With the advancement of automation technology, in industrial equipment, in addition to liquid column pressure gauges and elastic pressure gauges, pressure transmitters and pressure sensors that can convert pressure into electrical signals are more commonly used. So how do these pressure transmitters and pressure sensors convert pressure signals into electrical signals? What are the characteristics of different conversion methods? Today, Xiaobian compiled a summary of the working principles of several common pressure sensors, and hopes to help everyone.
First, the piezoelectric pressure sensor
The piezoelectric pressure sensor is mainly based on the piezoelectric effect (Piezoelectric effect), which uses electrical components and other machinery to convert the pressure to be measured into electricity, and then performs measurement related precision measurement instruments such as many pressure transmitters and pressure sensors. Piezoelectric sensors should not be used in static measurements because the charge from external forces can be preserved when the loop has an infinite input resistance. But this is not the case. Therefore, piezoelectric sensors can only be used in dynamic measurements. Its main piezoelectric materials are: dihydrogen phosphate, sodium potassium tartrate and quartz. The piezoelectric effect is found on quartz.
When the stress changes, the change in the electric field is very small, and some other piezoelectric crystals will replace the quartz. Sodium potassium tartrate, which has a large piezoelectric coefficient and piezoelectric sensitivity, but it can only be used where the humidity and temperature in the room are relatively low. Dihydrogen phosphate is an artificial crystal that can be used in environments with high humidity and high temperatures, so its application is very extensive. With the development of technology, the piezoelectric effect has also been applied to polycrystals. For example, piezoelectric ceramics, barium magnesium titanate piezoelectric ceramics, tantalate-based piezoelectric ceramics, and barium titanate piezoelectric ceramics are included.
Sensors that operate on the piezoelectric effect are electromechanical conversion and self-generating sensors. Its sensitive component is made of piezoelectric material, and when the piezoelectric material is subjected to an external force, its surface will form a charge, and the charge will be amplified by the charge amplifier, the measuring circuit, and the impedance will be transformed. It is converted into a power output that is proportional to the external force received. It is used to measure force and non-electrical quantities that can be converted into forces, such as:
Acceleration and pressure. It has many advantages: light weight, reliable operation, simple structure, high signal-to-noise ratio, high sensitivity, and wide signal bandwidth. However, it also has some shortcomings: some voltage materials are not wet, so a series of moisture-proof measures are needed, and the response of the output current is relatively poor. Then use a charge amplifier or a high input impedance circuit to make up for this shortcoming. Let the instrument work better.
Second, piezoresistive pressure sensor
The piezoresistive pressure sensor is mainly based on the piezoresistive effect. The piezoresistive effect is used to describe the change in electrical resistance of a material under mechanical stress. Unlike the piezoelectric effect described above, the piezoresistive effect only produces an impedance change and does not generate a charge.
Most metallic materials and semiconductor materials have been found to have a piezoresistive effect. Among them, the piezoresistive effect in semiconductor materials is much larger than that of metals. Since silicon is the mainstay of today's integrated circuits, the use of piezoresistive components made of silicon has become very interesting. The change in resistance is not only due to the stress-dependent geometric deformation, but also from the stress-dependent resistance of the material itself, which makes the degree factor greater than several hundred times that of the metal. The change in the resistance of N-type silicon is mainly due to the redistribution of the carriers between the different band gaps caused by the displacement of the three conduction band valley pairs, which causes the mobility of electrons in different flow directions to change. This is followed by a change in the effective mass associated with a change in the shape of the conduction band valley. In P-type silicon, this phenomenon becomes more complicated and also leads to equivalent mass changes and hole switching.
The piezoresistive pressure sensor is typically connected to the Wheatstone bridge via a lead. Usually, the sensitive core has no external pressure, and the bridge is in equilibrium (called zero position). When the sensor is pressed, the resistance of the chip changes and the bridge will lose its balance. If a constant current or voltage supply is applied to the bridge, the bridge will output a voltage signal corresponding to the pressure, so that the resistance change of the sensor is converted into a pressure signal output through the bridge. The bridge detects the change of the resistance value. After amplification, it is converted into a corresponding current signal through the conversion of the voltage and current. The current signal is compensated by the nonlinear correction loop, that is, the input voltage is linearly corresponding. ~20mA standard output signal.
In order to reduce the influence of temperature change on the core resistance value and improve the measurement accuracy, the pressure sensor adopts temperature compensation measures to maintain its high level of zero drift, sensitivity, linearity and stability.
Third, capacitive pressure sensor
A capacitive pressure sensor is a pressure sensor that uses a capacitor as a sensitive component to convert a measured pressure into a change in capacitance. The pressure sensor generally adopts a circular metal film or a metallized film as an electrode of the capacitor. When the film is deformed by pressure, the capacitance formed between the film and the fixed electrode changes, and the output voltage and voltage can be measured by the measuring circuit. A certain relationship with the electrical signal. Capacitive pressure sensors are pole-to-change capacitive sensors that can be divided into single-capacitor pressure sensors and differential capacitive pressure sensors.
The single-capacitance pressure sensor consists of a circular film and a fixed electrode. The film is deformed under pressure to change the capacity of the capacitor, and its sensitivity is roughly proportional to the area and pressure of the film and inversely proportional to the tension of the film and the distance from the film to the fixed electrode. Another type of fixed electrode adopts a concave spherical shape, and the diaphragm is a peripherally fixed tensioning plane, and the diaphragm can be made by a plastic metal plating layer. This type is suitable for measuring low pressure and has a high overload capacity. It is also possible to use a single-capacitance pressure sensor with a piston moving pole diaphragm to measure high voltage. This type reduces the direct compression area of ​​the diaphragm to increase sensitivity with a thinner diaphragm. It is also packaged with various compensation and protection sections and amplifier circuits to improve immunity to interference. This sensor is suitable for measuring dynamic high pressures and telemetry of aircraft. Single-capacitor pressure sensors are also available in microphone (ie microphone) and stethoscope models.
