Motion detector

Amotion detectoris an electrical device that utilizes asensorto detect nearby motion (motion detection). Such a device is often integrated as acomponentof a system that automatically performs a task oralerts a userof motion in an area. They form a vital component of security,automated lighting control,home control,energy efficiency,and other useful systems. It can be achieved by eithermechanicalorelectronicmethods.^[1]When it is done by natural organisms, it is calledmotion perception.

Overview

Anactiveelectronic motion detector contains an optical, microwave, or acoustic sensor, as well as a transmitter. However, apassivecontains only a sensor and only senses a signature from the moving object via emission or reflection. Changes in the optical, microwave or acoustic field in the device's proximity are interpreted by the electronics based on one of several technologies. Most low-cost motion detectors can detect motion at distances of about 4.6 metres (15 ft). Specialized systems are more expensive but have either increased sensitivity or much longer ranges.Tomographicmotion detection systems can cover much larger areas because the radio waves it senses are at frequencies which penetrate most walls and obstructions, and are detected in multiple locations.

Motion detectors have found wide use in commercial applications. One common application is activating automatic door openers in businesses and public buildings. Motion sensors are also widely used in lieu of a trueoccupancy sensorin activating street lights or indoor lights in walkways, such as lobbies and staircases. In suchsmart lightingsystems, energy is conserved by only powering the lights for the duration of a timer, after which the person has presumably left the area. A motion detector may be among the sensors of aburglar alarmthat is used to alert the home owner or security service when it detects the motion of a possible intruder. Such a detector may also trigger asecurity camerato record the possible intrusion.

Motion controllersare also used forvideo game consolesasgame controllers.A camera can also allow the body's movements to be used for control, such as in theKinectsystem.

Sensor technology

A passive infrared detector mounted on circuit board (right), along with photoresistive detector for visible light (left). This is the type most commonly encountered in household motion sensing devices and is designed to turn on a light only when motion is detectedandwhen the surrounding environment is sufficiently dark.

Motion can be detected by monitoring changes in:

Infrared light (passive and active sensors)
Visible light (video and camera systems)
Radio frequency energy (radar,^[2]microwave andtomographicmotion detection)
Sound (microphones, other acoustic sensors)
Kinetic energy (triboelectric,seismic, and inertia-switch sensors)
Magnetism (magnetic sensors, magnetometers)
Wi-Fi Signals (WiFi Sensing)

Several types of motion detection are in wide use:

Passive infrared (PIR)

Passive infrared (PIR) sensorsare sensitive to a person's skin temperature through emittedblack-body radiationatmid-infraredwavelengths, in contrast to background objects at room temperature. No energy is emitted from the sensor, thus the namepassive infrared.^[3]This distinguishes it from theelectric eyefor instance (not usually considered amotion detector), in which the crossing of a person or vehicle interrupts a visible or infrared beam. These devices can detect objects, people, or animals by picking up one's infrared radiation.^[4]

Mechanical

The most basic forms of mechanical motion detection utilize a switch or trigger. For example, the keys of atypewriteruse a mechanical method of detecting motion, where each key is a switch that is either off or on, and each letter that appears is a result of the key's motion.

Microwave

These detect motion through the principle ofDoppler radar,and are similar to aradar speed gun.Acontinuous waveofmicrowaveradiation is emitted, and phase shifts in the reflected microwaves due to motion of an object toward (or away from) the receiver result in aheterodynesignal at a lowaudio frequency.

Ultrasonic

Anultrasonic transduceremits anultrasonic wave(sound at a frequency higher than a human ear can hear) and receives reflections from nearby objects.^[5]Exactly as inDoppler radar,heterodyne detection of the received field indicates motion. The detecteddoppler shiftis also at low audio frequencies (for walking speeds) since the ultrasonicwavelengthof around a centimeter is similar to the wavelengths used in microwave motion detectors. One potential drawback of ultrasonic sensors is that the sensor can be sensitive to motion in areas where coverage is undesired, for instance, due to reflections of sound waves around corners.^[6]Such extended coverage may be desirable for lighting control, where the goal is the detection of any occupancy in an area, but for opening an automatic door, for example, a sensor selective to traffic in the path toward the door is superior.

