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Optical and electronic design for a field prototype of a laser-based vehicle delineation detection system
Optics and Lasers in Engineering 36 (2001) 11–27
Optical and electronic design for a field prototype of a laser-based vehicle delineation detection system Bin Lina, Harry H. Chenga,*, Benjamin D. Shawa, Joe Palenb a
Integration Engineering Laboratory, Department of Mechanical and Aeronautical Engineering, University of California, Davis, CA 95616, USA b Office of New Technology and Research, California Department of Transportation, Box 942873, MS 83, Sacramento, CA 94273, USA Received 1 January 2001; accepted 1 May 2001
Abstract A field prototype of a laser-based non-intrusive vehicle detection system has been developed for the measurement of delineations of moving vehicles on the highway. This prototype is based on our previous research on the principle of the measurement. The detection system uses two laser lines that are projected onto the ground as probes. The reflected light is collected and focused onto a photodiode array by an optical system. Vehicle presence is detected based on the absence of reflected laser light. By placing two identical laser/sensor pairs at a known distance apart, the speed of both the front and rear of a vehicle can be calculated based on the times when each sensor is triggered. The detector data are acquired and processed by a realtime system to obtain speed, acceleration, and length of a detected vehicle. The travel time of a vehicle can be acquired by detecting a vehicle at the beginning of a link and re-identifying the same vehicle at the end of the link. Several tests have been done with the field prototype system on the highway. The testing results show that the system can obtain the accuracy of measurement necessary to distinguish between moving vehicles on the highway. This article describes the design and implementation of each functional component of an advanced version of the field prototype system. r 2001 Elsevier Science Ltd. All rights reserved. Keywords: Diode laser; Photodiode array; Sensor electronics; Intelligent transportation system
*Corresponding author. Tel.: +1-530-752-5020; fax: +1-530-752-4158. E-mail address: [email protected] (H.H. Cheng). 0143-8166/01/$ - see front matter r 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 1 4 3 - 8 1 6 6 ( 0 1 ) 0 0 0 4 6 - X
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1. Introduction In an Intelligent Transportation System, or ITS, travel time is known as one of the most important parameters. Travel time is a good indicator of other direct constraints on ITS efficiency: cost, risk, and attentive workload [1,2]. Because trip travel time is the parameter the public most wants to minimize, this is the parameter that is most important for transportation service providers to measure and minimize. Speed is commonly used as an indicator of the travel time across a link. Any mechanism to measure travel time, by definition, is only determining the ‘‘past state’’ of the transportation system. This may or may not be a reasonable inference. Travelers want to know the ‘‘state’’ of the system (in the future) when they traverse it. In the simplest case, this is just a straight extrapolation of current ‘‘state’’. A major benefit of ITS will be to provide travelers with a much more valid and comprehensive ‘‘look ahead’’ model of the (short-term) future state of the transportation system. Validation of any traffic model requires (either implicitly or explicitly) traffic origination/destination (O/D) data. The lack of valid O/D data has been the major impediment in the calibration, validation, and usage of traffic models. In this research project we have developed a roadway detection system that can directly determine O/D data non-intrusively without violating the public’s privacy (as in license plate recognition systems). In current practice, vehicle features are most commonly measured using inductive loops or video image processing. An advantage of our system over loop detectors is the relative ease of installation and maintenance. Because loops are buried beneath the pavement, installation requires heavy equipment, and traffic must be re-routed [3]. It is for this reason that loops are expensive to install and repair. Because our system is mounted above the road, once installed, it can be maintained without disrupting the flow of traffic. More importantly, loop detectors cannot be relied upon to produce accurate speed (and therefore length) measurements because the inductive properties of the loops and loop detectors vary. Video can be used to directly measure the length of vehicles, however the use of real time video image processing is problematic due to its computationally intensive nature. Our system operates on a simple ‘‘on/off’’ basis, requiring much less computation for vehicle detection, and consequently much less computational hardware. Because video is a passive system (gathering ambient light), video images are dependent on the lighting conditions. Vehicle length measurements taken from video, even on the same vehicle, may not produce consistent results depending on time of day and weather conditions. For truly site and time independent vehicle length measurements, the video would require an external source of illumination. Because our system is active, it produces its own signals to be sensed and does not suffer from these limitations. One system that bears some similarity to the system we have developed is the Automatic Vehicle Dimension Measurement System (AVDMS) developed by the University of Victoria [4]. The AVDMS uses laser time-of-flight data to classify vehicles based on length, width, or height, and is based on the Schwartz ElectroOptics Autosense III sensor [5–8]. The Schwartz systems are entirely dependent on time-of-flight laser measurements with moving parts, similar to conventional lidar
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(laser radar) in the principle of measurement. There are some significant functional differences between our system and Schwartz’s. For example, the fundamental mechanism of detection of the Schwartz detector is that it determines the range (or distance) from the detector to the objects being detected. Our detector does not determine the range (or distance) from the detector to the objects being detected. The laser of Schwartz’s detector reflects off the vehicle to determine the size, shape, and ‘‘presence’’ of the vehicle. In our detector, the laser light reflected off the pavement is sensed. The lack of a reflection determines the size, shape, and ‘‘presence’’ of the vehicle. Therefore, our system will be more reliable because of its simplicity. The laboratory and the outdoor prototype of the detection system were reported in [2,12]. These previous systems were developed to verify the principle of the detection method. This paper presents our recent research results on improvements in the optics and electronics to make the system suitable for measurements in realtraffic environments with required accuracy.
