1. Field of the Invention
The present invention relates generally to object sensors and related methods, and in particular relates to electronic object sensors and methods useful in detecting vehicle speed and shape for classification and input to Intelligent Vehicle Highway Systems (IVHS).
2. Background Art
A vehicle sensor detecting the presence of a vehicle in a traffic lane and indicating the vehicle speed as it passed the sensor is described in U.S. Pat. No. 5,321,490 referenced above. A time-of-flight laser rangefinder is used to measure the normal distance to a road surface from a fixed point above the road surface and then measures the distance to a vehicle which either passes or stops under the sensor. Two laser beams pulsing at a high rate are projected across the road surface at a fixed angle between them. Because of the high repetition rate of the pulsed beam, the system is able to determine vehicle speed with an accuracy within one mph and using this calculated speed, develop a longitudinal profile of the vehicle using consecutive range measurements as the vehicle moves under the sensor. Such active near-field object sensors are relatively low in cost, are accurate and have utility in a wide variety of applications. A laser diode capable of emitting pulses of coherent infrared radiation is used together with collimating optics and a beam splitter to provide two diverging output beams directed toward the road surface under observation.
The sensor receives a portion of the energy reflected from either the area, or an object located within the area, such as a vehicle. The returned pulse energy is then provided as an input to a receiver for determining a time of flight change for pulses emitted and received, which may be caused by the presence of an object within the area. The sensor is also provided with various features useful in providing outputs which indicate either the speed, census, size or shape of one or more objects in the area. For example, the sensor is provided with means for receiving an input from the time of flight determining means and for providing an output indicating whether the object meets one of a plurality of classification criteria (e.g., is the object an automobile, truck or motorcycle). A receiver includes two detectors and alternately selects between outputs of the two detectors for providing the time-of-flight measurements. The time interval between interceptions of the two diverging outputs by a vehicle provides the speed of the vehicle passing through the area.
U.S. Pat. No. 5,278,423 referenced above discloses the generation of three-dimensional images of objects by rotating or scanning a laser beam rangefinder, operating at a high pulse rate, in a plane where there is relative motion between the rangefinder and the object to be sensed or imaged in a direction perpendicular to the laser beam plane of rotation. The laser rangefinder rotating beam covers the object being sensed permits a three-dimensional image of the object to be determined. By way of example, the '423 patent discloses a sensor traveling between rows of trees with the laser rangefinder scanning on either side of a moving vehicle carrying the sensor. Beam scanning is within a plane perpendicular to the motion of the vehicle. When the sensor detects the presence of foliage, it provides a signal activating a spraying system for the efficient spraying of the tree. This operation ensures that spraying takes place only when there is foliage present to intercept the sprayed materials. Economic and environmental benefit is thus realized.
The agricultural sprayer employs a pulsed time-of-flight range measuring system having separate apertures for a laser transmitter and receiver. The laser beam and receiver field-of-view are continuously scanned by a rotating mirror in a vertical plane which is perpendicular to the forward motion axis of the sprayer vehicle. The position of the mirror, and correspondingly the laser beam, is determined by a shaft encoder attached to the mirror drive motor shaft. With this embodiment, a single sensor makes range measurements on both sides of the vehicle as the vehicle moves the sensor between rows of trees. Since the sensor only needs to detect the presence of trees, range measurements are only made within elevation angles of plus and minus 45 degrees on each side of the sensor. Data is collected within 180 degrees out of the 360 degrees of a revolution or circular scan. As the vehicle moves along, the scan trace advances on consecutive revolutions of the mirror. Employing a distance traveled input from the vehicle, the sensor creates a panorama of images. An algorithm then determines whether trees are present from the measured range data as a function angle. Spray units are grouped in zones and the sensor provides foliage images for the zones and thus an indication of the amount of spray necessary for a particular tree zone.
There is a continuing demand for accurate, low-cost sensors useful in a wide variety of applications, including equipment used in the home, as well as for security, military and transportation applications. Traffic signal controllers utilizing overhead sensors are known as described by way of example in U.S. Pat. No. 3,167,739 to Girard et al; U.S. Pat. No. 3,436,540 to Lamorlett; U.S. Pat. No. 3,516,056 to Matthews; U.S. Pat. No. 3,532,886 to Kruger et al; U.S. Pat. No. 3,680,047 to Perlman; and U.S. Pat. No. 4,317,117 to Chasek. Likewise referenced, near-field sensors have also been utilized as intruder alarms and as automatic door operators. Examples of such arrangements are disclosed in U.S. Pat. No. 3,605,082 to Matthews; U.S. Pat. No. 3,644;917 to Perlman; U.S. Pat. No. 3,719,938 to Perlman; U.S. Pat. No. 3,852,592 to Scoville et al; U.S. Pat. No. 3,972,021 to Leitz et al; and U.S. Pat. No. 4,433,328 to Saphir et al. U.S. Pat. No. 4,768,713 discloses the use of an ultrasonic sonar-type sensor to detect the presence of tree foliage, as do U.S. Pat. Nos. 4,823,268 and 5,172,861. Optical dimensioning techniques have been incorporated in industrial uses as disclosed in U.S. Pat. Nos. 4,179,216 and 4,490,038.
Vehicle detection and classification provided by the Intelligent Vehicle Highway System and Method of the above-referenced related inventions, and as herein described, have proven to be very successful using rule-based algorithms to do shape-based vehicle classification. With the current class structure, the sensors have achieved 98.5% accuracy on a random 10,000 vehicle test. Using the National Academy of Sciences (NAS) vehicle database, which was collected under the ITS-6 IDEA program, the sensors achieved 96.5% accuracy on a 50,000 vehicle database including a range of weather conditions and traffic conditions.
However, in spite of this success, most applications in the U.S. still require vehicle classifications based on the number of axles a vehicle has as opposed to just the shape-based classification. This implies that tolling applications in the U.S. must deploy an axle counter, such as a treadle, along with a host of other sensors to perform the other tasks that the axle counter cannot do, such as vehicle separation. Along with the disadvantages of embedding a sensor in the road, the higher system cost of using several sensors, and added complexity of integrating different sensors into a single system, transportation professionals at all levels have expressed an increased interest in an overhead sensor that will count axles.