1. Definition of sensor
The national standard GB7665-87 defines a sensor as A device or device that can sense the specified measurement and convert it into a usable signal according to a certain rule, usually composed of sensitive components and conversion components.
A sensor is a detection device that can feel the information to be measured and can transform the detected and felt information into electrical signals or other required forms of information output according to certain rules to meet the needs of information transmission, processing, storage, and Display, record and control requirements. It is the first link to realize automatic detection and automatic control.
2. Classification of sensors
Sensors can be classified from different perspectives: their conversion principles (the basic physical or chemical effects of sensor work); their purpose; their output signal types and the materials and processes used to make them.
According to the working principle of the sensor, it can be divided into two categories: physical sensor and chemical sensor.
Classification of sensor working principles Physical sensors apply physical effects, such as piezoelectric effect, magnetostriction, ionization, polarization, pyroelectric, photoelectric, magnetoelectric, and other effects. Small changes in the measured signal volume will be converted into electrical signals.
Chemical sensors include those that take chemical adsorption, electrochemical reactions, and other phenomena as causal relationships. Small changes in the measured signal will also be converted into electrical signals.
Some sensors can neither be classified into physical nor chemical categories. Most sensors operate on the basis of physical principles. There are many technical problems with chemical sensors, such as reliability issues, the possibility of mass production, price issues, etc. If such problems are solved, the application of chemical sensors will increase tremendously.
According to its purpose, sensors can be classified as below.
Pressure-sensitive and force-sensitive sensors, position sensors
Liquid level sensor, energy consumption sensor
Speed sensor, a thermal sensor
Acceleration sensor, radiation sensor
Vibration sensor, humidity sensor
A magnetic sensor, gas sensor
Vacuum sensor, biosensor, etc.
Based on its output signal as the standard, the sensors can be divided into
An analog sensor converts the measured non-electrical quantity into an analog electric signal.
The digital sensor converts the measured non-electrical quantity into a digital output signal (including direct and indirect conversion).
Fake digital sensor, which converts the measured signal quantity into a frequency signal or short-period signal output (including direct or indirect conversion).
Switch sensor, when a measured signal reaches a certain threshold, the sensor correspondingly outputs a set low or high-level signal.
Under the influence of external factors, all materials will make corresponding and characteristic responses. Among them, those materials that are most sensitive to external effects, that is, those materials with functional characteristics, are used to make the sensor’s sensitive components.
From the point of view of applied materials, sensors can be divided into the following categories.
(1) Divided into metals, polymers, ceramics, and mixtures according to the types of materials used
(2) According to the physical properties of the material, it is divided into conductors, insulators, semiconductors, and magnetic materials
(3) According to the crystal structure of the material, it is divided into a single crystal, polycrystalline, and amorphous materials.
The sensor development work closely related to the use of new materials can be summarized into the following three directions.
(1) Explore new phenomena, effects, and reactions in known materials, and then enable them to be practically used 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 reactions on the basis of researching new materials, and implementing them in sensor technology.
The progress of modern sensor manufacturing depends on the intensity of the development of new materials and sensitive components for sensor technology. The basic trend of sensor development is closely related to the application of semiconductors and dielectric materials.
According to its manufacturing process, the sensors can be divided into integrated sensors, thin-film sensors, thick-film sensors, and ceramic sensors.
The integrated sensor is manufactured using standard process technology for the production of silicon-based semiconductor integrated circuits. Usually, part of the circuit used for preliminary processing of the signal under test is also integrated on the same chip.
Thin-film sensors are formed by depositing a thin film of corresponding sensitive materials on a dielectric substrate (substrate). When using a hybrid process, part of the circuit can also be manufactured on this substrate.
The thick film sensor is made by coating the slurry of the corresponding material on a ceramic substrate, the substrate is usually made of Al2O3 and then subjected to heat treatment to shape the thick film.
Ceramic sensors are produced using standard ceramic processes or some variant processes (sol-gel, etc.).
After completing the appropriate preparatory operations, the formed components are sintered at high temperatures. There are many common features between the thick film and ceramic sensor processes. In some respects, the thick film process can be considered a variation of the ceramic process.
Each process technology has its own advantages and disadvantages. Due to the low capital investment required for research, development, and production, as well as the high stability of sensor parameters, it is more reasonable to use ceramic and thick film sensors.
3. Sensor static characteristics
The static characteristic of the sensor refers to the correlation between the output and input of the sensor for the static input signal.
