Abstract:
Apparatus and method are provided for detecting vehicles which are moving through a predetermined zone. The apparatus includes a plurality of acousto-electric transducers trained on the zone. A bandpass filter is provided for processing electrical signals from the plurality of acousto-electric transducers. A correlator having at least two inputs and an output is provided for correlating filtered versions of the electrical signals originating from at least two of the plurality of acousto-electric transducers. An integrator is provided for integrating the output of the correlator means over time. Finally, a comparator is provided for indicating detection of a vehicle when the integrated output exceeds a predetermined threshold.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This invention is a continuation-in-part of application Ser. No. 08/069,957, filed May 28, 1993, now U.S. Pat. No. 6,021,364, the entire contents of which are incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to highway monitoring systems in general and, more specifically, to systems which detect and signal the existence of a motor vehicle within a predetermined detection zone on the roadway. 
     BACKGROUND OF THE INVENTION 
     Highway departments use a variety of techniques to monitor traffic in an effort to detect, mitigate, and prevent congestion. Typically, each highway department has a command center that receives and integrates a plurality of signals which are transmitted by monitoring systems located along the highway. Although different kinds of monitoring systems are used, the most prevalent system employs a roadway metal detector. In such system, a wire loop is embedded in the roadway and its terminals are connected to detection circuitry that measures the inductance changes in the wire loop. Because the inductance in the wire loop is perturbed by a motor vehicle (comprising a quantity of ferromagnetic material) passing over it, the detection circuitry can detect when a motor vehicle is over the wire loop. Based on this perturbation, the detection circuitry creates a binary signal, called a “loop relay signal,” which is transmitted to the command center of the highway department. The command center gathers the respective loop relay signals and from there makes a determination as to the likelihood of congestion. The use of wire loops is, however, disadvantageous for several reasons. 
     First, a wire loop system will not detect a motor vehicle unless the motor vehicle comprises a sufficient ferromagnetic material to create a noticeable perturbation in the inductance in the wire loop. Because the trend is to fabricate motor vehicles with non-ferromagnetic alloys, plastics and composite materials, wire loop systems will increasingly fail to detect the presence of motor vehicles. It is already well known that wire loops often overlook small vehicles. Another disadvantage of wire loop systems is that they are expensive to install and maintain. Installation and repair require that a lane be closed, that the roadway be cut and that the cut be sealed. Often too, harsh weather can preclude this operation for several months. 
     Other non-invasive systems have been suggested. U.S. Pat. No. 5,060,206, patented Oct. 22, 1991, by F. C. de Metz, Sr., entitled “Marine Acoustic Aerobuoy and Method of Operation,” provided a marine acoustic detector for use in identifying a characteristic airborne sound pressure field generated by a propeller-driven aircraft. The detector included a surface-buoyed resonator chamber which was tuned to the narrow frequency band of the airborne sound pressure field and which had a dimensioned opening formed into a first endplate of the chamber for admitting the airborne sound pressure field. Mounted within the resonator chamber was a transducer circuit comprising a microphone and a preamplifier. The microphone functioned to detect the resonating sound pressure field within the chamber and to convert the resonating sound waves into an electrical signal. The preamplifier functioned to amplify the electrical signal for transmission via a cable to an underwater or surface marine vehicle to undergo signal processing. The sound amplification properties of the resonator air chamber were exploited in the passive detection of propeller-driven aircraft at airborne ranges exceeding those ranges of visual or sonar detection to provide 44 dB of received sound amplification at common aircraft frequencies below 100 Hz. However, this patent used only a single electro-acoustic transducer for receiving acoustic signals within a detection zone, and did not teach spatial discrimination circuitry for representing acoustic energy emanating from a detection zone. 
