In industrial production, we often encounter in the use of measuring instruments such as E + H electromagnetic flowmeter and input level meter ‘4-20mA output’ such a term.
In industrial production, we often encounter in the use of measuring instruments such as E + H electromagnetic flowmeter and input level meter ‘4-20mA output’ such a term, in the end, this is what kind of technical standards, such a technical standard and the development of what is the reason why the signal phase to use this The power supply range, what are the benefits, in actual production practice, how do we use this technical standard for standardised operation, which is a lot of people just in touch with the industrial measuring instruments need to learn and master.
A two-wire transmitter structure and principle The principle of the two-wire transmitter is the use of the 4-20mA signal to provide their own power. If the transmitter's own power consumption is greater than 4mA, then it will not be possible to output the lower limit 4mA value. Therefore, it is generally required that the power consumption of the two-wire transmitter itself (all circuits including the sensor) is not greater than 3.5 mA, which is one of the fundamental principles of the design of the two-wire transmitter. From the overall structure, two-wire transmitter consists of three major components: sensor, conditioning circuit, two-wire V / I converter composition. Sensor temperature, pressure and other physical quantities into electrical parameters, conditioning circuit will be the sensor output of weak or non-linear electrical signals for amplification, conditioning, into a linear voltage output. A two-wire V/I converter circuit controls the overall current consumption based on the output of the signal conditioning circuit; it also obtains a voltage from the loop and regulates it for use by the conditioning circuit and the sensor. With the exception of the V/I converter circuit, each part of the circuit has its own current draw, and the core design idea of the two-wire transmitter is to include all of the currents in the V/I converter feedback loop. As shown in the figure, the sampling resistor Rs is connected in series at the low end of the circuit, and all current will flow through Rs back to the negative terminal of the power supply. The feedback signal taken from Rs contains all the power dissipated by the circuit. In a two-wire transmitter, the total power consumption of all circuits cannot be greater than 3.5mA, so the low power consumption of the circuit becomes the main design difficulty. The following will be analysed one by one the principle of each part of the circuit and design points.
Second, industry generally need to measure various types of non-electric physical quantities, such as temperature, pressure, speed, angle, etc., need to be converted to analogue electrical signals to be transmitted to hundreds of metres away from the control room or display equipment. This physical quantity will be converted into an electrical signal device called transmitter. Widely used in industry is to use 4 ~ 20mA current to transmit analogue. The reason for using current signals is that they are not susceptible to interference. And the current source of internal resistance is infinite, wire resistance in series in the loop does not affect the accuracy, in the ordinary twisted pair of wires can be transmitted hundreds of metres. The upper limit is taken as 20mA because of the requirement of explosion-proof: the spark energy caused by the current breakage of 20mA is not enough to ignite the gas. The reason why the lower limit is not taken as 0mA is in order to be able to detect the disconnection: it will not be lower than 4mA in normal operation, when the transmission line is disconnected due to a fault, the loop current will be reduced to 0. It is often taken as a disconnection alarm value of 2mA. Current type transmitter will be converted into physical quantities 4 ~ 20mA current output, there must be an external power supply for its power supply. Typically, the transmitter needs two power lines, plus two current output lines, a total of four wires to be connected, called four-wire transmitter. Of course, the current output can be connected to a common wire (common VCC or GND) with the power supply, which saves one wire and is called a 3-wire transmitter. In fact, you may notice that the 4-20mA current itself can power the E+H electromagnetic flowmeter transmitter, as shown in Figure 1C. The transmitter in the circuit is equivalent to a special load, the special thing is that the transmitter current consumption between 4 ~ 20mA according to the sensor output and change. The display instrumentation only needs to be strung in the circuit. This type of transmitter requires only 2 external wires and is therefore called a 2-wire transmitter. The lower limit of the industrial current loop standard is 4mA, so the transmitter is supplied with at least 4mA as long as it is within the range. This makes the design of two-wire sensors possible. In industrial applications, the measurement point is usually in the field, while the display or control device is usually in the control room or on the control cabinet. The distance between the two may be tens to hundreds of metres. At a distance of one hundred metres, the elimination of two wires means a cost reduction of nearly one hundred dollars! Therefore in the application of the two-wire sensor is bound to be.
