Passive infrared detectors are one of the most commonly used devices in intrusion anti-theft alarm systems. Since its inception, the technologies used have been continuously developed, and the problems that each technology targets are not the same, and some are mainly to filter white light. Some focus on reducing the effects of high-frequency interference, and some specifically address false alarms caused by small animal activities. There are two technologies that precisely emphasize two opposing aspects, such as pulse counting technology and one-step trigger technology. Some of the two technologies are applied in the opposite direction, but the result is the same, such as the spread spectrum technology and ultra narrowband technology in wireless anti-theft systems. As the construction maintenance and marketing department personnel of the alarm service industry, they should have a certain understanding of these technologies, so that according to the use of the environment, targeted design and construction plans can be formulated to achieve the use of the material and reduce the number of false alarms. According to the model of infrared detectors that have been actually installed in China in recent years, the author has initially summarized the techniques used in the following categories.
Selecting Mirror Material Using High-Quality Materials: Improving the materials used in Fresnel lenses to make them have good wavelength selection characteristics. While allowing the infrared rays emitted by the human body to pass through, blocking the other wavelengths of light in white light as much as possible, eliminating white light interference. A good quality lens can eliminate white light interference of up to 20,000 candelas. This is equivalent to an airport runway light illuminated at a distance of 2 meters. The detector will not cause false alarms.
Double Shielding Film: In front of the sensor, special conductive filter film is attached, which is called double shielding film. The role of this film, on the one hand, can reduce the impact of sunlight, car headlights, fluorescent lamps and other light sources. When the sun shines into the room in the morning or evening, the illuminance can reach 50000 Lux. With this shielding, the light intensity can reach 100,000 Lux and it will not produce false alarms; on the other hand, this film also has the effect of resisting RF radio frequency interference, making the detector With 20V/M or 30V/M anti-interference ability. (20V/M is equivalent to 10W wireless transmitter will not produce false alarm within 1m, 30V/M is equivalent to 10W transmitter distance is 0.3m-0.5m, no false alarm will occur.)
Magnetic protection relay: This relay can prevent wireless electromagnetic interference and external damage. When there is a strong magnetic field near the detector, the external relay generally lacks magnetic field shielding and may not work properly, resulting in failure of the detector. The use of magnetic protection relays, plus a strong magnetic field no longer affect the work of the relay.
Improved Optical System Ball Lens: When the detector is working, its detection range should be opened at a certain angle in the up and down direction. In the effective detection range, its sensitivity should be consistent as much as possible. In order to improve the uniformity of the detection sector, a ball lens is used so that the focal length of each detection area on the lens remains the same and the sensitivity remains unchanged.
Sealed optical system: Insulation plastic box between the lens and circuit board, the sensor is placed in a sealed box, in a relatively constant temperature condition, can reduce the interference caused by the circuit component heating; carbon black sponge around the sensor against white light Absorption can reduce the external white light interference, but also play a role in dust, moisture, and insect invasion.
Control the detection area: use curtains, pet-proof mirrors, avoid the scope of small animal 綷-regular activities, do not detect the area where it is active (such as a certain distance above the ground in the detection area), and does not affect the detection of intruders. .
Mirror adjustment and zoom function: Due to the different roof heights, the requirements for ceiling infrared detectors are also different. By adjusting the rotating mirror, the focal length and detection height can be changed to suit different environmental requirements.
Infrared Reflective Focusing: There are three types of optical systems that are compatible with infrared sensors: reflective, transmissive, and refractive. The reflection type has the highest sensitivity and far detection distance. The Fresnel lens is transmissive and has the lowest sensitivity and centered refraction. Fresnel lenses are small, easy to seal, have good stability, and have low prices. Therefore, transmissive systems are widely used at home and abroad. However, the use of mirrors to collect infrared energy is more effective than Fresnel lenses. Therefore, the gain of the detector signal amplification circuit can be made lower, and the signal-to-noise ratio is higher, which can better prevent false alarms caused by interference signals. A few passive infrared detectors imported from abroad have used the reflection method for focusing, and thus have achieved better anti-false alarm effects.
