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Analog quantity transmission of equipment such as temperature and humidity transmitter in industrial control
The emergence and promotion of industrial control technology has brought about the third industrial revolution, which has improved the safety and accuracy of manufacturing, construction and other industries, and has removed the temperature, pressure, and gas that originally required semi-automated or manual monitoring. Such environmental factors evolve into automation, which not only increases efficiency but also saves costs. Related industrial control equipment includes temperature and humidity transmitters, pressure transmitters, gas transmitters, etc.
Before entering the text, insert a piece of popular science: people have always not distinguished between transmitters and sensors. Because of the similar functions, they often confuse the two. However, transmitters and sensors There are still big differences between.
Transmitter converts the output signal of the sensor into a signal that can be recognized by the controller (or converts the non-electricity input from the sensor into an electrical signal and amplifies the signal source for remote measurement and control) converter. The sensor and transmitter together constitute a monitoring signal source for automatic control. Different physical quantities require different sensors and corresponding transmitters. There are many types of transmitters. The transmitters used in industrial control instruments mainly include temperature transmitters, pressure transmitters, flow transmitters, current transmitters, voltage transmitters, and so on.
In simple terms, a sensor is a device that senses the measured variable (temperature, pressure, gas, etc.) and converts it into a non-standard electrical signal; and the transmitter relies on the sensor to convert this The non-standard electrical signal is transformed into a standard analog quantity and uploaded to the industrial control system. Among them, what is the classification of the analog quantity and how to upload it is the question to be discussed in this article.
First of all, we must know that the transmitter signal and the analog quantity 4-20mA should be two concepts. The transmitter signal is generally an analog quantity signal, and the analog quantity signal can be 4-20mA The current can also be a voltage of 0-5V/0-10V. However, workers prefer to use 4-20mA to transmit analog signals in industrial control situations, and rarely use 0-5V and 0-10V.
Why do you prefer to use 4-20mA to transmit analog quantities in industrial control situations, but rarely use 0-5V or 0-10V?
First of all, electromagnetic interference in factories or construction sites is generally very serious, and voltage signals are more susceptible to interference than current signals. And the transmission distance of the current signal is farther than that of the voltage signal without signal attenuation.
Secondly, the signal current of general instruments is 4-20mA. 4-20mA means that the minimum current is 4mA and the maximum current is 20mA. So why choose 4-20mA instead of 0-20mA? 4mA instead of 0mA is used to detect the open circuit of the circuit. If 0 is the minimum, then the open circuit fault will not 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 the on-off of the 20mA current signal is not enough to ignite the explosion point of the combustible gas. If it exceeds 20mA, there is a danger of explosion.
Finally, when transmitting a signal, consider that there is resistance on the wire. If voltage transmission is used, a certain voltage drop will be generated on the wire, and then a certain error will occur in the signal at the receiving end. Therefore, in industrial control systems, current signal transmission is usually used for long distances less than 100 meters, and for short distance transmission, such as between instruments in the control room, 0-5V voltage signal transmission can be considered.
In today's industrial control system, the transmitter has become an indispensable existence, and choosing a suitable transmitter should not only consider the function of the transmitter, but also consider the transmission mode of the analog quantity. Analog quantity transmission is divided into current (4-20mA) and voltage (0-5V/0-10V). The biggest difference between the two is the anti-interference strength of the two, and the transmission distance between the two. In terms of anti-interference ability, the current signal is stronger than the voltage signal, and in the transmission distance, the attenuation of the current is much smaller than the voltage signal. Therefore, most instruments now choose current transmission when selecting signals.
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