Sensor is a kind of detection device that can sense the measured information, and can convert the sensed information into electrical signals or other required forms of information output according to certain rules to meet the requirements of information transmission, processing, storage, display, recording and control.
This is the first step to realize automatic detection and automatic control.
A "sensor" is defined as:
A device that receives power from one system and usually sends it to another system in another form.
According to this definition, the function of a sensor is to convert one kind of energy into another, so many scholars also use "transducer-transducer" to refer to "sensor-sensor".
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function
The functions of sensors are often compared with the five sensory organs of human beings:
Photosensitive sensor-visual sound sensitive sensor-hearing
Gas sensor-olfactory chemical sensor-taste
Pressure-sensitive, temperature-sensitive and fluid sensors-touch.
Classification of sensitive parts:
① Physics, based on physical effects such as force, heat, light, electricity, magnetism and harmonic.
② Chemistry, based on the principle of chemical reaction.
(3) Biology, based on the molecular recognition functions of enzymes, antibodies and hormones.
Generally, according to its basic sensing function, it can be divided into ten categories, such as heat sensitive element, light sensitive element, gas sensitive element, force sensitive element, magnetic sensitive element, humidity sensitive element, sound sensitive element, radiation sensitive element, color sensitive element and taste sensitive element (some people once divided sensitive elements into 46 categories).
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classify
Sensors can be classified from different angles: their conversion principle (the basic physical or chemical effect of sensor work); Their uses; Their output signal types, materials and processes for manufacturing them, etc.
According to the working principle of sensors, they can be divided into physical sensors and chemical sensors:
Classification of sensor working principle Physical sensors apply physical effects, such as piezoelectric effect, magnetostrictive phenomenon, ionization, polarization, thermoelectric, photoelectric, magnetoelectric and other effects.
Small changes in the measured signal amount will be converted into electrical signals.
Chemical sensors include sensors with chemical adsorption, electrochemical reaction and other phenomena as causal relations, and small changes in the measured signal amount will also be converted into electrical signals.
Some sensors can neither be classified as physical sensors nor as chemical sensors.
Most sensors work according to physical principles.
Chemical sensors have many technical problems, such as reliability, possibility of mass production, price and so on. After solving these problems, the application of chemical sensors will greatly increase.
The following table lists the application fields and working principles of common sensors.
1. Sensors are classified according to their uses.
Pressure and force sensor, position sensor.
Liquid level sensor energy consumption sensor
Speed sensor acceleration sensor
X-ray radiation sensor thermal sensor
24GHz radar sensor
2. Sensors are classified according to their principles.
Vibration sensor humidity sensor
Magnetic sensor gas sensor
Vacuum sensor, biosensor, etc.
3. Sensors are classified according to their output signals.
Analog sensor-converts measured non-electric quantity into analog electric signal.
Digital sensor-converts the measured non-electric quantity into digital output signal (including direct and indirect conversion).
Pseudo-digital sensor-converts the measured signal into frequency signal or short-period signal for output (including direct or indirect conversion).
Switch sensor-When the measured signal reaches a certain threshold, the sensor outputs a set low-level or high-level signal accordingly.
4. Sensors are classified according to their materials.
Under the influence of external factors, all substances will make corresponding and characteristic reactions.
Among them, those materials that are most sensitive to external effects, that is, materials with functional characteristics, are used to make sensitive elements of sensors.
From the perspective of application materials, sensors can be divided into the following categories:
(1) According to the type of materials used.
Metal polymer ceramic mixture
(2) According to the physical properties of the material: conductor insulator semiconductor magnetic material.
(3) According to the crystal structure of the material:
Single crystal polycrystalline amorphous material
The development of sensors closely related to the adoption of new materials can be summarized into the following three directions:
(1) Explore new phenomena, effects and reactions in known materials, and then apply them in sensor technology.
(2) Explore new materials and apply those known phenomena, effects and reactions to improve sensor technology.
(3) Explore new phenomena, new effects and new reactions on the basis of studying new materials, and realize them in sensor technology.
The progress of modern sensor manufacturing industry depends on the development intensity of new materials and sensitive components used in sensor technology.
The basic trend of sensor development is closely related to the application of semiconductors and dielectric materials.
Table 1.2 gives some materials that can be used in sensor technology and can convert energy forms.