Tomographic motion detector

These systems sense disturbances to radio waves as they pass from node to node of a mesh network. They have the ability to detect over large areas completely because they can sense through walls and other obstructions. RF tomographic motion detection systems may use dedicated hardware, other wireless-capable devices or a combination of the two. Other wireless capable devices can act as nodes on the mesh after receiving a software update.^[7]

Video camera software

With the proliferation of low-costdigital camerasable to shoot video, it is possible to use the output of such a camera to detect motion in its field of view usingsoftware.^[8]^[9]This solution is particularly attractive when the intent is to record video triggered by motion detection, as no hardware beyond the camera and computer is needed. Since the observed field may be normally illuminated, this may be considered anotherpassivetechnology. However, it can also be used together withnear-infraredillumination to detect motion in thedark,that is, with the illumination at a wavelength undetectable by a human eye.

More complex algorithms are necessary to detect motion when the camera itself ispanning,or when a specific object's motion must be detected in a field containing other, irrelevant movement—for example, a painting surrounded by visitors in anart gallery.With a panning camera, models based onoptical floware used to distinguish between apparent background motion caused by the camera's movement and that of independently moving objects.^[10]

Gesture detector

Photodetectors and infrared lighting elements can support digital screens todetect hand motions and gestureswith the aid of machine learning algorithms.^[11]

Dual-technology motion detectors

Many modern motion detectors use combinations of different technologies. While combining multiple sensing technologies into one detector can help reduce false triggering, it does so at the expense of reduced detection probabilities and increased vulnerability.^{[citation needed]}For example, many dual-tech sensors combine both a PIR sensor and a microwave sensor into one unit. For motion to be detected, both sensors must trip together.^{[citation needed]}This lowers the probability of a false alarm since heat and light changes may trip the (passive infrared) PIR but not the microwave, or moving tree branches may trigger the microwave but not the PIR. If an intruder is able to fool either the PIR or microwave, however, the sensor will not detect it.^{[citation needed]}

Often, PIR technology is paired with another model to maximize accuracy and reduce energy use.^{[citation needed]}PIR draws less energy than emissive microwave detection, and so many sensors are calibrated so that when the PIR sensor is tripped, it activates a microwave sensor.^{[citation needed]}^{[citation needed]}If the latter also picks up an intruder, then the alarm is sounded.

References

^"motion detection".Retrieved26 January2015.
^Amin, Moeness (2017-09-14).Radar for Indoor Monitoring: Detection, Classification, and Assessment.CRC Press.ISBN 978-1-4987-8200-5.
^Ultrasonicand Passive Infrared SensorIntegrationfor Dual TechnologyUser Detection Sensors
^"Why motion detectors react to animals and how to avoid it | Ajax Systems Blog".Ajax Systems.Retrieved2020-02-03.
^"What Is an Ultrasonic Motion Detector? (with picture)".Wisegeek.com. 2016-01-19.Retrieved2016-01-27.
^"Technology comparison of Motion Sensors".ecosirius.com.Retrieved19 July2014.
^"XANDEM – People sensing with wireless networks".Retrieved2020-12-14.
^Video motion detection (VMD)Archived2011-10-23 at theWayback Machine
^Mechanisms of visual motion
^Bewley, A., Guizilini, V., Ramos, F., & Upcroft, B. (2014).Online Self-Supervised Multi-Instance Segmentation of Dynamic Objects.In International Conference on Robotics and Automation (pp. 1296–1303). Hong Kong, China: IEEE.doi:10.1109/ICRA.2014.6907020
^Cho, Youngjun (2014)."US patent: Electronic device having proximity touch function and control method thereof".

External links

[1] "motion detection".Retrieved26 January2015.

[2] Amin, Moeness (2017-09-14).Radar for Indoor Monitoring: Detection, Classification, and Assessment.CRC Press.ISBN 978-1-4987-8200-5.

[3] Ultrasonicand Passive Infrared SensorIntegrationfor Dual TechnologyUser Detection Sensors

[4] "Why motion detectors react to animals and how to avoid it | Ajax Systems Blog".Ajax Systems.Retrieved2020-02-03.

[5] "What Is an Ultrasonic Motion Detector? (with picture)".Wisegeek.com. 2016-01-19.Retrieved2016-01-27.

[6] "Technology comparison of Motion Sensors".ecosirius.com.Retrieved19 July2014.

[7] "XANDEM – People sensing with wireless networks".Retrieved2020-12-14.

[8] Video motion detection (VMD)Archived2011-10-23 at theWayback Machine

[9] Mechanisms of visual motion

[10] Bewley, A., Guizilini, V., Ramos, F., & Upcroft, B. (2014).Online Self-Supervised Multi-Instance Segmentation of Dynamic Objects.In International Conference on Robotics and Automation (pp. 1296–1303). Hong Kong, China: IEEE.doi:10.1109/ICRA.2014.6907020

[11] Cho, Youngjun (2014)."US patent: Electronic device having proximity touch function and control method thereof".

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