Classification and introduction of SICK sensors - pressure sensors

Pressure Sensor Classification:

There are great differences in the skills, design, function and adaptability of pressure sensors, as well as the conditions and price of pressure sensors. According to preliminary estimates, there are more than 60 types of pressure sensors in the world and at least 300 companies produce pressure sensors.

Pressure sensors can be classified by the size of the pressure they can measure, the operating temperature and the type of pressure; The most important of these is the type of pressure. If the classification method according to the type of pressure, pressure sensors can be divided into the following five categories:

(1) Absolute pressure sensor:

This type of pressure sensor measures the true pressure of the fluid, which is relative to the pressure at the vacuum pressure. The definite atmospheric pressure at sea level is 101.325 kPa (14.7?). PSI）。

(2) gauge pressure sensor:

This type of pressure sensor is able to measure the pressure relative to atmospheric pressure at a given position, such as a tire pressure gauge, when the tire pressure gauge shows a reading of 0 PSI, indicating that the pressure inside the tire is equal to the atmospheric pressure, which is 14.7 PSI.

(3) Vacuum pressure sensor:

This type of pressure sensor is used to measure the pressure of less than one atmosphere, and some vacuum pressure sensors in the industry read relative to one atmosphere (the reading value is negative), while some are based on their positive pressure.

(4) Differential pressure gauge:

This instrument is used to measure the pressure difference between two pressures, for example, to measure the pressure difference between the two ends of an oil filter, and the differential pressure gauge is also used to measure the flow rate or to measure the liquid level height in a pressure vessel.

(5) Sealed pressure sensor:

This instrument is similar to a gauge pressure sensor, but it is specially calibrated to measure the pressure relative to sea level.

According to the different structures and principles, it can be divided into: strain gauge, piezoresistive, capacitive, piezoelectric, vibrating pressure sensors, etc. In addition, there are photoelectric, fiber-optic, ultrasonic pressure sensors, etc.

1. Strain gauge pressure sensor

A strain gauge pressure sensor is a type of sensor that directly measures pressure by measuring the strain of various elastic elements. Depending on the manufacturing material, strain gauges can be divided into two categories: metals and semiconductors. The principle of operation of strain gauges is based on the "strain effect" of conductors and semiconductors, i.e., when the conductors and semiconductors are mechanically deformed, their resistance values will change.

When the wire is affected by an external force, its length and cross-sectional area will be changed, and its resistance value will be changed, assuming that the wire is elongated by the effect of external force, its length will increase, and the cross-sectional area will be reduced, and the resistance value will increase. When the wire is tightened by the effect of an external force, the length decreases and the cross-section increases, and the resistance value decreases. The strain of the strain wire can be obtained by measuring the change in the voltage applied to both ends of the resistor.

2. Piezoresistive pressure sensor

Piezoresistive pressure sensors are sensors made using the piezoresistive effect of monocrystalline silicon materials and integrated circuit skills. After the effect of the force on the monocrystalline silicon material, the resistivity changes, and the electrical signal output proportional to the force change can be obtained by measuring the circuit. It is also known as a dispersed silicon piezoresistive pressure sensor, which is different from the mounted strain gauge, which directly senses the external force through the elastic sensitive element, but directly senses the measured pressure through the silicon diaphragm.

Piezoresistive pressure sensors are mainly based on the piezoresistive effect. The piezoresistive effect is used to describe the change in electrical resistance of a data in response to mechanical stress. Unlike the piezoelectric effect, the piezoresistive effect only changes the impedance and does not produce an electric charge.

Most metal and semiconductor materials have been found to have a piezoresistive effect. Among them, the piezoresistive effect in semiconductors is much greater than that of metals. Since silicon is the main raw material for today's integrated circuits, the use of piezoresistive components made of silicon becomes very interesting. The resistance of silicon is changed not only from a few stress-related deformations, but also from the stress-related resistance of the data itself, which makes it hundreds of times more factor than metal. The resistance of N-type silicon is mainly due to the de novo dispersion of carriers with different mobility between the conduction valleys due to the displacement of its three conduction-valley pairs, which then changes the mobility of electrons in different activity directions. The second is due to changes in the effective mass associated with changes in the shape of the band valleys. In P-type silicon, this phenomenon becomes more complex and also leads to equivalent mass changes and electrocavitation.

