A motion detector refers to a device that is used to detect objects on motion, specifically people. This device is sometimes integrated as a system component that performs a task automatically or alerts the user of any motion detected in an area. In addition, motion detectors are very important component of home control, security, energy efficiency, automated lighting control, and other vital systems. There are four different types of detectors/sensors that are used in a motion sensor spectrum, and they include passive infrared (PIR), Ultrasonic, Microwave and Tomographic motion detector.
The earlier designs were very unreliable. For example, the automatic sensor-based lighting would sometimes time-out, and thus leaving the user in darkness trying to reach the switch to turn it on again. But now better sensors, materials, lenses, system designs, algorithms and processors enable motion detectors to be much more reliable, inexpensive, and more usable than those used in the past. The design of motion detectors needs a very keen planning as well as implementation. For instance, the design of ultrasonic motion detector is built under different subsystem as follows: A 40 kHz ultrasonic frequency transmitters; A 40 kHz ultrasonic frequency receiver; power supply unit; and a modulated audible alert tone generator. On the other hand, PIR detector is built on a printed circuit board that contains all the necessary electronics needed for interpreting detector’s signals. The entire assemble is mainly contained inside a housing that is built in a region where the detector is able to cover monitored area. Usually, the housing has a “window” made of plastic that enables the infrared energy to penetrate. The penetration of infrared passes through the window to the detector because the plastic is transparent to the radiation of the infrared. The plastic window also prevents the entrance of foreign objects (insects, dust, etc.) from interfering with the detector’s field of view, or leading to false alarms. In addition, the window can serve as a filter for wavelengths (regulating it to 8-14 micrometres) so as to make it almost the same as the infrared radiation released by humans.
Theoretically, the detection mechanisms of local motion must satisfy a given minimum requirements so as to signal motion through a directionally selective way. In summary, a motion detector has to satisfy the following (Borst and Egelhaaf 297):
It needs more that two inputs: The two inputs are required because motion is a vector, and hence it requires two points for it to be represented. It is not possible for a single photoreceptor to differentiate a dark bar that passes through its receptive surface from the left to the right.
A non-linear interaction between the two inputs: This is required to ensure that the averaged time for a detector output would not be equal to that of detector response to its averaged time input signals. However, in the averaged time input signals, all the information on temporal sequence gets lost. Therefore, motion detectors having linear interaction could never be directionally selective.
It has to be asymmetrical: The way the two signal inputs of a motion detector are processed should be slightly different. In case the detector was symmetrical, the input channels of the detector could be replaced for each other without causing effect to its output. This could make it difficult to discriminate which channel was initiated first and which last. Therefore, motion detectors that are symmetrical could never be directionally selective.
In order to locate the visual motion in wide-angle visual fields, the following process takes place. The module of motion detection receives a digital image from a wide-angle camera, as input, and it then calculates the difference between successive images that is within a local surface/field. Later, the motion segmenter uses a technique of region-growing to determine contiguous motion blocks within the image difference (MIT, n. pag.).
The first limitation for using these detectors is that it might not fully cover the room. Secondly, unwanted things may interfere with the motion detector. For instance, external detector that is mounted closely to a light that is kept on throughout the night will continuously be interfered by bugs. Thirdly, the reliability of a motion detector can be interrupted by rapid changes in environmental condition as well as direct sunlight. Lastly, things like wind blowing direct from heater and an air conditioner and a fireplace may also affect the detector. The reason is that PIR sensor detects variations in infrared energy, particularly heat energy released from the temperature of a human skin.
Cost of the equipment
The cost of this device varies from one market to another, and it also depends with the kind of motion detector to be bought. For example, in home depot, the prices of some motion detectors are given as follows: PIR Detector (Bosch Professional Series, 50 ft. x 70 ft.) costs $61.80; Panoramic PIR Detector (Bosch, 24 ft. Low-profile) costs $77.25; Panoramic Detector (Bosch TriTech, 70 ft.) costs $133.90; and Quad PIR Motion (Bosch Blue Line Gen2, 40 ft. x 40 ft.) costs $35.02 (The Home Depot 1).
Borst, A. and Egelhaaf, M. “Principles of visual motion detection.” TINS 12.8 (1989): 297-306. Web. 12 April 2014. <http://pub.uni-bielefeld.de/luur/download?func=downloadFile&recordOId=1774131&fileOId=2311405>.
MIT. Motion Detection and Segmentation. MIT, n. d. Web. 12 April 2014. <http://www.ai.mit.edu/projects/cog/VisionSystem/motion_detection.html>.
The Home Depot. Motion Detectors. The Home Depot, n. d. Web. 12 April 2014. <http://www.homedepot.com/b/Electrical-Home-Security-Video-Surveillance-Security-Systems-Motion-Detectors/N-5yc1vZc20t?Ns=P_REP_PRC_MODE%7C1>.