     U.S. Pat. No. 3,445,637, patented May 20, 1969, by J. M. Auer, Jr., entitled “Apparatus For Measuring Traffic Density” provided apparatus for measuring traffic density in which a sonic detector produced a discrete signal which was inversely proportional only to vehicle speed for each passing vehicle. A meter, which was responsive to the discrete signals, produced a measurement representative of traffic density. However, this patent used only a single electro-acoustic transducer for receiving acoustic signals within a detection zone, and did not teach spatial discrimination circuitry for representing acoustic energy emanating from a detection zone. 
     U.S. Pat. No. 3,047,838, patented Jul. 31, 1962, by G. D. Hendricks, entitled “Traffic Cycle Length Selector” provided a traffic cycle length selector which automatically related the duration of a traffic signal cycle to the volume of traffic in the direction of heavier traffic along a throughfare. The Hendricks system did not teach the use of electro-acoustic transducers, but instead used pressure-sensitive detectors. While Hendricks employed plural, non-electro-acoustic transducers, the traffic cycle length selector system did not include spatial discrimination circuitry. Hendricks merely described the use of the output of several spatially discriminate detectors to generate a spatially indiscriminate signal. 
     SUMMARY OF THE INVENTION 
     Aims of the Invention 
     One object of the present invention is to provide apparatus and method to monitor highway traffic while avoiding many of the costs and restrictions associated with prior techniques. 
     Another object of the present invention is to provide such apparatus which can be installed and maintained in any weather and which does not require that the roadway be closed, torn-up or repaved. 
     Statements of Invention 
     The present invention provides apparatus for detecting vehicles moving through a predetermined zone, comprising: (a) a plurality of acousto-electric transducers trained on that zone; (b) bandpass filtering means for processing electrical signals from the plurality of acousto-electric transducers; (c) correlator means having at least two inputs and an output for correlating filtered versions of the electrical signals originating from at least two of the plurality of acousto-electric transducers; (d) integrator means for integrating the output of the correlator means over time; and (e) comparator means for indicating detection of a vehicle when the integrated output exceeds a predetermined threshold. 
     The present invention also provides a method for detecting vehicles moving through a predetermined zone, comprising the steps of: (a) training a plurality of acousto-electric transducers on that zone; (b) filtering electrical signals from the plurality acousto-electric transducers; (c) correlating at least two of the filtered electrical signals with one another; (d) integrating the results of correlation in step (c) over time; and (e) comparing the integrated result of step (d) to a predetermined threshold and indicating detection of a vehicle when the threshold is exceeded by the integrated result. 
     Other Features of the Invention 
     By one feature of this invention, the apparatus further includes a plurality of analog-to-digital converter means for converting said electrical signals to digital representations prior to the processing thereof. 
     By a further feature of this invention, the integrator and the comparator means are each microprocessor-based programs. 
     By still another feature of this invention, the plurality of acousto-electric transducers comprises two vertical and two horizontal multiple-microphone elements, and the correlator means has one of the at least two inputs receiving a sum of the two multiple-microphone vertical elements, and the other of the at least two inputs receiving a sum of the two horizontal multiple-microphone elements. 
     By one feature of the method of this invention, the method further includes the step of converting said electrical signals to digital representations prior to said filtering. By a feature of such feature, the steps of integrating and comparing are each computational routines. 
     By another feature of the method of this invention, the plurality of acousto-electric transducers comprises two vertical and two horizontal multiple-microphone elements, and the correlating step continuously correlates the sum of the two vertical multiple-microphone elements with sums of the two horizontal multiple-microphone elements. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a drawing of an illustrative embodiment of the present invention as it is used to monitor the presence or absence of a motor vehicle in a predetermined detection zone; 
     FIG. 2 is a drawing of an illustrative microphone array as can be used in the illustrative embodiment of the present invention; 
     FIG. 3 is a block diagram of the internals of an illustrative detection circuit as shown in FIG. 1; 
     FIG. 4 is a detailed block diagram of a preferred embodiment of the acoustic highway monitor according to the present invention; and 
     FIG. 5 is a flowchart showing the operation of the controller block shown in FIG.  4 . 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENT 
