First, recognize the sensor
1. The sensor
(1) Definition: sensor refers to such a kind of element: it can feel physical quantities such as force, temperature, light, sound, chemical composition and so on, and can convert them into another physical quantity (usually voltage, current and other electrical quantities) which is convenient for transmission and processing according to certain laws, or convert them into circuit on-off.
(2) Basic characteristics: convert the non-electrical quantity into electrical quantity, which can be easily measured, transmitted, processed and controlled.
2. The working principle of the sensor: the sensor through the sensitive element is usually non-electrical quantity, and it uses the conversion element output is usually electrical quantity, such as voltage, current, charge and so on.
The sensor is generally composed of four parts: sensitive element, conversion element, conversion circuit and auxiliary power supply. Its working principle is shown in the figure.
The sensitive element senses the measured directly and outputs the physical quantity signal which has definite relation with the measured; The conversion element converts the physical quantity signal output by the sensitive element into electrical signal; The conversion circuit is responsible for amplifying and modulating the electrical signal output by the conversion element; Conversion elements and conversion circuits generally also need auxiliary power supply.
It consists of reeds made of two soft magnetic materials sealed with glass tubes. When the magnet is near the reed tube, the two reeds are magnetized and connected, so the reed tube acts as a switch, which is operated by the invisible “hand” of the magnetic field. Reed pipe is a kind of magnetic field sensing sensor, widely used in electrical equipment and electronic equipment.
3. Characteristics of the sensor
Miniaturization, digitalization, intelligence, multi-function, systematization and networking are the first steps to realize automatic detection and control. The existence and development of sensors make objects have “touch”, “taste” and “smell”, etc., so that objects slowly “live”.
4. Sensor classification
(1) According to its use can be divided into: pressure sensor, position sensor, liquid level sensor, energy consumption sensor, speed sensor, acceleration sensor, ray radiation sensor, heat sensor, radar sensor, etc.
(2) According to its principle can be divided into: vibration sensor, humidity sensor, magnetic sensor, gas sensor, vacuum sensor, biological sensor, etc.
(3) According to its output signal can be divided into: analog sensor – the measured non-electrical quantity into analog electrical signal;
Digital sensor – converts measured non-electrical quantity into digital output signal (including direct and indirect conversion);
False digital sensor — converts the measured semaphore into a frequency signal or a short period signal (including direct and indirect conversion);
Switch SENSOR – WHEN a measured signal reaches a certain threshold, the sensor outputs a set low or high level signal accordingly.
(4) According to its measurement purpose can be divided into: physical sensor, chemical sensor, biological sensor.
Second, the understanding of sensitive components
1. Photoresistor: it is a resistor whose resistance value changes with the intensity of the incident light.
(1) Characteristics: When the photoresistor is irradiated with different light, different resistance will be obtained. According to the experimental data, the stronger the light intensity, the smaller the resistance.
(2) Essence: generally constitute the photoresistor material for semiconductor materials, when there is no light carrier is very few, conductive performance is not good; With the increase of illumination, the number of carriers increases, the conductivity becomes stronger, and the resistance decreases.
(3) Function: the optical amount of light intensity is converted into the electrical amount of resistance, just like human eyes, can perceive the intensity of light, the application of photosensitive resistance can be made into a photoelectric counter.
2. Thermistors and metal thermistors
(1) Made of semiconductor materials, the use of temperature changes in the conductive properties of the semiconductor of electronic components general thermistor resistance value decreases with the increase of temperature.
② Classification: thermistor is a kind of sensitive element, according to the different temperature coefficient is divided into positive temperature coefficient thermistor (PTC), negative temperature coefficient thermistor (NTC) and critical temperature thermistor (CTR). The positive temperature coefficient thermistor increases with increasing temperature. Negative temperature coefficient thermistors decrease in resistance with increasing temperature (this is the most common type of thermistor, such as the one shown in sidebar R-T); Critical temperature thermistors have the characteristic of negative resistance mutation. At a certain temperature, the resistance value decreases sharply with the increase of temperature, and has a large negative temperature coefficient. Their resistivity varies with temperature.
(2) metal thermal resistance: the resistivity of the metal increases with the increase of temperature, using this characteristic, the wire can also be made into a thermal sensor, known as the thermal resistance of the general metal thermal resistance sensitivity is poor.
(3) Comparison of manganese oxide thermistor and metal thermistor
Three, Hall element
1, Hall element: On a small rectangular semiconductor (such as indium arsenide) sheet, make four electrodes E, F, M, N, it becomes a Hall element.
2. Hall voltage
(1) Expression: Constant current I is applied between E and F, and a magnetic field with magnetic induction intensity B perpendicular to the sheet is applied, then Hall voltage UH, UH = kIB/d appears between MN.
(2) Principle: Taking the charge carriers as free electrons for example, the Hall voltage is derived as follows: according to the left hand rule, let the magnetic inductive line pass vertically through the palm, and the four fingers point to the opposite direction of the electron movement (namely, the direction of the current).
Thumb pointing in the direction which electrons by lorentz force and electronic deflect under the effect of lorentz force, and accumulated in left and right sides of the surface, the accumulation of negative charge on the left side of the surface, the right side of the surface is the same amount of positive charge accumulation, which on the right side of the surface of the electric potential is high, this will form the electric field, when the electronic balance in the electric field force and lorentz force, the left and right sides of voltage stability.
Hall elements can be divided into two classes: one is a metal Hall element whose charge carriers are free electrons; The other is a semiconducting Hall element whose charge carriers are holes (which can be thought of as positively charged particles).
Let the distance between the left and right plates of M and N be H, and the distance between the upper and lower plates of E and F be D, then the eE field =eU/ H = evB, and the current in the conductor I = nevS = Nev · HD, and the simultaneous equations obtain U = IB/ Ned. Since NE is determined by the material of the Hall element itself, we refer to kIB/ D as the Hall coefficient, denoted by k, so that we have UH = kIB/ D, where d is the thickness of the sheet.
3, Hall potential high and low judgment
The force direction of charged particles is judged by the left hand rule, and the deflection direction of charged particles is obtained. The surface where positive charge accumulates is the surface of high electric potential, and the surface where negative charge accumulates is the surface of low electric potential.
In judging the Hall potential, it is important to pay attention to whether the charge carriers are positive or negative. Whether the charge carrier is positive or negative, the four fingers refer to the direction of the current, namely the direction of the directional movement of positive charge and the opposite direction of the directional movement of negative charge (when the current direction is fixed, the force direction of the charge carrier is the same regardless of whether the charge carrier is positive or negative).
4. The function of Hall element
The thickness D and the Hall coefficient k of a hall element are constant values. If the current I is kept constant, the Hall voltage U is proportional to the magnetic induction intensity B. Therefore, the Hall element can convert the magnetic induction intensity into the electrical voltage, so the Hall element is also called the magnetic sensitive element.