Because the input and output are not related to time at this time, the relationship between them, that is, the static characteristics of the sensor can be an algebraic equation without time variables, or the input as the abscissa, and the corresponding output as the It is described by the characteristic curve drawn on the ordinate. The main parameters that characterize the static characteristics of the sensor are linearity, sensitivity, resolution, and hysteresis.
4. Sensor dynamic characteristics
The dynamic characteristic of the sensor refers to the characteristic of the output of the sensor when the input changes.
In actual work, the dynamic characteristics of the sensor are often expressed by its response to certain standard input signals. This is because the sensor’s response to the standard input signal is easily obtained by experimental methods, 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 sinusoidal signal, so the dynamic characteristics of the sensor are also often expressed in step response and frequency response.
5. The linearity of the sensor
Under normal circumstances, the actual static characteristic output of the sensor is a curve rather than a straight line. In actual work, in order to make the meter have a uniform scale reading, a fitting straight line is often used to approximate the actual characteristic curve, and linearity (non-linear error) is a performance indicator of this degree of approximation.
There are many ways to select the fitted straight line. For example, the theoretical straight line connecting the zero input and the full-scale output point is used as the fitting straight line; or the theoretical straight line with the smallest sum of square deviations from each point on the characteristic curve is used as the fitting straight line, this fitting straight line is called the least-squares straight fitting line.
6. Sensitivity of the sensor
Sensitivity refers to the ratio of the output change △y to the input change △x of the sensor under steady-state working conditions.
It is the slope of the output-input characteristic curve. If the sensor’s output and input show a linear relationship, the sensitivity S is a constant. Otherwise, it will change with the input quantity.
The dimension of sensitivity is the ratio of the dimensions of output and input. For example, for a certain displacement sensor, when the displacement changes by 1mm, the output voltage changes by 200mV, and its sensitivity should be expressed as 200mV/mm.
When the dimensions of the sensor’s output and input are the same, the sensitivity can be understood as magnification.
Improve the sensitivity and obtain higher measurement accuracy. However, the higher the sensitivity, the narrower the measurement range and the worse the stability.
7. The resolution of the sensor
The resolution of a sensor refers to the ability of the sensor to experience the smallest change in the measurement.
If the input quantity changes slowly from a certain 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 change exceeds the resolution, the output will change.
Generally, the resolution of each point in the full-scale range of the sensor is not the same, so the maximum change value in the input that can cause a step-change in the output in the full-scale range is commonly used as an index to measure the resolution. If the above index is expressed as a percentage of full scale, it is called resolution.
8. Resistive sensor
A resistive sensor is a device that converts physical quantities to be measured, such as displacement, deformation, force, acceleration, humidity, temperature, etc., into resistance. There are mainly resistance strain sensors, piezoresistive sensors, thermal resistance sensors, thermal sensors, gas sensors, and humidity sensors.
9. Resistance 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 so that the resistance value changes accordingly. Resistance strain gauges are mainly divided into two types, metal and semiconductor. Metal strain gauges are divided into wire type, foil type, and thin-film type. Semiconductor strain gauges have the advantages of high sensitivity (usually dozens of times that of wire and foil types) and small lateral effects.
10. Piezoresistive sensor
The piezoresistive sensor is a device made by diffusion resistance on a semiconductor material substrate based on the piezoresistive effect of the semiconductor material. The substrate can be directly used as a measuring sensor element, and the diffusion resistance is connected in the form of a bridge in the substrate. When the substrate is deformed by an external force, the resistance value will change, and the bridge will produce a corresponding unbalanced output.
The substrate (or diaphragm) materials used as piezoresistive sensors are mainly silicon wafers and germanium wafers. Silicon piezoresistive sensors made of silicon wafers as sensitive materials have attracted more and more attention, especially for pressure measurement. The application of solid-state piezoresistive sensors with speed and speed is the most common.
11. Thermal resistance sensor
The thermal resistance sensor mainly uses the characteristic that the resistance value changes with temperature to measure temperature and temperature-related parameters. This kind of sensor is more suitable for occasions where the temperature detection accuracy is relatively high.
At present, the more widely used thermal resistance materials are platinum, copper, nickel, etc., which have the characteristics of large resistance temperature coefficient, good linearity, stable performance, wide operating temperature range, and easy processing. It is used to measure the temperature in the range of -200℃~+500℃.
12. The hysteresis characteristics of the sensor
The hysteresis characteristic characterizes the inconsistency of the output-input characteristic curve between the forward direction (increased input amount) and reverse (decreased input amount) of the sensor. Usually, the maximum difference between the two curves is △MAX and full The percentage of range output F·S is expressed.
Hysteresis can be caused by the absorption of energy from the internal components of the sensor.
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