Overhead Inlkared Vehicle Sensor for TmcfffIcControl

Transcription

1 Overhead nlared Vehicle Sensor for TmcfffcControl BY TARK M. HUSSAN, TAREK N. SAADAW AND SAMR A. AHMED To meet traffic control needs, detection systems must first have a fundamental capability to count vehicles passing in each direction along a road. For multilane roads, this detection capability is needed on an individual lane basis. As traffic control technology becomes more sophisticated, particularly in urban areas, measuring vehicular speed and type (truc or passenger) as well as numbers of vehicles approaching intersections or other control points becomes desirable. Vehicle detectors have been in use for more than 50 years. The most common are inductive loops or ellipses of wire buried in the roadway.1,2 These detectors record a vehicle passing over the loop. They wor because most vehicles are made, at least partially, of ferro-magnetic materials that induce an inductive blip of electric current in the buried loop as they pass over it. These detectors are relatively expensive and can suffer from severe weather hazards (freezing and so forth) that might deteriorate pavements and affect detection loops. nductive loop detectors also have indeterminate zones of influence that mae them difficult for multilane use and for speed or length detection. By contrast the infrared laser system described in this article has the potential for being low cost, safe, easily installed overhead, reliable and weather resistant. t also has the potential for vehicular speed and length determination and, because of its defined geometric properties, is suitable for multilane usage. The infrared laser system described is based on the use of laser sources and photodiode detectors that can be mounted above roadways to monitor traffic passage (Figure 1). The system does this by distinguishing between the reflection of infrared laser light from the road surface and the interruption and changes in such reflections by the passage of traffic. Furthermore, by using several laser beams illuminating successive sections of roadway vehicle passage, speed and length are obtainable for each traffic lane by comparing reflected laser signals with a computerized correlation scheme. This article describes the development, design and experimental testing of a system operating on these principles. nfrared Laser Traffic Monitoring System Elements The basic elements of the system consist of two infrared laser transceivers. Each transceiver is a laser source and an optical detector. These are mounted on lamp standards or other elevated structures above the roadway so they have a line of sight to the approaching traffic on the lane they are monitoring (Figure 1). The optical outputs of the laser beam are made narrowly elliptical in cross section, so that a vehicle passing through the illuminated region maes a sharp, well-defined interruption from the normal roadway surface that the laser beam would otherwise strie. The photodetec- 1 SENSOB 1 SENSOR 2 Figure 1. Signal interruption. tor receivers are aligned to view the same region of roadway illuminated by the lasers. The output laser beams are pulsed at a 1 ilohertz (hz) repetition rate. The pulsed output permits the use of tuned narrow band photodetector receivers and results in higher signal to noise ratios. Because of the different reflections of the infrared laser light reflected from the surface of a passing vehicle compared to the steady state reflection from the road surface, passage of a vehicle would cause significant and abrupt consecutive interruptions of the beams and hence changes in the reflected light piced up by the optical photodetector receivers. This situation can be represented (Figure 2) by the two identical transmitted optical signals Z() from each of the laser transmitters, T1 and 72. The reflected signals received by receivers R and R2 are denoted by X() and Y(), respectively. For simple vehicle counting, the interruption of any one of the beams and consequently of its reflected signal can be used to detect and count passing vehicles. 38 TE JOURNAL SEPTEMBER 1993