Temperature compensation and photosensitive adjustment temperature compensation circuit: The strength of the signal detected by the passive infrared detector has a great relationship with the difference between the human body and the ambient temperature. Under normal circumstances, there is a large difference between the human body temperature and the ambient temperature. When the intruder moves, the sensor receives a large amplitude of the infrared change signal and triggers an alarm; when the ambient temperature is close to the human body temperature, the infrared change signal received by the sensor when the intruder is in motion has a small amplitude, which may be due to the signal Less than the trigger threshold without alarm. Therefore, the detector needs to compensate for the gain of the circuit according to the change of the ambient temperature.
The conventional temperature compensation is a linearly increasing shape, ie, the higher the temperature, the greater the compensation. When the ambient temperature is higher than the human body temperature, excessive compensation is obviously unreasonable, and it is prone to false alarms. The better compensation method is to use a parabolic shape, so that the detector can maintain the same sensitivity in a relatively large temperature variation range.
Photosensitive adjustment: Due to the large changes in day and night illumination, if the detector works day and night, the day and night work characteristics, will have a greater change. The detector circuit utilizes the characteristics that the photosensitive resistance changes with the illumination and the resistance changes accordingly. According to the brightness of the surrounding area and the working period, the receiving sensitivity is automatically adjusted to determine the different day/night working modes, which can effectively prevent the strong light interference during the day. False positives.
Optimize circuit design Detector operating voltage: The operating voltage range is the basic parameter of the detector. It seems simple but it is very important. One of the principles for the detection of the operating voltage of the detector is that it should be lower than the operating voltage of the control host. When the AC power is interrupted and the system is operated by the backup battery, the supply voltage will gradually decrease. If the detector operating voltage design range is small, the host can still work normally, and the detector can no longer maintain the normal state, and the system will continue to Generate false positives.
Power Filter: Using an electronic filter can better filter out the noise caused by the clutter in the power supply. The clutter to be filtered out by the electronic filter is mainly 50 Hz/60 Hz alternating current frequency and its harmonics, thereby reducing the false alarm caused by power interference.
One-step triggering technology: It is mainly used in applications where there is no interference (such as a well-closed basement, etc.) and high sensitivity requirements. On the basis of not increasing false positives, applying this technique, a moving object passes through the detection area and a pulse is generated to trigger an alarm. The response is very sensitive.
Pulse counting technology: The Fresnel lens divides the coverage area into a certain number of detection areas. When a temperature change occurs between two adjacent areas, a pulse signal is triggered. Setting multi-pulse start-up mode can, to some extent, reduce false alarms caused by accidental causes. In this mode, the detector counts the number of trigger edges triggered by the intruder from the beginning through digital counting, and triggers an alarm signal according to the number of digital pulses set in advance. However, this method will reduce the sensitivity.
Energy accumulation technology: According to the detected signal strength, determine the amount of energy, when the threshold is reached, generate an alarm. Small objects have a small coverage area, such as occupying only a part of a detection area of ​​a mirror, or occupying only one or even less than one area in far, middle, and near-field detection areas. Therefore, the signal strength does not meet the threshold requirements. No alarm will be generated.
Anti-high-frequency interference technology: When there is high-frequency interference, the intensity of the interference signal increases. If the trigger threshold of the detector does not change and the signal-to-noise ratio decreases, it is prone to false alarms. With the use of anti-high frequency interference technology, the detector's threshold will increase correspondingly with the enhancement of the interference signal, so that the signal-to-noise ratio will remain basically unchanged, thereby reducing false alarms caused by high-frequency interference.
Multivariate detection technology Dual sensor detection: In a detector, the two sensors are connected in a positive and negative manner. This has the advantage that when the ambient temperature, lighting or other factors change, each component produces the same The signals of change, but the opposite polarity, cancel each other out. When there is an external intrusion, the infrared source must appear in one zone for a given period of time and then appear in the other zone. The two components generate signals in succession to form a probe pulse. For dual infrared detectors, the dual sensor balance represents the detector's anti-jamming capability. The better the balance, the stronger the anti-jamming capability.
