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1. Pyroelectric Infrared Sensor Principles 1.1 Principle Characteristics of Pyroelectric Infrared Sensors Pyroelectric infrared sensors and thermocouples are pyroelectric infrared sensors based on the thermoelectric effect principle. The difference is that the thermoelectric coefficient of the pyroelectric infrared sensor is much higher than that of the thermocouple. The internal thermoelectric element is composed of a high thermoelectric coefficient lead-lead mercury-titanium-iron-titanate ceramic and a combination of lithium niobate and ferric sulphate with a filter lens window. Its polarization changes with temperature. In order to suppress the interference caused by the change of its own temperature, the sensor reversely connects two thermoelectric elements with the same characteristics in series or in a differential balanced circuit manner, so that the infrared energy change emitted by the object can be detected in a non-contact manner. Convert it to electrical signal output. The purpose of pyroelectric infrared sensor in the structure of the introduction of field effect tube is to complete the impedance transformation. Since the pyroelectric element outputs a charge signal, it cannot be directly used and it needs to be converted into a voltage form using a resistor. The resistance of the resistor is as high as 104 MΩ. Therefore, the introduced N-channel junction field effect transistor should be connected to a common-drain form, ie source follower. To complete the impedance transformation. The pyroelectric infrared sensor is composed of three parts: a sensing element, an interference filter and a field effect tube matching device. The design should be made of high-temperature thermoelectric materials made of thin slices, and on both sides of the metal electrode plated, and then electrify its polarization, so that it made a pyroelectric detector. Since the voltage applied to the electrode is polar, the detector element after polarization is also positive and negative.
1.2 Principles and Characteristics of Passive Pyroelectric Infrared Sensors The human body has a constant body temperature, typically at 37 degrees, so it emits infrared light with a specific wavelength of about 10 um. The passive infrared sensor works by detecting infrared radiation emitted by the human body, which is about 10 um. of. About 10 um of infrared radiation emitted by the human body is reinforced by the Philippine Muller filter and collected on the infrared induction source. Infrared sensors usually use pyroelectric elements, which lose their charge balance when they receive a change in the temperature of infrared radiation from the human body, releasing charge outward, and the subsequent circuit can generate an alarm signal after detection and processing.
1) The probe is aimed at detecting human radiation. Therefore, pyroelectric elements must be very sensitive to infrared radiation with a wavelength of about 10um.
2) In order to be sensitive only to human body's infrared radiation, a special Phillips filter is usually covered on its radiation surface, so that the environmental interference is obviously controlled.
3) Passive infrared sensors, whose sensors contain two pyroelectric elements connected in series or in parallel. In addition, the two polarization directions produced are the opposite. The ambient background radiation has almost the same effect on the two pyroelectric elements, causing them to cancel each other's charge generation effect. Thus, the detector has no signal output.
4) Once the person invades the detection area, the infrared radiation of the human body is partially focused by the mirror and received by the pyroelectric element. However, the heat received by the two pyroelectric elements is different, and the pyroelectricity is also different and cannot be cancelled. And the alarm.
5) Philippine mud filter according to different performance requirements, with different focal length (sensing distance), resulting in a different monitoring field of view, the more the field of view, the more rigorous control.
Advantages and Disadvantages of Passive Pyroelectric Infrared Probes:
advantage:
It does not emit any kind of radiation itself, the device consumes little power and is well concealed. Low price.
Disadvantages:
1), susceptible to various heat sources, light source interference 2), passive infrared penetrating power is poor, the body's infrared radiation is easily obstructed, not easily received by the probe.
3), susceptible to radio frequency radiation interference.
4) When the ambient temperature and human body temperature are close to each other, the detection and sensitivity decrease significantly, sometimes causing short-term failure.
Anti-jamming performance:
1) Anti-small-animal interference detectors are installed at the recommended use altitude, and generally do not generate alarms for small animals on the ground within the detection range.
2) The anti-electromagnetic interference performance of the anti-electromagnetic interference detector meets the requirements of 4.6.1 of GB10408. Generally, the electromagnetic interference of the mobile phone will not cause false alarms.
3) The anti-light interference detector is within the range of normal sensitivity, and the H4 halogen lamp of 3 meters is transmitted through the glass without alarm.
