Sick Sensors – A detailed introduction to the principles of the five common pressure sensors

1. Piezoelectric pressure sensor

Based on the piezoelectric effect, electrical components and other machinery are used to convert the pressure to be measured into electricity, and then perform related measurement work. Piezoelectric sensors can only be used for dynamic measurements. The main piezoelectric materials are: dihydroamine phosphate, potassium sodium tartrate and quartz. With the development of technology, the piezoelectric effect has also been applied to polycrystals. For example: piezoelectric ceramics, niobium-magnesium acid piezoelectric ceramics, niobate 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 elements are made of piezoelectric materials. When the piezoelectric material is subjected to an external force, an electric charge is formed on the surface, and the charge is converted into an electric output proportional to the external force after the charge amplifier, the measurement circuit amplification, and the transformation of the impedance. It is used to measure force and non-electrophysical quantities that can be converted into force, such as acceleration and pressure.

The advantages are: light weight, reliable operation, simple structure, high signal-to-noise ratio, high sensitivity and wide signal bandwidth.

The disadvantages are: some voltage materials are not wet, so a series of moisture-proof measures need to be taken; However, the output current response is relatively poor, so it is necessary to use a charge amplifier or a high input impedance circuit to compensate for this shortcoming.

2. Piezoresistive pressure sensor

The piezoresistive effect is used to describe the change in electrical resistance of a material when subjected to mechanical stress. Unlike the piezoelectric effect, the piezoresistive effect only produces a change in impedance and does not produce an electric charge. Most metallic and semiconductor materials have been found to have a piezoresistive effect. Since silicon is the main material for integrated circuits today, the application of piezoresistive components made of silicon becomes very interesting. Resistance variation is not only due to stress-related geometric deformation, but also from the stress-related resistance of the material itself, which makes it hundreds of times more factor than metal.

Piezoresistive pressure sensors are typically fed into Wheatstone bridges via leads. Normally, the sensitive core has no external pressure, and the bridge is in equilibrium (called zero), and when the chip resistance changes after the sensor is pressurized, 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, so that the resistance change of the sensor is converted into a pressure signal output through the bridge. The bridge detects the change of resistance value, after amplification, and then converts the voltage and current into the corresponding current signal, and the current signal is compensated by the nonlinear correction loop, that is, the 4-20mA standard output signal that corresponds linearly to the input voltage is generated.

In order to reduce the influence of temperature change on the resistance value of the core and improve the measurement accuracy, the pressure sensor adopts temperature compensation measures to keep its zero drift, sensitivity, linearity, stability and other technical indicators at a high level.

3. Capacitive pressure sensor

Using a capacitor as a sensitive element, the measured pressure is converted into a pressure sensor with a changing capacitance value. This kind of pressure sensor generally uses a round metal film or a metalized film as an electrode of the capacitor, and when the film is deformed by the pressure, the capacitance formed between the film and the fixed electrode changes, and the electrical signal with a certain relationship with the voltage can be output through the measurement circuit. Capacitive pressure sensors belong to the capacitive sensors with pole distance variation, which can be divided into single capacitive pressure sensors and differential capacitive pressure sensors.

A single capacitive pressure sensor consists of a round thin film with fixed electrodes. The sensitivity of the film 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 fixed electrode. The other type of fixed electrode is concave spherical, and the diaphragm is a tension plane fixed at the periphery, and the diaphragm can be made by the method of plastic metal plating. This type is suitable for measuring low pressures and has a high overload capacity. Single-capacitive pressure sensors with piston moving diaphragms can also be used to measure high pressures. This type reduces the direct pressure area of the diaphragm to allow for a thinner diaphragm to increase sensitivity. It is also packaged in a monolithic package with various compensation and protection parts and amplification circuits to improve immunity to interference. This sensor is suitable for measuring dynamic high pressures and telemetry of aircraft. Single-capacitive pressure sensors are also available in microphone type (i.e., microphone type) and stethoscope type.

The pressure diaphragm electrode of a differential capacitive pressure sensor is located between two fixed electrodes and forms two capacitors. Under the action of pressure, the capacitance of the capacitor increases and the other decreases correspondingly, and the measurement result is output by a differential circuit. Its fixed electrodes are made by plating a metal layer on a concave glass surface. In the event of overload, the diaphragm is protected by a concave surface from breaking. Differential capacitive pressure sensors have higher sensitivity and better linearity than single capacitive types, but they are difficult to process (especially difficult to ensure symmetry), and cannot achieve isolation of the gas or liquid to be measured, so they are not suitable for working in corrosive or impurity fluids.