5. Sensors are classified according to their manufacturing process.
Integrated sensor thin film sensor thick film sensor ceramic sensor
Integrated sensors are manufactured by standard technology for producing silicon-based semiconductor integrated circuits.
Usually, some circuits used for preliminary processing of measurement signals are also integrated on the same chip.
The thin film sensor is formed by a thin film of a corresponding sensitive material deposited on a dielectric substrate (substrate).
When a hybrid process is used, a part of the circuit can also be fabricated on the substrate.
Thick-film sensor is made by coating the slurry of the corresponding material on a ceramic substrate usually made of Al2O3, and then heat-treating it to form a thick film.
Ceramic sensors are produced by standard ceramic technology or some variant technologies (sol-gel, etc.). ).
After the proper preparation operation is completed, the molded parts are sintered at high temperature.
There are many similarities between thick film technology and ceramic sensor technology. In some aspects, thick film technology can be considered as a variant of ceramic technology.
Each technology has its own advantages and disadvantages.
Due to the low investment in R&D and production and the high stability of sensor parameters, it is more reasonable to use ceramic and thick film sensors.
(Provided by HVAC experts of air travel network)
6. Sensors are classified according to the purpose of measurement.
The physical sensor is made by using the characteristics that some physical properties of the measured substance have changed obviously.
Chemical sensors are composed of sensitive elements, which can convert chemical quantities such as composition and concentration of chemical substances into electrical quantities.
Biosensor is a sensor that uses the characteristics of various organisms or biological substances to detect and identify chemical components in organisms.
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characteristic
Static characteristics of sensor
The static characteristic of sensor refers to the relationship between output and input of sensor for static input signal.
Because the input and output have nothing to do with time, the relationship between them, that is, the static characteristics of the sensor can be described by an algebraic equation without time variables, or by a characteristic curve drawn with the input as the abscissa and the corresponding output as the ordinate.
The main parameters that characterize the static characteristics of the sensor are linearity, sensitivity, hysteresis, repeatability and drift.
(1) Linearity: refers to the degree to which the actual relationship curve between sensor output and input deviates from the fitting straight line.
It is defined as the ratio of the maximum deviation between the actual characteristic curve and the fitted straight line to the full-scale output value within the full-scale range.
(2) Sensitivity: Sensitivity is an important indicator of the static characteristics of the sensor.
It is defined as the ratio of the output increment to the corresponding input increment that causes the increment.
Use s to indicate sensitivity.
(3) Hysteresis: In the process of changing the input from small to large (positive stroke) and the input from large to small (reverse stroke), the phenomenon that the input-output characteristic curves of the sensor do not coincide is called hysteresis.
For the same input signal, the output signals of the sensor before and after the stroke are not equal, and this difference is called hysteresis difference.
(4) Repeatability: Repeatability refers to the degree to which the obtained characteristic curves are inconsistent when the input of the sensor changes in the same direction for many times.
(5) Drift: The drift of the sensor means that the output of the sensor changes with time when the input is unchanged, which is called drift.
There are two reasons for the drift: one is the structural parameters of the sensor itself; The second is the surrounding environment (such as temperature and humidity, etc.). ).
Sensor dynamic characteristics
The so-called dynamic characteristics refer to the characteristics of the output of the sensor when its input changes.
In practical work, the dynamic characteristics of the sensor are often expressed by its response to some standard input signals.
This is because the response of the sensor to the standard input signal can be easily obtained by experiments, and there is a certain relationship between its response to the standard input signal and its response to any input signal, and the latter can often be inferred by knowing the former.
The most commonly used standard input signals are step signal and sine signal, so the dynamic characteristics of the sensor are also commonly expressed by step response and frequency response.
Linearity of sensor
Usually, the actual static characteristic output of the sensor is a curve rather than a straight line.
In practical work, in order to make the instrument have a unified calibration reading, a fitting straight line is often used to approximate the actual characteristic curve, and linearity (nonlinear error) is a performance index of this approximation.
There are many ways to choose a fitting straight line.
For example, the theoretical straight line connecting zero input and full-scale output points is used as a fitting straight line; Or the theoretical straight line with the smallest sum of squares of deviations of each point on the characteristic curve is regarded as the fitting straight line, which is called the least square fitting straight line.
The following are schematic diagrams of several fitting methods.
