In-Pavement Wireless Sensor Network for Vehicle Classification

1. In-Pavement Wireless Sensor Network for VehicleClassification Authors:RavneetBajwa, Ram Rajagopal, PravinVaraiya and Robert Kavaler Presenter: XiangyiGu

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 (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 to 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

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 (classifying vehicles) 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. Sensor needs to be insensitive to the vehicles traveling in the neighboring lanes Sample fast enough to capture the transient vibrations Insensitive to the truck engine and environmental noise Challenges The sensor has to be well coupled to the road way and be resistant to heavy vehicle traffic 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 resolution target • is 500ug • Bandwidth 50Hz • Sampling frequency 512 Hz( > 5 times NyquistFrequency) • -Power consumption increases • for higher sampling rates

11. A B . 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: The vibration signals from successive axles tend to blend. In wide highway lanes, vehicles can experience significant wander

12. Sensor Design

13. Resolution:Selectingan 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

14. Noise: 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

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

16. 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

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

18. 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 informationand other commands to the sensors • AP assigns every node unique time slots and a node ID to communicate with it.

19. 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

20. 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

21. 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

22. 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

23. 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

24. 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

25. Wide Lane ADET Algorithm Wander movement in a lane Combining vibration readings from multiple sensors Delay  Di = di / v sennor2 will measures the peak enery a little later than 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 sensor i.

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

27. 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 The WSN setup at Suno; site. DHMN are vehicle detection sensors shereas I,J,K,L are vibration sensors

28. 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

29. Deployment Challenges • Packet Drops • Drop rate was low(1%) (compare 50 feet away) retransmit packets with a delay of 1 packet  drop rate is almost 0 • Packet 1, 2, 1again, 2again • Vehicle Wander Because vehicles are not tavelingstright in a lane we would like 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

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

31. 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

32. 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( 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.

33. Axle Spacing Left: for tandem axle Middle: pick up trucks, small two axle commercial 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

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

35. 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

36. 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 Thank you very much!