In-Pavement Wireless Sensor Network for Vehicle Classification
In-Pavement Wireless Sen sor Network for Vehicle Classification A U T H O R S : R AV N E E T B A J WA , R A M R A J A G O PA L , P R AV I N VA R A I YA A N D R O B E R T KAVA L E R PRESENTER: XIANGYI GU Outline Motivation Introduction Description Communication Protocol Design Experiment Setup
Performance Conclusion & Future Work Outline Motivation Introduction Description Communication Protocol Design Experiment Setup Performance Conclusion & Future Work Motivation
Intrusive technologies Piezoelectric sensors, inductive loops (examples) High installation and maintenance costs Non-intrusive technologies Infrared, video imaging(examples) Sensitive to traffic and weather condition Propose an alternative system base on a WSN that is both cost effective and insensitive t
o environmental conditions Transportation agencies collect vehicle classification information to plan highway maintenance programs, evaluate highway usage and optimize the deloyment of various resources on the road Motivation Introduction Description Communication Protocol Design Experiment Setup Performance Conclusion & Future Work
Problem Statement Cars, buses, three-axle single unit trucks, and five-axle single trailer trucks (classifying vehi cles) A vehicle travels in a traffic lane at some vary ing speed and we wish to count the number o f axles and the spacing between each axle in an accurate manner Proposed WSN System Vibration sensor (accelerometer) embedded in the r
oad Calculate the axle spacings Vehicle detection sensor (magnetometers) Report the arrival and departure times of a vehicle Access point (AP) Send commands to sensors Log the incoming data
First in-pavement, easy to deploy, WSN based system for counting axles and axle spacing Outline Motivation Introduction Description Communication Protocol Design Experiment Setup
Performance Conclusion & Future Work Wireless Vehicle Detection Sensor Measures the changes in magnetic field to infer the local presence of a vehicle Synchronous Nanopower Protocol(SNP), a TDMA based protocol Last 10 years with a single 7200 mAhr battery
Given the arrival times tai and taj at the two sensors i and j, the speed v will be v = dij / |taj tai| Estimate the length(L) of the vehicle L = v(tdj - taj) Wireless Vibration Sensor Sample the analog output of an accelerometer and transmit the data via a radio Designing a sensor that measures pavement vibrations for axle detection have many unique challenges Sensor needs to be insensitive
to the vehicles traveling in the neighboring lanes Sample fast enough to apture the transient vibrations nsensitive to the truck engine nd environmental noise The sensor has to be well coupled to the road way and be resistant to heavy vehicle traffic
Challenges Sensor resolution target is 500ug Bandwidth 50Hz Sampling frequency 512 Hz( > 5 times Nyquist Frequency) -Power consumption increases for higher sampling rates
Axle detection and counting Given vehicle speed measurement and reliable measurement from the wireless vibration sensor, we still need to construct an axle detection algorithm that has good performance There are two important challenges in detecting individual axles: B A The vibration signals from successive axles tend to blend.
. In wide highway lanes, vehicles can experience significant wander Sensor Design Resolution:Selecting an accelerometer
SD1221-005 has higher sensitivity and lower noise de nsity However, it consumes more than 20 times the current than MS9002.D and has to be operated at higher volta ge Both devices achieved the aimed minimum resolution of 500 ug Select MS9002.D due to its low operating voltage and low cu rrent consumption
Noise: Filters for mitigating sound noi se Accelerometer is sensitive to sound MS9002.D behaves like a microphone under the devices bandwidth 3rd order low-pass filter with cutoff frequenc y of 50 Hz is sufficiently aggressive to filter out most of the sound in the audible spectru m Casing Sound isolation
Protect the electronics from rain water and oil spill on the road Circuit Description 2.5 V supply voltage Amplifier with gain 10 The gain of 10 reduces the range of the accelerometer to 225mg This is necessary in order to ensure that the quantization noise from the ADC is less than the noise from the accelerometer
Otherwise, the resolution of the system will be limited by A DC noise The reduced range is still sufficient For heavy trucks 200 mg Outline Motivation Introduction Description Communication Protocol Design
Experiment Setup Performance Conclusion & Future Work Communication Protocol Design MAC Layer TDMA based Time is divided into multiple frames with each fra mes about 125 ms long Each frame is further divided into 64 time slots Slot 0 is used by AP to send clock synchronization information and other commands to the sensors
AP assigns every node unique time slots and a no de ID to communicate with it. Application Layer Sync Application AP sends sync packets on a periodic basis Sensor node listens to sync packets every 125 ms When the clock converges to steady state, then is listens for a sync packet only once in 30 s Sync application is also used to send commands Set Mode, Reset, Set Timeslot, Set RF, Download