2 For velocity measurements and vehicle characterization, it is necessary to consider the time the vehicle taes to traverse the distance (d in Figure 1) between the two laser footprints on the roadway. Since the distance between the optical footprints of the laser beams from T1 to T2 is fixed (the lasers remain fixed in the direction in which they point) the delay, T~ (in Figure 2), between the two interruptions detected by the received reflected signals X() and Y() will depend on the speed of the passing vehicle, and can in principle be used to measure this speed. The actual interruption duration, TXand TY, in each of the received signals depends on the speed and length of the vehicles as they pass through each of the optical footprints on the laser beams on the roadway. Thus if there is acceleration or deceleration between the two points then TX and T will be different, and together with #d can be used to determine speed, acceleration and length. Vehicle Counting For simple counting of passing vehicles, it is only necessary to record and count the episodes when the reflected laser signals undergo abrupt and sustained changes from their normal road surface reflection. t is, however, important that the circuitry and logic doing that be capable, through proper programming, of recognizing reflection signal changes characteristic of vehicles and hence distinguishing between changes due to passage of vehicles and other spurious noise changes (for example, a bird flying across the optical beam). Figure 3 shows a bloc diagram of the system arrangement used for simple counting. n this the received signal is fed to a preamplifier to improve signal to noise ratio, then to a low pass filter to produce a dc component. This dc component is then mounted and compared, in the comparator, with the steady-state reflection values from the roadway. Any abrupt and sufficiently sustained change in the reflected signal, such as that caused by the passage of a vehicle, is then detected and used (through a Schmitt invertor) to trigger a counter and hence count passage of a vehicle. The effect of sudden spurious signals, such as those due to the passage of a bird, are filtered out by filtering with appropriate time constants. Td m Z () transmitted signal at both sensors ruin X() Xo detected signal at the 1st sensor Y() detected 2nd sensor TX 4 b N nnmnn muuuul_l Yo signed at the Figure 2. Transmitted and reflected signal. Speed and Length Measurements As discussed in the previous section, the speed of a vehicle V can be determined by dividing the roadway distance between the laser footprints of two laser beams signals by the time T~ taen by the vehicle to cross the distance (see Figures 1 and 2). The speed of the vehicle is given by: v=: d [1] Td =( YO- XO), wherexo represents the starting time of interruption of reflected signal to the first receiver, and YOrepresents the starting time of interruption of reflected signal to the second receiver (see Figure 2). To determine the delay time T~ between the interruptions in each of the reflected signals (X() and Y()), a cross-correlation technique is used, the cross correlation between the outgoing or reference signal and reflected signals is examined. Cross-correlation for two signals, (X() and Z()), is given where T, is the window size (in number of pulses) in which the correlation function is performed. e R.,(i) = ~ Z()X( + i) [2] ={> This cross-correlation can be determined electronically by first digitizing then combining the relevant outgoing and reflected signals into a digital Figure 3. Bloc diagram of vehicle counting system. signal-processing integrated circuit chip. The magnitude of the output signal from the chip is then proportional to the extent that correlation exists between the reference and reflected signals. By examining the cross correlation between the transmitted signals, Z() and the reflected signals, X() and Y(), the start of the interruption times XO andyo for each of the reflected beams, and the duration of the interruptions, TX and TY respectively can be determined. When these values have been determined, they can be used to measure the speed and length of passing vehicles as well as their numbers. The use of a correlation function is basically a process of shifting the X() train of pulses by one pulse, multiplying with the corresponding Z() pulses and summing the results. The process repeats for the duration of the window. TE JOURNAL SEPTEMBER

3 x(t) J-LF + b Z() X() t X(+3) h ii,, Hlllii, 2222 Rzx() u t X(+l) 2222 X(+2) Figure 4. Calculation Llu Lll- of cross correlation. w Rzxo t Equation 2 to yield RXZ(1)=6. At steady state and if there is no vehicle passing, the cross correlation R,Z() is equal to a value of no vehicle passing s correlation RXZO(). When a vehicle interrupts the beam, RXz() increases and reaches the maximum value when the car is completely under the beam and then it decreases as the car leaves the beam footprint. From the cross-correlation calculations between the transmitted signals, Z() and the reflected signals, X() and Y(), the start of the interruption times, XO and Yo, and the duration of the interruptions, TX and TY, can be determined for