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Principles and Applications of Pyroelectric Sensors

Principles and Applications of Pyroelectric Sensors

Pyroelectric infrared sensor is a kind of sensor with great potential for application. It can detect infrared rays emitted by people or certain animals and convert them into electrical signal output. It is a new type of high-sensitivity infrared detection element that can detect infrared rays emitted by the human body. It can detect the change of infrared energy radiated by the human body in a non-contact form, and convert it into a voltage signal for output. Amplify the output voltage signal to drive various control circuits, such as power switch control and anti-theft and fire alarm.


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As early as 1938, someone proposed to use the pyroelectric effect to detect infrared radiation, but it did not receive much attention. It was not until the 1960s that with the rapid development of laser and infrared technology, the research on pyroelectric effects and the application and development of pyroelectric crystals were promoted.


Ⅰ. Pyroelectric effect of pyroelectric infrared sensor


When some crystals are heated, an equal number of charges with opposite signs will be generated at both ends of the crystal. This electric polarization caused by thermal changes is called the pyroelectric effect.


Usually, the bound charge generated by the spontaneous polarization of the crystal is neutralized by the free electrons attached to the surface of the crystal from the air, and the spontaneous polarization moment cannot be expressed. When the temperature changes, the center of gravity of the positive and negative charges in the crystal structure is relatively shifted, and the spontaneous polarization changes. Then the charge depletion occurs on the crystal surface, and the charge depletion is proportional to the degree of polarization.


The crystal that can produce a pyroelectric effect is called pyroelectrics or pyroelectric elements. The commonly used materials for pyroelectric elements are single crystal (LiTaO3, etc.), piezoelectric ceramics (PZT, etc.) and polymer films (PVFZ, etc.).


The pyroelectric sensor uses the pyroelectric effect, which is a temperature-sensitive sensor. It is composed of ceramic oxide or piezoelectric crystal elements. Electrodes are made on the two surfaces of the element. When the temperature changes in the sensor monitoring range ΔT, the pyroelectric effect will generate charge ΔQ on the two electrodes, that is, A weak voltage ΔV is generated between the two electrodes. Because of its extremely high output impedance, there is a field-effect tube in the sensor for impedance conversion. The charge ΔQ generated by the pyroelectric effect will be combined by the ions in the air and disappear, that is, when the ambient temperature is stable, ΔT=0, the sensor has no output. When the human body enters the detection area, due to the difference between the human body temperature and the ambient temperature, ΔT is generated, and then there is a ΔT output; if the human body does not move after entering the detection area, the temperature does not change, and the sensor does not output. Therefore, this kind of sensor detects human or animal activity sensing.


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Ⅱ. Structure of pyroelectric infrared sensor


The appearance of an ordinary pyroelectric infrared sensor for human body is shown in the figure. The D pin and S pin are the drain and source terminals of the internal FET respectively, and the G pin is the ground terminal of the internal sensitive components. Because the gap between the S pin and G pin is suspended, the output resistance R should be connected during use to output the sensing signal. In order to enhance the anti-interference ability, a capacitor C should be added to this resistor. The sensor consists of a sensitive unit, a filter window and a Fresnel lens.


Sensitive unit


The equivalent circuit of the sensitive unit is shown in the figure. The internal sensitive material is made of very thin sheets and each sheet has an electrode on the opposite sides. An equivalent small capacitance is formed at both ends of the electrode because these two small capacitors are made on the same silicon wafer, and the formed equivalent small capacitors can generate polarization by themselves, and generate positive and negative charges with opposite polarities at both ends of the capacitor. But the polarities of these two capacitors are opposite in series. This is the unique design of the sensor, which makes it unique anti-interference.


When the sensor does not detect the infrared signal radiated by the human body, because C1 and C2 are polarized by themselves, positive and negative charges with opposite polarities and equal power are generated at both ends of the capacitor. In addition, the polarities of the two capacitors are opposite in series, so the positive and negative charges cancel each other out, no current is generated in the loop, and the sensor has no output.


