Application selection of pressure transmitter in industry

At present, domestic small compressors generally use low-cost pressure switches for simple pressure control, and its main feature is low cost. There are two kinds of pressure switches, mechanical and electronic. Generally, there is only one or several switch points. For mechanical pressure switches, it is necessary to adjust the pressure switch manually when the pressure switch points need to be adjusted. Most large compressors and screw compressors in China use pressure transmitters, and the imported pressure transmitters equipped with original compressors are generally used.

1. Introduction of pressure transmitter Pressure transmitter, also called pressure transmitter, is an automatic control front-end component which is used to detect the pressure of fluid (actually pressure) and can transmit signals remotely, and the signals are transmitted to secondary instruments or computers for pressure control or monitoring. It has a very wide application in the field of industrial control.

2. Advantages of using pressure transmitter for pressure control. Product application table Industry recommended model features Screw compressor PT110S low cost, high reliability and accuracy 0.5% fs ~ 1% fs large air compressor system PT110S, PT210.

3. Present situation of pressure control technology in domestic compressor industry At present, domestic small compressors generally use low-cost pressure switches for simple pressure control, and its main feature is low cost. There are two kinds of pressure switches, mechanical and electronic. Generally, there is only one or several switch points. For mechanical pressure switches, it is necessary to adjust the pressure switch manually when the pressure switch points need to be adjusted. The performance of pressure transmitter mainly depends on the pressure sensitive element (i.e. pressure sensitive resistor), amplifier circuit, calibration and aging process in production. Multi-point control is realized, and it is convenient to use the pressure transmitter. The detected pressure signal is a continuous signal, so the pressure can be set at will, and it is convenient to set, and it is not allowed to adjust the pressure transmitter. The design of FPGA transmitter only needs to set the electric control part to realize the advanced control of compressor operation, and after using the pressure transmitter, the advanced compressor operation control can be realized, making the compressor run more stably. At the same time, if frequency conversion technology is applied, the number of compressor starts can be greatly reduced, the service life can be prolonged, and it is more energy-saving.

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.

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