Four-element sensor detection: The detector is equipped with four dual infrared sensors, two with positive sensitivities, and the moving heat source generates positive signals; two have negative sensitivities, and the moving heat source generates negative signals, which are positive and negative. Connected. The detector is designed to trigger an alarm only when a signal is detected by each part. Due to the small size of mice, such as mice, only one or two of the components can be triggered to generate signals when moving. Therefore, alarms are not caused. However, the large size of humans makes it easy to satisfy this condition. Therefore, the four-element infrared detector has a good effect on preventing false alarms from pets.
Sensor array detection: Eight independent infrared sensors are used, and array-type processing is adopted. Each sensor has a function of real-motion recognition of the human body. The signals received by the sensor arrays are compared and processed in a unified manner to accurately distinguish the interference signals generated by the human body movement signals or other environmental factors.
Intelligent judgment technology Intelligent judgment: Internally install a microprocessor chip, use the principle of fuzzy mathematics, analyze the signal's period, amplitude, duration, and polarity. Through extensive analysis of the human and animal reflection spectrum, a rich content recognition database is established to compare the difference between human activities and other factors to determine whether there is a change in the alarm mode or not, in order to improve the accuracy of the alarm.
Direction judgment: The direction detection function refers to the direction in which the detector can judge the movement of the object. Only the moving object that meets the pre-specified direction will cause an alarm. The general method is to install two sensors in the detector, judge according to the order of their triggering, and then determine whether it is an intrusion. For example, when a person enters the room from the outside, an alarm may be triggered. If the person walks outdoors from the room, no alarm will be given.
Wireless System Technology Rolling Code Technology: Rolling code is the use of advanced non-linear bit encryption technology before the transmission of code, resulting in a highly confidential scrolling code. The code sent each time is unique, irregular, and non-repeating, making it difficult to achieve any deciphering means such as illegal capture and scan tracking. Its main advantage is its good security. At present, the rolling code communication integrated circuit uses the serial number, the encryption key, and the synchronization count value to encrypt and encrypt the 32-bit highly confidential scrolling code. The 16-bit synchronization counter value is updated after each transmission, so the encrypted data is different each time, and without knowing the key, the subsequent password cannot be inferred from the previous password. The 32-bit password plus 34-bit plaintext will be sent together by the remote controller. This combination has reached as many as 700 billion, and under normal circumstances cannot be cracked.
Spread spectrum communication technology: In the wireless anti-jamming technology, it can include many means, such as high-frequency adaptive anti-jamming technology, high-speed frequency hopping technology, ultra-narrow band technology, but due to the cost and volume limitations, really have used wireless detection The device is spectrum expansion technology, referred to as spread spectrum technology. Its working principle is to hide the communication signal in the noise and use a wide frequency band to form pseudo-noise communication. As long as the power is effectively controlled, the synthesized noise waveform can be encoded and decoded. Therefore, it is also called pseudo noise. Communication. This kind of technology has strong anti-interference ability.
Detector protection against shading: In order to prevent human passive infrared detectors from shielding and rendering them ineffective, the general method is to add active infrared detection function in passive infrared detectors, if within a few tens of centimeters, there is cover As a result, the emitted infrared light will be reflected back into the detector, causing it to alarm.
Anti-spray adhesive: Anti-spray adhesive and anti-blocking have similarities, both require a short distance active infrared detection. When the surface of the detector is sprayed, the refractive index of the lens will change, which will cause the reflection of the active infrared energy, return to the detector, and cause an alarm.
Anti-smashing: In the detector is equipped with a shock sensor, which will trigger a shock sensor to trigger an alarm.
PCB Design and Manufacturing Process Surface Mount Technology: Using Surface Mount (SMD) technology to reduce the parasitic reactance (including parasitic inductance, parasitic capacitance, and parasitic conductance) caused by the leads, the process can better improve the detector's RF protection. Interference ability.