When the human body is still in the detection area of the sensor, the energy of the infrared light irradiated on C1 and C2 is equal and balanced, and the photocurrents with opposite polarities and equal energy cancel each other out in the loop. The sensor still has no signal output. In the same way, under the light or sunlight, because the sunlight moves very slowly, the infrared light energy on C1 and C2 can still be regarded as equal, and they cancel each other out in the loop; in addition, the response frequency of the sensor is very low (Generally 0.1~10Hz), that is, the sensitive range of the sensor to infrared light wavelength is very narrow (generally 5~15um), hence the sensor is not sensitive to them.


When the temperature of the sensor itself changes due to the environmental temperature change because C1 and C2 are made on the same silicon wafer, the photocurrents with opposite polarities and equal energy generated by it still cancel each other out in the loop, and the sensor has no output.


Only when the human body moves, the infrared radiation induces the two equivalent capacitances of the sensor's sensitive unit to produce different polarized charges, the electrical signal will be output. Therefore, this kind of sensor is only sensitive to the motion or movement of the human body, insensitive to the static or slow-moving human body. Besides, it has a good anti-interference ability to visible light and most infrared rays.


Filter window


It is made of a thin glass plate coated with a multi-layer filter film. The filter window can effectively filter out infrared rays with wavelengths other than 7.0~14μm. For example, the penetration of SCA02-1 to infrared rays with wavelengths of 7.5 to 14μm is 70%, which drops to 65% at 6.5μm, and drops to 0.1% at 5.0μm. It effectively ensures the selectivity of infrared rays to the human body.


Because the relationship between the strongest wavelength and temperature of the infrared radiation energy emitted by the object satisfies λm×T=2989(μmk) (where λm is the maximum wavelength and T is the absolute temperature). The normal body temperature of the human body is 36~37.5℃, that is, 309~310.5K, the wavelength of its strongest infrared is λm=2989/(309~310.5)=9.67~9.64μm, and the center wavelength is 9.65μm. Therefore, the wavelength of the strongest infrared radiation radiated by the human body falls exactly in the center of the response wavelength (7~14μm) of the filter window. Therefore, the filter window can effectively allow the infrared rays radiated by the body to pass through, and prevent the infrared rays in the visible light such as sunlight and lights from passing through, so as not to cause interference.


Fresnel lens


When the Fresnel lens is not used, the detection radius of the sensor is less than 2 meters, and the maximum effect can be exerted only when used with the Fresnel lens. When equipped with a Fresnel lens, the detection radius of the sensor can reach 10 meters.


The Fresnel lens is made of polyethylene plastic sheet, the color is milky white or black and it is semi-transparent. However, it is transparent to infrared light with a wavelength of about 10um. Its shape is a hemisphere, the lens is divided into 3 parts in the horizontal direction, and each part is equally divided into several different areas in the vertical direction. Each equal part of the uppermost part is a lens unit, which is composed of concentric circles, and the centers of the concentric circles are in the lens unit. Each equal part of the middle and lower half is also a separate lens unit, which is also composed of concentric circles, but the center of the concentric circles is not in the lens unit.


When light passes through these lens units, a visible area and a blind area oflight and dark are formed. Since each lens unit has only a small angle of view, the inside of the angle of view is the visible area, and the outside of it is the blind area. Any two adjacent lens units are separated by a blind zone and a visible zone, and they are intermittent and do not overlap or cross. In this way, when the lens is placed in an appropriate position directly in front of the sensor, once a moving human body appears in front of the lens, the infrared rays radiated by the human body pass through the lens to form a constantly changing shadow area (blind area) and bright area (visible area). So that the temperature of the sensor surface continuously changes to output electrical signals.


It can also be understood that when the human body moves in the detection area, once it leaves the field of view of one lens unit, the body will immediately enter the field of view of another lens unit (because adjacent lens units are very close). Blind areas and visible areas that move with the body then appear on the sensor, causing the temperature of the sensor to change and thus output an electrical signal.


