Motion detection
Mechanical devices
A tripwire is a simple form of motion detector. When an object steps into the tripwire’s sensory field (i.e., trips the wire), then a simple sound device (e.g., bells) might alert the user. Similarly, a glass filled to the brim so that surface tension causes a convex meniscus can be placed on top of an object to detect if the object has moved.
Mechanical motion detection devices can be simple to circumvent by interrupting the devices’ mechanics (e.g., by “cutting the wire” or “emptying the water”). Electronic motion sensing devices, such as motion detectors, can be used to make mechanical intervention more difficult.
Electronic devices
See also: Motion estimation
The principal methods by which motion can be electronically identified are optical detection and acoustical detection. Infrared light or laser technology may be used for optical detection. Motion detection devices, such as motion detectors, have sensors that detect movement and send a signal to a sound device that produces an alarm or switch on an image recording device. There are motion detectors which employ cameras connected to a computer which stores and manages captured images to be viewed later or viewed over a computer network.
The chief applications for such detection are (a) detection of unauthorized entry, (b) detection of cessation of occupancy of an area to extinguish lighting and (c) detection of a moving object which triggers a camera to record subsequent events. The motion detector is thus a linchpin of electronic security systems, but is also a valuable tool in preventing the illumination of unoccupied spaces.
A simple algorithm for motion detection by a fixed camera compares the current image with a reference image and simply counts the number of different pixels. Since images will naturally differ due to factors such as varying lighting, camera flicker, and CCD dark currents, pre-processing is useful to reduce the number of false positive alarms.
More complex algorithms are necessary to detect motion when the camera itself is moving, or when the motion of a specific object must be detected in a field containing other movement which can be ignored. An example might be a painting surrounded by visitors in an art gallery.
Occupancy Sensors for Lighting Control
Motion sensors are often used in indoor spaces to control electric lighting. If no motion is detected, it is assumed that the space is empty, and thus does not need to be lighted. Turning off the lights in such circumstances can save substantial amounts of energy. In lighting practice occupancy sensors are sometime also called “presence sensors” or “vacancy sensors”.
System design and components
Occupancy sensors for lighting control use infrared (IR) or acoustic technology, or a combination of the two. The field of view of the sensor must be carefully selected/adjusted so that it responds only to motion in the space served by the controlled lighting. For example, an occupancy sensor controlling lights in an office should not detect motion in the corridor outside the office. Sensors and their placement are never perfect, therefore most systems incorporate a delay time before switching. This delay time is often user-selectable, but a typical default value is 15 minutes. This means that the sensor must detect no motion for the entire delay time before the lights are switched. Most systems switch lights off at the end of the delay time, but more sophisticated systems with dimming technology reduce lighting slowly to a minimum level (or zero) over several minutes, to minimize the potential disruption in adjacent spaces. If lights are off and an occupant re-enters a space, most current systems switch lights back on when motion is detected. However, systems designed to switch lights off automatically with no occupancy, and that require the occupant to switch lights on when they re-enter are gaining in popularity due to their potential for increased energy savings. These savings accrue because in a spaces with access to daylight the occupant may decide on their return that they no longer require supplemental electric lighting.
References
Motion Detection Algorithms In Image Processing
Motion Detection and Recognition Research
Categories: SecurityHidden categories: Articles to be merged from November 2009 | All articles to be merged
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