2. Principle and method This section describes briefly the principle of the measurement. The details of the measurement principle are described in [2]. In our laser-based system, vehicle lengths along the cross section are measured by the laser lines to form the outline profiles of bumpers. The outline profile is used as a primary identifying feature. The system operates in the following manner, as illustrated in Fig. 1. The basic detector unit consists of a laser and a spatially offset photodetector positioned above the plane of detection. The laser is a pulsed infrared diode laser that utilizes line-generating optics, which project to a flat surface where objects are to be detected. The detector consists of imaging optics and a linear photodiode array. The offset photodiode array receives the laser light that is reflected back from the region of detection. The signal from the photodiode is amplified and sent to a computer for processing. Vehicle presence is detected based on the absence of reflected laser light. Two of these units are integrated and placed at a known distance apart, allowing the velocity of the object and its residence time under each detector to be measured, giving the object’s length and top–down outline profile. The detector is mounted at a distance of about 6.4 m (the height of a typical highway overpass) above the highway. The distance between each component of a laser/sensor pair is 60 cm. The sensors are mounted in a fixed vertical position, pointing downward, and are focused on the ground, forming two detection zones. The lasers are pointed towards the detection zones and are mounted at an adjustable angle, allowing the system to be mounted at different heights. The detection zones stretch across the width of the lane and are each about 5 mm wide in the direction of traffic flow. In this configuration, the minimum detectable object height, also called the critical height, is about 46 cm. This is lower than the bumper height of most common vehicles. Because the lasers are pointing downward at an angle, the distance between the first and the second blocking of the laser by vehicle depends on the height of vehicle,
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Fig. 1. System overview.
the higher the vehicle the smaller the blocking distance. Higher vehicles produce larger difference between measured and actual vehicle lengths. This error can be of the order of a few cm, implying that speed measurement will be slightly in error. However, since the vehicle data we measure are used as an identifier of vehicles, the absolute accuracy is not important. What we need is reproducible data for the same vehicle measured at a different location. By knowing the height of the sensor pair above the road, the reproducible vehicle data can be obtained at different locations by calibration of the detector system. When a vehicle moves into a detection zone, it blocks the laser from being received by the sensor. When the first beam is blocked, the current time is recorded. When the second beam is blocked, a second time is recorded. These times give the speed of the front of the car. In a similar manner, when each of the beams is no longer blocked, the times are recorded and the speed of the rear of the vehicle can be calculated. The time that each detector is blocked is also recorded and is used to calculate the vehicle length, assuming constant vehicle acceleration. Let t0 and t1 be the times at which the
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laser lines are blocked by the front and rear of the vehicle and t2 and t3 the times at which the laser lines are unblocked. The software uses the time interval and the distance between them to calculate the front velocity v0 ; d v0 ¼ ; t0 t1 where d is the distance between the two sensors. The rear velocity is calculated in a similar fashion: d v1 ¼ : t2 t3 The velocities are used to calculate the average acceleration of the vehicle as it passed under the two laser sensors. The calculation is based on the front velocity, v0 ; the rear velocity, v1 ; the time of the first rising edge, t0 ; and the time of the second rising edge, t2 ; v1 v0 a¼ : t2 t0 The length of the vehicle is determined from the front velocity, the timing of the first edges and the average acceleration: l ¼ v0 ðt2 t0 Þ þ 12aðt2 t0 Þ2 : The software obtains the current time and then groups all of the above-calculated parameters into one block of data and makes the data available for reading by an application. The group contains the timing information, the front and the rear velocities, the average acceleration, and finally the calculated length of the vehicle.
3. Field prototype system Even though the principle of detection described in the previous section is simple, the system needs to be designed carefully to meet the technical requirements of the detection system for the field test. Detection based on the critical height requires high-resolution sensor image optics with different magnifications in two dimensions. Since the laser power is limited by cost and safety considerations, and the laser beam is projected onto a line, the laser light reflection from a road is very weak. A welldesigned high-speed low-noise sensor electronics system is necessary. This section will describe the optical and electronic design that achieves these requirements. 3.1. Laser system Two off-the-shelf integrated diode laser systems, ML20A15-L2 high power diode laser systems from Power Technology are used as the laser sources. This is an integrated laser system that incorporates a DC/DC voltage converter, voltage regulator, pulse generator, laser diode, and line generation optics into a single unit. The system has a peak power output of 20 W at 905 nm (nominal), with a pulse width
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of 15 ns. It can be pulsed at a maximum rate of up to 10 kHz. The line generating optics produces a beam with a full fan angle of 301. A wavelength of 905 nm for the laser was chosen for a number of reasons. Infrared light has good transmittance through fog, giving the system better performance under a larger range of weather conditions. Furthermore, the intensity of sunlight around the wavelength of the laser is a local minimum, giving the system better rejection of noise due to sunlight. An infrared laser was also thought to be more appropriate for outdoor use because it is invisible to the human eye, and would therefore cause no distraction to passing motorists. The laser system has been shown to be eye safe [2,11]. A bandpass filter that is matched with the wavelength of the laser is used to reduce the level of ambient light received by the sensor. The wavelength of most diode lasers varies with temperature with the rate of 0.2–0.5 nm/1C. When a filter is used to eliminate the ambient light, it is important to ensure that the wavelength of laser is within the window of the band-pass filter. According to the specifications of the manufacturer, the wavelength range of the laser diodes we used is 90575 nm at room temperature. In order to operate our system in wide temperature ranges, i.e. in the hot summer and cold winter, the filter window should cover the whole range of laser wavelength when the temperature is changing. Due to the nature of wavelength shifting with temperature, it is necessary to measure the real spectrum of the laser diode before the filter is chosen. To this end, the laser spectrum was measured by a portable fiber optic spectrometer from Ocean Optics. The spectrometer was calibrated using known spectral lines from a Hg–Ar light source. The laser was guided into the spectrometer by an optical fiber. The measurements were carried out for temperatures of 25–471C. Fig. 2 shows a typical spectrum of a diode laser with an advertised wavelength of 905 nm; the actual peak (central) wavelength was about 916 nm at a temperature of 471C. The spectra were also measured with two different filters, as shown in Fig. 3. The filters A and B are 904-DF-15 and 904-DF-30 with a
Fig. 2. Typical spectrum of diode laser at temperature of 471C.
