Communication protocol of intelligent pressure transmitter

Intelligent pressure transmitter has three forms: voltage output (generally 0-5V), current output (4-20mA) and digital output. The reliability and stability of intelligent transmitter are very high, but communication problems are often encountered. Because there is no unified communication protocol standard in the world when the intelligent transmitter was born, the communication protocols used by the transmitters produced by various manufacturers are different. Typical examples, such as Honeywell's ST3000 uses DE protocol, Fisher-Rosement's 3051 uses HART protocol, and there are all-digital intelligent transmitters that follow the fieldbus communication standard.

(I) ㈠HART protocol

Hart (Highway Addressable Remote Transducer) is a frequency shift keying (FSK) technology based on Bell 202 communication standard, which realizes digital communication by superimposing frequency signals on 4 ~ 20mA current. Two different frequencies: 1200 Hz and 2,200 h, representing "0" and "1", are superimposed on the 4 ~ 20 mA DC signal in the form of sine waves. Because the average value of these sine waves is zero, no DC component is generated, which will not affect the 4 ~ 20 mA process signal. It completes the real synchronous communication without interrupting the transmission signal.

(ii) DE agreement

The DE protocol of intelligent pressure transmitter represents "1" and "0" by the pulse current. Digital signal and analog signal are transmitted separately. When transmitting digital signal, analog signal must be interrupted [1]. The frequency of HART communication is high (about 1 ~ 2 kHz), and the time constant of the network is required to be no more than 0.65μs, so the large zui capacitance of HART communication equipment and network limits the large zui distance of communication, which is generally no more than 400m m..

(iii) Fieldbus communication protocol

IEC (International Electrotechnical Commission) defines Fieldbus as "the digital, serial and multipoint communication data bus between the field devices installed in the manufacturing and process areas and the automatic control devices in the control room is called fieldbus." It is a fully digital, bidirectional, serial and multi-station communication network, and a pair of wires can transmit a variety of information.

Selection method of capacitive pressure transmitter

What is the selection method of capacitive pressure transmitter? It is very important for users to choose a suitable and practical capacitive pressure transmitter, so that the problems in future use will be relatively small, so what kind of selection method can be used for reference? Today, Xiaobian will introduce the selection method of capacitive pressure transmitter for everyone. 　　

1. What kind of pressure should the transmitter measure? First, determine that the zui value of measuring pressure should be recognized in the system. Generally speaking, it is necessary to select a transmitter with a pressure range about 1.5 times larger than the zui value. This is mainly because in many systems, especially in water pressure measurement and processing, there are peaks and continuous irregular fluctuation, which can damage the pressure sensor, and the continuous high pressure value or slightly exceeding the calibrated zui value of the transmitter will shorten the service life of the sensor, but because of this, the accuracy will decrease. Therefore, a buffer can be used to reduce the pressure burr, but this will slow down the response speed of the sensor. Therefore, when selecting the transmitter, we should fully consider the pressure scale, accuracy and stability. 　　

2. What kind of pressure medium: We should think about the medium measured by the capacitive pressure transmitter. Viscous liquid and mud will block the pressure interface, and whether the solvent or corrosive substances will damage the data transmitted by Lv Zhong to these media. These factors will determine whether to select the direct barrier film and the data that directly touches the medium. Usually, some raw materials for the touch medium of capacitive pressure transmitter are 318 stainless steel. If your medium is not corrosive to 318 stainless steel, then basically all capacitive pressure transmitters are suitable for you to measure the pressure of the medium. If your medium is corrosive to 318 stainless steel, then we should choose chemical seal, which can not only measure the pressure of the medium, but also hinder the contact between the medium and the capacitive pressure transmitter, and then maintain the capacitive pressure transmitter and prolong its life.

3. How much accuracy does the transmitter need? The accuracy depends on nonlinearity, hysteresis, non-repeatability of electromechanical business network, temperature, zero offset scale and the influence of temperature. But it is mainly due to nonlinearity, hysteresis and non-repetition. The higher the accuracy, the higher the price. Every kind of electronic measuring instrument will have accuracy errors, but because the accuracy levels of the standards in different countries are different, for example, the accuracy of the standards in China and the United States is better than that of the sensor's linear zui, which is the accuracy between 12% and 92% of the measurement scale; On the other hand, the accuracy of European standard is a little worse than that of linear zui, which is the accuracy between 0 and 10% and 90% and 100% of the measurement inverse. 　　

4. Temperature scale of transmitter: Usually, a transmitter will calibrate two temperature scales, namely, the normal operation temperature scale and the temperature compensation scale. The normal operating temperature scale refers to the temperature scale of the transmitter when it is not damaged in the working state. If it exceeds the temperature compensation scale, it may not reach its performance target. The temperature compensation scale is a typical scale smaller than the operating temperature scale. Working in this scale, the transmitter will definitely reach its due performance target. Temperature change affects its output from two aspects, one is zero drift; Second, it affects the full-scale output. 　　

5. What kind of output signals are needed: digital output of mV, V, mA and frequency. The choice of output depends on many factors, including the interval between transmitter and system controller or display, and whether there are "noise" or other electronic interference signals. Whether the amplifier is required, the orientation of the amplifier, etc. For many OEM devices with short intervals between transmitters and controllers, it is an economical and useful solution to choose the transmitter with mA output. If the output signal needs to be enlarged, the transmitter with built-in amplification is selected for the zui stick. Regarding the transmission or existence of strong electronic interference signals at a long distance, the ZI rod adopts mA-level output or frequency output. If the RFI or EMI target is very high, we should not only pay attention to the selection of mA or frequency output, but also consider special maintenance or filters. 　　

6. What kind of excitation voltage to choose: The type of output signal determines what kind of excitation voltage to choose. Many extended transmitters have built-in voltage conditioning equipment, so their power supply voltage scale is large. Some game transmitters are quantitative equipment and need a stable operating voltage. Therefore, an available operating voltage determines whether sensors with regulators can be selected. When selecting transmitters, the operating voltage and system cost should be considered. 　　

7. The transmitter needs to maintain stability after overtime operation: Most transmitters will drift after passing overtime operation, so it is necessary to know the stability of the transmitter before purchasing. This kind of pre-operation can reduce all kinds of troubles in future operation. 　　

8. Packaging of transmitter: The packaging of transmitter, especially its frame, is often simply neglected, but this will gradually expose its defects in future application. When choosing a transmitter, you must consider the working environment of the transmitter in the future, how the humidity is, how to install the transmitter, and whether there will be fierce collision or oscillation. 

Link-B2B is a professional supplier listing service platform which will provide high quality and trustworthy supplier list to international buyers. If you are looking for pressure transmitter suppliers, feel free to visit Link-B2B.com and send a request.