Firmware, Set ID Application Layer Accelerometer Application Idle Mode: accelerometer and related circuitry are tu rned off by disabling the voltage regulator Once every 30 s, the microcontroller and the transceiver wake up and acquire the sync packet Application Layer
Raw Data Mode: microcontroller wake up every 1/51 2 s, and samples the analog output from acceleromete r 32 samples at a sampling freq. 512Hz, and each sample c ontaining 12 bits of information
In every frame(125ms) we accumulate 96 bytes of inform ation to transmit To have a reasonable packet size, we fragment the data i n two parts, 48 bytes each, and transmit it using two diffe rent time slots 62.5ms apart Application Layer Download Firmware Application Reprogram the entire flash memory of a sensor n ode over the air AP transmits new code repeatedly and the node u pdating its code in small pieces
Only the data that do not overwrite the current ru nning program are updated by the node Axle Detection(ADET) Algorithm Results of ADET on truck49( two single axles and one tandem axle), a(n) is the measured acceleration in mg, e(n) is the scaled energy in mg2, and s(n) is smooth energy in mg2. The red asterisks on s(n) are the axle locations found by ADET. By reducing the minimum axle sepatation, the individual axles in the tandem axle can also be detected as shown by black circle Axle Detection(ADET) Algorithm
Using data from 4 trucks at different speeds, we observed the bandwidth of the energy sig nal and empirically defined by M(v) = 900/v Low-pass filter is optional Minimum time separation (v) was chosen by assuming that the axles are at least 6ft apart Wide Lane ADET Algorithm Wander movement in a lane Combining vibration readings from multiple sens ors
Delay Di = di / v sennor2 will measures the peak enery a little later tha n sensor 1, so the individual energy measurements nedd to be appropriately delayed System representation of the adjustment made to correct vehicle wander. The energy if the total signal at time n is the maximum of the energy of the individual signals ei(n) is each Outline Motivation Introduction Description
Communication Protocol Design Experiment Setup Performance Conclusion & Future Work Experiment Setup 4 vibration sensors and 4 vehicle detection sensor were installed on California Highway I-680 Vehicles come from Sunol Weigh Station Slow down at weigh station
Easy to collect ground truth Data from 53 different trucks, ranging from pickup trucks to 5-axle commercial trucks The WSN setup at Suno; site. DHMN are vehicle detection sensors shereas I,J,K,L are vibration sensors Installation
Boring a 4-inch diameter hole approximately 2.25 i nches deep Installed on a road in less than 20 minutes Installation of a small sensor is much cheaper and convenient than installing special material paveme nts required for piezoelectric sensors Deployment Challenges Packet Drops Drop rate was low(1%) (compare 50 feet away) ret
ransmit packets with a delay of 1 packet drop rate is almost 0 Packet 1, 2, 1again, 2again Vehicle Wander Because vehicles are not taveling stright in a lane we would lik e to choose data from vehicle sensor that was closest to the tires use Wide Lane ADET algorithm Sensor failure Sensor k did not work
Vibration data was available from 3 sensors Outline Motivation Introduction Description Communication Protocol Design Experiment Setup Performance Conclusion & Future Work Vibration Sensor Performance Noise with no vehicle in vicinity
414 ug RMS Truck was parked on top of the sensor with e ngine were on vs. truck blew its horn 7% vs. 4% With a heavy truck traveled in the closed lane Sensor did not register any noticeable peaks Axle Count Error difference between the ground truth axle c
ount and the estimated axle count By combining the measurements from all sensors, t he algorithm always gives the correct axle count Error results form the wander movement( Strongly affected by truck wander Performance of ADET using individual sensors and combinations of sensors. Count Err. Is the difference between the ground truth and ADET estimate. Under each sensor column is the observed frequency of the errors. Axle Spacing Left: for tandem axle
Middle: pick up trucks, small two axle comm ercial trucks Right: axles of trailers Distribution of estimated axle spacings. There are three clusters in the data separated by empty bins. The dotted lines represent
the means of there clusters Outline Motivation Introduction Description Communication Protocol Design Experiment Setup Performance Conclusion & Future Work Conclusion
A novel algorithm that estimates the axle count and spaci ng from pavement acceleration was designed and tested o n the collected data ADET is simple enough to implement a sensor node with li mited processing power Majorities of the existing technologies are wired solutions Both the sensors and the AP are powered by batteries and consume much less power than other technologies The installation procedure and sensors themselves are mu ch cheaper There is minimal maintenance compared to other technol ogies
Future Work Find an optimal arrangement of sensors in order to minimize the number of sensors de ployed Reduce the amount of data transmitted Reduce the sensor power consumption Thank you very much!
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