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Fig. 3. Central wavelength of diode laser vs. temperature.
central wavelength of 904 nm from Omega Optical, respectively. The central wavelength increased at the rate of 0.3 nm/1C when the temperature increased. The central wavelength was almost unchanged when the temperature was lower than 251C. In Fig. 3, the central wavelength with the filter is lower than without the filters. This is due to the fact that the filters blocked the higher part of the spectrum. We can see from this figure that the tolerance of the central wavelength and half bandwidth is large and the filter can block some of the laser light when the temperature increases. In order to overcome this problem, two filters with central wavelengths of 910 nm and half bandwidths of 30 nm were chosen according to the measurements of the laser spectrum. It is worthwhile to consider the effect of laser beam quality. A diode laser is multimode and sometimes the beam profile is not a real Gaussian distribution. There are high-order beams beside the main laser beam. In our system, the high-order side beams could cause some unwanted reflection. Even though the intensities of highorder beams are much lower than that of the main beam, the reflection from them could be comparable to that from the road, when they are reflected by vehicles with good surface conditions. The setup shown in Fig. 4 was designed to verify this. We placed a reflector A, which has good diffusive reflectance, in the field of view of the sensor. The position of the reflector is much higher than the critical height, but does not block the laser. In this case, we can get the reflection signal of the sensor on the same side of the laser. This reflection signal is comparable to the signal reflected from the road. When we move the laser closer to the reflector, we get higher reflection. Fig. 5 shows the relationship between the amplitude of the reflection signal and H; the distance between the reflective material and the laser beam. Fig. 6 indicates the amplitude of the reflection signal vs. d; the distance of the reflector from the detection system. In the highway situation it is possible the reflectance of some vehicles is higher than that of the reflector we are using in the lab.
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Fig. 4. Experimental setup for side-laser test (not to scale).
Fig. 5. Amplitude of the reflection measurement as a function of distance between laser and reflector.
The solution for reflection from the higher-order laser is to place a slot at a certain distance from the opening of laser. The slot will block the high-order beam as long as the distance is suitable. The distance depends on the beam shape and the mode of the laser. Fortunately, for our laser the high-order laser beam can be blocked by placing
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Fig. 6. Amplitude of reflection vs. distance between reflector and detection system.
Fig. 7. Configuration of system for side-laser blocking.
the slot about 8 in away from the laser diode. Fig. 7 shows the new system configuration we have designed to solve the problem caused by high-order beams. 3.2. Sensor system 3.2.1. Sensor optics The sensor optics consists of an imaging lens system and a telescopic lens system [2]. The imaging lens system focuses the reflected laser light onto the active area of the sensor array. The imaging lens was selected based on the criteria that it should have an adjustable focal length within a range around the desired focal length, that it
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should have a field-of-view large enough to capture the width of an entire lane and that it should be compact for easy integration into the outdoor system. The telescopic lens system is mounted in front of the imaging lens system, as shown in Fig. 7. It is designed to restrict the field-of-view of the imaging lens along the width of the laser line, but not alter the field-of-view along the length of the line. Because the laser line is much longer than it is wide, use of the imaging lens alone would result in a much wider strip of pavement being visible to the sensor than is desired. The telescopic lens system is used to match the dimensions of the laser line image with those of the sensor array. The telescopic system does not alter the position or focus of the image. Objects that are properly focused by the imaging lens remain in focus when the telescopic system is added. The mechanical design for the optics should be flexible and allows the optical components to be adjusted to optimize alignment and focus of the reflected laser beam. The mechanical configuration of system is shown in Fig. 8. The cylindrical lenses, image lenses, and APD sensors are mounted on optical rails, which are placed in the base plate. Each optical component and sensor is clamped by a holder which is adjustable in two dimensions. The image lenses can be swung around an axis with an optical cell. The sensor is clamped by a ring which is mounted on a rotation lens holder. As a result the sensor can be adjusted with both translation and rotation. After adjustment, a clear image of the reflecting laser can be obtained. The optimization of the optics significantly improves, i.e. reduces the critical height and eliminates the unexpected reflections when some reflective surface of a vehicle is present under the detector. A visible laser and a CCD are used to verify the image quality of the optical system.
Fig. 8. Mechanical configuration of the detection system.
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Fig. 9. Image of visible laser on the sensor at various reflection heights.
Fig. 9 shows the image of a reflected visible laser line on the position of a sensor from different heights. The principle of critical height is clearly demonstrated in this figure by noting the changes in the location of the bright vertical line, as the position of a reflective board is varied. Moving the board away from its base position (in this case a wall) causes the image of the laser line to move away from the sensor array location. 3.2.2. Sensor electronics According to the principle of detection, the system needs only to distinguish if a vehicle is present or not under the laser lines. Using a digital signal as output from the hardware will significantly simplify the signal processing in the software, therefore reducing the requirements of the computer system. The implementation of this method is based on the high signal-to-noise ratio of the new circuitry. A block diagram of the field prototype hardware is shown in Fig. 10. The hardware consists of seven parts: the diode lasers, the APD arrays, the power supply, the clock generator, the laser diodes, the amplifiers, the peak detectors and the digital outputs, and the digital I/O board and computer. A switch-type power supply is used to power both drivers of the diode lasers and the sensor circuits. A high voltage DC/DC converter changes 12 V DC to 250–350 V DC which is used to bias the sensor array from 290 to 310 V. The high voltage pulse generator for the laser diode is built into the laser unit, which is powered by
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Fig. 10. Diagram of electronics hardware.