each of the reflected beams. Speed is given by: v=+= d [3]., Y{, x,, Determination of the vehicle s length is accomplished by using TX, which represents the time required for the vehicle to pass the first beam, and combining that with the speed information obtained above, thus: Lve,,<,e= V. Tx [4] These correlation techniques provide an alternative approach described earlier for counting vehicles. This can be done by counting the number of interruptions in either beam as evidenced by the abrupt and sufficiently sustained change in the autocorrelation function. VEHCLE \ \ rfi7$m)=n/ OWERSUPPLY DRVER LASER~lODE w Yfl / col mtor Y- y Figure 5. System bloc diagram. Figure 4 gives a typical sequence of in Equation 2 yields RZX(0)=5. When operation. For example, when i=o the i=l, the correlation function can be correlation function can be calculated found in the same way by examining by looing at X() and Z() (see Figure Z() and X(+l) and the value of 4). Substituting the appropriate values R,Z(l) is again given by substituting in Practical System Parameters, Optical Laser Transmitter and Receiver System The infrared optical transmitter and receiver system are designed to be low maintenance and easy to install. Approximate cost estimates call for $500 in material and labor in quantity production. This could be compared to installed inductive loop costs of $1,800- $3,200 and considerable disruption of traffic during loop installation. The system needs to have sufficient laser output and receiver sensitivity to give satisfactory signal-to-noise ratios in order to be reliable and effective. To optimize signal-to-noise ratios, an infrared transceiver system operating in a pulsed mode at an approximately 1 hz repetition rate was designed. This permits tuned detectors to be used to 40 TE JOURNAL SEPTEMBER 1993

4 reduce noise. The repetition rate is high enough so that the passage of vehicles causes long interruption compared to the rate. That way the vehicles readily can be identified and differentiated from spurious signals. The experimental system used is shown in Figure 5. A single solid-state gallium arsenide injection laser diode is used in conjunction with a collimator and a beam splitter to provide the two transmitted signals. Two photodiodes are aligned to detect reflected signals from the road surface as well as reflected signals from passing vehicles. The photodiodes outputs go into an automatic gain control amplifier (AGC) and then to a counter and displayers to count the passing vehicles. The amplifier output is then fed to subsequent signal processing steps. Part of the outgoing signal is coupled, to serve as a reference for comparison purposes, into an analog-to-digital converter (A/D) and then to the signal-processing system for further speed and length measurements. The main components in the signal processing system are the A/D, the signal-processor chip TMS320C1X (for speed and length measurements) and the worstation 1/0 card model PO-12 parallel digital 1/0 interface. The A/D is a PM-0820 chip with eight-bit resolution with digital outputs designed for ease of use in microprocessor-based systems. This CMOS device offers 1.3 psec conversion time. t is ideally suited to a variety of A/D applications with high speed, low power and ease of use. No external sample and hold amplifier is needed in conjunction with input signals.3 The digital-signal processor is a TMS320 16/32-bit single chip. This combines the flexibility of a high-speed controller with the numerical capability of an array processor, offering an inexpensive alternative to multichip bit-slice processors. The high parallel architecture and efficient instruction set provide the necessary speed and flexibility to produce a MOS microprocessor family capable of executing 6.4 MPS.4 MetraByte s parallel digital 1/0 card provides a flexible interface between the data gathering and computational functions.5 Experimental Results Anexperimental prototype system was buift and tested in both the laboratory and p auto lrtoo.rlna. --.,.....O , ,, ,, , ,-* *-&+---.1,, :. *- +- -* -- 9-* -* v , L ,,.-- ~ 2*00000 s Soo.000 Uo 3,00000 co 500 u8/dlv current tlt nu@ wtlaua ov~rago p-pc 11 S67,590@V W BV 324. S9BV b OU1O rloaorlna (a) Transmitted signal , l-o +-?,.,.*.., ~..+b.+.a.. t......,,$.,<.,.>-.,.,-* ~..+~~..p,.,4. * ,.. - -~. _ ,-,. -,.---, ,., , , , , -.-.,., -..-.,,..,,, , ,-, , < s Soo.000 us , 500 Usllllv current sinlauo mol(lmua av.raga p-p( , 5@mV 2S.000 V S aV W (b) Reflected Signal Figure 6. Signals displayed on the oscilloscope s screen. in the field in at least six separate tests. Each Yor. The system readify detected passage field test lasted between two to four hours. of cars, vans and pedestrians under a variety For field testing the prototype was mounted of conditions. The test results demonstrate on a pedestrian bridge over a main street in that the sensor reliably detects vehicles. the Upper Manhattan campus of the Chy Typicafly, counting error was less than one College of the City University of New in two hundred vehicles. TEJOURNAL SEPTEMBER