Theoretical fitting, zero-crossing rotation fitting and end-point connection fitting
Sensitivity of sensor
Sensitivity refers to the ratio of output change △y to input change △x of the sensor under steady-state working conditions.
It is the slope of the output-input characteristic curve.
If there is a linear relationship between the output and the input of the sensor, the sensitivity S is constant.
Otherwise it will change with the change of input.
The dimension of sensitivity is the ratio of the dimensions of output and input.
For example, when the displacement of the displacement sensor changes 1mm and the output voltage changes by 200mV, its sensitivity should be expressed as 200 mv/mm.
When the output and input of the sensor are the same size, the sensitivity can be understood as the magnification.
Improve the sensitivity and obtain higher measurement accuracy.
But the higher the sensitivity, the narrower the measuring range and the worse the stability.
Resolution of sensor
Resolution refers to the sensor's ability to feel the smallest change being measured.
That is, if the input quantity changes slowly from a non-zero value.
When the input change value does not exceed a certain value, the output of the sensor will not change, that is, the sensor cannot distinguish the change of this input.
Only when the input changes beyond the resolution will its output change.
Usually, the resolution of each point of the sensor is different in the full-scale range, so the maximum change value of the input quantity that can make the output change step by step in the full-scale range is often used as the index to measure the resolution.
If the above indicators are expressed as a percentage of full scale, it is called resolution.
The resolution is negatively correlated with the stability of the sensor.
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24GHz radar sensor
The 24GHz radar sensor senses objects by transmitting and receiving microwaves with a frequency of about 24. 125GHz.
24GHZ radar sensor
Existence, measuring the moving speed, static distance, angle, etc. Using planar microstrip technology, the volume is small.
High integration, sensitive sensing and no contact.
24GHz radar sensor is a kind of loading and unloading device that can convert microwave echo signals into electrical signals, and it is the core chip of radar speedometer, water level gauge, automobile ACC auxiliary cruise system, automatic door sensor and so on.
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resistance type transducer
Resistance sensor is a device that converts the measured physical quantities such as displacement, deformation, force, acceleration, humidity and temperature into resistance values.
There are mainly resistance strain, piezoresistance, thermal resistance, thermal sensitivity, gas sensitivity, humidity sensitivity and other resistance sensing devices.
weighing cell
The weighing sensor is a electromechanical conversion device that converts gravity into electrical signals, and it is a key component of electronic weighing instrument.
There are many kinds of sensors to realize electromechanical conversion, such as resistance strain type, electromagnetic force type and capacitance type.
Electromagnetic force type is mainly used in electronic balance, capacitance type is used in some electronic hanging scales, and most weighing instruments use resistance strain type weighing sensors.
The invention has the advantages of simple structure, high precision and wide application, and can be used in relatively harsh environment.
Therefore, the resistance strain gauge load cell has been widely used in weighing instruments.
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Strain sensor
The resistance strain gauge in the sensor has the strain effect of metal, that is, it produces mechanical deformation under the action of external force, which makes the resistance value change accordingly.
There are two kinds of resistance strain gage: metal and semiconductor. Metal strain gauges are divided into wire type, foil type and membrane type.
Semiconductor strain gauges have the advantages of high sensitivity (usually dozens of times that of wire and foil) and small lateral effect.
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Piezoresistive sensor
Piezoresistive sensor is a device that makes diffusion resistance on semiconductor substrate according to the piezoresistive effect of semiconductor material.
Its substrate can be directly used as a measuring sensor, and the diffusion resistor is connected in the substrate in the form of a bridge.
When the substrate is deformed by external force, the resistance value will change and the bridge will produce corresponding unbalanced output.
Substrates (or diaphragms) used as piezoresistive sensors are mainly silicon wafers and germanium wafers. Silicon piezoresistive sensor with silicon wafer as sensitive material has attracted more and more attention, especially the solid-state piezoresistive sensor used to measure pressure and speed is the most widely used.
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Thermal resistance sensor
Thermal resistance temperature measurement is based on the characteristic that the resistance value of metal conductor increases with the increase of temperature.
Thermal resistors are mostly made of pure metal materials. At present, platinum and copper are the most widely used materials. In addition, materials such as nickel, manganese and rhodium have been used to manufacture thermal resistors.
Thermal resistance sensor mainly uses the characteristics of resistance value changing with temperature to measure temperature and temperature-related parameters.
This sensor is suitable for occasions with high temperature detection accuracy.