12 V DC. As the pulse generator is isolated by a transformer and is well shielded, there is very low noise induced by the pulse. The power supply for the laser units is well isolated by filters. As a result, even though we used only one power supply for both the lasers and the sensor electronics, there is no interference between. This reduces the cost of the system. A clock generator provides a clock signal that is used to trigger the laser. An NE555 is used as the oscillator. The frequency can be adjusted over the range 1–50 kHz, though the lasers are limited to a maximum pulse rate of 10 kHz. A 25-element avalanche photodiode (APD) array is used as the sensor in our detection system. The sensor converts the reflected laser light into a current signal. The sensor circuit is the main part of the electronics hardware in the detection system. Low-cost general-purpose amplifier chips were used to amplify the signal from the APD array. The bandwidth of the amplifier was chosen to match the pulse width of the laser. The high-frequency signal can be amplified effectively without oscillation. A new method using a TTL logic circuit, instead of a sampleand-hold amplifier as previously used for the peak detection [2,9,10], has been used to handle the short signal pulse. Using this method, a TTL logic circuit is triggered by the amplified pulse and the output is a digital signal. Usually, the sample-and-hold amplifier is the bottleneck of the time response of the circuitry, so this method will improve the time response reliability of the system and allow us to gather more channels of the signal, for example, up to 24 channels. Using a digital signal as output from the hardware will significantly simplify the signal processing in the software. The implementation of this method is based on the high signal-to-noise ratio of the new circuitry. The new circuitry can be divided into three stages: signal amplification, signal interface, and digital output, as shown in Fig. 11. The current generated by the laser light reflected onto the sensor element is amplified by video amplifier U1. The output of this amplifier is about 1 V. A highspeed transistor T1 is used as an interface between analog and digital parts of the circuit. A multi-vibration mono-stable oscillator U2 is triggered by the pulses from the amplifier and generates a high-level output when there is no vehicle under the system. When a vehicle blocks the laser, there is no pulse to trigger the U2, and the output will be low. Capacitors are used to isolate DC paths between different
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parts of the circuit, so that a continuous signal cannot pass through the circuit. Normally, ambient lights generate continuous signals. After intensive tests and adjustment, the interference between different parts of the circuit has been reduced to the lowest levels. Fig. 12 shows a typical signal from the amplifiers. The transition time of the signal is an important factor regarding the accuracy and consistency of the data measured. Because this electronic circuitry generates signals directly in the digital mode, there is no problem of slow transition time as appears in some sampleand-hold circuits. Because of the use of surface mounted chips and the simplicity of the circuitry, the new version of the electronics has a compact size. This makes it possible to place all amplifiers for 24 channels on a small printed circuit board without long connection wires. This is important because the signal from the photodiode array is very weak. The longer the connection wires between the photodiode array and amplifiers, the more the noise and oscillation that will be introduced into the signal. Fig. 13 illustrates the modularized printed circuit board mount. This structure allows us to use all 24 elements of the sensor array and is easy to maintain.
Fig. 11. Circuitry of sensor electronics.
Fig. 12. Output of amplifier.
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Fig. 13. Mount of printed circuit boards.
3.3. Data acquisition and computer system Since the output of the sensor circuit is a TTL-compatible digital signal, the A/D converter is not needed. A general-purpose low-cost digital I/O board PCI-DIO-96 from National Instruments is used an input interface between the sensor circuitry and the computer system. This digital I/O has 96 of channels configurable input/ output, so it is suitable for our system when all 48 channels of signal are required. Normally digital I/O is fast enough to transfer data of the order of kHz. Real-time software on LYNX real-time operation system and Windows NT with RTX realtime extension has been developed to acquire the data from hardware without losing any data, and to calculate and display the vehicle data at the same time.
4. Field testing results Field tests have been conducted on the highway with real traffic. Fig. 14 is a picture of the test site and the detection system mounted on a bridge across the highway. In the current phase only four of the 24 elements of each sensor were used for testing. The front and rear speeds, length, and acceleration were obtained in the tests according to the measured data. The results indicate that the signals of the new version of the system are clear and the transition is fast enough in the system. Fig. 15 demonstrates typical signals and measurements from the highway test. The limited rates of data sampling and laser pulse repetition caused errors in these measurements. The asynchronicity between sampling in the computer and the clock rate of the laser pulse also contributed to the errors. In the highway tests, the data acquisition rate and pulse rate of the laser were set to 10 kHz. These rates are limited by the diode laser and the data acquisition software. Since data sampling was not synchronized with laser pulses, the timing uncertainty due to the limited sampling and laser pulse rate is the sum of the periods of sampling and laser pulsing, which is 0.2 ms. If the speed of the vehicle passing the detector is 65 mile/h, the time for a
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Fig. 14. Detection system mounted above the highway.
Fig. 15. Test results.
vehicle to cross two laser lines is about 21 ms. Considering two sensor units were used to calculate the vehicle parameters, the timing uncertainty was 0.4 ms, which caused 2% uncertainty. This error can be reduced to a factor of 2 by synchronizing
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the laser and data acquisition. The speed, acceleration, and length are calculated from the same data source, so their relative errors should be the same. It is also noted that the system was tested outdoor for more than 24 h in different lighting and weather conditions, such as rain and light fog. The signal was slightly influenced by the temperature, but still within the threshold required for reliable performance.
5. Conclusions We have developed and tested an improved version of field prototype of the laserbased real-time, non-intrusive detection system for measurement of delineations of moving vehicles in real traffic environments. An infrared diode laser with a line generator is used as a probe to detect the presence of vehicles. The reflected laser is received by a linear APD array through the sensor optics, and amplified by the integrated amplifiers. With properly designed sensor optics and electronic circuitry, a signal with adequate signal/noise ratio can be obtained to detect, with high resolution, the blocking of a laser line by vehicles. The test results quantitatively verified that the principle of our detection system is technically sound and indicated that the algorithm implemented in the software works in most cases. This simple method of detecting vehicle presence based on the absence of a reflected laser beam works reliably in a real traffic environment. The real-time data acquisition and processing system can be used to gather and process the data from the system hardware and to calculate vehicle data, such as front and rear velocities of vehicles, vehicle lengths, and average accelerations.
Acknowledgements This project is funded by the California Department of Transportation through the PATH Program. The authors would like to thank X. Hu, B. Chen, J. Parks, J. Korach, C. Perez, Y. Zhu for their contribution to the project described in the paper.
