Infrared sensor is a kind of sensor that uses infrared ray to process data. It has the advantage of high sensitivity. The infrared sensor can control the operation of the drive device.
Infrared sensors are commonly used in non-contact temperature measurement, gas composition analysis and nondestructive testing, and are widely used in medicine, military, space technology and environmental engineering.For example, the infrared sensor is used to measure the thermal image of the human body surface temperature remotely, and the parts with abnormal temperature can be found.
A sensor that uses the physical properties of infrared light to measure。Infrared light, also known as infrared light, has the properties of reflection, refraction, scattering, interference, absorption, etc.Any substance, as long as it has a certain temperature (above absolute zero), can radiate infrared ray.The infrared sensor does not directly contact the measured object when measuring, so there is no friction, and it has the advantages of high sensitivity, fast response, etc.
Infrared sensor includesoptical system , detection element and conversion circuit.Optical systems can be divided into transmissive and reflective systems according to their structures.According to the working principle, the detection elements can be divided into thermal detection elements and photoelectric detection elements.Thermistors are the most widely used thermistors.When the thermistor is exposed to infrared radiation, the temperature rises and the resistance changes (this change may be larger or smaller, because the thermistor can be divided intoPositive temperature coefficient thermistorAnd negative temperature coefficient thermistor), which is converted into an electrical signal output through a conversion circuit.Photoelectric detection element is commonly used as photosensitive element, which is usually composed ofLead sulfide, lead selenideIndium arsenide, antimony arsenide, mercury cadmium tellurideTernary alloy, germanium and silicon doped materials.
Infrared sensors are often used to diagnose and treat diseases (see thermal imager);The use of infrared sensors on satellites to monitor the earth's clouds can achieve large-scale weather forecasting;The infrared sensor can be used to detect the overheating of the running engine on the aircraft.
haveinfrared sensor The telescope can be used for military operations, forest warfare detects enemies in dense forests, and urban warfare detects enemies behind walls. All of the above use infrared sensors to measure the temperature of human body surface to know where the enemy is.
type
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Infrared sensors can be divided into:
(1) Transform part of the infrared ray into heat, and take out the thermal type of the output signal such as resistance value change and electromotive force by heat.
(2) A quantum type that uses the semiconductor migration phenomenon to absorb the photoelectric effect of energy difference and the photoelectromotive force effect due to PN bonding.
The phenomenon of thermal type is commonly known as the pyrothermal effect, among which the most representative ones are the thermal bolometer, thermopile and pyroelectric components.
The advantages of thermal type are: it can operate at normal temperature, wavelength dependence (wavelength sensitivity varies greatly) does not exist, and the cost is cheap;
Disadvantages: low sensitivity and slow response (mS spectrum).
Advantages of quantum type: high sensitivity and fast response（μSpectrum of S);
Disadvantages: Must be cooled (liquid nitrogen), wavelength dependent, high price;
In particular, infrared sensors use the sensitivity of the far infrared range for human detection. The wavelength of infrared is longer than that of visible light and shorter than that of radio wave.Infrared ray makes people think that it is only emitted by hot objects, but in fact it is not the case. All objects existing in nature, such as human beings, fire, ice, etc., will emit infrared ray, but their wavelengths are different due to the temperature of their objects.AnthropogenictemperatureIt is about 36~37 ° C, and the emitted peak value is 9~10μM far-infrared ray, and the object heated to 400~700 ° C can emit a peak value of 3~5μM middle infrared ray.