5 f L WS&M Effects of Vehicle Signal Passage on Reflected The definition of quality signs: Wells Signs & Manufacturing. sign (sin) n. [< L. signum] 4. a publicly displayed board, placard, etc. bearing information, usually manufactured by Wells Signs & Manufacturing because no other company maes Fiber Optic, Blanout or lluminated signage that last and perform as well. Figure 6a shows the typical laser signal transmitted by the system and detected at street level. Figure 6b shows the corresponding signal reflected from the street and detected at the overpass and amplified further. t is this received signal that is interrupted by the passage of vehicles. The uninterrupted received signal is essentially the same in all tests. Figure 7 shows the received signal as a van passes through the laser beam footprint. Figure 8 shows the passage of a car. The signal received from a van is stronger because of the van s height and hence shorter distance to the detector. n addition the duration of the reflection disruption is related to the length of vehicle. These type of tests have been done repeatedly and the results were consistent and readily interpretable. The effects of cloudy, rainy and humid conditions were checed. There were no significant impacts. No test measurements were done in fog, because locally there were no foggy days when field tests were being done. However, the signal-to-noise ratio typically obtained with the system was greater than 53 decibels. This means that even a several hundred-fold reduction in signal because of fog could be tolerated and the system would still operate. (Several hundred-fold reductions in visibility also would probably mean no traffic could circulate). As described previously, the detection system incorporates automatic gain control amplifier. Thus, as the received signal is diminished because of deteriorating visibility, the receiver gain automatically is increased to compensate and ensure that the detector system output is essentially unchanged for the correlation and signal-processing stages. Road surfaces covered with a layer of snow would have no detrimental effect on the system since all they would do is enhance the steady-state signal. Falling snow would result in an increase in the noise in the system as reflections are obtained from individual flaes. However. with the lhz tuned detector approach (which could be easily increased to 10hz) this effect is expected to be negligible. Mu? + rufmlng [!!! Signs & Manufacturing ONLYmWells 6206East 18th Street,#4 Vancouver,WA O*9 1 1 Phone(206) Fax (206) ), M ~....,..--,...., i : $..7 -i L- 4/, -. w. <, r: :: ~,. \.. L-- $, -M+-+..,+$ l-,.++ t -. + t , , eoo.000 U9 S.000oo s S,aooo s 500 lm)dlv ~urreflt hlnlaum en1 ua worogo Vp-pt 1 ) c ppdl! W 64S. 750mV dl Figure 7. Reflected signal from surface of a van. 42 TE JOURNAL SEPTEMBER 1993

6 Vehicle Counting Figure 9 shows the results for 120, 90 and 80 minutes of operation of the counting system, during which 110, 44 and 39 vehicles passed, respectively. A vehicle passage is represented by a step jump in signal. Absence of vehicles is represented by a straight line. Results of tests were checed, and vehicle counting corroborated by human visual observation. Additional tests are now underway to chec and evolve the speed and length measurement capabilities of the system based on the cross-correlation approach described previously. The results of this wor will be reported in a subsequent paper. Summary and Conclusions , ,s,,...,,,......,, :. The basic elements of a system using infrared lasers and computerized correlation techniques to monitor and detect road vehicles was developed. t was demonstrated in the laboratory and in preliminary road tests that confirmed the basic capabilities of the system. The results of this wor indicate that a prod , $-..? , / ,...,-.++F...*., ~ , : { 1--_J._-_LoJ J -.-.