Briefly describes the electromagnetic flow meter and other industrial instrument used 4 - 20 ma signal as the output standard

In industrial production, we often put into type level gauge in measuring instrument such as electromagnetic flowmeter and the use of the meet & other 4 - 20 ma output & throughout; Such a noun, whether this is a what kind of technical standard, such a technical standard to produce and make the reason, why the signal phase to use this power range, so what are the benefits of, in the actual production practice, we how to use the technical standards for normative operation, this is just come into contact with many industrial measuring instrument personnel need to learn and master. A,. Two wire transducer structure and the principle of the wire transducer principle is to use the 4 ~ 20 ma signal provides power for themselves. If the transmitter power is greater than 4 ma, so will not be able to output the lower 4 ma. So generally require two wire system transmitter power consumption itself ( All circuit), including sensors, No more than 3. 5mA。 This is one of two wire transducer design principles. On the whole structure of it, the two wire transducer is composed of three parts: sensor, modulation circuit, two wire system of V/I converter. Sensors will physical quantity into electric parameters, such as temperature, pressure, regulate the output circuit will be weak or nonlinear signal amplification, conditioning, converted to a linear voltage output. Two wire system V/I conversion circuit according to the output of the signal conditioning circuit control overall power consumption current; From the loop on the voltage and voltage stability at the same time, for the use of modulation circuit and sensors. Besides V/I conversion circuit, the circuit of each part has its own power current, the core of the two wire transducer design thought is by including all of the current in the V/I conversion within the feedback loop. As shown in figure, the low end of the sampling resistance Rs in series circuit, all current will flow through Rs back to power the cathode. To the feedback signal is taken from the Rs, contains all the power consumption of the circuit. In two wire transducer, all of the total power consumption is not greater than 3. 5 ma, so the design of low power consumption of the circuit become major difficulty. The following parts will analyze circuit principle and design points. Second, the industry generally need to measure all kinds of non-electric physical quantity, such as temperature, pressure, speed, Angle and so on, needs to be converted into analog signal to transfer to a few hundred meters outside of the control room or the display device. This transform physical quantity into electric signal device called a transducer. Is widely used in the industrial used to transmit analog 4 ~ 20 ma current. By the reason of current signal is not susceptible to interference. And current source internal resistance infinity, does not affect the accuracy of circuit conductor resistance in series, in the ordinary twisted pair transmission hundreds of meters. Ceiling take 20 ma because of explosion-proof requirements: 20 ma electrical current caused by broken spark to ignite the gas energy. Floor did not pick up 0 ma reason is that in order to detect disconnection: normal not less than 4 ma at work, when the transmission line due to the fault circuit, the loop current drop to zero. Often take 2 ma as a disconnection alarm value. Type current transducer to convert physical quantities to 4 ~ 20 ma output current, is bound to have external power for the power supply. Typically require two power cord in the transmitter, plus two output current thread that altogether it will take 4 wires, called four wire transducer. Of course, can output current and power utility a thread ( Public VCC or GND) , can save a thread, called three wire transducer. In fact, you may notice, 4 - 20 ma current itself can for electromagnetic flow meter transducer power supply, as shown in figure 1 c. Transmitter is equivalent to a special load in the circuit, is special in the transmitter power consumption between 4 ~ 20 ma current according to the sensor output. Display instrument you just need to string in the circuit. The transmitter only external 2 lines, therefore is called the two wire transducer. Industrial current loop the lower limit is 4 ma, so as long as within the scope of the range, at least 4 ma power transmitter. This makes the design of the two wire sensor possible. In industrial applications, the measurement point generally at the scene, and display devices or control equipment are generally in control room or control on the ark. Distance between the two may dozens to hundreds of meters. According to the one hundred meters distance calculation, save 2 wire means lower costs nearly hundred dollars! So two wire system in the application of sensor must be * *. Three,. Two wire system V/I converter V/I converter is a circuit of voltage signal to control the output current can be used. Two wire system V/I converter with general difference in V/I conversion circuit, voltage is not directly control the output current, but electricity current control of the circuit itself. At the same time, also from the current loop extraction and stability of the power supply voltage for the modulation circuit and sensors.

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