Method and apparatus for measuring the speed of a vehicle

Abstract: The invention relates to a method for measuring the speed of a vehicle by successively detecting the passage of the vehicle in at least two locations lying at a predetermined mutual distance, determining the time lag between the detections at the successive locations and deriving the speed therefrom. A number of measuring points are established during each detection and the speed is calculated by determining the time lag between corresponding measuring points of the successive detections. A wave-form detection signal is generated each time the vehicle passes and the measuring points are formed by corresponding characteristic points of the wave-form signal. An accurate speed measurement can thus be obtained with simple means. The invention further relates to an apparatus for performing this method.

Description: The invention relates to a method for measuring the speed of a vehicle by successively detecting the passage of the vehicle in at least two locations lying at a predetermined mutual distance establishing a number of measuring points during each detection, determining the time lag between corresponding measuring points of the detections at the successive locations and deriving the speed therefrom. Such a method is known, e.g. from EP-A-0 629 861, and is used to perform speed measurements at fixed points. Such measurements are particularly important at places where too high a speed can cause a dangerous traffic situation such as for instance at intersections. In the known method to three dimensional magnetic field detectors are used, which are arranged near a road at a certain fixed mutual distance. Each of these magnetic field detectors generates a wave form signal when a vehicle passes. This signal is compared to a threshold value, and both the moment when the signal strength exceeds the threshold and the moment that the signal strength again falls below the treshold are established as measuring points. The time elapsed between corresponding measuring points in the signals of the two detectors is measured. As there are always two measuring points, there are two values for the time lags, from which a single mean value is determined. The speed of the vehicle is eventually determined simply by deviding the known distance between the detectors by the mean time measured. When this speed is higher than the maximum speed a camera may for instance be activated. In order to enhance the accuracy this document also proposes to compare each signal with a plurality of treshold values, in order to obtain more measuring points per signal. The document further proposes to determine the median of the measured times, which could then again be averaged. For this known measurement, like in other known methods wherein use is generally made of detection loops arranged in the road surface, only two detection loops are used. However, there is increasing objection to this. It has been found that such a measurement is not always completely reliable, whereby there is a danger that road-users are being penalized for offences which they have not actually committed, or conversely that traffic offences go unpunished. It is therefore already required in some countries that at least three independent measurements be performed when speed measurements are carried out in this way, wherein there is only a question of an offence when this is clearly indicated by all three measurements. It has already been proposed for this purpose to simply increase the number of detection loops, whereby a plurality of successive speed measurements could be performed. Arranging detection loops in the road surface is however relatively expensive, while it moreover results in traffic disruption. In addition, as stated above, such speed measurements are often performed in the vicinity of intersections, wherein the detection loops also serve to detect offences relating to traffic lights placed at that junction. Now, if more than two detection loops have to be arranged, each of which must have a sufficiently large interspacing to enable an accurate measurement, the final loops will be situated in the middle of the intersection area. This will entail the danger of interference occurring as a result of the crossing traffic, while interference problems will moreover occur when speed measurements have to be performed on both intersecting roads. The invention now has for its object to provide a method of the above described type with which a number of independent speed measurements can be performed without additional detection loops having to be arranged for this purpose. According to the invention this is achieved in that per detection at least three measuring points are determined and the speed derived therefrom is only accepted as valid measurement value if the time lag determined for each of these measuring points corresponds with the time lag determined for the other measuring points. By establishing a number of measuring points during each detection and by using these points as independent measuring points, a plurality of independent speed measurements can be performed with two detections which are for instance carried out with two detection loops. Preferably applied variants of the method according to the invention are described in the dependent claims 2-4. The invention also relates to an apparatus for performing the above described method. Such an apparatus, which is an apparatus for measuring the speed of a vehicle provided with means arranged at at least two locations lying at a predetermined mutual distance for detecting a passage of the vehicle, said detection means being adapted for establishing a number of measuring points during each passage, means connected to the detection means for measuring the time lag between corresponding measuring points established by the detection means, and processing and control means connected to the time measuring means for deriving the speed therefrom, is characterized according to the invention by means connected to the processing and control means for recording an image of the vehicle, wherein the processing and control means are adapted for transmitting a control signal to the recording means if a corresponding time lag is determined for at least three measuring points and the speed derived therefrom is greater than the maximum allowed speed at that location. Preferred embodiments of the measuring apparatus according to the invention form the subject-matter of the dependent claims 6-8. The invention is now elucidated on the basis of an embodiment, wherein reference is made to the annexed drawing, in which: fig. 1 shows a schematic perspective view of an intersection of two roads where a speed measuring apparatus according to the present invention is present, fig. 2 is a graphic representation of the detection signals generated at two successive locations when a passenger vehicle passes there, fig. 3 is a representation corresponding with fig. 2 of the passage of a truck, and fig. 4 is a schematic flow diagram in which the different steps of the method according to the invention are shown. At a crossroads 1 (fig. 1) which is provided with a number of traffic lights 2, an apparatus 10 is arranged for measuring the speed of passing vehicles 9. This speed measuring apparatus 10 comprises detection means in the form of a plurality of detection loops 5A, 6A and 5B, 6B arranged in the road surface 3. Detection loops 5A, 6A, 5B, 6B are arranged on either side of a halt line 4 painted on the road surface, whereby they can also serve to detect offences relating to traffic lights 2, such as