At present, platinum, copper, nickel and other thermal resistance materials widely used have the characteristics of large temperature coefficient of resistance, good linearity, stable performance, wide temperature range and easy processing.
Used to measure the temperature in the range of -200℃ ~+500℃.
Classification of thermal resistance sensors:
1.NTC thermal resistance sensor:
This sensor is a negative temperature coefficient sensor, that is, the resistance of the sensor decreases with the increase of temperature.
2.PTC thermal resistance sensor:
This sensor is a positive temperature coefficient sensor, that is, the resistance of the sensor increases with the increase of temperature.
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Laser sensor
A sensor that uses laser technology to measure.
It consists of a laser, a laser detector and a measuring circuit.
Laser sensor is a new type of measuring instrument, which has the advantages of non-contact long-distance measurement, high speed, high precision, large measuring range and strong anti-photoelectric interference ability.
When the laser sensor works, the laser emitting diode is aimed at the target to emit laser pulses.
The laser is reflected by the target and scattered in all directions.
Part of the scattered light returns to the sensor receiver and is imaged on the avalanche photodiode after being received by the optical system.
Avalanche photodiode is an optical sensor with internal amplification function, so it can detect extremely weak optical signals and convert them into corresponding electrical signals.
Using the characteristics of high directivity, high monochromaticity and high brightness of laser, non-contact long-distance measurement can be realized.
Laser sensors are often used to measure physical quantities such as length (ZLS-Px), distance (LDM4x), vibration (ZLDS 10X), speed (LDM30x) and orientation, and can also be used for flaw detection and monitoring of air pollutants.
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temperature sensor
1. Room temperature tube temperature sensor:
The room temperature sensor is used to measure the indoor and outdoor ambient temperature, and the tube temperature sensor is used to measure the tube wall temperature of evaporator and condenser.
The room temperature sensor and the tube temperature sensor have different shapes, but the temperature characteristics are basically the same.
According to the temperature characteristics, there are two kinds of room temperature tube temperature sensors currently used in the United States: 1. The value of constant b is 4100k 3%, and the reference resistance is10k ω 3% at 25℃.
The higher the temperature, the smaller the resistance; The lower the temperature, the greater the resistance.
The farther away from 25℃, the greater the tolerance range of the corresponding resistance; The corresponding resistance tolerance is about 7% at 0℃ and 55℃. However, the resistance tolerance of different suppliers will be different below 0℃ and above 55℃.
The higher the temperature, the smaller the resistance; The lower the temperature, the greater the resistance.
The farther away from 25℃, the greater the tolerance range of the corresponding resistance.
2. Exhaust temperature sensor:
The exhaust temperature sensor is used to measure the exhaust temperature at the top of the compressor. The value of constant b is 3950k 3%, and the reference resistance is 90℃, corresponding to a resistance of 5k Ω 3%.
3. Module temperature sensor: The module temperature sensor is used to measure the temperature of frequency conversion module (IGBT or IPM). At present, the model of the temperature sensor used is 602F-3500F, and the reference resistance at 25℃ is 6kΩ1%.
The resistance values corresponding to several typical temperatures are-10 ℃→ (25.897-28.623) kω; 0 ℃→( 16.3248─ 17.7 164)kω； 50 ℃→( 2.3262─2.5 153)kω； 90 ℃→( 0.667 1─0.7565)Kω.
There are many kinds of temperature sensors, including thermal resistance: PT 100, PT 1000, Cu50, Cu100; ; Thermocouple: B, E, J, K, S, etc.
There are not only many kinds of temperature sensors, but also various combinations. Choose the right products according to different places.
Principle of temperature measurement: According to the principle that the resistance value of resistor and the potential of thermocouple change regularly with different temperatures, we can get the temperature value we need to measure.
(Provided by HVAC experts of air travel network)
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Photosensitive sensor
Photosensitive sensor is one of the most common sensors, which has many kinds, mainly including: photocell, photomultiplier tube, photoresistor, phototransistor, solar cell, infrared sensor, ultraviolet sensor, optical fiber photoelectric sensor, color sensor, CCD and CMOS image sensor.
Its sensitive wavelength is near the wavelength of visible light, including infrared wavelength and ultraviolet wavelength.
Optical sensors are not only limited to detecting light, but also can be used as detection elements to detect many non-electric quantities, as long as these non-electric quantities are converted into changes of optical signals.