Circuit board design: Properly arrange the wiring of the circuit board, paying particular attention to the layout of the ground wire, which can significantly reduce the influence of external electromagnetic interference and surge on the detector.
Concluding Remarks There are many techniques used in the detectors, such as double-detection detection that is coupled with microwave detection or air vibration detection, quantum-type infrared detection due to different detection principles, etc., which are difficult to cover in this article. Due to limited space, some of the technical principles involved can not be fully explained. The intention is to allow users to have a preliminary understanding in order to correctly select and apply.
Selecting Mirror Material Using High-Quality Materials: Improving the materials used in Fresnel lenses to make them have good wavelength selection characteristics. While allowing the infrared rays emitted by the human body to pass through, blocking the other wavelengths of light in white light as much as possible, eliminating white light interference. A good quality lens can eliminate white light interference of up to 20,000 candelas. This is equivalent to an airport runway light illuminated at a distance of 2 meters. The detector will not cause false alarms.
Double Shielding Film: In front of the sensor, special conductive filter film is attached, which is called double shielding film. The role of this film, on the one hand, can reduce the impact of sunlight, car headlights, fluorescent lamps and other light sources. When the sun shines into the room in the morning or evening, the illuminance can reach 50000 Lux. With this shielding, the light intensity can reach 100,000 Lux and it will not produce false alarms; on the other hand, this film also has the effect of resisting RF radio frequency interference, making the detector With 20V/M or 30V/M anti-interference ability. (20V/M is equivalent to 10W wireless transmitter will not produce false alarm within 1m, 30V/M is equivalent to 10W transmitter distance is 0.3m-0.5m, no false alarm will occur.)
Magnetic protection relay: This relay can prevent wireless electromagnetic interference and external damage. When there is a strong magnetic field near the detector, the external relay generally lacks magnetic field shielding and may not work properly, resulting in failure of the detector. The use of magnetic protection relays, plus a strong magnetic field no longer affect the work of the relay.
Improved Optical System Ball Lens: When the detector is working, its detection range should be opened at a certain angle in the up and down direction. In the effective detection range, its sensitivity should be consistent as much as possible. In order to improve the uniformity of the detection sector, a ball lens is used so that the focal length of each detection area on the lens remains the same and the sensitivity remains unchanged.
Sealed optical system: Insulation plastic box between the lens and circuit board, the sensor is placed in a sealed box, in a relatively constant temperature condition, can reduce the interference caused by the circuit component heating; carbon black sponge around the sensor against white light Absorption can reduce the external white light interference, but also play a role in dust, moisture, and insect invasion.
Control the detection area: use curtains, pet-proof mirrors, avoid the scope of small animal 綷-regular activities, do not detect the area where it is active (such as a certain distance above the ground in the detection area), and does not affect the detection of intruders. .
Mirror adjustment and zoom function: Due to the different roof heights, the requirements for ceiling infrared detectors are also different. By adjusting the rotating mirror, the focal length and detection height can be changed to suit different environmental requirements.
Infrared Reflective Focusing: There are three types of optical systems that are compatible with infrared sensors: reflective, transmissive, and refractive. The reflection type has the highest sensitivity and far detection distance. The Fresnel lens is transmissive and has the lowest sensitivity and centered refraction. Fresnel lenses are small, easy to seal, have good stability, and have low prices. Therefore, transmissive systems are widely used at home and abroad. However, the use of mirrors to collect infrared energy is more effective than Fresnel lenses. Therefore, the gain of the detector signal amplification circuit can be made lower, and the signal-to-noise ratio is higher, which can better prevent false alarms caused by interference signals. A few passive infrared detectors imported from abroad have used the reflection method for focusing, and thus have achieved better anti-false alarm effects.
Temperature compensation and photosensitive adjustment temperature compensation circuit: The strength of the signal detected by the passive infrared detector has a great relationship with the difference between the human body and the ambient temperature. Under normal circumstances, there is a large difference between the human body temperature and the ambient temperature. When the intruder moves, the sensor receives a large amplitude of the infrared change signal and triggers an alarm; when the ambient temperature is close to the human body temperature, the infrared change signal received by the sensor when the intruder is in motion has a small amplitude, which may be due to the signal Less than the trigger threshold without alarm. Therefore, the detector needs to compensate for the gain of the circuit according to the change of the ambient temperature.