The pressurized diaphragm electrode of the differential capacitive pressure sensor is located between the two fixed electrodes to form two capacitors. Under the action of pressure, the capacity of one capacitor increases and the other decreases accordingly, and the measurement result is output by the differential circuit. Its fixed electrode is made by plating a metal layer on the concave curved glass surface. When overloaded, the diaphragm is protected by a concave surface without breaking. Differential capacitive pressure sensors have higher sensitivity and better linearity than single-capacitance type, but they are difficult to process (especially difficult to ensure symmetry), and can not achieve isolation of the gas or liquid to be measured, so it is not suitable for working in corrosion. Sexual or impurity fluid.
Fourth, electromagnetic pressure sensor
A variety of sensors using electromagnetic principles, including inductor pressure sensors, Hall pressure sensors, eddy current pressure sensors.
Inductive pressure sensor
The working principle of the inductive pressure sensor is due to the difference in magnetic material and magnetic permeability. When the pressure acts on the diaphragm, the size of the air gap changes. The change of the air gap affects the change of the inductance of the coil. The processing circuit can transform the change of the inductance. Corresponding signal output, so as to achieve the purpose of measuring pressure. The pressure sensor can be divided into two types according to the magnetic circuit change: variable magnetic resistance and variable magnetic permeability. Inductive pressure sensors have the advantages of high sensitivity and large measurement range; the disadvantage is that they cannot be applied to high-frequency dynamic environments.
The main components of the variable reluctance pressure sensor are the iron core and the diaphragm. They form a magnetic circuit with the air gap between them. When there is pressure, the size of the air gap changes, that is, the magnetoresistance changes. If a certain voltage is applied to the core coil, the current will change as the air gap changes, thereby measuring the pressure.
In the case where the magnetic flux density is high, the magnetic permeability of the ferromagnetic material is unstable, and in this case, the variable permeability magnetic pressure sensor can be used for measurement. The variable permeability pressure sensor replaces the core with a movable magnetic element, and the change in pressure causes the movement of the magnetic element, so that the magnetic permeability changes, thereby obtaining a pressure value.
Hall pressure sensor
Hall pressure sensors are based on the Hall effect of certain semiconductor materials. The Hall effect refers to the phenomenon that when a solid conductor is placed in a magnetic field and a current is passed, the charge carriers in the conductor are biased to one side by the Lorentz force, which in turn generates a voltage (Hall voltage). The electric field force caused by the voltage will balance the Lorentz force. By the polarity of the Hall voltage, it can be confirmed that the current inside the conductor is caused by the movement of the negatively charged particles (free electrons).
Applying a magnetic field perpendicular to the direction of the current on the conductor causes the electrons in the wire to be concentrated by the Lorentz force, thereby generating an electric field in the direction in which the electrons are concentrated, which will cause the later electrons to be balanced by the action of electricity. The Lorentz force caused by the magnetic field makes the subsequent electrons pass smoothly without offset, which is called the Hall effect. The built-in voltage generated is called the Hall voltage.
When the magnetic field is an alternating magnetic field, the Hall electromotive force is also an alternating electromotive force of the same frequency, and the time for establishing the Hall electromotive force is extremely short, so the response frequency is high. The material of an ideal Hall element requires higher resistivity and carrier mobility in order to obtain a larger Hall electromotive force. The materials of commonly used Hall elements are mostly semiconductors, including N-type silicon (Si), indium antimonide (InSb), indium arsenide (InAs), germanium (Ge), gallium arsenide GaAs) and multilayer semiconductor structural materials, N The Hall coefficient, temperature stability and linearity of the silicon are good, and the temperature drift of the gallium arsenide is small, which is currently applied.
Eddy current pressure sensor
Pressure sensor based on eddy current effect. The eddy current effect is caused by a moving magnetic field intersecting a metal conductor or by a moving metal conductor perpendicular to the magnetic field. In short, it is caused by the electromagnetic induction effect. This action produces a current that circulates within the conductor.
The eddy current characteristics make the eddy current detection have characteristics such as zero frequency response, so the eddy current pressure sensor can be used for static force detection.
Five, vibrating wire pressure sensor
The vibrating wire pressure sensor is a frequency sensitive sensor. This frequency measurement has a high accuracy, because time and frequency are physical quantity parameters that can be accurately measured, and the frequency signal can ignore the resistance and inductance of the cable during transmission. The influence of factors such as capacitance. At the same time, the vibrating wire pressure sensor also has strong anti-interference ability, small zero drift, good temperature characteristics, simple structure, high resolution, stable performance, convenient data transmission, processing and storage, easy to digitize the instrument, so vibrating the strings Pressure sensors can also be used as one of the directions in the development of sensing technology.
The sensitive component of the vibrating wire pressure sensor is a tensioned steel string, and the natural frequency of the sensitive component is related to the magnitude of the tension. The length of the string is fixed, and the amount of vibration frequency change of the string can be used to measure the magnitude of the tension, that is, the input is a force signal, and the output is a frequency signal. The vibrating wire pressure sensor is composed of two upper and lower parts, and the lower part is mainly a combination of sensitive components. The upper member is an aluminum case that contains an electronic module and a terminal block that is placed in two small chambers so that the sealing of the electronic module chamber is not affected when wiring.
The vibrating wire pressure sensor can be selected from the current output type and the frequency output type. The vibrating wire pressure sensor is in operation, the vibrating wire keeps vibrating at its resonant frequency. When the measured pressure changes, the frequency changes. This frequency signal can be converted into a 4~20mA current signal through the converter.
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