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In-Pavement Wireless Sensor Network for Vehicle Classification

In-Pavement Wireless Sensor Network for Vehicle Classification. Ravneet Bajwa , Ram Rajagopal , Pravin Varaiya and Robert Kavaler IPSN’11. Outline. Motivation Introduction Description Communication Protocol Design Experiment Setup Performance Conclusion & Future Work. Outline.

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In-Pavement Wireless Sensor Network for Vehicle Classification

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  1. In-Pavement Wireless Sensor Network for VehicleClassification RavneetBajwa, Ram Rajagopal, PravinVaraiya and Robert Kavaler IPSN’11

  2. Outline • Motivation • Introduction • Description • Communication Protocol Design • Experiment Setup • Performance • Conclusion & Future Work

  3. Outline • Motivation • Introduction • Description • Communication Protocol Design • Experiment Setup • Performance • Conclusion & Future Work

  4. Motivation • Intrusive technologies • Piezoelectric sensors, inductive loops • High installation and maintenance costs • Non-intrusive technologies • Infrared, video imaging • Sensitive to traffic and weather condition • Propose an alternative system base on a WSN that is both cost effective and insensitive to environmental conditions

  5. Motivation • Introduction • Description • Communication Protocol Design • Experiment Setup • Performance • Conclusion & Future Work

  6. Problem Statement • Cars, buses, three-axle single unit trucks, and five-axle single trailer trucks • A vehicle travels in a traffic lane at some varying speed and we wish to count the number of axles and the spacing between each axle in an accurate manner

  7. Proposed WSN System • Vibration sensor (accelerometer) embedded in the road • 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, easyto deploy, WSN basedsystem for counting axlesand axle spacing

  8. Outline • Motivation • Introduction • Description • Communication Protocol Design • Experiment Setup • Performance • Conclusion & Future Work

  9. Wireless Vehicle Detection Sensor • Measures the changes in magnetic field to infer the local presence of a vehicle • Synchronous Nanopower Protocol(SNP), aTDMA based protocol • Last 10 years with a single 7200 mAhr battery • Given the arrival times tai and taj at the twosensors i and j, the speed v will bev = dij / |taj – tai| • Estimate the length(L) of the vehicleL = v(tdj - taj)

  10. Wireless Vibration Sensor • Sample the analog output of an accelerometer and transmit the data via a radio • Sample fast enough to capture the transient vibrations • Sensor needs to be insensitive to the vehicles traveling in the neighboring lanes • Insensitive to the truck engine and environmental noise • Sensor resolution target is 500 ug • Bandwidth 50Hz • Sampling frequency 512 Hz( > 5 times Nyquist Frequency) • Power consumption increases for higher sampling rates

  11. Selecting an accelerometer • SD1221-005 has higher sensitivity and lower noise density • However, it consumes more than 20 times the current than MS9002.D and has to be operated at higher voltage • Both devices achieved the aimed minimum resolution of 500 ug • Select MS9002.D due to its low operating voltage and low current consumption

  12. Filters for mitigating sound noise • Accelerometer is sensitive to sound • MS9002.D behaves like a microphone under the device’s bandwidth • 3rd order low-pass filter with cutoff frequency of 50Hz is sufficiently aggressive to filter out most of the sound in the audible spectrum

  13. Casing • Sound isolation • Protect the electronics fromrain water and oil spill on theroad

  14. 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 ensurethat the quantization noise from the ADC is less than the noise from the accelerometer • Otherwise, the resolution of the system will be limited by ADC noise • The reduced range is still sufficient • For heavy trucks  ± 200 mg

  15. Outlin • Motivation • Introduction • Description • Communication Protocol Design • Experiment Setup • Performance • Conclusion & Future Work

  16. Communication Protocol Design • MAC Layer • TDMA based • Time is divided into multiple frames with each frames 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 node ID to communicate with it.

  17. 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

  18. Application Layer • Accelerometer Application • Idle Mode: accelerometer and related circuitry are turned off by disabling the voltage regulator • Once every 30 s, the microcontroller and the transceiver wake up and acquire the sync packet

  19. Application Layer • Raw Data Mode: microcontroller wake up every 1/512 s, and samples the analog output from accelerometer • 32 samples at a sampling freq. 512Hz, and each sample containing 12 bits of information • In every frame(125ms) we accumulate 96bytes of information to transmit • To have a reasonable packet size, we fragment the data in two parts, 48 bytes each, and transmit it using two different time slots 62.5ms apart

  20. Application Layer • Download Firmware Application • Reprogram the entire flash memory of a sensor node over the air • AP transmits new code repeatedly and the node updating its code in small pieces • Only the data that do not overwrite the current running program are updated by the node

  21. Axle Detection(ADET) Algorithm

  22. Axle Detection(ADET) Algorithm • Using data from 4 trucks at different speeds, we observed the bandwidth of the energy signal and empirically defined by M(v) = 900/v • Low-pass filter is optional • Minimum time separation ζ(v) was chosen by assuming that the axles are at least 6ft apart

  23. Wide Lane ADET Algorithm • Wander movement in a lane • Combining vibration readings from multiple sensors • Delay  Di = di / v

  24. Outline • Motivation • Introduction • Description • Communication Protocol Design • Experiment Setup • Performance • Conclusion & Future Work

  25. 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, rangingfrom pickup trucks to 5-axle commercial trucks

  26. Installation • Boring a 4-inch diameter hole approximately 2.25 inches deep • Installed on a road in less than 20 minutes • Installation of a small sensor is much cheaper and convenient than installing special material pavements required for piezoelectric sensors

  27. Deployment Challenges • Packet Drops • Drop rate was low(1%)  retransmit packets with a delay of 1 packet  drop rate is almost 0 • Packet 1, 2, 1, 2 • Vehicle Wander • use Wide Lane ADET algorithm • Sensor failure • Sensor k did not work • Vibration data was available from 3 sensors

  28. Outline • Motivation • Introduction • Description • Communication Protocol Design • Experiment Setup • Performance • Conclusion & Future Work

  29. Vibration Sensor Performance • Noise with no vehicle in vicinity • 414 ug RMS • Truck was parked on top of the sensor with engine 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

  30. Axle Count • Error  difference between the ground truth axle count and the estimated axle count • By combining the measurements from all sensors, the algorithm always gives the correct axle count • Error results form the wander movement

  31. Axle Spacing • Left: for tandem axle • Middle: pick up trucks, small two axle commercial trucks • Right: axles of trailers

  32. Outline • Motivation • Introduction • Description • Communication Protocol Design • Experiment Setup • Performance • Conclusion & Future Work

  33. Conclusion • A novel algorithm that estimates the axle count and spacing from pavement acceleration was designed and tested on the collected data • ADET is simple enough to implement a sensor node with limited 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 much cheaper • There is minimal maintenance compared to other technologies

  34. Future Work • Find an optimal arrangement of sensors in order to minimize the number of sensors deployed • Reduce the amount of data transmitted • Reduce the sensor power consumption

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