     Each motor vehicle using a highway radiates acoustic energy from the power plant (e.g., the engine block, pumps, fans, belts, etc.) and from its motion along the roadway (e.g., tire noise due to friction, wind flow noise, etc.). While the energy fills the frequency band from DC up to approximately 16 KHz, there is a reliable presence of energy from about 3 KHz to about 8 KHz. Embodiments of the present invention exploit this observation for the purpose of highway surveillance. 
     Description of FIG. 1 
     FIG. 1 depicts an illustrative embodiment of the present invention that monitors a predetermined area of roadway, called a “predetermined detection zone,” for the presence of a motor vehicle within that area. The salient items in FIG. 1 are roadway  101 , motor vehicle  103 , motor vehicle  105 , detection zone  107 , microphone array  111 , microphone support  109 , detection circuit  115  and interface circuit  119  in a roadside cabinet (not shown), electrical bus  113 , electrical bus  117  and lead  121 . 
     Each omni-directional microphone in microphone array  111  receives an acoustic signal which comprises the sound radiated, inter alia, from motor vehicle  103 , motor vehicle  105  and ambient noise. Each microphone in microphone array  111  then transforms its respective acoustic signal into an analog electric signal and outputs the analog electric signal on a distinct lead on electrical bus  113  in ordinary fashion. The respective analog electric signals are then fed into detection circuit  115 . 
     To determine the presence or passage of a motor vehicle in predetermined detection zone  107 , the respective signals from microphone array  111  are processed in ordinary fashion to provide the sensory spatial discrimination needed to isolate sounds emanating from within predetermined detection zone  107 . The ability to control the spatial directivity of microphone array  111  is called “beam-forming.” It will be clear to those skilled in the art that electronically-controlled steerable beams can be used to form multiple detection zones. 
     Description of FIG. 2 
     As shown in FIG. 2, microphone array  111  preferably comprises a plurality of acoustic transducers (e.g., omni-directional microphones), arranged in a geometrical arrangement known as a Mill&#39;s Cross. For information regarding Mill&#39;s Cross arrays, the interested reader is directed to  Microwave Scanning Antenna , R. C. Hensen, E., Academic Press (1964), and  Principals of Underwater Sound  (3rd. Ed), R. J. Urick (1983). While microphone array  111  could comprise only one microphone, the benefits of multiple microphones (to provide signal gain and directivity, whether in a fully or sparsely populated array or vector), will be clear to those skilled in the art. It will also be clear to those skilled in the art how to baffle microphone array  111  mechanically so as to attenuate sounds coming from other than predetermined detection zone  107  and to protect microphone array  111  from the environment (e.g., rain, snow, wind, UV). Microphone array  111  is advantageously rigidly mounted on support  109  so that the predetermined relative spatial positionings of the individual microphones are maintained. A typical deployment geometry is shown in FIG.  1 . For this geometry, the horizontal distance of the sensor from the nearest lane with traffic is assumed to be less than about 15 feet. The vertical height above the road is advantageously between about 20 and about 35 feet, depending on performance requirements and available mounting facilities. It will be clear to those skilled in the art that the deployment geometry is flexible and can be modified for specific objectives. Furthermore, it will also be clear to those skilled in the art how to position and orient microphone array  111  so that it is well suited to receive sounds from predetermined detection zone  107 . 
     Description of FIG. 3 
     Referring to FIG. 3, detection circuit  115  advantageously comprises bus  301 , vertical summer  305 , analog-to-digital converter  313 , finite-impulse-response filter  317 , bus  303 , horizontal summer  307 , analog-to-digital converter  315 , finite-impulse-response filter  319 , multiplier  321  and comparator  325 . The electric signals from microphone  201 , microphone  203 , microphone  205 , microphone  207  and microphone  209  (as shown in FIG. 2) are fed, via bus  301 , into vertical summer  305  which adds them in well-known fashion and feeds the sum into analog-to-digital converter  313 . While in the illustrative embodiment, vertical summer  305  performs an unweighted addition of the respective signals, it will be clear to those skilled in the art that vertical summer  305  can alternately perform a weighted addition of the respective signals so as to shape and steer the formed beam (i.e., to change the position of predetermined detection zone  107 ). It will also be clear to those skilled in the art that illustrative embodiments of the present invention can comprise two or more detection circuits, so that one microphone array can gather the data for two or more detection zones, in each lane or in different lanes. 