Example
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Infrared sensor application circuit
The alarm can detect the infrared ray emitted by the human body. When a person enters the monitoring area of the alarm, the alarm will sound. It is suitable for anti-theft alarm in homes, offices, warehouses, laboratories and other important occasions.The circuit principle of the device is shown in Figure 1.By infrared sensor, signal amplification circuitVoltage comparator, delay circuit and audible alarm circuit.When the infrared detection sensor IC1 detects the infrared signal radiated from the front human body, the foot ② of IC1 outputs a weak electrical signaltriodeVT1, etc. form the first stage amplifier circuit for amplification, and then input to the operational amplifier IC2 through C2 for high gain and low noise amplification. At this time, the signal output by the IC2 ① pin is strong enough.IC3 workVoltage comparatorThe reference voltage of the ⑤ pin is provided by R10 and VD1. When the signal voltage output from the ① pin of IC2 reaches the ⑥ pin of IC3, the voltage of the two input terminals is compared. At this time, the ⑦ pin of IC3 changes from the original high level to the low level.IC4 is an alarm delay circuit, R14 and C6 form a delay circuit, and the time is about 1 minute.When pin ⑦ of IC3 becomes low level, C6 discharges through VD2. At this time, pin ② of IC4 becomes low level, which is compared with the reference voltage of pin ③ of IC4. When it is lower than its reference voltage, pin ① of IC4 becomes high level, VT2 is on, and the buzzer BL is powered up to give an alarm sound.After the infrared signal of the human body disappears, the ⑦ pin of IC3 returns to high level output, and VD2 is cut off.Since the voltage at both ends of C6 cannot change suddenly, charge C6 slowly through R14. When the voltage at both ends of C6 is higher than its reference voltage, the ① pin of IC4 becomes low level for about 1 minute, that is, the alarm lasts for 1 minute.
The flame sensor makes use of the characteristic that infrared ray is very sensitive to flame. It uses a special infrared receiver tube to detect the flame, and then converts the brightness of the flame into a level signal with high and low changes, which is input toa central processorThe central processing unit makes corresponding program processing according to the change of signal.
Flame sensorThe infrared light with a wavelength of 700 nm~1000 nm can be detected, and the detection angle is 60 °. The sensitivity of infrared light with a wavelength of 880 nm reaches the maximum.
The far-infrared flame probe converts the intensity change of the external infrared light into the change of the currentA/D converterIt is reflected as the change of value in the range of 0~255.The stronger the external infrared light, the smaller the value;The weaker the infrared light, the greater the value.
Infrared range sensorUsing the principle that the intensity of infrared signal reflected from different distances to obstacles is also different, the distance of obstacles is detected.The infrared ranging sensor has a pair of infrared signal transmitting and receiving diodes. The transmitting tube transmits infrared signals of a specific frequency, and the receiving tube receives infrared signals of this frequency. When the infrared detection direction encounters obstacles, the infrared signals are reflected back and received by the receiving tube. After processingDigital sensorThe interface returns to the CPU host,a central processorThe infrared return signal can be used to identify the changes in the surrounding environment.
Infrared temperature instrument
Infrared temperature instrumentThe structure of the system mainly consists of optical system, modulator, infrared sensor amplifier, indicator and other parts.infrared sensor It is a device that receives target radiation and converts it into electrical signals.[1]
infrared imaging
In many cases, people not only need to know the average temperature of the object surface, but also need to know the temperature distribution of the object to analyze, study the structure of the object, and detect internal defects.Infrared imaging can directly display the temperature distribution of objects in the form of images.
Application considerations
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The infrared sensor is a very important part of the infrared detection system, but it is very delicate. If you do not pay attention to it in use, the infrared sensor may be damaged.Therefore, the following points should be paid attention to in the use of infrared sensors:
(1) Must first pay attention to understandinfrared sensor Its performance indicators and application range, and its use conditions.
(2) Attention must be paid to the working temperature of the sensor. Generally, infrared sensors that can work at room temperature should be selected for easy maintenance.
(3) Properly adjust the operating point of the infrared sensor.Generally, the sensor has an optimal operating point.The signal to noise ratio of the infrared sensor is the largest only when it works at the optimal working point.
(4) Proper selectionPreamplifierCooperate with the infrared sensor to obtain the best detection effect.
(5) The modulation frequency matches the frequency response of the infrared sensor.
(6) The optical part of the sensor cannot be touched or wiped by hand to prevent damage and contamination.
(7) When storing sensors, pay attention to moisture-proof, vibration proof and anti-corrosion.[2]