driving through a red light. Detection loops 5A and 6A serve to measure the speed in the left-hand lane, while detection loops 5B and 6B measure the speed of vehicles in the right-hand lane. Detection loops 5A, 5B, 6A, 6B are connected via lines 11A, 11B, 12A, 12B to a system unit 7 in which time measuring means and processing and control means are accommodated. In contrast to known measuring systems based on detection loops, which perform a scanning measurement, the detection means, time measuring means and processing and control means 7 in the measuring apparatus according to the invention are adapted to perform measurements continuously. The system unit 7 is further connected via lines 13 to traffic lights 2, whereby the system unit can also be used to control the traffic lights and optionally to register offences relating to these traffic lights. System unit 7 is further connected via a line 14 to means 8 for recording one or more images of an offending vehicle 9. These recording means 8 can take the form of a photo or video camera. The system unit 7, which can take the form of a programmable logic circuit (PLC) or a small PC, has means for measuring the time lag between detections by successive detection loop 5A and 6A or 5B and 6B. These detections herein take the form of a wave-form signal, the progression of which in time (plotted along the horizontal axis 17) is shown schematically in fig. 2 and 3. The wave-form detection signal 15, 16 represents the percentual change of the inductance in the loops 5A, 5B, 6A, 6B (plotted along the vertical axis 18) which, in accordance with Maxwell's law, is the result of the displacement of a conductor, i.e. the strongly metallic vehicle 9, relative to the geomagnetic field. The wave form of the detection signal is therein characteristic of a determined vehicle, while the magnitude of the inductance change is related to the speed of the vehicle. When fig. 2 and 3 are compared, it is noticeable that the signal of a passenger car is generally stronger than that of a truck, because the metal parts of a car move closer to the road surface than those of a truck. In addition, the length of the signal is shorter as a result of the normally shorter length of a passenger car. The detection signal 15 respectively 16 has a number of characteristic points 15a-15j respectively 16a-16n. The point 15a respectively 16a can herein for instance represent driving onto the detection loop, while the point 15j respectively 16n indicates departure from the loop. Therebetween can be designated a number of characteristic points of inflection or extremes 15b-15i respectively 16b-16m which are related to the form and the speed of the vehicle. By now comparing the detection signal 15 respectively 16 of the first loop 5A or 5B to the detection signal 15' respectively 16' of the second loop 6A or 6B, corresponding characteristic points in both signals can be identified. The time which has elapsed between each set of corresponding characteristic points can be measured and a speed can be determined therefrom. In this manner ten measurements can for instance be performed for the passenger vehicle in the example shown, while in the example shown for the truck as many as fourteen measurements could be performed. It is thus possible to comply in relatively simple manner with the more and more frequently imposed requirement that at least three measurements must be performed. Since as stated the wave-form of the signal 15 or 16 is related to the shape, dimensions and speed of the vehicle, it is always possible to determine in simple manner that the signals which are compared to each other come from one and the same vehicle. Incorrect measurements are therefore practically impossible. When however the speed of the vehicle changes greatly between the two detection locations, for instance because the driver of the vehicle suddenly brakes or accelerates, the wave-form of the signal will also change, whereby no or not enough corresponding points are found. Insufficient measuring points will then be available to perform a valid measurement. Performing of a speed measurement in the manner according to the invention therefore proceeds as follows (fig. 4). Starting from a ready for operation or standby mode (block 19), the detection means determine that a vehicle 9 is passing the first location 5A or 5B (block 20). The passage of the vehicle over the second location 6A or 6B is subsequently also detected (block 21). The signals obtained from these two detections are each then first sampled (blocks 22 and 23), whereby the analog signal 15 respectively 16 shown in fig. 2 and 3 is converted into a histogram. The histogram bars are then successively compared to each other. A search is made for this purpose in block 24 as to whether any histogram bars remain which have not yet been subjected to a comparison. Should this be the case, the first subsequent, as yet unexamined part of the histogram of the first signal is then taken (block 25) and the corresponding part of the second signal is matched therewith (block 26). The height of these histogram bars, which represents the inductance change of the original signal, is compared (block 27). If the heights do not correspond, block 24 is re-entered. If the heights do correspond, such a bar is accepted as valid measuring point. The time lag between the compared histogram parts is then determined (block 28) and stored as measuring point (block 29), whereafter block 24 is re-entered. In this manner a number of measuring points in the signal are determined. When no further unexamined parts of the sampled signal remain, a check is made in block 30 as to whether enough measuring points have been found, therefore at least three according to the new criteria. If this is not the case, the starting situation is restored (block 19). If however sufficient measuring points have been found, the measured values for the time lag between each set of corresponding measuring points are compared to each other (block 31). A check is subsequently made in block 32 as to whether these values correspond sufficiently with each other. Should this not be the case, for instance because the speed of the vehicle has changed greatly during the measurement, this constitutes an invalid measurement and the starting situation is restored. If the values do correspond sufficiently, the speed is then determined therefrom by dividing the distance between the detection loops by the measured time lag (block 33) and compared with the speed limit in force at that location (block 34). When the measured speed is greater than the maximum allowed speed a warning signal is generated (block 35). In the embodiment shown this warning signal can be transmitted directly to the recording device 8, whereby a photo or video recording of vehicle 9 is made. The warning signal can also be used to bring the offender to the attention of a supervising police officer. The method and apparatus according to the invention thus nevertheless enable a very accurate and reliable speed measurement with a minimum number of detection loops, whereby the danger of unjustified penalties or unpunished offences is minimized without large investments being required for this purpose. Although the invention is described above with reference to a speed measurement with two detection loops at an intersection, it will be apparent to the skilled person that it is not limited thereto. The speed measurement can after all take place at any location where at least two such loops can be arranged, while it is also possible to apply more than two loops, for instance as extra safeguard against errors.

