Volumetric-Motion Sensors

Volumetric-motion sensors are designed to detect intruder motion within the interior of a protected space. Volumetric sensors may be active or passive. Active sensors (such as microwave sensors) fill the volume to be protected with an energy pattern and recognize disturbances in the pattern that will be caused by anything moving within the detection zone. Passive sensors (such as infrared sensors) detect energy generated by an intruder. Some sensors, known as dual-technology sensors, use a combination of two different technologies—usually one sensor is active and the other is passive. If CCTV assessment or surveillance cameras are installed, video-motion sensors can be used to detect intruder movement within the area. Because ultrasonic motion sensors are seldom used, they will not be discussed here.

Interior microwave motion sensors typically house the transmitter and the receiver in the same enclosure (transceiver). The transmitter and receiver may have separate antennas or they may share a common antenna. The shape of the transmitted beam is a function of the antenna configuration. The range of the transmitted beam can be controlled with a range adjustment.

A variety of detection patterns can be generated. The frequency of the transmitted signal is compared with the frequency of the signal reflected back from objects in the protected area. If there is no movement within the area, the transmitted and received frequencies will be identical and no alarm will be generated. Movement in the area will generate a Doppler frequency shift in the reflected signal, producing an alarm if the signal satisfies the sensor's alarm criteria. Microwave energy can pass through glass doors and windows as well as lightweight walls or partitions constructed of plywood, plastic, or fiberboard. False alarms are possible because of the reflection of the microwave signals from people or vehicles moving outside the protected area. Sophisticated microwave motion sensors may be equipped with electronic range gating. This feature allows the sensor to ignore the signals reflected beyond the adjustable detection range. Range gating may be used to minimize unwanted alarms from activity outside the protected area.

Interior microwave sensors are most effective when the target to be detected is moving directly toward (rather than perpendicular to) the sensor during an intrusion.

Passive-infrared motion sensors detect a change in the thermal energy pattern caused by a moving intruder and send an alarm when the change in energy satisfies the detector's alarm criteria. These sensors are passive devices that monitor the energy radiated by the surrounding environment.

The infrared energy must be focused onto a sensing element, somewhat as a camera lens focuses light onto a film. Two techniques are commonly used. One technique uses reflective focusing; parabolic mirrors focus the energy. The other uses an optical lens. Fresnel lenses are preferred because they do not have to be as thick for short focal lengths. Because glass attenuates infrared energy, lenses usually are made of plastic.

The sensor's detection pattern is determined by the arrangement of lenses or reflectors. The pattern is not continuous, but consists of a number of rays or fingers, one for each mirror or lens segment. Numerous detection patterns are available. The passive infrared sensor is not provided with a range adjustment. The range can be adjusted somewhat by manipulating the sensor's position. Careful selection of the appropriate detection pattern is critical to proper sensor performance.

Most manufacturers use a dual-element sensor to minimize nuisance alarms caused by changes in ambient temperature. An intruder entering one of the detection fingers produces an imbalance between the two halves, causing an alarm condition. Quadelement sensors that combine and compare two dual-element sensors are also used. Pulse-count activation requires that a predefined number of pulses be produced within a specific interval before an alarm is generated.

Passive infrared sensors are most effective when the target to be detected is moving across the finger pattern (rather than toward or away from the pattern) during an intrusion.

To minimize false alarms, dual-technology sensors combine two different technologies in one unit. Ideally, each of the two sensors has a high probability of detection and does not respond to common sources of false alarms. Available dual-technology sensors combine an active ultrasonic or microwave sensor with a passive infrared sensor. The alarms from each sensor are combined in a logical “and” configuration. Nearly simultaneous alarms from both sensors are needed to produce a valid alarm. Although combined technology sensors have a lower false-alarm rate than individual sensors, the probability of detection also is slightly reduced.

Video motion sensors generate an alarm when an intruder enters a selected portion of a video camera's field of view. The sensor processes and compares changes in successive images to predefined alarm criteria.

There are two categories of video motion detectors—analog and digital. Analog detectors generate an alarm in response to changes in a picture's contrast. Digital devices convert selected portions of the analog video signal into digital data that are compared with data converted previously; if differences exceed preset limits, an alarm is generated.

The signal processor usually provides an adjustable window that can be positioned anywhere on the video image. The horizontal and vertical window size, window position, and window sensitivity can be adjusted. More sophisticated units provide several adjustable windows that can be individually sized and positioned. For example, in a scene containing six doorways leading into a long hallway, a sensor with two adjustable windows can be set to monitor just two critical doorways.

Some video motion sensor systems generate a “track,” a superimposed line that can show the path followed by an intruder that entered the field of view.