Sensor Principle

Publisher: Administrator   Date:2022-08-08



What is a sensor? In a broad sense, a sensor is a device that can sense external information and convert it into usable signals according to a certain law; In brief, a sensor is a device that converts external signals into electrical signals. Therefore, it is composed of sensitive components (sensing elements) and converter devices. Some semiconductor sensitive components can directly output electrical signals, which themselves constitute sensors. There are many kinds of sensitive components. According to the principle of sensing external information, they can be divided into ① physical types, based on physical effects such as force, heat, light, electricity, magnetism and sound. ② Chemistry, based on the principle of chemical reaction. ③ Biology, based on molecular recognition functions such as enzymes, antibodies, and hormones. Generally, according to its basic sensing functions, it can be divided into ten categories: thermal sensor, photosensitive sensor, gas sensor, force sensor, magnetic sensor, humidity sensor, acoustic sensor, radiation sensor, color sensor and taste sensor (some people have divided the sensors into 46 categories). The commonly used thermal, photosensitive, gas, force and magnetic sensors and their sensitive elements are described below.

(I) temperature sensor and thermal sensor
The temperature sensor is mainly composed of thermal sensors. There are many varieties of thermistors. Bimetal, copper thermistors, platinum thermistors, thermocouples and semiconductor thermistors are sold on the market. The temperature sensor with semiconductor thermistor as the detection element is widely used because within the allowable working conditions of the element, the semiconductor thermistor has the characteristics of small volume, high sensitivity and high accuracy, and the manufacturing process is simple and the price is low.

1. Working principle of semiconductor thermistor
According to the temperature characteristics, thermistors can be divided into two types. The thermistor with positive temperature coefficient increases with the temperature rise, and the thermistor with negative temperature coefficient conversely.
⑴ working principle of positive temperature coefficient thermistor
This kind of thermistor is made of barium titanate (BaTiO3) as the basic material, mixed with a proper amount of rare earth elements, and sintered at high temperature by ceramic technology. Pure barium titanate is a kind of insulating material, but after adding appropriate rare earth elements such as lanthanum (LA) and niobium (NB), it becomes a semiconductor material, which is called semiconducting barium titanate. It is a polycrystalline material, and there is a grain interface between grains. For conductive electrons, the grain interface is equivalent to a potential barrier. When the temperature is low, due to the effect of the electric field in the semiconducting barium titanate, the conductive electrons can easily cross the potential barrier, so the resistance value is small; When the temperature rises to the Curie point temperature (i.e., the critical temperature, the 'temperature control point' of this element is generally the Curie point of barium titanate is 120 ℃), the internal electric field is destroyed and cannot help the conductive electrons to cross the potential barrier. Therefore, the resistance value increases sharply. Because the resistance changes very slowly with the temperature before reaching the Curie point, this element has the functions of constant temperature, temperature regulation and automatic temperature control. It only generates heat, does not redden, has no open flame, is not easy to burn, and has a voltage of AC and DC of 3-440v. It has a long service life and is very suitable for overheating detection of electric devices such as motors.
⑵ working principle of negative temperature coefficient thermistor
The negative temperature coefficient thermistor is made of metal oxides such as manganese oxide, cobalt oxide, nickel oxide, copper oxide and aluminum oxide by ceramic process. These metal oxide materials have semiconductor properties and are completely similar to germanium and silicon crystal materials. The number of carriers (electrons and holes) in the body is small and the resistance is high; As the temperature increases, the number of carriers in the body increases and the natural resistance decreases. There are many types of negative temperature coefficient thermistors, which are used to distinguish low temperature (- 60-300 ℃), medium temperature (300-600 ℃) and high temperature (> 600 ℃). They have the advantages of high sensitivity, good stability, fast response, long service life and low price. They are widely used in temperature automatic control circuits requiring fixed-point temperature measurement, such as temperature control systems of refrigerators, air conditioners and greenhouses.
The thermistor combined with a simple amplification circuit can detect the temperature change of one thousandth of a degree, so it can complete high-precision temperature measurement by forming a thermometer with electronic instruments. The working temperature of general-purpose thermistors is - 55 ℃ ~ 315 ℃, and the working temperature of special low-temperature thermistors is lower than - 55 ℃ and can reach - 273 ℃.


