Sensor Smarts

Dec 1, 2006 12:00 PM, By Jim Paulson

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When people “request to exit” a facility, a request-to-exit or REX sensor triggers a door latch to allow them to exit. Most REX sensors do this job admirably - seldom do they fail to let a person out who wants or needs to leave.

The REX sensor is based on a motion detector - a type of electronic security device that senses movement and usually triggers an alarm, even in total darkness. Motion detectors use a variety of technologies to detect movement, each with its own advantages and disadvantages. Thus, one type of technology can make up for another's defects. For instance, infrared detectors sense heat and can detect body heat. Most infrared detectors are passive. This means that they do not send out signals of their own. They only receive signals acknowledging changes in temperature.

On the other hand, microwave detectors are active. They actually send out waves of energy and receive waves reflected back by objects. Disturbances in the reflected waves caused by moving objects trigger alarms.

The problem with REX sensors is that they can often create a false alarm when nobody is present. False alarms can be costly and can waste time. REX sensors based solely on passive infrared (PIR) technology are especially susceptible.

What is a PIR sensor?

Passive infrared, or PIR, detectors are the most common sensors used in the security industry to detect movement. They sense changes in temperature by monitoring infrared radiation. All living things produce heat energy, which a PIR sensor can detect. When an intruder enters an area guarded by a PIR detector, it catches any rapid changes in radiated heat. When positioned and adjusted correctly, the PIR sensor is not affected by any normal or gradual changes in the area's heat, such as those caused by HVAC (heating, ventilation and air conditioning) systems or sunlight.

From a more technical perspective, passive infrared motion detectors are sensitive to infrared wavelengths of energy, invisible to us. During intrusion applications, this energy is detected as body heat. To survey an entire room or to define areas to be surveyed, passive infrared motion detectors use lenses, which view an area as several distinct zones. As these zones spread out vertically and horizontally from the detector lens, they establish “defense lines” across the area, down to the floor.

Some PIR sensors employ a Fresnel lens: a thin, flat optical lens that consists of a series of small narrow concentric grooves on the surface of a lightweight plastic sheet in order to reduce the thickness, weight and cost. Each groove is at a slightly different angle than the next and with the same focal length in order to focus the light toward a central focal point. Every groove can be considered an individual small lens that bends parallel Fresnel light waves and focuses the light.

Using electronic circuits, the PIR detector recognizes the amount of heat that is usually present in the area. When a prowler enters, his or her body heat adds to the amount of heat usually observed. As the intruder moves from one zone (going through a “defense line”) to another within the detector's field of view, the PIR sensor notes an increase in energy and signals an alarm.

There can be problems with these sensor. For instance, warm water being poured under a door or sliding a coat hanger with material attached under or between doors can trigger a PIR sensor, thus creating a false alarm or worse. In some cases, it can trigger the sensor to release the door latch, allowing an intruder easy access. And, although infrared motion detectors are very sensitive to heat changes, they cannot see through solid objects, including glass.

What is a Range-Controlled Radar (RCR) sensor?

Motion detectors can also use the Doppler effect to detect motion by emitting electromagnetic energy via microwaves. The Doppler effect can be visualized as the ripples of water that form when a pebble is thrown into a pond. If a boat crosses the rings, it compresses them. That illustrates how motion detectors use the Doppler effect to detect movement. The detector compares the frequency of the microwaves that are emitted by the transmitter when no motion is present to the frequency that results when motion occurs. When there is no motion, the sound is sent out and bounces back in an even, steady pattern. When motion occurs, the microwaves are disturbed and the circuit detects the shift. The pattern is uneven. As opposed to PIR sensors, microwave energy will penetrate solid objects such as glass, walls and living things.

Range-controlled radar (RCR) motion detectors respond to motion only within the range specified and ignore incidental motion within that range. RCR sensors use radar technology to precisely time the return signal and calculate distance to a moving object. Installers quickly set a jumper to select tight coverage patterns for 35-, 27-, 18- or 9-ft. ranges. Range settings are exact because of the timed measurement of the return signal. The sensors do not respond to any return signal beyond the range selected and ignore spurious signal reflections within the protected area.

They prevent false alarms caused by rattling doors and activity in nearby areas. RCR sensors ignore false alarms from passing traffic and activity in adjoining areas, ceiling fans, rattling doors and water moving in pipes. The sensors can be set in radar-only mode for stealth installations behind barriers such as sheet rock, paneling, blinds and drapes. Pet-immune models are available for immunity to non-human movement.

The Dual-Technology REX sensor

Designed to prevent false alarms, a dual-technology motion detector combines a passive infrared device and a microwave device. The PIR sensor sees multiple 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. Thus, both heat and physical motion must be detected to trigger the REX device.

Both types of sensor technology complement themselves in another way. A PIR detector senses movement whether or not the target is moving across the field of view or toward the detector. But, by the very fact that the technology creates “defense lines” across the supervised area, the PIR sensor is more sensitive to movement across its field of view. Therefore, it will trigger more false alarms caused by disturbances when something, such as a moth, moves across its field of view than by coming toward it.

Microwave detectors are just the opposite. They are more sensitive to targets moving toward them than to targets moving across their field of view. For instance, if a book falls off a shelf, a microwave detector is more likely to detect this motion than a PIR sensor. But if a prowler moves past an outside window, it may detect it when a PIR sensor is unlikely to do so.

To protect users from false triggers of this type, dual-technology motion detectors employ a circuit that requires both devices to detect motion before it sends an alarm. A car passing the windows might activate the microwave device, but the infrared device will not even notice it.

Combining active radar with passive infrared, range-controlled radar creates a sensor with crisp detection and superior false alarm immunity. A dual-technology REX sensor is more immune to common attempts to defeat PIRs because both heat and physical motion are required to trigger the device. For maximum protection, the installer adjusts the radar so that the curtain is above the floor. Thus, objects inserted along the floor or under the door do not activate the sensor.

Jim Paulson is the general manager of residential and commercial solutions for GE Security.

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