Industry Knowleges: The past and present of transmitters and sensors

Transmitters are developed from sensors, and any sensor that can output standard signals. Standard signals refer to signals whose form and quantity range of physical quantities meet international standards. Since DC signals have the advantages of not being affected by the inductance, capacitance and load properties in the line, and there is no phase shift problem, the International Electrotechnical Commission (IEC) has determined the current signal 4mA~20mA (DC) and the voltage signal 1V~5V (DC) as the unified standard for analog signals in process control systems.

Transmitters work based on the principle of negative feedback, and they are mainly composed of a measuring part, an amplifier and a feedback part.

The measuring part is used to detect the measured variable x and convert it into an input signal Zi (voltage, current, displacement, force or torque, etc.) that can be accepted by the amplifier. The feedback part converts the output signal y of the transmitter into a feedback signal Zf, and then sends it back to the input end. The algebraic sum of Zi and the zeroing signal Zo is compared with the feedback signal Zf, and the difference ε is sent to the amplifier for amplification and converted into a standard output signal y.

Transmitters can be divided into two types according to the output signal type: current output type and voltage output type. <br />
The voltage output transmitter has 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 far, the tiny interference signal current will generate a higher 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 work of other instruments, and the withstand voltage requirements of the output stage are reduced, thereby improving the reliability of the instrument. The range of voltage signals is 1~5 V, 0~10 V, -10~10 V, 1~5 V, 0~10 V.

The current output transmitter has 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 remote 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, 4～20 mA, 0 mA is usually used as circuit fault or power supply fault indication signal.

Current signal transmission and voltage signal transmission have their own characteristics. Current signal is suitable for long-distance transmission, and voltage signal enables the instrument to adopt "parallel system" connection. 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 widely used in industry.