的原理有很多，不同的压力变送器，原理就有所不同。 There are many principles of pressure transmitters. Different pressure transmitters have different principles. The above explains the principle of several commonly used pressure transmitter tasks.
Capacitive pressure transmitter principle. Capacitive pressure transmitters are mainly composed of chamber sensitive components that complete pressure / capacitance conversion and electronic circuit boards that convert capacitance to two-wire 4-20mA. When the process pressure is measured from the measuring chamber, After the two sides (or one side) are applied to the isolation diaphragm, it is transferred to the center of gravity diaphragm of the chamber through the silicone oil filling liquid. The center of gravity diaphragm is a diaphragm with a tight edge. Under the action of pressure, the corresponding displacement occurs. This displacement constitutes a change in differential capacitance, and undergoes conditioning, oscillation, and reduction of the electronic circuit board, and is converted into a 4-20mA signal input. The input current is inversely proportional to the process pressure.
The principle of the dispersed silicon pressure transmitter and the pressure of the measured medium indirectly act on the diaphragm of the sensor (stainless steel or ceramic), causing the diaphragm to undergo a micro-displacement that is inversely proportional to the pressure of the medium, and changes the resistance value of the sensor, and This change is detected by an electronic circuit, and a regulated measurement signal corresponding to the pressure is input.
The principle of ceramic pressure transmitter and the use of anti-corrosion pressure transmitter have no liquid transmission. The pressure indirectly acts on the front surface of the ceramic diaphragm, causing huge deformation of the diaphragm. Thick film resistors are printed on the reverse side of the ceramic diaphragm. Connected to a Wheatstone bridge (closed bridge), due to the piezoresistive effect of the varistor, the bridge generates a voltage signal that is highly linear and inversely proportional to the pressure and inversely proportional to the encouraging voltage. The standard signal is based on the pressure gas. Different calibration of the process is 2.0 / 3.0 / 3.3mV / V, etc., which can be compatible with strain sensors. After laser calibration, the sensor has high temperature stability and stability at all times. The sensor has a temperature compensation of 0 to 70 ° C, and can be indirectly in contact with most media. Ceramics are a proven material with high elasticity, corrosion resistance, abrasion resistance, shock resistance, and vibration. The thermal stability of ceramics and its thick film resistance can make its task temperature range up to -40 ~ 135 ° C, and it has high accuracy and high stability of measurement. Electrical insulation level> 2kV, strong input signal, good temporary stability. High-performance, low-cost ceramic sensors will be the development direction of pressure transmitters. In Europe and the United States, there is a trend to replace other types of sensors. In China, more and more players are using ceramic sensors to replace dispersed silicon pressure transmitters. .
The pressure transmitter we often say is actually two kinds of pressure which are divided into the measuring medium and enter the high and low pressure chambers. They act on the isolation diaphragms on both sides of the delta element (that is, the sensitive element). The filling liquid in the sheet and element is transferred to both sides of the measuring diaphragm. The measuring diaphragm and the electrodes on the insulating sheets on both sides constitute a capacitor. When the pressure on the two sides is inconsistent, the measurement diaphragm is displaced. The displacement is proportional to the pressure difference, so the capacitance on both sides is not the same. Through the oscillation and demodulation links, it is converted into a signal proportional to the pressure. 和绝对压力变送器的工作原理和差压变送器相同，所不同的是低压室压力是大气压或真空。 The working principle of pressure transmitters and absolute pressure transmitters is the same as that of differential pressure transmitters, except that the pressure in the low-pressure chamber is atmospheric or vacuum. The A / D converter converts the current of the demodulator into a digital signal, and its value is used by the microprocessor to determine the input pressure value. The microprocessor controls the operation of the transmitter. In addition, it performs sensor linearization. Reset the measurement range. Engineering unit conversion, damping, square root, sensor trimming and other operations, as well as diagnostics and digital communications. This microprocessor has 16 bytes of program RAM and three 16-bit counters, one of which performs A / D conversion. The D / A converter fine-tunes the data from the microprocessor and the corrected digital signal. These data can be modified by the transmitter software. The data is stored in the EEPROM, and it is stored intact even if the power is off. The digital communication line provides the transmitter with a connection interface to external devices (such as a Model 275 intelligent communicator or a control system using the HART protocol). This line detects the digital signal superimposed on the 4-20mA signal and transmits the required information through the loop. The type of communication is frequency shift keying FSK technology and is based on the BeII202 standard.
The pressure sensor's sensitive principle is not only Hall type, capacitive type, metal strain type, piezoresistive type, vibrating cylinder type, etc., but capacitive type, piezoresistive type and metal strain type sensors are still the most common. Metal strain type pressure transmitter is a kind of pressure sensor with a long history, but because of its shortcomings such as hysteresis, creep and poor temperature performance, its application is greatly limited. A piezoresistive sensor is a new type of sensor manufactured by using the semiconductor piezoresistive effect. It has the characteristics of convenient manufacturing and low cost. However, because semiconductor materials are extremely sensitive to temperature, their performance is greatly affected by temperature and product consistency. Worse. Capacitive pressure transmitter is the most widely used type of pressure transmitter, and its principle is very simple. The capacitance of an infinite parallel plate capacitor can be expressed as: C = ε s / d (ε is the dielectric constant of the medium between the parallel plates, d is the distance between the plates, and s is the area covered by the plates) Change one of them Parameters, you can change the capacitance. Due to the simple structure, almost all capacitive pressure transmitters use the method of changing the gap to obtain variable capacitance. 的初始电容值较小，一般为几十皮法，它极易受到导线电容和电路的分布电容的影响，因而必须采用先进的电子线路才能检测出电容的微小变化。 Capacitive pressure transmitters have a small initial capacitance value, which is usually tens of picofarads. It is extremely susceptible to the capacitance of the wire and the distributed capacitance of the circuit. Therefore, advanced electronic circuits must be used to detect small changes in capacitance. It can be said that a good capacitive sensor should be the perfect combination of variable capacitor design and signal processing circuit. It can be said that the pressure transmitter produced by Beijing Outemei Automation Technology Co., Ltd. has done this.