2. Model of thermistor
The thermistor made in China is made according to the ministerial standard sj1155-82 and consists of four parts.
Part I: main name, with the letter'm 'to indicate the sensitive element.
Part II: category, use the letter 'Z' to indicate the positive temperature coefficient thermistor, or use the letter 'f' to indicate the negative temperature coefficient thermistor.
Part III: use or characteristics, expressed by one digit (0-9). General number '1' indicates general use, '2' indicates voltage stabilizing use (negative temperature coefficient thermistor), '3' indicates microwave measuring use (negative temperature coefficient thermistor), '4' indicates side heating type (negative temperature coefficient thermistor), '5' indicates temperature measuring use, '6' indicates temperature control use, '7' indicates demagnetization use (positive temperature coefficient thermistor), '8' indicates linear type (negative temperature coefficient thermistor), '9' indicates constant temperature type (positive temperature coefficient thermistor), '0' indicates special type (negative temperature coefficient thermistor)
Part IV: serial number, also represented by numbers, represents specification and performance.
Often, for the special needs of distinguishing this series of products, the manufacturer adds "derived serial number" after the serial number, which is composed of letters, numbers and "-".
Example: m Z 1 1

3. Main parameters of thermistor
The working conditions of various thermistors must be within the allowable range of their factory parameters. The main parameters of the thermistor include more than ten items: nominal resistance value, ambient temperature (maximum working temperature), measured power, rated power, nominal voltage (maximum working voltage), working current, temperature coefficient, material constant, time constant, etc. The nominal resistance value is the resistance value at 25 ℃ zero power. In fact, there is always a certain error, which should be within ± 10%. The working temperature range of ordinary thermistors is large, and can be selected from - 55 ℃ to 315 ℃ as required. It is worth noting that the maximum working temperature of different types of thermistors varies greatly, for example, the negative temperature coefficient thermistor mf11 is 125 ℃, while the negative temperature coefficient thermistor MF53-1 is only 70 ℃. Students should pay attention to it during experiments (generally not more than 50 ℃).

(4) selection of thermistor for experiment
The general-purpose negative temperature coefficient thermistor is preferred because its change with temperature is generally easier to observe than that of the positive temperature coefficient thermistor, and the resistance value continuously decreases significantly. If a positive temperature coefficient thermistor is selected, the experimental temperature shall be near the Curie point temperature of the element.
Example: mf11 common negative temperature coefficient thermistor parameters
Name of main technical parameters parameter value mf11 thermistor symbol outline drawing
Nominal resistance (K Ω) 10 ~ 15 sheet shape symbol
Rated power (W) 0.25
Range of material constant B (k) 1980 ~ 3630
Temperature coefficient (10-2 / ℃) - (2.23 ~ 4.09)
Dissipation coefficient (MW / ℃) ≥ 5
Time constant (s) ≤ 30
Maximum operating temperature (℃) 125
For rough measurement of the thermistor value, a multimeter with moderate range and small current measured through the thermistor should be selected. If the thermistor is about 10K Ω, the MF10 multimeter can be selected and the gear switch can be set to ohmic gear R × 100, clamp the two pins of the thermistor with an alligator clip instead of the lead. When the ambient temperature is significantly lower than the body temperature, the reading is 10.2k. Hold the thermistor with your hand, and you can see that the resistance value indicated by the needle decreases gradually; After releasing the hand, the resistance value increases and gradually recovers. Such thermistors can be selected (the maximum operating temperature is about 100 ℃).

Several practical temperature sensors
A special temperature control sensor in the air conditioner: the thermal sensor is sealed in copper metal.
B) air temperature measurement sensor


(II) optical sensor and photosensitive element
The light sensor is mainly composed of photosensitive elements. At present, photosensitive elements are developing rapidly, with a wide variety of applications. There are photosensitive resistors, photodiodes, phototriodes, photocouplers and photocells on the market.
1. Photosensitive resistor
The photoresist is composed of semiconductor photoelectric crystals that can transmit light. Due to the different composition of semiconductor photoelectric crystals, it is divided into visible light photoresist (cadmium sulfide crystal), infrared light photoresist (gallium arsenide crystal), and ultraviolet light photoresist (zinc sulfide crystal). When the sensitive wavelength light illuminates the surface of the semiconductor photoelectric crystal, the carriers in the crystal increase, making its conductivity increase (that is, the resistance decreases).
Main parameters of photoresist:
◆ photocurrent and luminous resistance: under a certain applied voltage, when there is light (100lx illumination), the current flowing through the photoresist is called photocurrent; The ratio of the applied voltage to the current is the light resistance, generally several K Ω ~ several tens of K Ω.
◆ dark current and dark resistance: under a certain applied voltage, when there is no light (0 LX illumination), the current flowing through the photoresist is called dark current; The ratio of the applied voltage to the current is the dark resistance, generally several hundred K Ω to several thousand K Ω.
◆ maximum working voltage: generally tens of volts to hundreds of volts.
◆ ambient temperature: generally - 25 ℃ to 55 ℃, and some models can be - 40 ℃ to 70 ℃.
◆ rated power (power consumption): the product of the bright current of the photoresist and the external voltage; There are 5MW to 300MW specifications.
◆ the main parameters of the photoresist include response time, sensitivity, spectral response, lighting characteristics, temperature coefficient, volt ampere characteristics, etc.
It is worth noting that the light characteristics (characteristics varying with light intensity), temperature coefficient (characteristics varying with temperature), and voltage current characteristics are not linear. For example, the photo resistance of CDs (cadmium sulfide) photoresist sometimes increases with the increase of temperature, and sometimes decreases with the increase of temperature.
Parameters of cadmium sulfide photoresist:
Model and specification: mg41-22, mg42-16, mg44-02, mg45-52
Ambient temperature (℃) - 40 ~ 60 - 25 ~ 55 - 40 ~ 70 - 40 ~ 70
Rated power (MW) 20 10 5 200
Bright resistance, 100lx (K Ω) ≤ 2 ≤ 50 ≤ 2 ≤ 2
Dark resistance, 0lx (m Ω) ≥ 1 ≥ 10 ≥ 0.2 ≥ 1
Response time (MS) ≤ 20 ≤ 20 ≤ 20 ≤ 20
Maximum operating voltage (V) 100 50 20 250