References [1] Palen J. The need for surveillance in intelligent transportation systems, Parts 1 and 2. Intellimotion 1997;6(1,2):1–3. [2] Cheng HH, Shaw BD, Palen J, Larson JE, Hu X, Katwyk KV, A real-time laser-based detection system for measurement of delineations of moving vehicles. CD-ROM Proceedings of the ASME 19th Computers in Engineering Conference. Paper # DETC99/CIE-9072, Las Vegas, NV. September 12– 15, 1999. IEEE/ASME Trans. Mech, in press. [3] Tyburski RM. A review of road sensor technology for monitoring vehicle traffic. ITE J 1989;59(8): 27–9. [4] Halvorson GA. Automated real-time dimension measurement of moving vehicles using infrared laser rangefinders. MS Thesis. University of Victoria, 1995. [5] Olson RA, Gustavson RL, Wangler RJ, McConnell RE. Active near-field object sensor and method employing object classification techniques. US Patent No. 5,321,490, 1994.
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Optical and electronic design for a field prototype of a laser-based vehicle delineation detection system Bin Lina, Harry H. Chenga,*, Benjamin D. Shawa, Joe Palenb a
Integration Engineering Laboratory, Department of Mechanical and Aeronautical Engineering, University of California, Davis, CA 95616, USA b Office of New Technology and Research, California Department of Transportation, Box 942873, MS 83, Sacramento, CA 94273, USA Received 1 January 2001; accepted 1 May 2001
Abstract A field prototype of a laser-based non-intrusive vehicle detection system has been developed for the measurement of delineations of moving vehicles on the highway. This prototype is based on our previous research on the principle of the measurement. The detection system uses two laser lines that are projected onto the ground as probes. The reflected light is collected and focused onto a photodiode array by an optical system. Vehicle presence is detected based on the absence of reflected laser light. By placing two identical laser/sensor pairs at a known distance apart, the speed of both the front and rear of a vehicle can be calculated based on the times when each sensor is triggered. The detector data are acquired and processed by a realtime system to obtain speed, acceleration, and length of a detected vehicle. The travel time of a vehicle can be acquired by detecting a vehicle at the beginning of a link and re-identifying the same vehicle at the end of the link. Several tests have been done with the field prototype system on the highway. The testing results show that the system can obtain the accuracy of measurement necessary to distinguish between moving vehicles on the highway. This article describes the design and implementation of each functional component of an advanced version of the field prototype system. r 2001 Elsevier Science Ltd. All rights reserved. Keywords: Diode laser; Photodiode array; Sensor electronics; Intelligent transportation system
*Corresponding author. Tel.: +1-530-752-5020; fax: +1-530-752-4158. E-mail address: [email protected] (H.H. Cheng). 0143-8166/01/$ - see front matter r 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 1 4 3 - 8 1 6 6 ( 0 1 ) 0 0 0 4 6 - X
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1. Introduction In an Intelligent Transportation System, or ITS, travel time is known as one of the most important parameters. Travel time is a good indicator of other direct constraints on ITS efficiency: cost, risk, and attentive workload [1,2]. Because trip travel time is the parameter the public most wants to minimize, this is the parameter that is most important for transportation service providers to measure and minimize. Speed is commonly used as an indicator of the travel time across a link. Any mechanism to measure travel time, by definition, is only determining the ‘‘past state’’ of the transportation system. This may or may not be a reasonable inference. Travelers want to know the ‘‘state’’ of the system (in the future) when they traverse it. In the simplest case, this is just a straight extrapolation of current ‘‘state’’. A major benefit of ITS will be to provide travelers with a much more valid and comprehensive ‘‘look ahead’’ model of the (short-term) future state of the transportation system. Validation of any traffic model requires (either implicitly or explicitly) traffic origination/destination (O/D) data. The lack of valid O/D data has been the major impediment in the calibration, validation, and usage of traffic models. In this research project we have developed a roadway detection system that can directly determine O/D data non-intrusively without violating the public’s privacy (as in license plate recognition systems). In current practice, vehicle features are most commonly measured using inductive loops or video image processing. An advantage of our system over loop detectors is the relative ease of installation and maintenance. Because loops are buried beneath the pavement, installation requires heavy equipment, and traffic must be re-routed [3]. It is for this reason that loops are expensive to install and repair. Because our system is mounted above the road, once installed, it can be maintained without disrupting the flow of traffic. More importantly, loop detectors cannot be relied upon to produce accurate speed (and therefore length) measurements because the inductive properties of the loops and loop detectors vary. Video can be used to directly measure the length of vehicles, however the use of real time video image processing is problematic due to its computationally intensive nature. Our system operates on a simple ‘‘on/off’’ basis, requiring much less computation for vehicle detection, and consequently much less computational hardware. Because video is a passive system (gathering ambient light), video images are dependent on the lighting conditions. Vehicle length measurements taken from video, even on the same vehicle, may not produce consistent results depending on time of day and weather conditions. For truly site and time independent vehicle length measurements, the video would require an external source of illumination. Because our system is active, it produces its own signals to be sensed and does not suffer from these limitations. One system that bears some similarity to the system we have developed is the Automatic Vehicle Dimension Measurement System (AVDMS) developed by the University of Victoria [4]. The AVDMS uses laser time-of-flight data to classify vehicles based on length, width, or height, and is based on the Schwartz ElectroOptics Autosense III sensor [5–8]. The Schwartz systems are entirely dependent on time-of-flight laser measurements with moving parts, similar to conventional lidar
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(laser radar) in the principle of measurement. There are some significant functional differences between our system and Schwartz’s. For example, the fundamental mechanism of detection of the Schwartz detector is that it determines the range (or distance) from the detector to the objects being detected. Our detector does not determine the range (or distance) from the detector to the objects being detected. The laser of Schwartz’s detector reflects off the vehicle to determine the size, shape, and ‘‘presence’’ of the vehicle. In our detector, the laser light reflected off the pavement is sensed. The lack of a reflection determines the size, shape, and ‘‘presence’’ of the vehicle. Therefore, our system will be more reliable because of its simplicity. The laboratory and the outdoor prototype of the detection system were reported in [2,12]. These previous systems were developed to verify the principle of the detection method. This paper presents our recent research results on improvements in the optics and electronics to make the system suitable for measurements in realtraffic environments with required accuracy.