Claims: Method for measuring the speed of a vehicle (9) by successively detecting the passage of the vehicle (9) in at least two locations (5A, 6A; 5B, 6B) lying at a predetermined mutual distance establishing a number of measuring points (15a-15j, 16a-16n; 15a'-15j', 16a'-16n') during each detection, determining the time lag between corresponding measuring points (15a-15j, 16a-16n; 15a'-15j', 16a'-16n') of the detections at the successive locations (5A,6A; 5B, 6B) and deriving the speed therefrom, characterized in that per detection at least three measuring points are determined and the speed derived therefrom is only accepted as valid measurement value if the time lag determined for each of these measuring points corresponds with the time lag determined for the other measuring points. Method as claimed in claim 1, characterized in that each time the vehicle (9) passes a wave-form detection signal (15, 15'; 16, 16') is generated and the measuring points (15a-15j, 16a-16n; 15a'-15j', 16a'-16n') are formed by corresponding characteristic points of the wave-form signal (15, 15'; 16, 16'). Method as claimed in claim 1 or 2, characterized in that the wave-form detection signal (15, 16) is sampled, the strength of each thus sampled part of the signal (15, 16) is compared with that of a corresponding part of the following (15', 16') or foregoing signal and a sampled signal part is only accepted as measuring point if the compared signal strengths mutually correspond. Method as claimed in any of the foregoing claims, characterized in that the speed measurements are performed continuously. Apparatus (10) for measuring the speed of a vehicle (9) provided with means (5A, 6A; 5B, 6B) arranged at at least two locations lying at a predetermined mutual distance for detecting a passage of the vehicle (9), said detection means (5A, 6A; 5B, 6B) being adapted for establishing a number of measuring points during each passage, means (7) connected to the detection means (5A, 6A; 5B, 6B) for measuring the time lag between corresponding measuring points established by the detection means (5A, 6A; 5B, 6B), and processing and control means (7) connected to the time measuring means (7) for deriving the speed therefrom, characterized by means (8) connected to the processing and control means (7) for recording an image of the vehicle (9), wherein the processing and control means (7) are adapted for transmitting a control signal to the recording means (8) if a corresponding time lag is determined for at least three measuring points and the speed derived therefrom is greater than the maximum allowed speed at that location. Measuring apparatus (10) as claimed in claim 5, characterized in that the detection means (5A, 6A; 5B, 6B) are adapted for generating a wave-form detection signal (15, 16) during passage of the vehicle (9), wherein characteristic points (15a-15j, 16a-16n) of this wave-form signal (15, 16) function as measuring points. Measuring apparatus (10) as claimed in claim 5 or 6, characterized in that the processing and control means (7) are adapted for sampling each wave-form detection signal (15, 16) and for comparing the strength of each thus sampled part of a signal (15, 16) with that of a corresponding part of the following (15', 16') or foregoing signal. Measuring apparatus (10) as claimed in claim 7, characterized in that the processing and control means are adapted for establishing a sampled signal part as measuring point if the strength thereof corresponds with that of a corresponding signal part of a following or foregoing detection signal (15, 15'; 16, 16'). Measuring apparatus (10) as claimed in any of the claims 5-8, characterized in that the detection means (5A, 6A; 5B, 6B), the time measuring means (7) and the processing and control means (7) are adapted for continuous operation.

Inventors: GATSONIDES TOM, GATSONIDES, TOM
Applicants: GATSOMETER B V, GATSOMETER B.V.
Application year: 1997
Main classification: G01P3
IPC codes: G01P3