Three, two-wire V/I converter V/I converter is a kind of circuit that can control the output current with voltage signal. Two-wire V/I converter and general V/I converter circuit is different in: the voltage signal is not directly control the output current, but to control the entire circuit itself current consumption. At the same time, a stable voltage is extracted from the current loop to power the conditioning circuits and sensors. The attached figure is a two-wire V / I conversion circuit of the basic principle diagram: OP1, Q1, R1, R2, Rs constitute the V / I converter. Analyse the negative feedback process: if point A is higher than 0V for some reason, the output of op-amp OP1 rises, the voltage at both ends of Re rises, and the current through Re becomes larger. This corresponds to the overall power consumption becoming larger, the current through the sampling resistor Rs also becomes larger, and the voltage at point B becomes lower (more negative). The result is that the point A voltage is pulled down through R2. Conversely, if point A is below 0V for some reason, it will also be raised by negative feedback back to 0V. In short, the result of negative feedback is that the op-amp OP1 is falsely shorted, and the voltage at point A = 0V. The following is an analysis of the principle of control of Vo on the total power dissipation: Assuming that the output voltage of the conditioning circuit is Vo, the current flowing through R1, I1=Vo/R1, the op-amp inputs are not likely to absorb current, so that all of the I1 flowing through R2, then the voltage at point B VB= -I1*R2 = -Vo*R2/R1 When taking R1=R2, there is VB=-Vo There are only two resistors, Rs and R2, between the negative power supply and the whole circuit, so all the currents flow through Rs and R2. the upper end of R2 is the virtual ground (0V), and the upper end of Rs is the GND. therefore, the voltages of the two terminals of R2 and Rs are the same, and they are all equal to VB . Rs and R2 are connected in parallel as current sampling resistors. Therefore, the total current of the circuit: Is=Vo/(Rs//R2) If we take R2>>Rs, Is=Vo/Rs Therefore, in Fig. 3, we take Rs=100 ohm, and when the output of the conditioning circuit is 0.4~2V, the total current consumption will be 4~20mA. If we can't meet the requirement of R2>>Rs, it doesn't matter, Rs is in parallel with R2 and the voltage of Rs is fixed, and the Is is still linearly proportional to the VB, so the error scale factor is equal to VB. Rs and R2 in parallel (Rs//R2) is a fixed value, Is and Vo are still linearly related, and the error scaling factor can be eliminated on time. In addition to the correct circuit, there are two other conditions for the circuit to work properly: firstly, it must consume as little power as possible, and the saved current must be supplied to the conditioning circuit and to the transmitter. Secondly, the op-amp must be able to work with a single power supply, i.e., in the absence of a negative power supply, the input is still able to accept a 0V input and work properly. The LM358/324 is a common and low-priced single-supply op-amp that consumes 400uA/per op-amp, which is basically acceptable. When powered from a single supply, the input works fine from -0.3V to Vcc-1.5V. If replaced with a precision amplifier such as OP07, because the input is not allowed to go as low as 0V, it will not work in this circuit instead. R5 and U1 form a reference source to produce a stable reference voltage of 2.5 V. The LM385 is a low-cost, micro-power reference that operates above 20 uA, and the curve given in the manual is flat around 100 uA, so the current is controlled at about 100 uA by R5. OP2 forms an isotropic amplifier that amplifies the reference to supply power to the conditioning circuit and the sensor. Because wide input voltage, low-power regulators are scarce and costly; amplifying the reference as a regulated power supply is an inexpensive solution. Off-the-shelf integrated circuits are also available for this part of the circuit. For example, XTR115/116/105, etc. The accuracy and stability are better than homemade ones, and their own power consumption is lower (meaning that more current can be left for the conditioning circuit, and the conditioning part is easier to design). But the cost is more than 10 times higher than the above mentioned solutions.
