A passive infrared sensor ( PIR sensor ) is an electronic sensor that measures infrared (IR) light radiating from an object in its field of view. They are most commonly used in PIR-based motion detectors.
Video Passive infrared sensor
Principle of operation
All objects with temperatures above zero absolute radiate heat energy in the form of radiation. Usually this radiation is not visible to the human eye as it radiates at infrared wavelengths, but can be detected by electronic devices designed for such purpose.
The term passive in this case refers to the fact that the PIR device does not generate or emit energy for detection purposes. They work entirely by detecting infrared radiation emitted by or reflected from the object. They do not detect or measure "heat".
Maps Passive infrared sensor
Construction
Infrared radiation enters through the front of the sensor, known as the 'face sensor'. The core of the PIR sensor is a solid state sensor or sensor set, made of a pyroelectric material - material that generates energy when exposed to heat. Typically, the sensor is about 1/4 inch square (40 mm 2 ), and takes the form of a thin film. Materials commonly used in PIR sensors include gallium nitride (GaN), cesium nitrate (CsNO 3 ), polyvinyl fluoride, derivatives of phenylpyridine, and cobalt phthalocyanine. Sensors are often made as part of integrated circuits.
PIR motion detector
PIR-based motion detectors are used to sense the movement of people, animals, or other objects. They are commonly used in burglar alarms and lighting systems that are activated automatically. They are generally called simply "PIR", or sometimes "PID", for "passive infrared detectors".
Operation
Individual PIR sensors detect changes in the amount of infrared radiation that overwrites, which vary depending on the temperature and surface characteristics of the object in front of the sensor. When an object, like a human, passes in front of the background, like a wall, the temperature at that point in the field of view of the sensor will rise from room temperature to body temperature, and then back again. The sensor changes the resulting changes in the infrared radiation that goes into the change in output voltage, and this triggers the detection. Objects with the same temperature but different surface characteristics may also have different infrared emission patterns, and thus removing them with attention to the background may trigger the detector as well.
PIRs come in a variety of configurations for a wide range of applications. The most common models have many Fresnel lenses or mirror segments, the effective range is about ten meters (thirty feet), and the field of view is less than 180 degrees. Models with a wider field of view, including 360 degrees, are available - usually designed to be mounted on the ceiling. Some of the larger PIRs are made with single-segment mirrors and can sense changes in infrared energy more than thirty meters (one hundred feet) from the PIR. There are also PIRs that are designed with reversible orientation mirrors that allow wide coverage (110 Â ° wide) or very narrow coverage "drapes", or with segments that can be individually selected to "shape" coverage.
Differential detection
The pair of sensor elements can be transferred as opposite inputs to the differential amplifier. In such configurations, the PIR measurements cancel each other so that the field average temperature is removed from the electrical signal; the increase of IR energy across the sensor is self-canceling and will not trigger the device. This allows the device to reject any indication of incorrect change in terms of exposure to a brief flash of light or field lighting. (Continuous high energy exposures may still saturate the sensor material and make the sensor unable to register more information.) At the same time, this differential setting minimizes common-mode interference, allowing the device to withstand triggers due to the nearest electric field. However, a pair of differential sensors can not measure the temperature in this configuration, and therefore only useful for detecting motion.
Practical Implementation
When the PIR sensor is configured in differential mode, it specifically becomes valid as a motion detector device. In this mode when human motion is detected in the "line of sight" of the PIR sensor, a pair of complementary pulses are processed at the output pin of the PIR sensor. To implement this output signal for practical triggering loads such as relays or data registers, or alarms, differential signals are rectified using the bridge rectifier and fed to the transistorized relay driver circuit. These relay contacts close and open in response to signals from the PIR, activating the load attached to its contacts, recognizing the detection of a human within a predetermined range of areas.
Product design
The PIR sensor is usually installed on the printed circuit board containing the electronics required to interpret the signal from the sensor itself. Complete assembly is usually contained in housing, installed in locations where the sensor can cover the area to be monitored.
The home will usually have a plastic "window" where infrared energy can enter. Although often only translucent visible, infrared energy is able to reach the sensor through the window because the plastic is used transparently for infrared radiation. Plastic windows reduce the possibility of foreign objects (dust, insects, etc.) from obscuring the field of view of the sensor, damaging mechanisms, and/or causing false alarms. The window can be used as a filter, to limit the wavelength to 8-14 micrometers, which is closest to the infrared radiation emitted by humans. It can also serve as a focus mechanism; See below.
Focus
Different mechanisms can be used to focus far infrared energy onto the sensor surface.
Lens
The plastic window coverings may have some aspect formed into them, to focus the infrared energy into the sensor. Each individual aspect is a Fresnel lens.
- Multi-Fresnel PIR lens type
Mirrors
Some PIRs are manufactured with internally segmented parabolic mirrors to focus the infrared energy. Where a mirror is used, the plastic window cover generally does not have a Fresnel lens formed into it.
- Segmented mirror type of PIR
Security app
When used as part of a security system, electronics in PIR usually control small relays. This relay completes the circuit in a pair of electrical contacts connected to the burglar alarm detection input control zone. The system is usually designed in such a way that if no motion is detected, the relay contact is closed - 'closed normal' relay (NC). If movement is detected, open relay, trigger alarm.
Placements
Manufacturers recommend their product placement carefully to prevent false alarms (ie, any detection not caused by intruders).
They suggested installing a PIR such that the PIR could not "see" out of the window. Although the wavelengths of infrared radiation where sensitive chips do not penetrate the glass very well, powerful infrared sources (such as from vehicle headlights or sunlight) can overload the sensors and cause false alarms. A person moving on the other side of the glass will not be "seen" by PID. It may be good for windows overlooking public sidewalks, or bad for windows on interior partitions.
It is also recommended that the PIR is not placed in such a position so that the HVAC ventilation will blow hot or cold air onto the plastic surface covering the housing window. Although air has very low emissivity (emits small amounts of infrared energy), air blowing on plastic window coverings can alter enough plastic temperatures to trigger false alarms.
Sensors are also often designed to "ignore" domestic pets, such as dogs or cats, by setting a higher sensitivity threshold, or by ensuring that the floor of the room remains unfocused.
Because the PIR sensor has a range of up to 10 meters (30 feet), a single detector placed near the entrance is usually all that is required for a room with only one entrance. PIR-based security systems are also feasible in outdoor security and motion-sensitive lighting; one of its hallmarks is their low attractiveness, which allows them to power solar.
Remote-based thermometer PIR
The design has been implemented in which the PIR circuit measures the temperature of the remote object. In such circuits, non-differential PIR output is used. The output signal is evaluated based on the calibration for the IR spectrum of the specific type of material to be observed. In this way, relatively accurate and precise temperature measurements can be obtained remotely. Without calibration of the observed type of material, the PIR thermometer device is capable of measuring changes in IR emissions that are directly related to temperature changes, but the actual temperature values ​​can not be calculated.
See also
- Heat detector
- Infrared point sensors
- Sensor list
Note
External links
- How infrared motion detector components work
- Design the advice and assembly instructions of the motion detector kit
- The PIR motion detector is mated to a Gunn 10 oscillator and a 10 GHz radar detector to create a remote, solar-powered remote gate.
- AS. Patent 3,703,718 issued November 25, 1969 to H. L Berman
Source of the article : Wikipedia
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