Piezoresistive pressure sensors are typically fed into Wheatstone bridges via leads. Normally, the sensitive core has no external pressure effect, 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 the resistance value, and after expansion, it is transformed into the corresponding current signal through the conversion of voltage and current, and the current signal is compensated by the nonlinear calibration loop, that is, the standard output signal of 4-20mA with the linear correspondence of the input voltage occurs.

In order to reduce the influence of temperature change on the resistance value of the core and improve the measurement accuracy, the temperature compensation method is used to make the zero drift, sensitivity, linearity, stability and other technical indicators of the pressure sensor adhere to a high level.

3. Capacitive pressure sensor

A capacitive pressure sensor is a pressure sensor that uses a capacitor as a sensitive element to convert the measured pressure into a change in capacitance. This kind of pressure sensor generally uses a round metal film or a metallized film as an electrode of the capacitor, when the film is deformed by the pressure, the capacitance formed between the film and the fixed electrode is changed, and the electrical signal that is bound to be connected with the voltage can be output through the measurement circuit. Capacitive pressure sensors are classified as capacitive sensors with pole distance modification, 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 film is deformed under the effect of pressure, and then the capacitance of the capacitor is changed, 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 separation from the film to the fixed electrode. The other type of fixed electrode is concave spherical, 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 pressure and has high overload ability. It is also possible to use a single capacitive pressure sensor with a piston moving diaphragm to measure high voltage. This type reduces the direct pressure area of the diaphragm, allowing for the use of thinner diaphragms to improve sensitivity. It is also encapsulated with various compensation and maintenance parts and enlarged circuits to improve anti-interference capabilities. This sensor is suitable for measuring dynamic high pressure 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 effect of pressure, the capacitance of one capacitor increases and the other decreases accordingly, and the measurement result is output by the 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 maintained by the concave surface without 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 the barrier of the gas or liquid to be measured, so they are not suitable for operation in corrosive or impurity fluids.

4. Piezoelectric pressure sensor

Piezoelectric pressure sensors are mainly based on the piezoelectric effect, using electrical components and other machinery to convert the pressure to be measured into electricity, and then carry out related measurement operations of precision instruments, such as many pressure transmitters and pressure sensors.

Piezoelectric sensors cannot be used in static measurements, because the charge after being subjected to the effect of an external force can only be preserved when the loop has an infinite input resistance. But that's not the case. Therefore, piezoelectric sensors can only be used for dynamic measurements. Its main piezoelectric materials are: dihydroamine phosphate, potassium sodium tartrate and quartz. The piezoelectric effect is found in quartz.

When the stress changes, the change in the electric field is very small, and some other piezoelectric crystals replace quartz. Potassium sodium tartrate, it has a large piezoelectric coefficient and piezoelectric sensitivity, but it can only be used in places where the humidity and temperature are relatively low indoors. Dihydroamine phosphate is a man-made crystal, which can be used in environments with high humidity and high temperatures, so its use is very widespread. With the development of skills, the piezoelectric effect has also been used in polycrystals. For example: piezoelectric ceramics, niobium-magnesium acid piezoelectric ceramics, niobate piezoelectric ceramics and barium titanate piezoelectric ceramics are included.

The piezoelectric effect can be divided into: positive piezoelectric effect and inverse piezoelectric effect.

The positive piezoelectric effect refers to the fact that when the crystal is subjected to the effect of an external force in a fixed direction, the internal polarization phenomenon occurs, and a charge with opposite signs occurs on two surfaces together; When the external force is withdrawn, the crystal recovers to an uncharged state; When the direction of the effect of the external force changes, the polarity of the charge also changes; The amount of charge generated by the force applied to the crystal is directly proportional to the magnitude of the external force. Piezoelectric sensors are mostly made using the positive piezoelectric effect.