Optical sensor is one of the most abundant and widely used sensors at present, which plays a very important role in automatic control and non-electric quantity measurement technology.
The simplest photosensitive sensor is a photosensitive resistor, which generates current when photons hit the joints.
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Humidity sensor information
Polymer capacitive humidity sensors are usually made on insulating substrates made of glass, ceramics, silicon and other materials by screen printing or vacuum coating, and then humidity-sensitive glue is coated on electrodes by dipping or other methods to make capacitive elements.
In the atmospheric environment with different relative humidity, the capacitance of the humidity sensor changes regularly because the humidity sensitive film absorbs water molecules, which is the basic mechanism of the humidity sensor.
The temperature characteristics of polymer capacitor elements are affected by temperature, not only the dielectric constant ε of polymer as medium and the dielectric constant ε of adsorbed water molecules, but also the geometric size of the elements is affected by the thermal expansion coefficient.
According to Debye's theory, the dielectric constant ε of liquid is a dimensionless constant related to temperature and frequency.
The ε of water molecule is 78.36 at T=5℃ and 79.63 at T=20℃.
The relationship between organic matter ε and temperature varies from material to material, and does not completely follow the proportional relationship.
In some temperature regions, ε increases with the increase of T, while in some temperature regions, ε decreases with the increase of T..
In the analysis of humidity-sensitive mechanism of polymer humidity-sensitive capacitive elements, most documents think that the dielectric constant of polymer is small, for example, the dielectric constant of polyimide is 3.0-3.8 at low humidity.
While that dielectric constant of wat molecules is ten of times that of polymer ε.
Therefore, the dielectric constant of the water-absorbing heterogeneous layer after moisture absorption is greatly improved due to the dipole distance of water molecules, which is determined by the additivity of the composite dielectric constant of multiphase media.
Due to the change of ε, the capacitance c of humidity-sensitive capacitive element is proportional to the relative humidity.
It is difficult to establish the full humidity range linearity of humidity sensing characteristics in the design and manufacturing process.
As a capacitor, the thickness d of polymer dielectric film and the effective area s of flat capacitor are also related to temperature.
The change of medium geometry caused by temperature change will affect C value.
The average thermal expansion coefficient of polymers can reach orders of magnitude.
For example, the average thermal expansion coefficient of nitrocellulose is 108x 10-5/℃.
With the increase of temperature, the thickness d of dielectric film increases, which has a negative contribution to c; However, the expansion of the humidity-sensitive film increases the adsorption of water by the medium, which is a positive contribution to C.
It can be seen that the temperature characteristics of humidity-sensitive capacitors are dominated by many factors, and the temperature drift is different in different humidity ranges. It has different temperature coefficients in different temperature regions; Different humidity-sensitive materials have different temperature characteristics.
In a word, the temperature coefficient of polymer humidity sensor is not a constant, but a variable.
Therefore, in general, the sensor manufacturer can linearize the sensor in the range of-10-60 degrees Celsius to reduce the influence of temperature on the humidity sensor.
High-quality products mainly use polyamide resin, and the product structure is summarized as follows: vacuum evaporation of gold-plated electrodes on borosilicate glass or sapphire substrate, then spraying a planar humidity-sensitive film in the form of humidity-sensitive dielectric material (as mentioned above), and then evaporation of gold-plated electrodes on the film.
The capacitance of the humidity sensor is proportional to the relative humidity, and the linearity is about 2%.
Although the humidity measurement performance is ok, the temperature resistance and corrosion resistance are not ideal. In the industrial field, the service life, temperature resistance, stability and corrosion resistance need to be further improved.
Ceramic humidity sensor is a new type of sensor developed vigorously in recent years.
The advantages are high temperature resistance, humidity lag, fast response speed, small volume and convenience for mass production. However, due to the porous material, which has a great influence on dust and frequent daily maintenance, it often needs to be cleaned by electric heating, which easily affects the product quality and humidity. The poor linearity in low humidity and high temperature environment, especially the short service life and poor long-term reliability, is an urgent problem for this kind of humidity sensor.
At present, in the development and research of humidity sensor, resistance humidity sensor should be the most suitable for humidity control. Its representative product, lithium chloride humidity sensor, has many important advantages such as stability, temperature resistance and long service life. Lithium chloride humidity sensor has more than 50 years of production and research history, and there are many product types and manufacturing methods, all of which apply the advantages of lithium chloride humidity sensitive liquid, especially the strongest stability.