The conventional temperature compensation is a linearly increasing shape, ie, the higher the temperature, the greater the compensation. When the ambient temperature is higher than the human body temperature, excessive compensation is obviously unreasonable, and it is prone to false alarms. The better compensation method is to use a parabolic shape, so that the detector can maintain the same sensitivity in a relatively large temperature variation range.
Photosensitive adjustment: Due to the large changes in day and night illumination, if the detector works day and night, the day and night work characteristics, will have a greater change. The detector circuit utilizes the characteristics that the photosensitive resistance changes with the illumination and the resistance changes accordingly. According to the brightness of the surrounding area and the working period, the receiving sensitivity is automatically adjusted to determine the different day/night working modes, which can effectively prevent the strong light interference during the day. False positives.
Optimize circuit design Detector operating voltage: The operating voltage range is the basic parameter of the detector. It seems simple but it is very important. One of the principles for the detection of the operating voltage of the detector is that it should be lower than the operating voltage of the control host. When the AC power is interrupted and the system is operated by the backup battery, the supply voltage will gradually decrease. If the detector operating voltage design range is small, the host can still work normally, and the detector can no longer maintain the normal state, and the system will continue to Generate false positives.
Power Filter: Using an electronic filter can better filter out the noise caused by the clutter in the power supply. The clutter to be filtered out by the electronic filter is mainly 50 Hz/60 Hz alternating current frequency and its harmonics, thereby reducing the false alarm caused by power interference.
One-step triggering technology: It is mainly used in applications where there is no interference (such as a well-closed basement, etc.) and high sensitivity requirements. On the basis of not increasing false positives, applying this technique, a moving object passes through the detection area and a pulse is generated to trigger an alarm. The response is very sensitive.
Pulse counting technology: The Fresnel lens divides the coverage area into a certain number of detection areas. When a temperature change occurs between two adjacent areas, a pulse signal is triggered. Setting multi-pulse start-up mode can, to some extent, reduce false alarms caused by accidental causes. In this mode, the detector counts the number of trigger edges triggered by the intruder from the beginning through digital counting, and triggers an alarm signal according to the number of digital pulses set in advance. However, this method will reduce the sensitivity.
Energy accumulation technology: According to the detected signal strength, determine the amount of energy, when the threshold is reached, generate an alarm. Small objects have a small coverage area, such as occupying only a part of a detection area of ​​a mirror, or occupying only one or even less than one area in far, middle, and near-field detection areas. Therefore, the signal strength does not meet the threshold requirements. No alarm will be generated.
Anti-high-frequency interference technology: When there is high-frequency interference, the intensity of the interference signal increases. If the trigger threshold of the detector does not change and the signal-to-noise ratio decreases, it is prone to false alarms. With the use of anti-high frequency interference technology, the detector's threshold will increase correspondingly with the enhancement of the interference signal, so that the signal-to-noise ratio will remain basically unchanged, thereby reducing false alarms caused by high-frequency interference.
Multivariate detection technology Dual sensor detection: In a detector, the two sensors are connected in a positive and negative manner. This has the advantage that when the ambient temperature, lighting or other factors change, each component produces the same The signals of change, but the opposite polarity, cancel each other out. When there is an external intrusion, the infrared source must appear in one zone for a given period of time and then appear in the other zone. The two components generate signals in succession to form a probe pulse. For dual infrared detectors, the dual sensor balance represents the detector's anti-jamming capability. The better the balance, the stronger the anti-jamming capability.
Four-element sensor detection: The detector is equipped with four dual infrared sensors, two with positive sensitivities, and the moving heat source generates positive signals; two have negative sensitivities, and the moving heat source generates negative signals, which are positive and negative. Connected. The detector is designed to trigger an alarm only when a signal is detected by each part. Due to the small size of mice, such as mice, only one or two of the components can be triggered to generate signals when moving. Therefore, alarms are not caused. However, the large size of humans makes it easy to satisfy this condition. Therefore, the four-element infrared detector has a good effect on preventing false alarms from pets.
