A motion sensor (or motion detector) is an electrical device that utilizes a sensor to detect nearby object's motion. Motion sensors are used primarily in security, automated lighting control, home control, energy efficiency, and other useful systems. They can be used to activate floodlights, trigger audible alarms, activate switches, and even alert the police.
There are several types of motion sensor technology in wide use:
Passive Infrared sensor (PIR)
The passive infrared sensor is the most commonly used motion sensor, especially in lighting control systems. PIR sensors are sensitive to a person's skin temperature through emitted black-body radiation at mid-infrared wavelengths, in contrast to background objects at ambient temperature. No energy wave is emitted from the sensor, thus the name passive infrared. The difference in temperature, as detected by the PIR, is the primary element in triggering a response.
Microwave sensor (MW)
The microwave sensor is based on the principle of Doppler radar, and is similar to a radar speed gun. A continuous wave of microwave radiation is emitted, and phase shifts in the reflected microwaves due to motion of an object toward (or away from) the receiver result in a heterodyne signal at a low audio frequency. MWs can monitor larger areas than infrared sensors can, but MWs are more vulnerable to object interference.
Ultrasonic sensor
An ultrasonic sensor emits an ultrasonic wave (sound at a frequency higher than a human ear can hear) and receives reflections from nearby objects. Exactly as in Doppler radar, heterodyne detection of the received field indicates motion. The detected doppler shift is also at low audio frequencies (for walking speeds) since the ultrasonic wavelength of around a centimeter is similar to the wavelengths used in microwave motion sensors.
Tomographic sensor
Tomographic sensor is a relatively new kind of technology. Instead of a single sensor, tomographic motion sensors use several nodes placed throughout a space. These nodes communicate with one another. When something enters the field between them, it disturbs the links, signaling movement. Tomographic sensors can be completely hidden because they don’t rely on line of sight the way other sensors do. Another advantage is that, because they use radio waves, tomographic sensors can sense through walls and other obstructions.
Summary
Microwave sensor, ultrasonic sensor, and tomographic sensor are all active motion sensors. They all have both a transmitter and a receiver.They detects motion by measuring changes in the amount of sound or radiation reflecting back into the receiver.
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