Lithium chloride humidity sensitive device belongs to electrolyte humidity sensitive material. Among many humidity-sensitive materials, lithium chloride electrolyte humidity-sensitive liquid first attracted people's attention and was used to manufacture humidity-sensitive devices. The equivalent conductance of lithium chloride electrolyte humidity-sensitive liquid decreases with the increase of solution concentration.
Electrolyte is dissolved in water to reduce the water vapor pressure on the water surface.
The substrate structure of lithium chloride humidity sensor is divided into columnar and dressing-like, and the humidity-sensitive liquid and gold electrode with lithium chloride polyvinyl alcohol coating as the main components are the three components of lithium chloride humidity sensor.
Over the years, product manufacturing has been continuously improved and product performance has been continuously improved. The unique long-term stability of lithium chloride humidity sensor is irreplaceable by other humidity sensitive materials, and it is also the most important performance of humidity sensor.
In the process of product production, the preparation of humidity-sensitive mixture and strict control of process are the keys to maintain and exert this characteristic.
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hysteresis characteristic
The hysteresis characteristic indicates the degree of inconsistency between the forward (increasing input) and reverse (decreasing input) strokes of the output-input characteristic curve of the sensor, which is usually expressed as the maximum difference △MAX between the two curves and the percentage of the full-scale output F S.
The energy absorption of the internal components of the sensor will lead to hysteresis.
Interface sensor
Veidemyuller sensor/actuator interface products can be directly connected to the field bus by adding corresponding bus protocol adapters.
Profibus-DP, CANopen, DeviceNet, Interbus and ASi fieldbus protocols can be supported.
Passive Sensor/Actuator Interface Product (SAI)
The protection level reaches IP68, so it can be installed directly without protection.
Save installation materials, time and space.
Provide 4.6.8-way distributors, each with 3-pin, 4-pin and 5-pin structures (providing unidirectional and bidirectional signals).
There are covered type with wiring (standard type) and cable prefabricated type.
Products with metal shell can be provided separately, which is suitable for food industry.
Signal and power indication.
Active Sensor/Actuator Interface Product (SAI)
SAI products can be directly connected to the field bus by adding corresponding bus protocol adapters.
Profibus-DP, CANopen, DeviceNet, Interbus and ASi fieldbus protocols can be supported.
Products with two levels of protection are provided: IP67 (bus connection mode is ring joint connection) and IP68 (bus connection mode is self-assembly).
It provides five input and output products: 8DI, 8DO, 8DI/4DO, 16DI and 8DI/8DO.
Development trend of sensors
Adopt new principles and develop new sensors
Vigorously develop physical sensors (because some structural sensors can't meet the requirements)
Sensor integration
Multifunctional sensor
Intelligent sensor (intelligent sensor)
Study biological senses and develop bionic sensors.
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Example of workflow
The sensor is provided with 15V power supply, and the crystal oscillator in the excitation circuit generates a square wave of 400Hz, which is generated by the TDA2030 power amplifier, and is transmitted from the static primary coil to the rotating secondary coil through the energy loop transformer T 1, and the obtained AC power supply is obtained by the on-axis rectification filter circuit as the working power supply of the operational amplifier AD822; A high-precision regulated power supply consisting of a reference power supply AD589 and a dual-channel operational amplifier AD822 generates a precision DC power supply of 4.5V, which is used as both a bridge power supply and a working power supply for amplifiers and V/F converters.
When the elastic shaft is twisted, the mV-level strain signal detected by the strain bridge is amplified into a strong signal of 1.5V 1V by the instrument amplifier AD620, and then converted into a frequency signal by the V/F converter LM 13 1, and transmitted from the rotating primary coil to the stationary secondary coil through the signal ring transformer T2. After filtering and shaping by the signal processing circuit on the shell, the frequency signal proportional to the torque on the elastic bearing can be obtained. The signal is TTL level, which can be provided to a special secondary meter or frequency meter for display or directly sent to a computer for processing.
Because the gap between the dynamic and static rings of the resolver is only a few tenths of a millimeter, and the upper part of the sensor shaft is sealed in the metal shell, it forms an effective shielding and has strong anti-interference ability.