Transmitter Principle and Industrial Application
Release time:
2013-07-12
It is generally necessary to measure various electrical quantities and non-electrical physical quantities, such as current (AD), voltage (VD), power (WD), frequency (FD), temperature (TT), weight (LD), position (PT), pressure, rotating speed (RT), angle, etc., which need to be converted into receivable DC analog electrical signals before being transmitted to a control room or display device hundreds of meters away. This device that converts the measured physical quantity into a transmittable DC electrical signal is called a transmitter. Industry is usually divided into power transmitter and non-power transmitter.
It is generally necessary to measure various electrical quantities and non-electrical physical quantities, such as current (AD), voltage (VD), power (WD), frequency (FD), temperature (TT), weight (LD), position (PT), pressure, rotating speed (RT), angle, etc., which need to be converted into receivable DC analog electrical signals before being transmitted to a control room or display device hundreds of meters away. This device that converts the measured physical quantity into a transmittable DC electrical signal is called a transmitter. Industry is usually dividedElectrical quantity transmitterand non-electrical transmitters.
TransmitterThe traditional output DC signals are 0-5V, 0-10V, 1-5V, 0-20mA, 4-20mA, etc. At present, the most widely used is to transmit analog quantity with 4 ~ 20mA current. The most widely used in industry is to use 4 ~ 20mA current to transmit analog.
The current signal is used because it is not susceptible to interference. 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. It can transmit hundreds of meters on the ordinary twisted pair. The upper limit of 20mA is due to explosion-proof requirements: the spark energy caused by the on-off of 20mA current is not enough to ignite gas. The reason why the lower limit is not 0mA is to detect disconnection: it will not be lower than 4mA during normal operation, and the loop current will drop to 0 when the transmission line is broken due to fault. Often take 2mA as the disconnection alarm value.
Current transmitterThe physical quantity is converted into 4 ~ 20mA current output, there must be an external power supply for its power supply. The most typical transmitter requires two power lines, plus two current output lines, a total of 4 lines, 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 and is called a three-wire transmitter.
In fact, you may notice that 4-20mA of current can itself power the transmitter, as shown in Figure 1C. The transmitter is equivalent to a special load in the circuit, and the special feature is that the power consumption current of the transmitter varies between 4 and 20mA according to the sensor output. The display meter only needs to be strung in the circuit. This transmitter only needs to be externally connected to 2 lines, so it is called a two-wire transmitter. The lower limit of the industrial current loop standard is 4mA, so as long as it is within the range, the transmitter has at least 4mA power supply. This makes the design of a two-wire sensor possible.
In industrial applications, the measuring point is generally in the field, and the display device or control device is generally in the control room or control cabinet. The distance between the two may be tens to hundreds of meters. According to the calculation of a distance of 100 meters, saving 2 wires means that the cost is reduced by nearly 100 yuan! Therefore, the two-wire sensor is more and more used in practical use.
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