted to hundreds of meters away. control room or Display Device. The most widely used in industry is to use 4~20mA current to transmit analog quantity.
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You may be very familiar with RS232, RS485, CAN and other commonly used industrial buses, which are all ways of transmitting digital signals. So, how do we transmit analog signals? The industry generally needs to measure various non-electrical physical quantities, such as temperature, pressure, speed, angle, etc., which all need to be converted into analog electrical signals before they can be transmitted to hundreds of meters away. control room or Display device. The most widely used in industry is to use 4~20mA current to transmit analog quantity.
The reason for using the current signal is that it is not easily disturbed, because the amplitude of the noise voltage in the industrial field may reach several V, but the power of the noise is very weak, so the noise current is usually less than nA level, so the error brought to 4-20mA transmission is very high Small; the internal resistance of the current source tends to be infinite, and the wire resistance in series in the loop does not affect the accuracy, so it can transmit hundreds of meters on the ordinary twisted pair; due to the large internal resistance and constant current output of the current source, at the receiving end we only need Placing a 250 ohm resistor to ground gives 0-5V. The benefit of a low input impedance receiver is that the nA level of input current noise produces only very weak voltage noise.
The upper limit of 20mA is due to the requirements of explosion-proof: the spark energy caused by the current on and off of 20mA is not enough to ignite the gas. The reason why the lower limit is not set to 0mA is to detect disconnection: it will not be lower than 4mA during normal operation. When the transmission line is disconnected due to a fault, the loop current will drop to 0. 2mA is often taken as the disconnection alarm value. The current type transmitter converts the physical quantity into a 4~20mA current output, which must be powered by an external Power Supply. The most typical is that the transmitter requires two power lines, plus two current output lines, a total of 4 lines to be connected, which is called a four-wire transmitter. Of course, the current output can share a line with the power supply (common VCC or GND), which can save a line, so now basically a four-wire transmitter is called a three-wire transmitter. In fact, you may have noticed that the 4-20mA current itself can supply power to the transmitter. The transmitter is equivalent to a special load in the circuit. This kind of transmitter only needs 2 external wires, so it is called two wires. control transmitter. The industrial current loop standard lower limit is 4mA, so in the range, the transmitter is usually only 24V, 4mA power supply (thus, high efficiency DC/DC power supply (TPS54331, TPS54160) under light load conditions, low power sensors and Signal chain products, and low-power processors (such as MSP430) are very important for two-wire 4-20mA transceivers). This makes the design of two-wire sensors both possible and challenging.
Generally need to design a VI converter, input 0-3.3v, output 4mA-20mA, can use op amp LM358, power supply +12v.
Let’s systematically take a look at why analog devices prefer to use 4~20mA to transmit signals~
4-20mA. DC (1-5V.DC) signal system is an analog signal transmission standard adopted by the International Electrotechnical Commission (IEC) process control system. my country also adopts this international standard signal system. The instrument transmits the signal using 4-20mA.DC, and the receiving signal adopts 1-5V.DC, that is, the signal system of current transmission and voltage reception.
The signal current of general instruments is 4-20mA, which means the minimum current is 4mA and the maximum current is 20mA. When transmitting a signal, because there is also resistance on the wire, if the voltage is transmitted, a certain voltage drop will be generated in the wire, and the signal at the receiving end will have a certain error, so the current signal is generally used as the standard transmission of the transmitter.
1. What is the 4~20mA.DC (1~5V.DC) signal system?
The 4~20mA.DC (1~5V.DC) signal system is the International Electrotechnical Commission (IEC): analog signal standard for process control systems. my country has adopted this international standard signal system from the DDZ-Ⅲ type electric instrument. The instrument transmission signal adopts 4~20mA.DC, and the contact signal adopts 1~5V.DC, that is, the signal system of current transmission and voltage reception is adopted.
4~20mA current loop working principle:
In the industrial field, using an instrumentation amplifier to complete signal conditioning and long-term transmission will cause the following problems: first, since the transmitted signal is a voltage signal, the transmission line will be disturbed by noise; second, the distributed resistance of the transmission line will generate voltage drop; third, how to provide the operating voltage of the instrumentation amplifier in the field is also a problem.
In order to solve the above problems and avoid the influence of related noise, we use current to transmit the signal, because current is not sensitive to noise. The current loop of 4-20mA uses 4mA to represent the zero signal, and 20mA to represent the full scale of the signal, and the signal below 4mA and higher than 20mA is used for various fault alarms.
Second, the advantages of 4~20mA.DC (1~5V.DC) signal system?
The field instrument can realize two-wire system. The so-called two-wire system means that the power supply and the load are connected in series, and there is a common point, while the signal connection and power supply between the field transmitter and the control room instrument only use two wires. Because the starting point current of the signal is 4mA.DC, it provides a static working current for the transmitter, and the electrical zero point of the instrument is 4mA.DC, which does not coincide with the mechanical zero point. This “live zero point” is conducive to identifying faults such as power failure and disconnection . In addition, the two-wire system is also convenient for the use of safety barriers, which is conducive to safe explosion-proof.
The instruments in the control room use voltage parallel signal transmission, and there are common terminals between instruments belonging to the same control system, which is convenient for testing instruments, adjusting instruments, computers, and alarm devices, and is convenient for wiring.
The reason for using 4~20mA.DC for the liaison signal between the field instrument and the control room instrument is: because the distance between the field and the control room is far, the resistance of the connecting wire is large. If the voltage signal is used for remote transmission, it is better than the wire. The voltage division between the resistance and the input resistance of the receiving instrument will cause a large error, and the constant current source signal is used as the remote transmission. As long as there is no branch in the transmission loop, the current in the loop will not change with the length of the wire, thus ensuring that Transmission accuracy.
