IntroductionThe main concept of high precision pressure transmitter
The main introduction of the transmitter is divided into the following aspects: the main concepts, the main characteristics (take the two-wire system as an example), classification and comparison, case analysis, the main parameters of the capacitance transmitter, and so on.
What is a transmitter? So first of all, what is a sensor? A sensor is a general term for a device or device that can be measured and converted into a usable output signal in accordance with a certain rule. It usually consists of a sensitive element and a transition element.
When the output of the sensor is a prescribed standard signal, it is called a transmitter. In addition to the sensing function, it also has the function of amplifying and shaping, and the output is a standard control signal. Such as: 4-20mA
Main features (take two-wire system as an example)
The advent of two-wire transmitters has solved this problem. We designed a small, low-cost, two-wire current transmitter based on the DH4-20 transmission module. It features low offset voltage (<30 μV), low voltage drift (<0.7 μV / C °), and ultra-low nonlinearity (<0.01%). It converts the current isolation of the field equipment power line into a 4-20 mA standard current signal output that varies linearly and then sends it to the input interface of the monitoring system through a pair of twisted pairs. The twisted pair will also be located at the same time. The 24V working power of the system is sent to the current transmitter. The measurement signal and power are transmitted on the twisted pair at the same time, which saves the expensive transmission cable, and the signal is transmitted in the form of current, which greatly enhances the anti-interference ability.
The two-wire system refers to the fact that the field transmitter communicates with the control room instruments using only two wires. These two wires are both power and signal wires. Compared with the two-wire system and the three-wire system (one positive power line, two signal lines, one of which is GND) and the four-wire system (two positive and negative power lines, two signal lines, one of which is GND), The advantages of the two-wire system are:
1. It is not susceptible to the influence of parasitic thermocouples and resistance voltage drop and temperature drift along the wire, and can use very cheap and thinner wires; it can save a lot of cable wires and installation costs;
2. When the output resistance of the current source is large enough, the voltage induced by the magnetic field coupling into the wire loop will not have a significant impact, because the current caused by the interference source is very small, generally using twisted pair can reduce the interference; three-wire system Shielded wire must be used with the four-wire system, and the shield of the shielded wire must be properly grounded.
3. Capacitive interference will cause errors related to the receiver resistance. For a 4 to 20 mA two-wire loop, the receiver resistance is usually 250 Ω (sampling Uout = 1 to 5 V). This resistance is small enough to cause a significant error. Allowable cable lengths are longer and longer than voltage telemetry systems;
4. Each single reading device or recording device can be exchanged between different channels with different wire lengths, and the accuracy difference is not caused by the different wire lengths. Decentralized collection is achieved. The advantage of decentralized collection is: Acquisition, centralized control ...
5. Using 4mA for zero level makes it very convenient to judge open circuit and short circuit or sensor damage (0mA state).
6, It is very easy to add one or two lightning protection and surge protection devices to the two-line output port, which is conducive to safety, lightning protection and explosion protection.
Both three-wire and four-wire transmitters do not have the above-mentioned advantages. They will be replaced by two-wire transmitters. You can see from the foreign industry trends and the supply and demand of transmitter chips. It is installed on the power line of the field equipment, and the monitoring system with the single-chip microcomputer as its core is located in the monitoring room farther away from the equipment site. The two are generally tens to hundreds of meters or even farther apart. The environment of the equipment site is relatively harsh. Strong electrical signals will generate various electromagnetic interferences, and lightning induction will generate strong surge pulses. In this case, a difficult problem encountered in single-chip computer application systems is how to distance them in harsh environments. Reliably transmit tiny signals.
Classification and comparison
A device that converts a physical measurement signal or an ordinary electrical signal into a standard electrical signal output or can be output in a communication protocol. Generally divided into: temperature / humidity transmitter, pressure transmitter, differential pressure transmitter, liquid level transmitter, current transmitter, power transmitter, flow transmitter, weight transmitter and so on.
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