     Analog-to-digital converter  313  receives the output of vertical summer  305  and samples it at 32,000 samples per second in well-known fashion. The output of analog-to-digital converter  313  is fed into finite-impulse response filter  317 . 
     Finite-impulse response filter  317  is preferably a bandpass filter with a lower passband edge of 4 KHz, an upper passband edge of 6 KHz and a stopband rejection level of 60 dB below the passband (i.e., stopband levels providing 60 dB of rejection). It will be clear to those skilled in the art how to make and use finite-impulse-response filter  317 . 
     The electric signals from microphone  211 , microphone  213 , microphone  205 , microphone  215 , and microphone  217  (as shown in FIG. 2) are fed, via bus  303 , into horizontal summer  307  which adds them in well-known fashion and feeds the sum into analog-to-digital converter  315 . While in the illustrative embodiments, horizontal summer  307  performs an unweighted addition of the respective signals, it will be clear to those skilled in the art that horizontal summer  307  can alternately perform a weighted addition of the respective signals so as to shape and steer the formed beam (i.e., to change the position of predetermined detection zone  107 ). 
     Analog-to-digital converter  315  receives the output of horizontal summer  305 , and samples it at 32,000 samples per second in well-known fashion. The output of analog-to-digital converter  313  is fed into finite-impulse response filter  319 . 
     Finite-impulse response filter  319  is preferably a bandpass filter with a lower passband edge of 4 KHz, an upper passband edge of 6 KHz and a stopband rejection level of 60 dB below the passband (i.e., stopband levels providing 60 dB of rejection). It will be clear to those skilled in the art how to make and use finite-impulse-response filter  319 . 
     Multiplier  321  receives, as input, the output of finite-impulse-response filter  317  and finite-response-filter  319  and performs a sample-by-sample multiplication of the respective inputs and then performs a coherent averaging of the respective products. The output of multiplier  321  is fed into comparator  325 . It will be clear to those skilled in the art how to make and use multiplier  321 . 
     Comparator  325  advantageously, on a sample-by-sample basis, compares the magnitude of each sample to a predetermined threshold and creates a binary signal which indicates whether a motor vehicle is within predetermined detection zone  107 . While the predetermined threshold can be a constant, it will be clear to those skilled in the art that the predetermined threshold can be adaptable to various weather conditions and/or other environmental conditions which can change over time. The output of comparator  325  is fed into interface circuitry  119 . 
     Interface circuitry  119  receives the output of detection circuitry  115  and preferable creates an output signal such that the output signal is asserted when a motor vehicle is within predetermined detection zone  107  and such that the output signal is retracted when there is no motor vehicle within the predetermined detection zone  107 . Interface circuitry  119  also makes any electrical conversions necessary to interface to the circuitry at the command center of the highway department. Interface circuitry  119  can also perform statistical analysis on the output of the detection circuitry  115  so as to output a signal which has other characteristics than those described above. 
     Description of FIG. 4 
     FIG. 4 of the drawings illustrates an exemplary implementation using digital processing components to a great extent. The microphone array  400  comprises two vertical elements V 1  and V 2 , and two horizontal elements H 1  and H 2 . As shown, each element has three microphones. Each of the four elements V 1 , V 2 , H 1 , and H 2  feeds a respective analog filter  401 - 404  to attenuate unwanted noise outside the maximal frequency band of interest, which is normally between about 4 and about 9 kHz. The filters  401 - 404  are each followed by a selectable gain preamplifier  405 - 408 , the gain of which is selectable in 3-dB steps ranging from 0 dB to 15 dB (hereto to be described more fully later). Four respective analog-to-digital converters  409 - 412  follow the preamplifiers  405 - 408 . Respective digital finite impulse response (FIR) filters  413 - 416  follow the A/D converters  409 - 412 . The FIR filters  413 - 416  determine the actual frequency band of operation, which is selected, e.g., from the following four bands: 
     Band 1: 4-6 kHz; 
     Band 2: 5-7 kHz; 
     Band 3: 6-8 kHz; and 
     Band 4: 7-9 kHz. 
     One value for the gain of all the preamplifiers  405 - 408  will exemplarily be selected for the four above bands as follows: 
     