Fourth, two-wire pressure transmitter design Pressure bridge, load cell output signal is weak, are mV-level signals. This type of small signals are generally required to use differential amplifiers to amplify its level. General selection of low-modulation, low temperature drift differential amplifier. In addition, in the two-wire application, low power consumption is also necessary. AD623 is commonly used in low-power precision differential amplifier, commonly used in the differential output preamplifier. AD623 large 200uV misregulation, temperature drift 1uV / degree, in general pressure transmission applications to ensure sufficient accuracy. R0 will be 0.4V superimposed on the AD623 REF feet (5 feet), in the case of pressure = 0 by adjusting R0 to make the output 4mA, and then adjust the RG output 20.00mA, complete the calibration. Circuit design should be noted that the pressure bridge sensor is equivalent to a kilo-ohm resistor, power consumption is generally large. Appropriately reducing the excitation voltage of the pressure bridge can reduce the power consumption current. However, the output amplitude also decreases and the gain of the AD623 needs to be increased. Figure 6 gives the sensor using a constant voltage supply, most of the semiconductor pressure sensors in practical applications require constant current supply to obtain better temperature characteristics, you can use an op-amp to form a constant current source to provide excitation.
V. Stability and security considerations in industrial environments, harsh environments and high reliability requirements, so the design of two-wire transmitters need to consider a certain degree of protection and enhance the stability of the measures. 1. Power protection. Power reverse, overvoltage, surge is a common industrial power problems. Reverse power supply is equipment installation and wiring errors are likely to occur, the input port string a diode can prevent reverse power supply damage to the circuit. If a full-bridge rectifier is added to the input, it can still work normally even if the power supply is reversed. In order to prevent lightning, electrostatic discharge, surge and other energy damage to the transmitter, the transmitter inlet can be installed a TVS tube to absorb the instantaneous overvoltage energy. Generally, the TVS voltage value is taken slightly lower than the limit voltage of the op amp in order to play a protective role. If you may suffer a lightning strike, TVS may not absorb enough capacity, varistor is also necessary, but the leakage of the varistor itself will bring a certain error. 2. Overcurrent protection. Equipment operation may have sensor disconnection, short circuit and other errors occur. Or the input itself is likely to exceed the range, the transmitter must ensure that under any circumstances the output will not rise without limit, otherwise there is a risk of damage to the transmitter itself, the power supply, or the remote display instrument. Rb and Z1 in the diagram form the overcurrent protection circuit. No matter what causes OP1 output is greater than 6.2V (1N4735 is a 6.2V regulator), will be clamped by Z1, the base of Q1 can not be higher than 6.2V. Therefore, the voltage on the Re can not be higher than 6.2-0.6 = 5.6V, so the total current will not be greater than the Ue/Re = 5.6V/200 = 28mA. 3. wide voltage adaptability. General two-wire transmitter can adapt to a wide range of voltage changes without affecting the accuracy. This can be applied to all types of power supply, while being able to adapt to large load resistance. Sensitive part of the power supply is the reference source, while the reference source is also the main components to determine the accuracy of the 3 floor diagram of the reference through the R5 current limitation, when the power supply voltage changes, the current on the R5 also changed, the stability of the reference has a great impact. Attachment in the use of constant-current source LM334 for the benchmark power supply, voltage changes over a wide range, the current is basically unchanged, to ensure the stability of the benchmark. 4. Retro-coupling capacitance General circuit design, each integrated circuit power supply will have a retrograde capacitance. In the two-wire transmitter power-up, the charging of these capacitors will lead to a large current in an instant, potentially damaging the remote instrument. Therefore, each decoupling capacitor should not exceed 10nF, and the total decoupling capacitance should not exceed 50nF. A 10nF capacitor at the inlet is necessary to ensure that the circuit does not oscillate under long inductive loads.
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