What is a piezoelectric pressure sensor?
The piezoelectric pressure sensor is a sensor that uses the piezoelectric effect of piezoelectric materials to convert the measured pressure into an electrical signal. Use electrical components and other machinery to convert the pressure to be measured into electricity. Measuring precision instrument for related measurement work. Such as pressure transmitters and pressure sensors.
Piezoelectric pressure sensor working principle
Piezoelectric pressure sensors are mainly based on the piezoelectric effect (Piezoelectric effect). Use electrical components and other machinery to convert the pressure to be measured into electricity. Then perform measurement precision instruments such as pressure transmitters and pressure sensors.
Piezoelectric sensors can not be used in static measurement. The reason is the electric charge after the action of external force. When the circuit has infinite input resistance, it can be preserved.
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 electric field changes very little, and some other piezoelectric crystals will replace quartz.
Potassium sodium tartrate, it has a large piezoelectric coefficient and piezoelectric sensitivity. However, it can only be used indoors where the humidity and temperature are relatively low.
Dihydrogen phosphate is a kind of artificial crystal. It can be used in high humidity and high temperature environment. Therefore, its application is very wide.
With the development of technology, the piezoelectric effect has also been applied to polycrystals. For example: piezoelectric ceramics, niobate magnesium acid piezoelectric ceramics, niobate series piezoelectric ceramics and barium titanate piezoelectric ceramics, etc. are included.
The sensors based on the piezoelectric effect are electromechanical conversion and self-generation sensors. Its sensitive components are made of piezoelectric materials.
When the piezoelectric material is subjected to an external force, an electric charge will be formed on its surface. The electric charge will be amplified by the charge amplifier, the measuring circuit and the impedance converted. It will be converted into an electrical output proportional to the external force received.
It is used to measure force and non-electrical physical quantities that can be converted into force. For example: acceleration and pressure.
It has many advantages: lighter weight, reliable work, simple structure, high signal-to-noise ratio, high sensitivity and signal bandwidth, etc.
But it also has some shortcomings: some voltage materials are protected from moisture. Therefore, a series of moisture-proof measures need to be taken. And the output current response is relatively poor. Then it is necessary to use a charge amplifier or a high input impedance circuit to make up for this shortcoming. Make the instrument work better.
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Piezoelectric pressure sensor advantages and disadvantages
- The advantages are bandwidth, high sensitivity, high signal-to-noise ratio, simple structure, reliable work, and lightweight.
- The disadvantage is that some piezoelectric materials need moisture-proof measures. And the output DC response is poor. It is necessary to use a high input impedance circuit or a charge amplifier to overcome this defect.
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The working principle of 5 common pressure sensors
In addition to the Piezoelectric Pressure Sensors described in this article, there are many pressure transmitters and pressure sensors that can convert pressure into electrical signals.
So how do these pressure transmitters and pressure sensors convert pressure signals into electrical signals? What are the characteristics of different conversion methods?
Here, I have summarized the measurement principles of several common pressure sensors for everyone. hope that it can help us.
Piezoresistive pressure sensors are mainly based on the piezoresistive effect (Piezoresistive effect). The piezoresistive effect is used to describe the resistance change of a material under mechanical stress.
Unlike the piezoelectric effect described above, the piezoresistive effect only produces impedance changes, and does not generate electric charges.
Most metal materials and semiconductor materials have been found to have a piezoresistive effect. Among them, the piezoresistive effect in semiconductor materials is much greater than that in metals.
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Because silicon is the mainstay of today’s integrated circuits. The application of piezoresistive elements made of silicon becomes very meaningful.
Piezoresistive pressure sensors are generally connected to Wheatstone bridges through lead wires.
Normally, the sensitive core has no applied pressure, and the bridge is in a balanced state (called zero position). When the sensor is pressed, the chip resistance changes, and the bridge will lose balance.
If a constant current or voltage power supply is added to the bridge. The bridge will output a voltage signal corresponding to the pressure. In this way, the resistance change of the sensor is converted into a pressure signal output by the electric bridge.
The bridge detects the change in resistance value, and after amplification, it undergoes voltage and current conversion. Converted into the corresponding current signal. The current signal is compensated by the nonlinear correction loop. That is, a standard output signal of 4-20mA with a linear corresponding relationship between the input voltage is produced.
In order to reduce the influence of temperature changes on the resistance of the core, and improve the measurement accuracy. The pressure sensor adopts temperature compensation measures to maintain a high level of technical indicators such as zero drift, sensitivity, linearity, and stability.
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The capacitive pressure sensor is a pressure sensor that uses capacitance as a sensitive element to convert the measured pressure into a change in capacitance value.
This kind of pressure sensor generally uses a circular metal film or a metal-plated film as an electrode of the capacitor. When the film deforms under pressure, the capacitance formed between the film and the fixed electrode changes.
The electrical signal that has a certain relationship with the voltage can be output through the measuring circuit.
The capacitive pressure sensor is a capacitive sensor with a variable pole pitch. It can be divided into a single capacitive pressure sensor and a differential capacitive pressure sensor.
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A variety of sensors that use electromagnetic principles are collectively referred to. They mainly include inductive pressure sensors, Hall pressure sensors, and eddy current pressure sensors.
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The working principle of inductive pressure sensors is due to the different magnetic materials and permeability.
When the pressure acts on the diaphragm, the size of the air gap changes, and the change of the air gap affects the change of the inductance of the coil. The processing circuit can convert the change of this inductance into the corresponding signal output. So as to achieve the purpose of measuring pressure.
This kind of pressure sensor can be divided into two types according to the change of the magnetic circuit: variable magnetic resistance and variable magnetic permeability. The advantages of inductive pressure sensors are high sensitivity and large measuring range. The disadvantage is that they cannot be used in high-frequency dynamic environments.
Hall pressure sensors are made based on the Hall effect of certain semiconductor materials.
The Hall effect refers to when a solid conductor is placed in a magnetic field and a current flows through it.
The charge carriers in the conductor are biased to one side by the Lorentz force. Then a voltage (Hall voltage) is generated. The electric field force caused by the voltage will balance the Lorentz force.
Through the polarity of the Hall voltage, it can be confirmed that the current inside the conductor is caused by the movement of negatively charged particles (free electrons).
Eddy current pressure sensor is a pressure sensor based on the eddy current effect.
The eddy current effect is caused by the intersection of a moving magnetic field and a metal conductor, or the intersection of a moving metal conductor and the magnetic field perpendicularly.
In short, it is caused by electromagnetic induction effects. This action creates a current circulating in the conductor.
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Vibrating wire pressure sensor is a frequency sensitive sensor.
This kind of frequency measurement has the desired high accuracy. Because time and frequency are physical parameters that can be accurately measured. And the frequency signal can ignore the influence of cable resistance, inductance, capacitance and other factors during the transmission process.
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. It is convenient for data transmission, processing and storage. It is easy to realize the digitalization of the instrument.
Therefore, vibrating wire pressure sensors can also be used as one of the development directions of sensing technology.
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