       
         
               
               
               
               
               
             
           
               
                   
                   
               
               
                   
                 Band 1 
                 Band 2 
                 Band 3 
                 Band 4 
               
               
                   
                   
               
             
             
               
                   
                 9dB 
                 11dB  
                 13dB 
                 15dB 
               
               
                   
                 6dB 
                 8dB 
                 10dB 
                 12dB 
               
               
                   
                 3dB 
                 5dB 
                  7dB 
                  9dB 
               
               
                   
                 0dB 
                 2dB 
                  4dB 
                   6dB. 
               
               
                   
                   
               
             
          
         
       
     
     The selection of the frequency band would normally depend on the general nature of the expected vehicle traffic at the particular location of the sensor. The selected gain would depend, in addition, on the distance of the sensor from the road surface. The outputs of the FIR filters  413  and  414  (the paths of V 1  and V 2 ) are summed in digital summer  417 , while the outputs of the FIR filters  415  and  416  (the paths of H 1  and H 2 ) are summed in digital summer  418 . The respective digital summers  417  and  418  are followed by digital limiters  419  and  420 , respectively, and the outputs of the latter are input to correlator  421 , the output of which is fed to a parallel-to-serial converter  422 , the serial output of which would normally be fed to a TDMA multiplexer (TDMA-MUX)  423  to be time-division multiplexed with other (conveniently four) processed microphone array signals originating from overhead locations near the array  400 . The multiplexed output of TDMA-MUX  423  is then normally relayed by cable  424  to roadside microprocessor-based controller  425 , where it is demultiplexed in DEMUX  426  into the original number of serial outputs representing the serial outputs of correlators, e.g.,  421 . After demultiplexing in DEMUX  426 , the cross-correlated digital output from the correlator  421  is intergrated in integrator  427  (which could be a software routine in the microprocessor/controller  425 ), and, depending on the correlated/integrated signal level, which is compared to a threshold in vehicle detector  428 , a “vehicle present” signal is issued for the duration above threshold. This information is processed by a flow parameter calculation routine  429  of the controller  425 , the output of which is an RS 232  standard in addition to hard-wired vehicle presence circuits or relays (not shown). 
     Description of FIG. 5 
     The operation of the controller  425 , whereby the demultiplexed signal from DEMUX  426  is processed, will be better explained by reference to the flow-chart shown in FIG.  5 . The signal is adjusted in gain/offset  500  depending on user specific parameters  501  and then sampled  502  and integrated  503 . The signal sampling  503  continues until enough samples  504  have been collected, upon which the integrator  503  is reset  505  and the mode (i.e., whether the controller is used to indicate only vehicle presence or to monitor traffic flow) is determined  506 . If the mode is to indicate vehicle presence (for example, to switch a traffic light from red to green), and a vehicle is detected  507 , the decision is immediately output  508 . If the mode  506  is “free flow,” then long-term speed average is calculated  508  from which variable thresholds are progressively calculated  509 . That is, the more vehicles there are, the more accurate will the average progressively become. This variable threshold is used to continue to determine vehicle presence  510 , and to calculate flow parameters  511 . The flow parameters  511  are stored in memory  512  and output  508  over the RS 232  serial link to (other) central traffic management systems (not shown), and where desired activate other interface circuits. As may be seen, the binary vehicle presence decision  507  is determined by a user-selected fixed threshold  513 .