1. Transmitters can be divided into two types according to the output signal type: current output type and voltage output type.
(1) Voltage output transmitters have the properties of a constant voltage source. The impedance of the voltage input terminal of the PLC analog input module is very high. If the transmission distance is long, the tiny interference signal current will generate a high interference voltage on the input impedance of the module, so the anti-interference ability of the remotely transmitted analog voltage signal is poor. However, it is suitable for sending the same signal to multiple instruments in parallel, and the installation is simple. The disassembly and installation of one of the instruments will not affect the operation of other instruments. The voltage resistance requirements of the output stage are reduced, thereby improving the reliability of the instrument. The voltage signal range is 1~5 V, 0~10 V, -10~10 V, first 1~5 V, 0~10 V.
(2) Current output transmitters have the properties of a constant current source, and the internal resistance of the constant current source is very large. When the input of the PLC analog input module is current, the input impedance is low, and the interference voltage generated by the interference signal on the line on the module is very low, so the analog current signal is suitable for long-distance transmission, which can reach hundreds of meters when using shielded cable signal lines. The standard of current signal is 0～10 mA, 0～20 mA, 4～20 mA, and 4～20 mA is preferred. 0 mA is usually used as a circuit fault or power failure indication signal.
Current signal transmission and voltage signal transmission have their own characteristics. Current signal is suitable for long-distance transmission, and voltage signal allows the instrument to be connected in "parallel". Therefore, in the control meter system, the transmission signal in and out of the control room adopts current signal, and the communication between the instruments in the control room adopts voltage signal, that is, the connection method is current transmission and parallel reception of voltage signal.
Transmitters are divided into two types: two-wire and four-wire. Four-wire transmitters have two power lines and two signal lines, and there is no strict requirement for the power consumption of the zero-point element of the current signal. Two-wire transmitters have only two external wirings, which are both power lines and signal lines. The lower limit of the current signal cannot be zero, but the two-wire transmitter has fewer wirings and a long transmission distance, and is most widely used in industry. [3]
2. According to the different energy sources used, transmitters are divided into pneumatic transmitters and electric transmitters.
(1) Pneumatic transmitter
Pneumatic transmitters use dry, clean compressed air as energy. They can convert various measured parameters (such as temperature, pressure, flow and liquid level, etc.) into air pressure signals of 0.02-0.1IMPa, so as to transmit them to the combined instrument of adjustment, display and other units for indication, recording or adjustment. Pneumatic transmitters have a relatively simple structure, relatively reliable operation, strong anti-interference ability against electromagnetic fields, radiation, temperature, humidity and other environmental influences, fireproof and explosion-proof, and relatively cheap; the disadvantages are slow response speed, limited transmission distance, and difficulty in connecting to computers.
(2) Electric transmitter
Electric transmitters use electricity as energy, and the signal connection is relatively convenient. They are suitable for long-distance transmission and easy to connect to electronic computers. In recent years, they can also be made explosion-proof for safe use. Its disadvantage is that the investment is generally high, and it is greatly affected by interference from temperature, humidity, electromagnetic fields and radiation: the electric transmitter can convert various measured parameters into 0~10V or 4~20mA (unified standard signal of DC current) for transmission to other units in the automatic control system.