Infrared pyroelectric sensor installation requirements:
The infrared pyroelectric human sensor can only be installed indoors, and its false alarm rate has a great relationship with the location and mode of installation. The correct installation should meet the following conditions:
1) The infrared pyroelectric sensor should be 2.0-2.2 meters from the ground.
2) Infrared pyroelectric sensors are far away from air-conditioning, refrigerators, stoves and other places where the air temperature changes sensitively.
3) Infrared pyroelectric sensors must not be screened, furniture, large bonsai or other partitions within the detection range.
4) The infrared pyroelectric sensor should not be directly facing the window. Otherwise, the disturbance of the hot air flow outside the window and the movement of people will cause false alarms. It is better to have the curtains on conditional conditions. Infrared pyroelectric sensors should also not be installed where there is strong airflow.
The sensitivity of infrared pyroelectric sensors to the human body is also related to the direction of human motion. Infrared pyroelectric sensors are the least sensitive to radial movements and are most sensitive to movement in the cross-cut direction (ie, perpendicular to the radius). Selecting the appropriate mounting location in the field is to avoid false alarms for the infrared probe and to obtain the best results. The sensitivity of the test is extremely important.
1.3 Pyroelectric Effect When some crystals are heated, an equal number of oppositely-charged charges will be generated at both ends of the crystal. This polarization due to thermal changes is called the pyroelectric effect. In general, the bound charge generated by the spontaneous polarization of the crystal is neutralized by the free electrons attached to the surface of the crystal in the air, and its spontaneous polarization electric moment cannot be expressed. When the temperature changes, the positive and negative charge center of gravity in the crystal structure shifts relative to one another, spontaneous polarization changes, and the charge on the crystal surface is depleted. The state of charge depletion is proportional to the degree of polarization.
The crystal which can produce a pyroelectric effect is called a pyroelectric body or a pyroelectric element. Commonly used materials include monolithic (LiTaO3, etc.), piezoelectric ceramics (PZT, etc.) and high molecular films (PVFZ, etc.).
Pyroelectric infrared sensor principle and application of the introduction made according to the Fresnel principle, the infrared light is divided into visible and blind areas, at the same time have the role of focus, so that the pyroelectric infrared sensor (PIR) greatly increased sensitivity. Fresnel lens refraction and reflection two forms, its role is a focus, the infrared signal thermal refraction (reflection) on the PIR; the second is the detection zone is divided into a number of light and dark areas, The moving object entering the detection zone can generate a change pyro-infrared signal on the PIR in the form of a temperature change so that the PIR can generate a changing electrical signal.
If we connect the thermoelectric element with appropriate resistance, when the element is heated, there will be a current flowing through the resistance and a voltage signal will be obtained at both ends.
2. Pyroelectric Infrared Sensor Application - Automatic Door 2.1 Automatic Door Design Principle In the field of automatic doors, the passive human body pyroelectric infrared sensor switch is widely used. It is widely used because of its stable performance and long-term stable and reliable work. Welcome, this switch is mainly composed of human pyroelectric infrared sensor, signal processing circuit, control and execution circuit, power supply circuit and other parts.
Pyroelectric infrared automatic doors are mainly composed of optical systems, pyroelectric infrared sensors, signal filtering and amplification, signal processing, and automatic gate circuits. The Fresnel lens can focus the infrared radiation of the human body radiation on the pyroelectric infrared detector element, and at the same time also produce alternately changing high sensitivity regions and blind regions of the infrared radiation, so as to adapt to the changing characteristics of the pyroelectric detection element request signal; 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 use by the signal processing part. The signal processing mainly amplifies, filters, delays, and compares the weak electric signal output by the sensor. To lay the foundation for the realization of the alarm function.
In this detection technique, the so-called "passive" means that the detector itself does not emit any form of energy, but merely receives natural energy or energy changes to complete the detection. Passive infrared automatic door is characterized by being able to respond to changes in infrared radiation caused by the movement of the human body in the detection area, and can make the monitoring alarm generate an alarm signal to complete the alarm function. Figure 6 shows the operating circuit schematic of the alarm.
