外文翻譯--車輛檢測技術(shù)在交通管理上的應(yīng)用-交通線路(文件)
【正文】 etter exploited than others at any time. Thus, a technology may show promise for future applications, but the stateoftheart of current hardware or software may be hampering its present deployment. The detectors that were used in the technology evaluations during the field tests are listed in Table 1. Not all detectors were available at all sites as shown in the footnotes to the table. A summary of the advantages and disadvantages of the detector technologies is given in Table 2. Some of them are application specific, implying that a particular technology may be suitable for some but not all applications. A factor not addressed in this table is detector cost. This issue is again application specific. For example, a higher cost detector may be appropriate for an application requiring specific data or multiple detection zones (suitable for multiple lane coverage) that are incorporated into the more expensive detector. Table 3 shows examples of overhead detector technology patibility with several traffic management applications. The assumptions shown concerning the application dictate, in part, the appropriateness of the technology. THEORY OF OVERHEAD DETECTOR OPERATION The following paragraphs give a brief explanation of the underlying operating principles for microwave, passive infrared, active infrared, ultrasonic, passive acoustic, and video image processor detectors. Microwave Radar Microwave radars used in the . for vehicle detection transmit energy at GHz, a frequency allocated by the FCC for this purpose. Their output power is regulated by the FCC and certified by the manufacturer to meet FCC requirements. No further certification is required of the transportation agencies for their deployment. Two types of microwave radar detectors are used in traffic management applications. The first transmits electromagic energy at a constant frequency. It measures the speed of vehicles within its field of view using the Doppler principle, where the difference in frequency between the transmitted and received signals is proportional to the vehicle speed. Thus, the detection of a frequency shift denotes the passage of a vehicle. This type of detector cannot detect stopped vehicles and is, therefore, not suitable for applications that require vehicle presence such as at a signal light or stop bar. The second type of microwave radar detector transmits a sawtooth waveform, also called a frequencymodulated continuous wave (FMCW), that varies the transmitted frequency continuously with time. It permits stationary vehicles to be detected by measuring the range from the detector to the vehicle and also calculates vehicle speed by measuring the time it takes for the vehicle to travel between two internal markers (range bins) that represent known distances from the radar. Vehicle speed is then simply calculated as the distance between the two range bins divided by the time it takes the vehicle to travel that distance. Since this detector can sense stopped vehicles, it is sometimes referred to as a truepresence microwave radar. Passive Infrared Detectors Passive infrared detectors can supply vehicle passage and presence data, but not speed. They use an energy sensitive photon detector located at the optical focal plane to measure the inf
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