Ultrasonic Motion Detector,

Ultrasonic Motion Detector,
type of electronic security device that senses movement and usually triggers an alarm. Many types of motion detectors can sense motion in total darkness, without an intruder becoming aware that an alarm has been triggered.

Motion detectors are an important part of most burglar alarm systems. They help alert security personnel, especially in situations where no obvious break-in has occurred. For instance, if an intruder steals a key to gain access to a protected site or hides within the site during normal business hours, the intruder’s entrance or presence could go unnoticed. A motion detector will detect the intruder’s movements as soon as he or she walks or otherwise moves within the area protected by the detector.

Motion detectors usually protect indoor areas, where conditions can be more closely controlled. Detectors for use in homes usually detect movement in spaces about 11 m × 11 m (35 ft × 35 ft) in area. Detectors for large warehouses can protect areas with dimensions as large as 24 m × 37 m (80 ft × 120 ft). Buildings with valuable or important assets, such as museums, also use motion detectors to detect break-ins at vulnerable points. Such points include walls, doors, windows, skylights, and even air ducts. Special motion detectors can protect the inside of exhibit cases where items such as diamonds are displayed. Others can be focused to a narrow area of coverage, somewhat like a curtain, that is projected in front of a painting to detect even the slightest touch.

II  TYPES AND OPERATION OF MOTION DETECTORS,

Motion detector systems use a variety of methods to detect movement. Each method has advantages and disadvantages. Infrared detectors sense heat, and can detect an intruder’s radiating body heat. Most infrared detectors are passive, meaning they do not send out signals but merely receive signals, such as changes in temperature. Microwave and ultrasonic detectors are active—that is, they send out waves of energy and receive waves reflected back by objects. Any disturbance in the reflected waves caused by a moving object will trigger an alarm. Video systems optically monitor an area, and movement is registered visually on a monitor. Often, these methods are combined to improve accuracy and reduce chances of a false alarm.

A  Infrared,

Passive infrared motion detectors are sensitive to infrared wavelengths of energy, which are invisible to the human eye. Infrared energy can be detected as heat, and this type of detector senses the heat that a human or other animal emits. Passive infrared motion detectors have lenses that allow them to “see” an area as several distinct zones within the range of the detector. These zones spread out vertically and horizontally from the detector lens as a series of fingerlike areas, fanning out from the lens across the protected area and down to the floor. Electronic circuits enable the detector to recognize the normal amount of heat that is usually present in the area. When an intruder enters, the heat from the intruder’s body adds to the amount of heat normally present in the area. As the intruder moves from one zone to another within the detector’s field of view, the detector senses an increase in infrared energy and signals an alarm. Infrared motion detectors are very sensitive to heat changes, but they cannot see through solid objects or even through glass.

Infrared energy is also used in active infrared motion detectors. These detectors project a beam of infrared energy from one unit (the transmitter) to another (the receiver). An intruder who moves through the beam breaks a circuit in the receiver, triggering an alarm. Active infrared motion detectors are a more advanced version of electric-eye motion detectors, which use a photoelectric cell to detect an interruption of a beam of light. Active infrared systems use invisible infrared energy, so an intruder is less likely to discover or avoid the system.

B  Ultrasonic,

Some older burglar alarm systems use ultrasound (sound of very high frequency) to detect motion. They are called ultrasonic motion detectors. In such a detector a transmitter sends out sound of a frequency that is too high for the human ear to hear. A receiver picks up the sound waves reflected from the room or area under protection. The motion of someone or something in the space between the receiver and transmitter will cause a change, or shift, in the frequency of the sound. A circuit in the device detects any unusual shift in the frequency. A small shift, such as that produced by an insect or rodent, is ignored. When a larger shift, such as one produced by a moving person, is detected, the device triggers the alarm. Ultrasonic detectors are extremely sensitive, and can sometimes be triggered by loud noises or air gusts from an open vent.

The frequency shift discussed above is also known as the Doppler effect, which results from the behavior of sound waves when they are compressed by a moving object. Ultrasonic motion detectors use the Doppler effect to detect movement. The detector’s circuitry compares the frequency of the sound that is emitted by the transmitter when no motion is present to the frequency that results when motion occurs. When no motion is present, the sound is emitted and bounces back in an even, steady pattern. When motion occurs, the sound waves are disturbed and the circuit detects the shift.

C  Microwave,

Microwave motion detectors also use the Doppler effect to detect motion. Instead of emitting sound, they emit electromagnetic energy in the form of microwaves. Unlike microwave ovens, burglar alarm systems use microwaves that will not cook food or harm people. One characteristic of microwave energy is that it will penetrate glass, thin walls, or even people. If microwave motion detectors are not properly installed in a protected space, they may actually detect motion outside the building by penetrating the walls or windows.

D  Video,  

Video motion detectors sense changes in ambient light, or the light that is normally present in a space. They are less sophisticated than the systems previously discussed, because they require ambient light to function. A video camera views an area, and sends the image to a monitor. The motion detector’s computer reads the pixels, or the tiny picture elements that make up a video image, that are displayed on the monitor. The detector notes whether the pixels are light or dark. When an intruder moves across the field of view, the monitor displays the movement, changing the lightness or darkness of the individual pixels. The detector notices the changes and signals an alarm.

III  REDUCING FALSE ALARMS,

A burglar alarm can initiate a considerable response by police or private security personnel, who may leave other important duties to race to the scene of the alarm. Therefore, it is important to prevent false alarms when designing and installing alarm systems. Modern burglar alarms make use of several different technologies to reduce false alarms.

Passive infrared detectors can be programmed to ignore the first movement detected, as in when the intruder moves from one detection zone to another, and to sound the alarm only when the movement passes through two or more detection zones within a specified period of time. In this way, an insect landing on the detector’s lens, or a sudden rise in background temperature caused by an activated furnace, is ignored.

Another means of preventing false alarms is the dual-technology motion detector. This is probably the most common type of detector used in more sophisticated burglar alarm systems. A dual-technology detector combines a passive infrared device and a microwave device in one small unit. The passive infrared device sees many detection zones and measures the change in background temperature as a target moves across them. At the same time, the detector projects microwaves and measures the Doppler shift when a target moves through the protected space.

An infrared motion detector will detect movement regardless of whether the target is moving across the field of view or toward the detector. But such a detector is more sensitive to movement across its field of view. Thus, it is more prone to false alarms caused by disturbances such as a mouse or rat moving across its field of view than by movement toward it. Microwave detectors are just the opposite: more sensitive to targets moving toward them than they are to targets moving across their field of view. If a large leaf falls off a plant in a room, a microwave detector is more likely to detect the motion than is an infrared detector. But if there is movement outside a window, a microwave detector might detect it when an infrared detector probably would not.

Dual-technology motion detectors use a circuit that requires both devices to detect motion before an alarm is sounded. A bird landing on an outside windowsill might trip the microwave device but not the infrared device, so no false alarm would be transmitted.

IV  OTHER USES, 

Motion detectors have other uses besides for security. In manufacturing, electric eye detectors can count products on an assembly line, and perform other tasks that people would find tedious. Some dangerous machinery is equipped with active infrared or electric eye beams of light as a safety device. Should an operator’s hand get dangerously close to the machinery, the beam is broken. The circuit is interrupted, and the machine shuts down. Garage doors with automatic door openers can be equipped with beams that sense a child under the door and stop the downward movement before the child is injured. Outdoor porch or patio lights fitted with motion detectors can be turned on automatically when motion is detected. One of the most common and convenient applications of motion detectors is found at modern grocery stores, where doors automatically open as customers enter and leave.