4. Electromagnetic pressure sensor

Sensors that use electromagnetic principles are collectively referred to as electromagnetic pressure sensors, mainly including inductive pressure sensors, Hall pressure sensors, eddy current pressure sensing, etc.

(1) Inductive pressure sensor

The working principle of the inductive pressure sensor is that due to the difference between the magnetic material and the 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 coil inductance, and the processing circuit can convert the change of the inductance into the corresponding signal output, so as to achieve the purpose of measuring the pressure. This kind of pressure sensor can be divided into two types according to the change of magnetic circuit: variable reluctance and variable permeability. The advantages of inductive pressure sensors are their high sensitivity and large measuring range; The disadvantage is that it cannot be used in high-frequency dynamic environments.

The main components of a variable reluctance pressure sensor are an iron core and diaphragm. The air gap between them forms a magnetic circuit. When there is pressure, the size of the air gap changes, i.e., the magnetic resistance changes. If a certain voltage is applied to the core coil, the current will change with the change of the air gap, and the pressure will be measured.

In the case of high magnetic flux density, the permeability of ferromagnetic materials is unstable, in which case variable permeability pressure sensors can be used. Variable permeability pressure sensors replace the iron core with a movable magnetic element, and the change in pressure causes the magnetic element to move, resulting in a change in the permeability and thus a pressure value.

(2) Hall pressure sensor

Hall pressure sensors are made based on the Hall effect of certain semiconductor materials. The Hall effect is a phenomenon in which a charge carrier in a solid conductor is placed in a magnetic field and an electric current is passed through it, and the charge carriers in the conductor are deflected to one side by the Lorentz force, resulting in a voltage (Hall voltage). The electric field force caused by the voltage balances the Lorentz force. The polarity of the Hall voltage proves that the current inside the conductor is caused by the movement of negatively charged particles (free electrons).

The magnetic field perpendicular to the direction of the current will cause the electrons in the wire to be gathered by the Lorentz force, thus generating an electric field in the direction of the electron aggregation, which will make the later electrons be affected by electricity and balance the Lorentz force caused by the magnetic field, so that the later electrons can 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 EMF is also an alternating EMF of the same frequency, and the time to establish the Hall EMF is very short, so its response frequency is high. Most of the commonly used materials for Hall elements are semiconductors, including N-type silicon (Si), indium antimonide (InSb), indium arsenide InAs, germanium (Ge), gallium arsenide GaAs) and multilayer semiconductor-structured materials.

(3) Eddy-current pressure sensor

Based on the eddy current effect, it is generated by a moving magnetic field intersecting a metallic conductor, or by a moving metallic conductor perpendicular to a magnetic field. In short, it is caused by electromagnetic induction. This action creates an electric current circulating through the conductor. The eddy-current characteristics give eddy-current testing characteristics such as zero-frequency response, so eddy-current pressure sensors can be used for static force testing.

5. Vibrating wire pressure sensor

Vibrating wire pressure sensor is a frequency-sensitive sensor, this kind of 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 influence of cable resistance, inductance, capacitance and other factors in 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, convenient data transmission, processing and storage, easy to realize the digitization of the instrument, so the vibrating wire pressure sensor can also be used as one of the directions of the development of sensing technology.

The sensing element of a vibrating wire pressure sensor is a tensile steel string, and the natural frequency of the sensing element is related to the tensioning force. The length of the string is fixed, and the vibration frequency of the string can be used to measure the magnitude of the tensile force, that is, the input is the force signal, and the output is the frequency signal. The vibrating wire pressure sensor is divided into two parts, and the lower component is mainly a combination of sensitive elements. The superstructure is an aluminum shell, which contains an electronic module and a terminal block, which is divided into two chambers and placed so that the sealing of the electronic module chamber is not affected when wiring.

Vibrating wire pressure sensors are available in both current output and frequency output types. The vibrating wire pressure sensor is in operation, the vibrating wire vibrates continuously with its resonant frequency, and when the measured pressure changes, the frequency will change, and this frequency signal is converted into a current signal of 4-20mA by the converter.

Sick Sensors – A detailed introduction to the principles of the five common pressure sensors

Sick Sensors – A detailed introduction to the principles of the five common pressure sensors

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