1. In-Pavement Wireless Sensor Network for Vehicle Classification Ravneet Bajwa, Ram Rajagopal, Pravin Varaiya 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, easy to deploy, WSN based system for counting axles and 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), a TDMA based protocol – Last 10 years with a single 7200 mAhr battery Given the arrival times t ai and t aj at the two sensors i and j, the speed v will be v = d ij / |t aj – t ai | Estimate the length(L) of the vehicle L = v(t dj - t aj )

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 3 rd order low-pass filter with cutoff frequency of 50 Hz is sufficiently aggressive to filter out most of the sound in the audible spectrum

13. Casing Sound isolation Protect the electronics from rain water and oil spill on the road

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 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 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 96 bytes 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. 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  D i = d i / 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, ranging from 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