2. Principle and method This section describes briefly the principle of the measurement. The details of the measurement principle are described in [2]. In our laser-based system, vehicle lengths along the cross section are measured by the laser lines to form the outline profiles of bumpers. The outline profile is used as a primary identifying feature. The system operates in the following manner, as illustrated in Fig. 1. The basic detector unit consists of a laser and a spatially offset photodetector positioned above the plane of detection. The laser is a pulsed infrared diode laser that utilizes line-generating optics, which project to a flat surface where objects are to be detected. The detector consists of imaging optics and a linear photodiode array. The offset photodiode array receives the laser light that is reflected back from the region of detection. The signal from the photodiode is amplified and sent to a computer for processing. Vehicle presence is detected based on the absence of reflected laser light. Two of these units are integrated and placed at a known distance apart, allowing the velocity of the object and its residence time under each detector to be measured, giving the object’s length and top–down outline profile. The detector is mounted at a distance of about 6.4 m (the height of a typical highway overpass) above the highway. The distance between each component of a laser/sensor pair is 60 cm. The sensors are mounted in a fixed vertical position, pointing downward, and are focused on the ground, forming two detection zones. The lasers are pointed towards the detection zones and are mounted at an adjustable angle, allowing the system to be mounted at different heights. The detection zones stretch across the width of the lane and are each about 5 mm wide in the direction of traffic flow. In this configuration, the minimum detectable object height, also called the critical height, is about 46 cm. This is lower than the bumper height of most common vehicles. Because the lasers are pointing downward at an angle, the distance between the first and the second blocking of the laser by vehicle depends on the height of vehicle,
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Fig. 1. System overview.
the higher the vehicle the smaller the blocking distance. Higher vehicles produce larger difference between measured and actual vehicle lengths. This error can be of the order of a few cm, implying that speed measurement will be slightly in error. However, since the vehicle data we measure are used as an identifier of vehicles, the absolute accuracy is not important. What we need is reproducible data for the same vehicle measured at a different location. By knowing the height of the sensor pair above the road, the reproducible vehicle data can be obtained at different locations by calibration of the detector system. When a vehicle moves into a detection zone, it blocks the laser from being received by the sensor. When the first beam is blocked, the current time is recorded. When the second beam is blocked, a second time is recorded. These times give the speed of the front of the car. In a similar manner, when each of the beams is no longer blocked, the times are recorded and the speed of the rear of the vehicle can be calculated. The time that each detector is blocked is also recorded and is used to calculate the vehicle length, assuming constant vehicle acceleration. Let t0 and t1 be the times at which the
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laser lines are blocked by the front and rear of the vehicle and t2 and t3 the times at which the laser lines are unblocked. The software uses the time interval and the distance between them to calculate the front velocity v0 ; d v0 ¼ ; t0 t1 where d is the distance between the two sensors. The rear velocity is calculated in a similar fashion: d v1 ¼ : t2 t3 The velocities are used to calculate the average acceleration of the vehicle as it passed under the two laser sensors. The calculation is based on the front velocity, v0 ; the rear velocity, v1 ; the time of the first rising edge, t0 ; and the time of the second rising edge, t2 ; v1 v0 a¼ : t2 t0 The length of the vehicle is determined from the front velocity, the timing of the first edges and the average acceleration: l ¼ v0 ðt2 t0 Þ þ 12aðt2 t0 Þ2 : The software obtains the current time and then groups all of the above-calculated parameters into one block of data and makes the data available for reading by an application. The group contains the timing information, the front and the rear velocities, the average acceleration, and finally the calculated length of the vehicle.
3. Field prototype system Even though the principle of detection described in the previous section is simple, the system needs to be designed carefully to meet the technical requirements of the detection system for the field test. Detection based on the critical height requires high-resolution sensor image optics with different magnifications in two dimensions. Since the laser power is limited by cost and safety considerations, and the laser beam is projected onto a line, the laser light reflection from a road is very weak. A welldesigned high-speed low-noise sensor electronics system is necessary. This section will describe the optical and electronic design that achieves these requirements. 3.1. Laser system Two off-the-shelf integrated diode laser systems, ML20A15-L2 high power diode laser systems from Power Technology are used as the laser sources. This is an integrated laser system that incorporates a DC/DC voltage converter, voltage regulator, pulse generator, laser diode, and line generation optics into a single unit. The system has a peak power output of 20 W at 905 nm (nominal), with a pulse width
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of 15 ns. It can be pulsed at a maximum rate of up to 10 kHz. The line generating optics produces a beam with a full fan angle of 301. A wavelength of 905 nm for the laser was chosen for a number of reasons. Infrared light has good transmittance through fog, giving the system better performance under a larger range of weather conditions. Furthermore, the intensity of sunlight around the wavelength of the laser is a local minimum, giving the system better rejection of noise due to sunlight. An infrared laser was also thought to be more appropriate for outdoor use because it is invisible to the human eye, and would therefore cause no distraction to passing motorists. The laser system has been shown to be eye safe [2,11]. A bandpass filter that is matched with the wavelength of the laser is used to reduce the level of ambient light received by the sensor. The wavelength of most diode lasers varies with temperature with the rate of 0.2–0.5 nm/1C. When a filter is used to eliminate the ambient light, it is important to ensure that the wavelength of laser is within the window of the band-pass filter. According to the specifications of the manufacturer, the wavelength range of the laser diodes we used is 90575 nm at room temperature. In order to operate our system in wide temperature ranges, i.e. in the hot summer and cold winter, the filter window should cover the whole range of laser wavelength when the temperature is changing. Due to the nature of wavelength shifting with temperature, it is necessary to measure the real spectrum of the laser diode before the filter is chosen. To this end, the laser spectrum was measured by a portable fiber optic spectrometer from Ocean Optics. The spectrometer was calibrated using known spectral lines from a Hg–Ar light source. The laser was guided into the spectrometer by an optical fiber. The measurements were carried out for temperatures of 25–471C. Fig. 2 shows a typical spectrum of a diode laser with an advertised wavelength of 905 nm; the actual peak (central) wavelength was about 916 nm at a temperature of 471C. The spectra were also measured with two different filters, as shown in Fig. 3. The filters A and B are 904-DF-15 and 904-DF-30 with a
Fig. 2. Typical spectrum of diode laser at temperature of 471C.