-l._..l. 1.,./ Soo. 000us s 5,50000 *S W us/dlv current minlwb aax Jaua avorogo Vp-pl 1 ) MV d06.250@j 481.2S0,V V Figure 8. Reflected signal from the surface of a car.,.,., Tips from Budget Heros Who Ron Scholfz, Traffic Engineering, St. Joseph, MO Ron has been en OMJC customer since At that time St. Joseph, MO (74,000 pop.) was taing bids to re-wor 35 downtown intersections. The old equipment consisted of sight inch signals and Eagle EF20 timers. OMJC won the signal section of the bid, which gave St. Joseph the quality of reconditioned aluminum 12 traffic signals at a savings of over $40 per signal compared to new polycarbonate. That amounted to over $5500 savings on signals alone. OMJC also provided reconditioned hardware, which resulted in more savings. Ron s sole complaint was that thare was some variation in installation since all 135 signals were not identical. n 1991 St. Joseph continued their downtown renovation, and bought a few replacement controllers from OMJC. The reconditioned Eagle EF 20 units that OMJC provided cost Ron about 1/4 the cost of new -- a savings of almost $1 0,000! They have been on the street for over a year, and are running strong. OMJC also provided reconditioned EF120 panels in cabinets and reconditioned steel M size ceinets to St. Joe at substantial savings. Ron says, Those steel cabinets are lie new! So far in our relationship, OMJC has saved Ron s budget nearly $ Ron sure nows how to stretch a dollar. UsedWh pennlmbn. n OMJC Signal,nc. PO, Box 1594 Waterloo, 1A Phone 1-SCC FAX 319-2*1 554 Bright Plastic Minicades and Narrowcades with sign legends or barricade stripes printed on 3M reflective sheeting (engineering grade or high intensity). They feature handles, detents to clic loc open and are molded hollow to be internally ballasted with sand NEWPORT DRVE, ROLLNG MEADOWS, LLNOS (800) FAX (708) TE JOURNAL SEPTEMBER

7 * > 60 6 s Time(mlnties) Testl Figure 9. Vehicle Test.2 Test Couuting. uct based on the system technology developed shows promise of being a practical, cost-effective device. Acnowledgment This wor was supported by the New Yor State Energy, Research and Development Authority. References 1. U.S. Department of Transportation. Traffic Detector Technical Appendix. Washington, DC: Federal Highway Administration, April U.S. Department of Transportation. Traffic Detector Handboo. Washington, DC: Federal Highway Administration, April PM Data Boo Texas nstruments. TMS3201O User s Guide. Texas nstruments, Keithley Metrabyte/ASYST/DAC. Data Acquisition and Control, Vol. 23,1990. Tari M. Hussain currently is woring toward his dissertation thesis at the City College of the City University of New Yor (CCNY). He earned a B.S. in electrical engineering from the University of Technology, raq, and an MSEE from CCNY. He is a Student Member of TE. Tare N. Saadawi has been with CCNY since 1980 and currently is a professor teaching courses in computer networs, local area networ and communications systems. He received a B. SC. and M. SC. from Cairo University, Egypt, and a Ph.D. from the University of Maryland. Samir A. Ahmed is chair of the electrical engineering department of CCNY. He earned a B.A. in engineering from Cambridge University and a Ph.D. from University College, London. He is a Member of the nstitute. H.,W **,,. - =::! HCM/Cinema 2.0 m,,3! lsm -!,., w m lef~~ #Popular, friendly interactive graphics software to analyze signalized intersections. + W,l. ~ and develop signal timings according to Chapter 9 of the 1985 HCM. Powerful......< =...= e, : h.- :3! ulsm, ; pp animation displays enable you to see how your design will function and clearly \!, 1 %. : % :,,,) ::::::; N demonstrate results to policy maers and to the public.,) mm,=, ow, N nr!, n- ::::::: ;::; j:fi. Fast, easy to use: Graphical Data Entry, On-Line Guidance & nteractive cmlmm - *,,,:,: ::::::: Diagnostic Testing.,,,,,,,,,,,,,,,,.,,,,,,,,,,,,,,,,,,,,,,,W, :~ ::,:::::,:,:,:,:,:,:.,.,...,,,,,,,,,,,,.,.,....,.,.,.,,,,,,,.,,,,... - Automatically draws yourinterseetion,volumes and phasing plan on screen as you Actual HCM/Cinema 2.0 Screens enter data. Save any graphical display and import direetly inta your final report! m9.-..btl. f. c..:1- - -r mnms=,+mmml -m-m mnaat m2.. m. ncludes a version of the TRAF-NETS@ simulation model ta evaluate buses, K.!* b- -.- (-. queues, turn-bay overflows, fuel consumption and emissions. **M- *U 0 L.17.X *1C.&,s tl. t,,,,, c Colorfirl 2D animation of simulated traftlc operations; great for presentations.... :::::::.,,,.,. These displays can be transformed into spectacular3d Videos by KLDAssociates. 19 L. S4196C..:.7! us c -! ~ ,:,:.:. ] 1 Q.9 c Professional text l graphical reports; exports Chapter 9 worsheets as Lotus 1-2-Wspreadsheets..f9.%,-lS6C!,,4,,,1 a J: *.- m.% *,,,e,,,.,,m 113 D ++ u, m.2,,1 2 Srz \% 9m!# KLD Associates, nc. Polytechnic Univ. *-=*:,rz,m m,. c # 300 Broadway & 333 Jay Street.,,. Huntington Station, NY Broolyn, NY Satisfaction Guaranteed Cdl today to order (800) TE JOURNAL SEPTEMBER