The reason for the use of 1~5V.DC for the communication signal between the instruments in the control room is that it is convenient for multiple instruments to receive the same signal together, and is conducive to wiring and forming various complex control systems. If a current source is used as the contact signal, when multiple instruments receive the same signal together, their input resistances must be connected in series, which will make the maximum load resistance exceed the load capacity of the transmitting instrument, and the negative terminal potential of the signal of each receiving instrument Different from each other, interference will be introduced, and a single centralized power supply cannot be achieved.
Using the voltage source signal connection, the current signal used for the connection with the field instrument must be converted into a voltage signal. The easiest way is to connect a 250Ω standard resistor in series in the current transmission loop to convert 4~20mA.DC into 1~5V .DC, which is usually done by the distributor.
3. Why does the transmitter choose 4~20mA.DC as the transmission signal?
1. The first is the safety consideration from the field application
The key point of safety is to consider the explosion-proof safety spark-type instrument, and to control the energy of the instrument as the premise, to reduce the static and dynamic power consumption to maintain the normal operation of the instrument to a minimum. For transmitters that output 4~20mA.DC standard signals, the power supply voltage is usually 24V.DC. The main reason for using DC voltage is that large-capacity capacitors and inductors are not needed, and only the transmitter and control room instruments are required. The distributed capacitance and inductance of the connecting wire, such as the distributed capacitance of the 2mm2 wire, is about 0.05μ/km; the inductance of the single wire is about 0.4mH/km; it is much lower than the value of detonating hydrogen, which is obviously very beneficial to explosion-proof.
2. The current source for signal transmission is better than the voltage source
Because the distance between the field and the control room is far, and the resistance of the connecting wire is large, if the voltage source signal is used for remote transmission, due to the voltage division between the wire resistance and the input resistance of the receiving instrument, a large error will occur. The source signal is used as a remote transmission, as long as there is no branch in the transmission loop, the current in the loop will not change with the length of the wire, thus ensuring the transmission accuracy.
3. Reasons for choosing 20mA for the maximum signal current
The choice of the maximum current of 20mA is based on the consideration of safety, practicality, power consumption and cost. The safety spark meter can only use low voltage and low current. 4~20mA current and 24V.DC are also safe for flammable hydrogen. For 24V.DC hydrogen, the detonation current is 200mA, which is far above 20mA. In addition, production should be comprehensively considered. Factors such as the connection distance between the field instruments, the load carried, and other factors such as power consumption and cost, requirements for Electronic components, and power supply requirements.
4. Reasons for choosing 4mA for the starting point current of the signal
Most transmitters with an output of 4~20mA are in two-wire system. The two-wire system means that the power supply and the load are connected in series, and there is a common point. The signal connection and power supply between the field transmitter and the control room instrument only use two. wire. Why is the starting point signal not 0mA? This is based on two points: First, the transmitter circuit will not work without static operating current, and the starting point current of the signal is 4mA.DC, which does not coincide with the mechanical zero point. This “live zero point” is conducive to identifying power failures and disconnection etc.
4. The origin of 4~20mA sensor?
The reason for using the current signal is that it is not easily disturbed, and the internal resistance of the current source is infinite, and the wire resistance is connected in series in the loop without affecting the accuracy, and it can transmit hundreds of meters on ordinary twisted pair lines.
The reason for using the current signal is that it is not easily disturbed, because the amplitude of the noise voltage in the industrial field may reach several V, but the power of the noise is very weak, so the noise current is usually less than nA level, so the error brought to 4-20mA transmission is very high Small; the internal resistance of the current source tends to be infinite, and the wire resistance in series in the loop does not affect the accuracy, so it can transmit hundreds of meters on the ordinary twisted pair; due to the large internal resistance and constant current output of the current source, at the receiving end we only need Placing a 250 ohm resistor to ground gives 0-5V. The benefit of a low input impedance receiver is that the nA level of input current noise produces only very weak voltage noise.
The upper limit of 20mA is due to the requirements of explosion-proof: the spark energy caused by the current on and off of 20mA is not enough to ignite the gas. The reason why the lower limit is not set to 0mA is to detect disconnection: it will not be lower than 4mA during normal operation. When the transmission line is disconnected due to a fault, the loop current will drop to 0. 2mA is often taken as the disconnection alarm value. The current type transmitter converts the physical quantity into a 4~20mA current output, which must be powered by an external power supply. The most typical is that the transmitter requires two power lines, plus two current output lines, a total of 4 lines to be connected, which is called a four-wire transmitter. Of course, the current output can share a line with the power supply and share VCC or GND, which can save a line, which is called a three-wire transmitter. In fact, you may have noticed that the 4-20mA current itself can power the transmitter. The transmitter is equivalent to a special load in the circuit. The special feature is that the power consumption current of the transmitter varies according to the sensor output between 4 and 20 mA. The display instrument only needs to be connected in the circuit. This kind of transmitter only needs 2 external wires, so it is called a two-wire transmitter. The standard lower limit of the industrial current loop is 4mA, so as long as it is within the range, the transmitter has at least 4mA power supply.
Therefore, the 4-20mA signal output is generally not susceptible to interference and is safe and reliable, so the two-wire 4-20mA power output signal is commonly used in the industry. But in order to better process the signal of the sensor, there are more output signals in other forms: 3.33MV/V; 2MV/V; 0-5V; 0-10V, etc.
A simple circuit diagram of a 4 to 20mA to voltage signal is attached:
This picture uses a 250 ohm resistor to convert the 4 to 20mA current signal into a 1 to 5V voltage signal, and then uses an RC filter and a diode (forgive my bad analog circuit, I don’t know what it means) to connect AD conversion pin of the microcontroller.
The Links: LQ104V1DG81 LQ10D41 LCD-SOURCE