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Fig. 3. Central wavelength of diode laser vs. temperature.
central wavelength of 904 nm from Omega Optical, respectively. The central wavelength increased at the rate of 0.3 nm/1C when the temperature increased. The central wavelength was almost unchanged when the temperature was lower than 251C. In Fig. 3, the central wavelength with the filter is lower than without the filters. This is due to the fact that the filters blocked the higher part of the spectrum. We can see from this figure that the tolerance of the central wavelength and half bandwidth is large and the filter can block some of the laser light when the temperature increases. In order to overcome this problem, two filters with central wavelengths of 910 nm and half bandwidths of 30 nm were chosen according to the measurements of the laser spectrum. It is worthwhile to consider the effect of laser beam quality. A diode laser is multimode and sometimes the beam profile is not a real Gaussian distribution. There are high-order beams beside the main laser beam. In our system, the high-order side beams could cause some unwanted reflection. Even though the intensities of highorder beams are much lower than that of the main beam, the reflection from them could be comparable to that from the road, when they are reflected by vehicles with good surface conditions. The setup shown in Fig. 4 was designed to verify this. We placed a reflector A, which has good diffusive reflectance, in the field of view of the sensor. The position of the reflector is much higher than the critical height, but does not block the laser. In this case, we can get the reflection signal of the sensor on the same side of the laser. This reflection signal is comparable to the signal reflected from the road. When we move the laser closer to the reflector, we get higher reflection. Fig. 5 shows the relationship between the amplitude of the reflection signal and H; the distance between the reflective material and the laser beam. Fig. 6 indicates the amplitude of the reflection signal vs. d; the distance of the reflector from the detection system. In the highway situation it is possible the reflectance of some vehicles is higher than that of the reflector we are using in the lab.
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Fig. 4. Experimental setup for side-laser test (not to scale).
Fig. 5. Amplitude of the reflection measurement as a function of distance between laser and reflector.
The solution for reflection from the higher-order laser is to place a slot at a certain distance from the opening of laser. The slot will block the high-order beam as long as the distance is suitable. The distance depends on the beam shape and the mode of the laser. Fortunately, for our laser the high-order laser beam can be blocked by placing
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Fig. 6. Amplitude of reflection vs. distance between reflector and detection system.
Fig. 7. Configuration of system for side-laser blocking.
the slot about 8 in away from the laser diode. Fig. 7 shows the new system configuration we have designed to solve the problem caused by high-order beams. 3.2. Sensor system 3.2.1. Sensor optics The sensor optics consists of an imaging lens system and a telescopic lens system [2]. The imaging lens system focuses the reflected laser light onto the active area of the sensor array. The imaging lens was selected based on the criteria that it should have an adjustable focal length within a range around the desired focal length, that it
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should have a field-of-view large enough to capture the width of an entire lane and that it should be compact for easy integration into the outdoor system. The telescopic lens system is mounted in front of the imaging lens system, as shown in Fig. 7. It is designed to restrict the field-of-view of the imaging lens along the width of the laser line, but not alter the field-of-view along the length of the line. Because the laser line is much longer than it is wide, use of the imaging lens alone would result in a much wider strip of pavement being visible to the sensor than is desired. The telescopic lens system is used to match the dimensions of the laser line image with those of the sensor array. The telescopic system does not alter the position or focus of the image. Objects that are properly focused by the imaging lens remain in focus when the telescopic system is added. The mechanical design for the optics should be flexible and allows the optical components to be adjusted to optimize alignment and focus of the reflected laser beam. The mechanical configuration of system is shown in Fig. 8. The cylindrical lenses, image lenses, and APD sensors are mounted on optical rails, which are placed in the base plate. Each optical component and sensor is clamped by a holder which is adjustable in two dimensions. The image lenses can be swung around an axis with an optical cell. The sensor is clamped by a ring which is mounted on a rotation lens holder. As a result the sensor can be adjusted with both translation and rotation. After adjustment, a clear image of the reflecting laser can be obtained. The optimization of the optics significantly improves, i.e. reduces the critical height and eliminates the unexpected reflections when some reflective surface of a vehicle is present under the detector. A visible laser and a CCD are used to verify the image quality of the optical system.
Fig. 8. Mechanical configuration of the detection system.
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Fig. 9. Image of visible laser on the sensor at various reflection heights.
Fig. 9 shows the image of a reflected visible laser line on the position of a sensor from different heights. The principle of critical height is clearly demonstrated in this figure by noting the changes in the location of the bright vertical line, as the position of a reflective board is varied. Moving the board away from its base position (in this case a wall) causes the image of the laser line to move away from the sensor array location. 3.2.2. Sensor electronics According to the principle of detection, the system needs only to distinguish if a vehicle is present or not under the laser lines. Using a digital signal as output from the hardware will significantly simplify the signal processing in the software, therefore reducing the requirements of the computer system. The implementation of this method is based on the high signal-to-noise ratio of the new circuitry. A block diagram of the field prototype hardware is shown in Fig. 10. The hardware consists of seven parts: the diode lasers, the APD arrays, the power supply, the clock generator, the laser diodes, the amplifiers, the peak detectors and the digital outputs, and the digital I/O board and computer. A switch-type power supply is used to power both drivers of the diode lasers and the sensor circuits. A high voltage DC/DC converter changes 12 V DC to 250–350 V DC which is used to bias the sensor array from 290 to 310 V. The high voltage pulse generator for the laser diode is built into the laser unit, which is powered by
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Fig. 10. Diagram of electronics hardware.
