You may be very familiar with RS232, RS485, CAN and other industries. They are all ways to transmit digital signals. So, what method do we use to transmit analog signals?
In the industry, various non-electrical physical quantities are generally needed to measure, such as temperature, pressure, speed, angle, etc. These need to be converted into analog electrical signals to be transmitted to a control room or display device several hundred meters away. is the most widely used in the industry to transmit analog quantity using currents of 4~20mA. The reason why
uses current signals is that it is not easily disturbed, because the amplitude of the noise voltage in the industrial site may reach several V, but the noise power is very weak, so the noise current is usually less than the nA level, so the error to the 4-20mA transmission is very small; the internal resistance of the current source tends to be infinite, and the wire resistance is connected in series in the loop does not affect the accuracy, so hundreds of meters can be transmitted on ordinary twisted pair ; due to the large internal resistance and constant current output of the current source, we only need to place a 250 ohm to ground resistor at the receiving end to obtain a voltage of 0-5V. The advantage of a low input impedance receiver is that the nA-level input current noise only produces very weak voltage noise. The upper limit of
is 20mA because of explosion-proof requirements: the spark energy caused by the on-off current of 20mA is not enough to ignite gas . The reason why has not taken 0mA at the lower limit is to detect the disconnection: it will not be less than 4mA during normal operation, and when the transmission line is broken due to a fault, the loop current drops to 0. 2mA is often used as the disconnection alarm value. The current-type transmitter converts the physical quantity into 4~20mA current output, and an external power supply must be used to power it. The most typical example is that the transmitter requires two power cords, plus two current output cords, and a total of 4 wires are connected, which is called a four-wire transmitter. Of course, the current output can be one wire (common VCC or GND) with the power supply, which can save one wire, so now basically the four-wire transmitter is called a three-wire transmitter. In fact, you may have noticed that 4-20mA current can power the transmitter itself. The transmitter is equivalent to a special load in the circuit. This transmitter only needs to be connected to two external wires, so it is called a two-wire transmitter. The lower limit of the industrial current loop standard is 4mA, so within the range, the transmitter is usually only 24V and 4mA powered (so, 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 transmission and reception). This makes the design of two-wire sensors possible and challenging.
generally requires designing a VI converter, input 0-3.3v, output 4mA-20mA, and use op amp LM358, power supply +12v.
Let’s systematically see why analog equipment prefers to use 4~20mA to transmit signals~
4-20mA. DC (1-5V.DC) signal system is an analog signal transmission standard used by International Electrotechnical Commission (IEC) process control system . my country also adopts this international standard signal system . The instrument transmits signals using 4-20mA.DC and the received signals using 1-5V.DC, which is a signal system that uses current transmission and voltage reception.
Generally, the signal current of instruments and meters is 4-20mA, which means that the minimum current is 4mA and the maximum current is 20mA. When transmitting signals, because there are resistors on the wire, if voltage is used to transmit, a certain voltage drop will be generated within the wire, and the signal at the receiving end will produce a certain error. Therefore, 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?
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 since the DDZ-III electric instrument. The instrument transmission signal uses 4~20mA.DC, and the contact signal uses 1~5V.DC, which is a signal system that uses current transmission and voltage reception.
4~20mA current loop working principle:
In the industrial site, using an instrument amplifier to complete the signal conditioning and perform long-term transmission, the following problems will arise: First, since the transmitted signal is a voltage signal, the transmission line will be disturbed by noise; second, the distribution resistance of the transmission line will generate a voltage drop; third, how to provide the working voltage of the instrument amplifier at the site is also a problem.
In order to solve the above problems and avoid the influence of related noise, we use current to transmit signals because current is not sensitive to noise. The current loop of 4 to 20mA is used to represent the zero signal with 4mA and the full scale of the signal with 20mA. The signals below 4mA and above 20mA are used to alarm various faults.
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2. Advantages of signal system of 4~20mA.DC (1~5V.DC)?
on-site instrument can realize a two-wire system. The so-called two-wire system means the power supply and load are connected in series, and there is a common point. The signal communication and power supply of the field transmitter and the control room instrument are only used to use two wires. Because the starting current of the signal is 4mA.DC, it provides the transmitter with a static working current, 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 wire breakage. In addition, the two-wire system also facilitates the use of security grid , which is conducive to safety and explosion protection.
control room instruments use voltage parallel signal transmission, and there are common ends between the instruments to which the same control system belongs, which is convenient for the use of detection instruments, adjustment instruments, computers, and alarm devices, and convenient for wiring.
The communication signal between the field instrument and the control room instrument uses 4~20mA.DC. The reason is that because the distance between the field and the control room is relatively long, the resistance to connecting the wire is larger. If the voltage signal is transmitted remotely, it is better than the voltage division between the wire resistance and the input resistance of the receiving instrument, which will produce a large error. Using the constant current source signal as the remote transmission, as long as the transmission circuit does not branch, the current in the loop will not change with the length of the wire, thus ensuring the transmission accuracy. The communication signal between the instruments in the control room of
uses 1~5V.DC reason is: in order to facilitate multiple instruments to receive the same signal together, and to facilitate wiring and forming various complex control systems. If a current source is used as a contact signal, when multiple instruments jointly receive the same signal, their input resistance must be connected in series, which will make the maximum load resistance exceed the load capacity of the transmitting instrument, and the negative potential of each receiving instrument is different, which will introduce interference and cannot achieve a single centralized power supply.
uses voltage source signal communication, and the current signal used to communicate with the on-site instrument must be converted into a voltage signal. The easiest way is to connect a 250Ω standard resistor in the current transmission loop, convert 4~20mA.DC to 1~5V.DC. This task is usually accomplished by distribution .
