外文翻譯--車輛檢測技術(shù)在交通管理上的應用-交通線路-全文預覽
【正文】 rared energy emitted by objects in the detector’s field of view. Passive detectors do not transmit energy of their own. When a vehicle enters the detection zone, it produces a change in the energy normally measured from the road surface in the absence of a vehicle. The change in energy is proportional to the absolute temperature of the vehicle and the emissivity of the vehicle’s metal surface (emissivity is equal to the ratio of the energy actually emitted by a material to the energy emitted by a perfect radiator of energy at the same temperature). The difference in energy that reaches the detector is reduced when there is water vapor, rain, snow, or fog in the atmosphere. For the approximately 20 ft ( m) distances typical of traffic monitoring applications with this type of detector, these atmospheric constituents may not produce significant performance degradation. Active Infrared Detectors Active infrared detectors function similarly to microwave radar detectors. The most prevalent types use a laser diode to transmit energy in the near infrared spectrum (approximately micrometer wavelength), a portion of which is reflected back into the receiver of the detector from a vehicle in its field of view. Laser radars can supply vehicle passage, presence, and speed information. Speed is measured by noting the time it takes a vehicle to cross two infrared beams that are scanned across the road surface a known distance apart. Some laser radar models also have the ability to classify vehicles by measuring and identifying their profiles. Other types of active infrared detectors use light emitting diodes (LEDs) as the signal source. Ultrasonic Detectors Ultrasonic vehicle detectors can be designed to receive range and Doppler speed data. However, the most prevalent and lowcost ultrasonic detectors are those that measure range to provide vehicle passage and presence data only. The ultrasonic Doppler detector that also measures vehicle speed is an order of magnitude more expensive than the presence detector. Ultrasonic detectors transmit sound at 25 kHz to 50 kHz (depending on the manufacturer). These frequencies lie above the audible region. A portion of the transmitted energy is reflected from the road or vehicle surface into the receiver portion of the instrument and is processed to give vehicle passage and presence. A typical ultrasonic presence detector transmits ultrasonic energy in the form of pulses. The measurement of the roundtrip time it takes for the pulse to leave the detector, bounce off a surface, and return to the detector is proportional to the range from the detector to the surface. A detection gate is set to identify the range to the road surface and inhibit a detection signal from the road itself. When a vehicle enters the field of view, the range from the detector to the top of the vehicle is sensed, and being less than the range from the detector to the road, causes the detector to produce a vehicle detection signal. Passive Acoustic Detectors Vehicular traffic produces acoustic energy or audible sound from a variety of sources within the vehicle and from the interaction of the vehicle’s tires with the road surface. Arrays of acoustic microphones are used to pickup these sounds from a focused area within a lane on a r
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