The Fresnel lens can not only form the visible zone and the blind zone but also has a focusing effect. Its focal point is generally about 5 cm. In actual application, the distance between the Fresnel lens and the sensor should be adjusted according to the actual situation or the instructions provided by the information. Generally, the lens is fixed 1~5 cm directly in front of the sensor.


Ⅲ. Working principle and characteristics of passive pyroelectric infrared sensor


In nature, any object will produce an infrared spectrum when the temperature is higher than absolute temperature (-273K). Objects at different temperatures have different wavelengths of infrared energy released. Therefore, infrared wavelengths are related to temperature. In addition, the size of the radiant energy is related to the surface temperature of the object.


The human body has a constant body temperature, generally at 37 degrees, so it emits infrared rays with a specific wavelength of about 10 microns. The passive infrared probe works by detecting the infrared rays emitted by the human body. The infrared rays are enhanced by the Fresnel filter and concentrated to the pyroelectric element. This element loses the charge balance when receiving the infrared radiation change of the human body, releases the charge outward, and then generates an alarm signal after detection. The sensor of the passive infrared probe contains two pyroelectric elements connected in series or parallel. Moreover, the two electrical polarization directions made are exactly opposite, and the environmental background radiation has almost the same effect on the two pyro-emission elements. Therefore, the discharge effect of the two pyroelectric elements is canceled by each other, and the detector has no signal output.


The advantages and disadvantages of passive pyroelectric infrared probes:


Advantages:


1. The passive infrared sensor itself does not emit any type of radiation and has good concealment;


2. The power consumption of the device is very small;


3. Its price is low.


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Disadvantages:


1. The signal amplitude is small and easily interfered with by various heat sources and light sources;


2. Passive infrared penetration is poor; the infrared radiation of the human body is easily blocked and difficult to be received by the probe.


3. It is susceptible to interference from radio frequency radiation.


4. When the ambient temperature is close to the human body temperature, the detection and sensitivity will decrease significantly, sometimes causing short-term failure.


5. The main detection movement direction of the passive infrared detector is the lateral movement direction. Besides, its ability to detect objects moving in the radial direction is relatively poor.


Ⅳ. The application of pyroelectric infrared sensor


Pyroelectric crystals have been widely used in infrared spectrometers, infrared remote sensing, thermal radiation detectors, etc. In addition to being used in corridor automatic switches and anti-theft alarms, it has also been used in other fields. For example: air conditioners and water dispensers can be automatically shut down when there is no one in the room; a circuit will automatically shut down the TV after it is judged that no one is watching or the audience is asleep; a monitor or an automatic doorbell can be automatically turned on when a person approaches; a camera or digital camera will automatically record the activities of animals or people;...


Automatic door


In the field of automatic doors, passive human body pyroelectric infrared sensor switches are widely used. Because of their stable performance and long-term reliable work, they are welcomed by users. This kind of switch is mainly composed of several parts such as human body pyroelectric infrared sensors and signal processing circuit, control and execution circuit, power circuit, etc.


The pyroelectric infrared automatic door is mainly composed of the optical system, pyroelectric infrared sensor, signal filtering, signal processing and automatic door circuit. The Fresnel lens can focus the infrared radiation radiated by the human body onto the pyroelectric infrared detection element. At the same time, it also produces alternating high-sensitivity and blind areas of infrared radiation to adapt to the constantly changing characteristics of the pyroelectric detection element; The electric infrared sensor is the core device in the design of the alarm. It can convert the infrared signal of the human body into an electric signal for the signal processing part; the signal processing mainly amplifies, filters, delays, and compares the weak electrical signal output from the sensor to lay the foundation for the implementation of the function.


Human body induction lamp


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Intelligent air conditioner


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The intelligent air conditioner can detect whether there are people in the room and whether the people are still or moving. Then it can automatically control the power on and off, the amount of cooling (heating), and the room temperature accordingly, so as to achieve the purpose of energy saving and humanization.

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