12 V DC. As the pulse generator is isolated by a transformer and is well shielded, there is very low noise induced by the pulse. The power supply for the laser units is well isolated by filters. As a result, even though we used only one power supply for both the lasers and the sensor electronics, there is no interference between. This reduces the cost of the system. A clock generator provides a clock signal that is used to trigger the laser. An NE555 is used as the oscillator. The frequency can be adjusted over the range 1–50 kHz, though the lasers are limited to a maximum pulse rate of 10 kHz. A 25-element avalanche photodiode (APD) array is used as the sensor in our detection system. The sensor converts the reflected laser light into a current signal. The sensor circuit is the main part of the electronics hardware in the detection system. Low-cost general-purpose amplifier chips were used to amplify the signal from the APD array. The bandwidth of the amplifier was chosen to match the pulse width of the laser. The high-frequency signal can be amplified effectively without oscillation. A new method using a TTL logic circuit, instead of a sampleand-hold amplifier as previously used for the peak detection [2,9,10], has been used to handle the short signal pulse. Using this method, a TTL logic circuit is triggered by the amplified pulse and the output is a digital signal. Usually, the sample-and-hold amplifier is the bottleneck of the time response of the circuitry, so this method will improve the time response reliability of the system and allow us to gather more channels of the signal, for example, up to 24 channels. Using a digital signal as output from the hardware will significantly simplify the signal processing in the software. The implementation of this method is based on the high signal-to-noise ratio of the new circuitry. The new circuitry can be divided into three stages: signal amplification, signal interface, and digital output, as shown in Fig. 11. The current generated by the laser light reflected onto the sensor element is amplified by video amplifier U1. The output of this amplifier is about 1 V. A highspeed transistor T1 is used as an interface between analog and digital parts of the circuit. A multi-vibration mono-stable oscillator U2 is triggered by the pulses from the amplifier and generates a high-level output when there is no vehicle under the system. When a vehicle blocks the laser, there is no pulse to trigger the U2, and the output will be low. Capacitors are used to isolate DC paths between different
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parts of the circuit, so that a continuous signal cannot pass through the circuit. Normally, ambient lights generate continuous signals. After intensive tests and adjustment, the interference between different parts of the circuit has been reduced to the lowest levels. Fig. 12 shows a typical signal from the amplifiers. The transition time of the signal is an important factor regarding the accuracy and consistency of the data measured. Because this electronic circuitry generates signals directly in the digital mode, there is no problem of slow transition time as appears in some sampleand-hold circuits. Because of the use of surface mounted chips and the simplicity of the circuitry, the new version of the electronics has a compact size. This makes it possible to place all amplifiers for 24 channels on a small printed circuit board without long connection wires. This is important because the signal from the photodiode array is very weak. The longer the connection wires between the photodiode array and amplifiers, the more the noise and oscillation that will be introduced into the signal. Fig. 13 illustrates the modularized printed circuit board mount. This structure allows us to use all 24 elements of the sensor array and is easy to maintain.
Fig. 11. Circuitry of sensor electronics.
Fig. 12. Output of amplifier.
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Fig. 13. Mount of printed circuit boards.
3.3. Data acquisition and computer system Since the output of the sensor circuit is a TTL-compatible digital signal, the A/D converter is not needed. A general-purpose low-cost digital I/O board PCI-DIO-96 from National Instruments is used an input interface between the sensor circuitry and the computer system. This digital I/O has 96 of channels configurable input/ output, so it is suitable for our system when all 48 channels of signal are required. Normally digital I/O is fast enough to transfer data of the order of kHz. Real-time software on LYNX real-time operation system and Windows NT with RTX realtime extension has been developed to acquire the data from hardware without losing any data, and to calculate and display the vehicle data at the same time.
4. Field testing results Field tests have been conducted on the highway with real traffic. Fig. 14 is a picture of the test site and the detection system mounted on a bridge across the highway. In the current phase only four of the 24 elements of each sensor were used for testing. The front and rear speeds, length, and acceleration were obtained in the tests according to the measured data. The results indicate that the signals of the new version of the system are clear and the transition is fast enough in the system. Fig. 15 demonstrates typical signals and measurements from the highway test. The limited rates of data sampling and laser pulse repetition caused errors in these measurements. The asynchronicity between sampling in the computer and the clock rate of the laser pulse also contributed to the errors. In the highway tests, the data acquisition rate and pulse rate of the laser were set to 10 kHz. These rates are limited by the diode laser and the data acquisition software. Since data sampling was not synchronized with laser pulses, the timing uncertainty due to the limited sampling and laser pulse rate is the sum of the periods of sampling and laser pulsing, which is 0.2 ms. If the speed of the vehicle passing the detector is 65 mile/h, the time for a
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Fig. 14. Detection system mounted above the highway.
Fig. 15. Test results.
vehicle to cross two laser lines is about 21 ms. Considering two sensor units were used to calculate the vehicle parameters, the timing uncertainty was 0.4 ms, which caused 2% uncertainty. This error can be reduced to a factor of 2 by synchronizing
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the laser and data acquisition. The speed, acceleration, and length are calculated from the same data source, so their relative errors should be the same. It is also noted that the system was tested outdoor for more than 24 h in different lighting and weather conditions, such as rain and light fog. The signal was slightly influenced by the temperature, but still within the threshold required for reliable performance.
5. Conclusions We have developed and tested an improved version of field prototype of the laserbased real-time, non-intrusive detection system for measurement of delineations of moving vehicles in real traffic environments. An infrared diode laser with a line generator is used as a probe to detect the presence of vehicles. The reflected laser is received by a linear APD array through the sensor optics, and amplified by the integrated amplifiers. With properly designed sensor optics and electronic circuitry, a signal with adequate signal/noise ratio can be obtained to detect, with high resolution, the blocking of a laser line by vehicles. The test results quantitatively verified that the principle of our detection system is technically sound and indicated that the algorithm implemented in the software works in most cases. This simple method of detecting vehicle presence based on the absence of a reflected laser beam works reliably in a real traffic environment. The real-time data acquisition and processing system can be used to gather and process the data from the system hardware and to calculate vehicle data, such as front and rear velocities of vehicles, vehicle lengths, and average accelerations.
Acknowledgements This project is funded by the California Department of Transportation through the PATH Program. The authors would like to thank X. Hu, B. Chen, J. Parks, J. Korach, C. Perez, Y. Zhu for their contribution to the project described in the paper.
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