(2) photodiode
Compared with ordinary diodes, except that its die is also a PN junction and has unidirectional conductivity, other diodes are very different. First, the PN junction in the die is shallow (less than 1 micron) to improve the photoelectric conversion ability; The second PN junction area is relatively large, and the electrode area is very small, so as to facilitate the photosensitive surface to collect more light; The third photodiode has a "window" sealed with a plexiglass lens and capable of concentrating light on the photosensitive surface; Therefore, the sensitivity and response time of the photodiode are much better than that of the photoresist.

Common photodiodes and symbols are as follows:
2du has three poles: front pole, rear pole and ring pole. The ring electrode is designed to reduce the dark current of the photodiode and increase the working stability. It needs to be connected to the positive electrode of the power supply during application. The main parameters of the photodiode include: maximum working voltage (10-50V), dark current (≤ 0.05-1 μ a), photocurrent (> 6-80 μ a), photoelectric sensitivity and response time (tens of ns-tens of NS) μ s) Junction capacitance and forward voltage drop, etc.
The advantages of photodiodes are good linearity, fast response speed, high sensitivity to light with a wide range of wavelengths and low noise; The disadvantage is that the output current (or voltage) used alone is very small, and an amplification circuit is required. Suitable for communication and photoelectric control circuits.
The photodiode can be detected with a multimeter R × At 1K gear, the forward resistance shall be 10K Ω ~ 200 K Ω, and the reverse resistance shall be ∞. The greater the right deflection angle after removing the light shield, the higher the sensitivity.
The phototransistor can be regarded as a combination element of a photodiode and a triode. Due to its amplification function, its dark current, photocurrent and photoelectric sensitivity are much higher than those of the photodiode. However, due to structural reasons, the junction capacitance increases and the response characteristics deteriorate. Widely used in low-frequency photoelectric control circuits.
Semiconductor optoelectronic devices also have MOS structures. For example, CCD (charge coupled device) commonly used in scanners and camera heads is an array of integrated photodiodes or MOS structures.