Sensor array detection: Eight independent infrared sensors are used, and array-type processing is adopted. Each sensor has a function of real-motion recognition of the human body. The signals received by the sensor arrays are compared and processed in a unified manner to accurately distinguish the interference signals generated by the human body movement signals or other environmental factors.
Intelligent judgment technology Intelligent judgment: Internally install a microprocessor chip, use the principle of fuzzy mathematics, analyze the signal's period, amplitude, duration, and polarity. Through extensive analysis of the human and animal reflection spectrum, a rich content recognition database is established to compare the difference between human activities and other factors to determine whether there is a change in the alarm mode or not, in order to improve the accuracy of the alarm.
Direction judgment: The direction detection function refers to the direction in which the detector can judge the movement of the object. Only the moving object that meets the pre-specified direction will cause an alarm. The general method is to install two sensors in the detector, judge according to the order of their triggering, and then determine whether it is an intrusion. For example, when a person enters the room from the outside, an alarm may be triggered. If the person walks outdoors from the room, no alarm will be given.
Wireless System Technology Rolling Code Technology: Rolling code is the use of advanced non-linear bit encryption technology before the transmission of code, resulting in a highly confidential scrolling code. The code sent each time is unique, irregular, and non-repeating, making it difficult to achieve any deciphering means such as illegal capture and scan tracking. Its main advantage is its good security. At present, the rolling code communication integrated circuit uses the serial number, the encryption key, and the synchronization count value to encrypt and encrypt the 32-bit highly confidential scrolling code. The 16-bit synchronization counter value is updated after each transmission, so the encrypted data is different each time, and without knowing the key, the subsequent password cannot be inferred from the previous password. The 32-bit password plus 34-bit plaintext will be sent together by the remote controller. This combination has reached as many as 700 billion, and under normal circumstances cannot be cracked.
Spread spectrum communication technology: In the wireless anti-jamming technology, it can include many means, such as high-frequency adaptive anti-jamming technology, high-speed frequency hopping technology, ultra-narrow band technology, but due to the cost and volume limitations, really have used wireless detection The device is spectrum expansion technology, referred to as spread spectrum technology. Its working principle is to hide the communication signal in the noise and use a wide frequency band to form pseudo-noise communication. As long as the power is effectively controlled, the synthesized noise waveform can be encoded and decoded. Therefore, it is also called pseudo noise. Communication. This kind of technology has strong anti-interference ability.
Detector protection against shading: In order to prevent human passive infrared detectors from shielding and rendering them ineffective, the general method is to add active infrared detection function in passive infrared detectors, if within a few tens of centimeters, there is cover As a result, the emitted infrared light will be reflected back into the detector, causing it to alarm.
Anti-spray adhesive: Anti-spray adhesive and anti-blocking have similarities, both require a short distance active infrared detection. When the surface of the detector is sprayed, the refractive index of the lens will change, which will cause the reflection of the active infrared energy, return to the detector, and cause an alarm.
Anti-smashing: In the detector is equipped with a shock sensor, which will trigger a shock sensor to trigger an alarm.
PCB Design and Manufacturing Process Surface Mount Technology: Using Surface Mount (SMD) technology to reduce the parasitic reactance (including parasitic inductance, parasitic capacitance, and parasitic conductance) caused by the leads, the process can better improve the detector's RF protection. Interference ability.
Circuit board design: Properly arrange the wiring of the circuit board, paying particular attention to the layout of the ground wire, which can significantly reduce the influence of external electromagnetic interference and surge on the detector.
Concluding Remarks There are many techniques used in the detectors, such as double-detection detection that is coupled with microwave detection or air vibration detection, quantum-type infrared detection due to different detection principles, etc., which are difficult to cover in this article. Due to limited space, some of the technical principles involved can not be fully explained. The intention is to allow users to have a preliminary understanding in order to correctly select and apply.
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