Rangefinder type non-imaging traffic sensor

Details Rangefinder type non-imaging traffic sensor Rangefinder type non-imaging traffic sensor

1999-04-30

2001-08-14

Wu, Daniel J. (Department: 2632)

Communications: electrical

Vehicle detectors

Photoelectric

C340S933000, C340S555000

Reexamination Certificate

active

06275171

ABSTRACT:

CROSS-REFERENCE TO RELATED APPLICATIONS
Not applicable.
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
Not applicable.
BACKGROUND OF THE INVENTION
Introduction
This invention relates to traffic control systems, and more particularly, to a rangefinder type non-imaging traffic sensor used in such a system.
In U.S. Pat. No. 5,764,163, there is described a non-imaging traffic sensor (NITS). As described therein, the sensor design relied upon filtering, both optical and electronic, to discriminate against shadows. While effective in operation, a drawback of a system employing the sensor was the requirement of a luminaire such as a mercury vapor light or lamp to illuminate the roadway monitored by the system. The luminaire illuminated the scene both day and night. The system also severely filters the sensor signal, greatly reducing signal strength. This, in turn, leads to bandwidth restrictions and reduces resolution time to a level far below that of which the sensor is inherently capable. Additionally, the system requires a spectral filter which is the most expensive item in the optics portion of the system.
As described in the patent, the system utilizes a dual sensor geometry (see FIG. 5 of the patent) to measure vehicle velocity. Because of this, an angle exists between the lines-of-sight (LOS) of the two sensors. Such an angle suggests that a rangefinder principle might be used to determine the height (above the pavement) of a passing disturbance. This would render all the filtering and the daytime artificial illumination unnecessary, since shadows have zero height. A number of advantages now result including lower system cost, an increase in the signal-to-noise ratio (SNR), and enhanced time resolution.
Other patents dealing with traffic flow also address shadow removal. See, for example, U.S. Pat. No. 5,467,402. However, the teachings of this '402 patent require a complicated process of shadow removal requiring information to be maintained concerning the season of the year, time of day, and weather condition information, in addition to information concerning shadow length, luminance, etc.
BRIEF SUMMARY OF THE INVENTION
Among the several objects of the invention may be noted the provision of a range finder type, non-imaging sensing system for use in traffic monitoring and roadway control systems;
the provision of such a system to readily detect passage of vehicles over the roadway and to determine the type of vehicle and its speed;
the provision of such a system which is useful with multilane roadways to sense vehicles moving through each lane as well as between lanes;
the provision of such a system which operates under a wide range of climatic conditions to readily identify passing vehicles and does not require artificial light sources to provide sufficient illumination for vehicle sensing;
the provision of such a system which operates equally as effectively in areas where artificial roadway lighting is used;
the provision of such a system to readily identify shadowing effects such as are predominant at sunrise and sunset, the passage of clouds, etc. so to distinguish a passing disturbance as being caused by a vehicle or a shadow and so provide only vehicle information; and,
the provision of such a system to be a relatively low cost system which is readily installed above the lanes of a roadway to provide timely and accurate information about traffic on the roadway.
In accordance with the invention, generally stated, a non-imaging traffic sensing system senses vehicular roadway traffic. The system employs three separate detectors each positioned above the roadway and spatially separated along a length of the roadway. The detectors detect light reflected off the roadway surface. Each detector has its own field of view of the roadway surface with a separate footprint being defined on the surface by intersection of the respective fields of view with the surface. Presence of a disturbance passing over the roadway changes the amount of reflected light sensed by the detectors and the detectors generate respective signals indicative of the amount of reflected light they receive. A first pair of the detectors are used for measuring the speed of a passing disturbance. A second pair of the detectors are used to identify shadows so to eliminate their effects. With respect to this second pair of detectors, the footprints are defined by the overlap of their fields of view. A processor processes the detector signals from the first detector pair to determine the speed of the disturbance. The processor further processes the signals from the second detector pair to determine the height of the disturbance. The disturbance is classified as caused by vehicular traffic if the height exceeds a predetermined threshold, but as caused by movement of a shadow if less than the threshold. This allows the effects of shadows on the roadway to be readily identified and distinguished from the movement of vehicular traffic. Other objects and features will be in part apparent and in part pointed out hereinafter.


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