Analysis on the classification basis of transmitter

1. The transmitter can be divided into current output type and voltage output type according to the output signal type.

The voltage output transmitter has the property of constant voltage source, and the impedance of the voltage input end of the analog input module of PLC is very high. Assuming that the transmission distance is long, the tiny disturbing signal current will generate a higher disturbing voltage at the input impedance of the module, so the anti-disturbing ability of the analog voltage signal transmitted over a long distance is poor. However, it is suitable for sending the same signal to multiple surfaces in parallel, and the installation is simple. During disassembly and assembly, one surface will not affect the operation of other surfaces, which reduces the voltage resistance requirement of the output stage, and then improves the reliability of the surfaces. The scale of voltage signal is 1 ~ 5 V, 0 ~ 10 V, 10 ~ 10 V, first of all, 1 ~ 5 V and 0 ~ 10 V..

Current output transmitter has the property of constant current source, and the internal resistance of constant current source is very large. When the input of PLC analog input module is current, the input impedance is low, and the disturbing voltage generated by the disturbing 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 the shielded cable signal line. The standard of current signal is 0 ~ 10 mA, 0 ~ 20 ma and 4 ~ 20 ma, and shou is 4 ~ 20 ma, and 0 ma is usually used as an indication signal of circuit fault or power failure.

Current signal transmission and voltage signal transmission have their own characteristics. The current signal is suitable for long-distance transmission, and the voltage signal enables the surface to be connected in parallel. Therefore, in the control table system, the transmission signal in and out of the control room is current signal, and the contact between the surfaces in the control room is voltage signal, that is, the connection method is current transmission and parallel reception of voltage signal.

Transmitters are divided into two-wire system and four-wire system. The four-wire transmitter has two power lines and two signal lines, so there is no strict requirement on the power consumption of the components of the current signal. Two-wire transmitter only needs two external wires, which are both power wires and signal wires, and the lower limit of current signal cannot be zero. However, two-wire transmitter has few wires and long transmission distance, so it is widely used in industry.

2. According to the different power used, transmitters can be divided into pneumatic transmitters and electric transmitters.

Pneumatic transmitter:

Pneumatic transmitter is powered by dry and clean compressed air, which can convert various measured parameters (such as temperature, pressure, flow rate and liquid level, etc.) into air pressure signals of 0.02 ~ 0.1 IMPA, so as to transmit them to the combined surfaces of conditioning and display units for indication, recording or conditioning. Pneumatic transmitter has simple structure, reliable operation, strong anti-interference ability to electromagnetic field, radiation, temperature, humidity and other environmental influences, fire prevention and explosion prevention, and low price; The disadvantage is that the response speed is slow, the transmission distance is limited and it is difficult to connect with the computer.

Electric transmitter:

The electric transmitter is powered by electricity, and the communication between signals is convenient. It is suitable for long-distance transmission and easy to connect with electronic computers. In recent years, it can also be explosion-proof to facilitate the full use of an. Its disadvantage is that the investment is generally high, which is greatly influenced by the disturbance of temperature, humidity, electromagnetic field 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.