When the infrared radiation emitted by the human body is focused on the detector element of the pyroelectric infrared sensor through the Fresnel lens, the sensor in the circuit will output the voltage signal, and then the signal is first passed through a combination of C1, C2, R1, and R2. A bandpass filter with an upper cutoff frequency of 16 Hz and a lower cutoff frequency of 0.16 Hz. Because the pyroelectric infrared sensor output detection signal voltage is very weak (usually only about 1mV), but also a changing signal, while the role of the Fresnel lens makes the output signal voltage in the form of pulses (pulse voltage frequency is Since the moving speed of the measuring object is usually about 0.1 to 10 Hz, the voltage signal output from the pyroelectric infrared sensor should be amplified. This design uses an integrated operational amplifier, the LM324, to perform two stages of amplification so that it gains enough gain. When the sensor detects the human body radiation infrared signal and sends it to the window comparator after being amplified, if the signal amplitude exceeds the upper and lower limits of the window comparator, the system will output a high-level signal; if there is no abnormal situation, it will output a low-level signal. In this comparator, R9, R10, and R11 are used as reference voltages. Two operational amplifiers are used for comparison. The main role of the two diodes is to make the output more stable. The upper and lower limit voltages of the window comparators are reference voltages of 3.8V and 1.2V, respectively. This high and low level signal rising edge signal is used as the trigger signal of the one-shot HEF4538B, and it is allowed to output a high level signal with a pulse width of about 10 s. Then use this pulse width signal as the input control signal of the alarm circuit KD9561 to make the circuit generate a 10s alarm signal, and finally use the triode VT1 and VT2 to amplify the electric signal again so that there is enough current to drive the horn. 10s alarms are issued continuously.
The first two LM324s are two-stage amplifiers. The sensing signal generated after the sensor detects the human body's infrared rays is weak, and many bypass capacitors are set in the circuit to suppress interference and prevent malfunction. The last two LM324s are upper and lower voltage comparators. Only the alternating signal generated by the sensor is amplified to a sufficient level to make the output high, so as to control whether the relay K1 is energized. K1 gets power and will enter the automatic door control section. When K1 is energized, KA2 is energized and KA2 is normally closed and self-locking. When the motor starts rotating forward, the door opens slowly. When the contact switch QS1 is touched, the previously actuated contact is reset, the motor is stopped, the door is not moved, and it is always in the open state. At the same time, the normally open contact of the limit switch is closed, and the time relay KT is delayed by 5 seconds. After 5S, KT is normally open and closed, KA is powered, and KA3 is normally closed and self-locking. The motor starts to reverse until it hits the limit switch QS2. The door is closed and the motor stops rotating. The above is all the circuit operation process.
2.2 Installation:
Component selection and parameter determination:
In this design, LM324 was mainly chosen as the main chip, because the two chips had contact in the previously learned books, and they were relatively familiar to use. The LM324 is a comparator with two-pole amplification. In the automatic door control circuit, I used relays, time relays, trip switches, and other strong electrical devices to achieve the normal operation of automatic doors.
3. Conclusions <br> <br> as the correlation signal processor performance and increasing the reliability of the pyroelectric crystal has been widely used in infrared spectroscopy, infrared and thermal radiation sensing detectors, because of its low cost, stable performance art It is welcomed by a large number of users and professionals and is widely used in various automatic control devices. It can be used as an ideal detector for infrared lasers, and it can also be applied to infrared fields such as burglar alarms and automatic doors.
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Pyroelectric infrared sensor principle and application introduction
With the development of society, various automatic control systems that are convenient for life have begun to enter people's lives. Automatic door systems based on pyroelectric infrared sensors are among them. The pyroelectric infrared sensor is a pyroelectric infrared sensor based on the thermoelectric effect principle. The internal thermoelectric elements are composed of high-thermocoefficient lead titanate-lead-mercury ceramics and lithium niobate and ferric-gallium sulfate. The polarization of these elements varies with temperature. The pyroelectric infrared sensor is composed of three parts: a sensing element, an interference filter and a field effect tube matching device. The design should be made of high-temperature thermoelectric materials made of thin slices, and on both sides of the metal electrode plated, and then electrify its polarization, so that made a pyroelectric detector.