Analog quantity transmission data of temperature and humidity transmitter and other equipment in industrial control!
Before entering the text, insert a popular science note: For a long time, people do not distinguish between transmitters and sensors. Because of their similar functions, they often mix them up. However, there are still large differences between transmitters and sensors.
The temperature and humidity transmitter is a converter that converts the output signal of the sensor into a signal that can be recognized by the controller (or a signal source that converts the non electric quantity input by the sensor into an electrical signal and simultaneously amplifies it for remote measurement and control). The sensor and transmitter together constitute the monitoring signal source for automatic control. Different physical quantities require different sensors and corresponding transmitters.
In short, a sensor is a device that senses the measured variables (temperature, pressure, gas, etc.) and converts them into non-standard electrical signals; The transmitter is based on the sensor to convert this non-standard electrical signal into standard analog quantity and upload it to the industrial control system.
First of all, we should know that transmitter signal RS485 and analog quantity 4-20mA should be two concepts. Generally, transmitter signal is analog quantity signal, and analog quantity signal can be RS485 4-20mA current or 0-5V/0-10V voltage.
Why do you prefer to use 4-20mA to transmit analog quantity in industrial control occasions, but rarely use 0-5V or 0-10V?
First of all, electromagnetic interference is very serious in factories or construction sites, and voltage signals are more vulnerable to interference than current signals. Moreover, the transmission distance of current signal is far greater than that of voltage signal, and it will not cause signal attenuation.
Secondly, the signal current of general instruments is 4-20mA. 4-20mA refers to the minimum current of 4mA and the maximum current of 20mA. So why choose 4-20mA instead of 0-20mA? 4mA instead of 0mA is used to detect the open circuit of the line. If 0 is the minimum, the open circuit fault cannot be detected. Another reason is that the minimum 4mA is used because the disconnection point can be detected. The maximum 20mA is used to meet the explosion-proof requirements, because the spark potential energy caused by 20mA current signal on and off is not enough to ignite the explosion point of combustible gas. If it exceeds 20mA, there will be an explosion risk.
Finally, when transmitting signals, it is necessary to consider that there is resistance on the wire. If voltage transmission is used, there will be a certain voltage drop on the wire, and then the signal at the receiving end will produce a certain error. Therefore, in the industrial control system, current signal transmission is usually used for long-distance transmission less than 100m, while in the case of short distance transmission, such as between instruments in the control room, 0-5V voltage signal transmission can be considered.
The selection of a suitable transmitter should not only consider the function of the transmitter, but also consider the transmission mode of analog quantity. Analog quantity transmission is divided into current (4-20mA) and voltage (0-5V/0-10V). The biggest difference between the two is their anti-interference strength and their transmission distance. In terms of anti-interference ability, the current signal is stronger than the voltage signal, and in terms of transmission distance, the attenuation of current is much smaller than the voltage signal. Therefore, most instruments now choose RS485 transmission when selecting signals