3. Why does the transmitter choose 4~20mA.DC as the transmission signal?
1. First of all, consider the security of on-site applications
. The security focus is to consider explosion-proof and safe spark-type instruments, and to control the energy of the instrument, the static and dynamic power consumption that maintains the normal operation of the instrument is reduced to a minimum.The power supply voltage of the transmitter that outputs a standard signal of 4~20mA.DC is usually 24V.DC. The main reason for using DC voltage is that you do not use large-capacity capacitors and inductor . You only need to consider the distribution capacitance and inductor of the wire connecting the transmitter to the control room instrument. For example, the distribution capacitance of the 2mm2 wire is about 0.05μ/km; the inductance of a single wire is about 0.4mH/km; it is much lower than the value of detonating hydrogen , which is obviously very beneficial for explosion protection.
2. The current source for transmitting signals is better than the voltage source
. Because the distance between the field and the control room is relatively long, if the resistance to connecting the wire is large, if the voltage source signal is used to transmit remotely, due to the voltage division between the wire resistance and the input resistance of the receiving instrument, a large error will occur. If the current source signal is used as 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 selecting the maximum current of 20mA for signal
The maximum current of 20mA is based on safety, practicality, power consumption and cost considerations. Safety spark instruments can only use low voltage and low current. 4~20mA current and 24V.DC are also safe for flammable hydrogen. The detonation current of 24V.DC is 200mA, which is far above 20mA. In addition, factors such as the connection distance between the instruments on the production site, the load, etc. should be comprehensively considered; there are also factors such as power consumption and cost issues, requirements for electronic components, and power supply power requirements.
4. Reason for selecting 4mA of signal starting point current
transmitters with output of 4~20mA are mostly two-wire systems. The two-wire systems, namely the power supply and load, are connected in series, and there is a common point. The signal communication and power supply between the on-site transmitter and the control room instrument are only used to provide two wires. Why is the starting point signal not 0mA? This is based on two points: First, the transmitter circuit will not be able to operate without a static working current, and the signal starting point current is 4mA.DC, which does not coincide with the mechanical zero point. This "live zero point" is conducive to identifying faults such as power outages and wire breakages.
4. The origin of the 4~20mA sensor?
The reason why it uses a current signal is that it is not easily disturbed, and the internal resistance of the current source is infinite. The wire resistance is connected in series in the loop without affecting the accuracy. It can be transmitted hundreds of meters on ordinary twisted pair wires. The reason why
uses current signals is that it is not easily disturbed, because the amplitude of the noise voltage in the industrial site may reach several V, but the noise power is very weak, so the noise current is usually less than the nA level, so the error to the 4-20mA transmission is very small; the internal resistance of the current source tends to be infinite, and the wire resistance is connected in series in the loop does not affect the accuracy, so it can be transmitted hundreds of meters on ordinary twisted pair wires; due to the large internal resistance and constant current output of the current source, we only need to place a 250 ohm to ground resistor at the receiving end to obtain a voltage of 0-5V. The advantage of a low input impedance receiver is that the input current noise at the nA level only produces very weak voltage noise. The upper limit of
is 20mA because of explosion-proof requirements: the spark energy caused by the on-off current of 20mA is not enough to ignite gas. The reason why the lower limit is not taken 0mA is to detect the disconnection: it will not be less than 4mA during normal operation. When the transmission line is broken due to a fault, the loop current will drop to 0. 2mA is often used as the disconnection alarm value. The current-type transmitter converts the physical quantity into 4~20mA current output, and an external power supply must be used to power it. The most typical thing is that the transmitter requires two power cords, plus two current output cords, and a total of 4 wires are connected, which is called a four-wire transmitter. Of course, the current output can be one wire with the common power supply VCCh or GND, which can save one wire, called a three-wire transmitter. In fact, you may have noticed that 4-20mA current can power the transmitter itself. The transmitter is equivalent to a special load in the circuit, and the special thing is that the current consumption of the transmitter varies between 4~20mA according to the sensor output. The display meter only needs to be serially connected to the circuit. This type of transmitter only needs two external wires, so it is called a two-wire transmitter. The lower limit of the industrial current ring standard is 4mA, so the transmitter has at least 4mA power supply as long as it is within the range.
Therefore, the signal output of 4-20mA is generally not easily disturbed and is safe and reliable. Therefore, the two-wire 4-20mA power output signal is commonly used in the industry. However, in order to better process the sensor signals, there are currently more other forms of output signals: 3.33MV/V; 2MV/V; 0-5V; 0-10V, etc.
also has a simple circuit diagram of the voltage signal of 4 to 20mA:
This diagram uses a 250 ohm resistor to convert the current signal of 4 to 20mA into a voltage signal of 1 to 5V, and then use an RC filter to add an diode (forgive me of the bad analog circuit, I don’t know what it means) to connect it to the AD conversion pin of the microcontroller.
Original link: https://mp.weixin.qq.com/s/4XIiPvDNZc4O5y1tUhjC4w
Reprinted from: The last bug
Original link: It is simple to transmit digital signals, so what is commonly used to transmit analog signals?
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