(III) gas sensor and gas sensor
The textbook only requires simple thermistor and photoresist characteristic experiments. As gas is closely related to human daily life, the detection of gas has become an indispensable means to protect and improve the ecological living environment, and gas sensors play an extremely important role. For example, when the concentration of carbon monoxide in the living environment reaches 0.8-1.15 ml / L, there will be shortness of breath, accelerated pulse, and even syncope. When the concentration reaches 1.84 ml / L, there is a risk of death within a few minutes. Therefore, the detection of carbon monoxide must be rapid and accurate. Using SnO2 metal oxide semiconductor gas sensing material, SnO2 nanoparticles were prepared by ultrafine particle refinement and doping process, and then doped with a certain catalyst as the substrate. After proper sintering process and surface modification, a side heated sintered co sensing element was made, which can detect 0.005% ~ 0.5% CO gas. There are also many sensors for detecting toxic gases such as explosive combustible gas, alcohol gas and automobile exhaust gas. Commonly used gas sensors include contact combustion gas sensors, electrochemical gas sensors and semiconductor gas sensors. The detection element of the contact combustion type gas sensor is generally platinum wire (platinum, palladium and other rare metal catalytic layers can also be coated on the surface). During use, a current is applied to the platinum wire to maintain a high temperature of 300 ℃ ~ 400 ℃. At this time, if the combustible gas is contacted, the combustible gas will burn on the rare metal catalytic layer, so the temperature of the platinum wire will rise, and the resistance value of the platinum wire will also rise; The concentration of combustible gas can be known by measuring the change of the resistance value of the platinum wire. Electrochemical gas sensors generally use liquid (or solid, organic gel, etc.) electrolytes. The output form can be the current generated by direct oxidation or reduction of gas, or the electromotive force generated by ions acting on ion electrodes. Semiconductor gas sensor has the characteristics of high sensitivity, fast response, good stability and simple use, and is widely used; The following focuses on the semiconductor gas sensor and its gas sensing elements.
Semiconductor gas sensors can be divided into n-type and p-type. The resistance of n-type decreases with the increase of gas concentration; The p-type resistance increases with the increase of gas concentration. Like SnO2 metal oxide semiconductor gas sensitive material, it belongs to n-type semiconductor. At the temperature of 200-300 ℃, it adsorbs oxygen in the air to form negative ion adsorption of oxygen, reducing the electron density in the semiconductor and increasing its resistance value. When encountering a combustible gas (such as CO) capable of supplying electrons, the originally adsorbed oxygen is desorbed, and the combustible gas is adsorbed on the metal oxide semiconductor surface in a positive ion state; Oxygen desorption releases electrons, and the adsorption of combustible gas in the state of positive ions also releases electrons, so that the electron density of the conduction band of the oxide semiconductor increases and the resistance value decreases. When the combustible gas does not exist, the metal oxide semiconductor will automatically recover the negative ion adsorption of oxygen, so that the resistance value rises to the initial state. This is the basic principle of detecting combustible gas with semiconductor gas sensor.
At present, there are 2 kinds of gas sensors made in China. One is a direct heating type, in which the heating wire and the measuring electrode are sintered together in the metal oxide semiconductor die; The side heating gas sensing element is based on a ceramic tube, with heating wires inside the tube and two measuring electrodes outside the tube. The metal oxide gas sensing material is used between the measuring electrodes and is sintered at high temperature. The parameters of the gas sensor mainly include heating voltage, current, measuring circuit voltage, sensitivity, response time, recovery time, voltage in calibration gas (0.1% butane gas), load resistance value, etc. Qm-n5 type gas sensor is applicable to the detection of natural gas, gas, hydrogen, alkane gas, alkene gas, gasoline, kerosene, acetylene, ammonia, smoke, etc. it is an n-type semiconductor element. It has high sensitivity, good stability, short response and recovery time, and is widely used in the market. The parameters of qm-n5 gas sensor are as follows: the voltage in the calibration gas (0.1% butane gas, optimal working conditions) is ≥ 2V, the response time is ≤ 10s, the recovery time is ≤ 30s, the optimal working conditions are heating Voltage 5V, measuring circuit voltage 10V, load resistance RL is 2K, and the allowable working conditions are heating voltage 4.5-5.5v, measuring circuit voltage 5-15v, and load resistance 0.5-2.2k. The following figure shows the simple test circuit of the gas sensor (composed of sensor). The greater the change of the voltmeter pointer, the higher the sensitivity; As long as a simple circuit is added, the alarm can be realized. Common gas sensors include mq-31 (dedicated to detecting CO), qm-j1 wine sensor, etc.

(IV) force sensor and force sensor
There are many kinds of force sensors. The traditional measurement method is to use the deformation and displacement of elastic materials to express. With the development of microelectronic technology, a force sensitive sensor with small volume, light weight and high sensitivity has been developed by using the piezoresistive effect of semiconductor materials (that is, when pressure is applied to a certain direction, its resistivity changes) and good elasticity, which is widely used for the measurement of physical and mechanical quantities such as pressure and acceleration.

(V) magnetic sensor and magnetic sensor
At present, magnetic sensors include Hall devices (based on Hall effect), magnetoresistive devices (based on magnetoresistive effect: the resistance of semiconductors increases with the increase of the magnetic field due to the external magnetic field) Magneto sensitive diodes and triodes. Magnetic sensors based on magnetic sensors are widely used in the measurement of some electrical, magnetic and mechanical quantities.
In a certain sense, the sensor has a corresponding relationship with human senses, and its sensing ability has far exceeded human senses. For example, the infrared imaging system (night camera) that uses the infrared radiation of the target itself to observe can find people at 1000 meters and vehicles at 2000 meters in the dark; The core component of the thermal imager is the infrared sensor. In the 1991 Gulf War, the detection distance of the night vision instrument equipped with Iraqi tanks was only 800 meters, which was less than half of that of the United States and British coalition forces. It was inevitable that Iraq was defeated in the dark. At present, all countries in the world regard sensor technology as the priority of high-tech development. In order to greatly provide the performance of sensors, new structures, new materials and new processes will be continuously adopted to develop towards miniaturization, integration and intelligence.
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