The inverse piezoelectric effect refers to the phenomenon of mechanical deformation of the crystal caused by the application of an alternating electric field to the crystal, also known as the electroelastic effect. Transmitters made with the inverse piezoelectric effect can be used in electroacoustic and ultrasonic engineering. There are five basic forms of force deformation of piezoelectric sensitive elements: thickness deformation type, length deformation type, volume deformation type, thickness shear type, and plane shear type. Piezoelectric crystals are anisotropic, and not all crystals can undergo piezoelectric effects in these 5 states. Quartz crystals, for example, do not have volumetric deformation piezoelectric effects, but have outstanding thickness deformation and length deformation piezoelectric effects.

5. Inductive pressure sensor

Electromagnetic pressure sensor is a general term for a variety of sensors that use electromagnetic principles, mainly including inductive pressure sensors, Hall pressure sensors, eddy current pressure sensors, etc.

The operating principle of the inductive pressure sensor is because the magnetic data and permeability are different, when the pressure effect is applied to 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, and then achieve the purpose of measuring the pressure. This kind of pressure sensor can be divided into two types according to the magnetic circuit change: variable reluctance and variable permeability. The advantages of inductive pressure sensors are high sensitivity and large measurement scale; 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 a pressure effect, 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 according to the change of the air gap, and then the pressure will be measured.

In the case of high magnetic flux density, the permeability of ferromagnetic data is unstable, in this case, a variable permeability pressure sensor can be used to measure. Variable permeability pressure sensors replace the iron core with a movable magnetic element, and the change in pressure causes the magnetic element to move, and then the permeability changes, from which the pressure value is derived.

6. Hall pressure sensor

Hall pressure sensors are based on the Hall effect of certain semiconductor sources. The Hall effect is a phenomenon in which a charge carrier in a solid conductor is placed in a magnetic field and there is a period when an electric current passes through it, and the charge carriers in the conductor are tilted to one side by the Lorentz force, and then a voltage (Hall voltage) occurs. 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 formed 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, and then an electric field will occur in the direction of the electron aggregation, which will make the later electrons be affected by the electric effect and balance the Lorentz force formed by the magnetic field, so that the later electrons can pass smoothly without offset, which is called the Hall effect. The built-in voltage that occurs 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 to establish the Hall EMF is very short, so its response frequency is high. The data for the aspiring Hall element requires a high resistivity and carrier mobility in order to obtain a large Hall EMF. Most of the commonly used Hall element data are semiconductors, including N-type silicon (Si), indium antimonide (InSb), indium arsenide InAs), germanium (Ge), gallium arsenide GaAs) and multilayer semiconductor structure data.

7. Eddy current pressure sensor

Pressure sensor according to the eddy current effect. The eddy current effect occurs when a moving magnetic field intersects a metallic conductor, or when a moving metallic conductor crosses perpendicular to a magnetic field. In short, it is caused by electromagnetic induction. This action takes place with an electric current circulating in the conductor.

The eddy current characteristics make the eddy current test have characteristics such as zero frequency response, so the eddy current pressure sensor can be used for the detection of static forces.

8. Vibrating wire pressure sensor

Vibrating wire pressure sensors are classified as frequency-sensitive sensors, and this frequency measurement has the highest 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 during transmission. Together, the vibrating wire pressure sensor also has a strong anti-interference ability, small zero drift, good temperature characteristics, simple structure, high resolution, stable function, convenient data transmission, processing and storage, and simple realization of instrument digitization, so the vibrating wire pressure sensor can also be used as one of the directions of sensing skills.

The sensitive element of a vibrating wire pressure sensor is a tensioned steel string, and the natural frequency of the sensitive element is related to the size of the tensioning force. The length of the string is fixed, and the oscillation frequency of the string can be used to measure the size 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 can be selected between current output type and frequency output type. The vibrating wire pressure sensor is in operation, the vibrating wire is constantly oscillating with its resonant frequency, when the measured pressure is changed, the frequency will change, and this frequency signal can be converted into a current signal of 4-20mA through the converter.

Classification and introduction of SICK sensors - pressure sensors

Classification and introduction of SICK sensors - pressure sensors

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