Structural Health Monitoring

1. Structural Health Monitoring Sukun Kim, David Culler James Demmel, Gregory Fenves, Steve Glaser, Shamim Pakzad UC Berkeley CENTS Retreat

2. Structure Monitoring Data Acquisition Data Collection Data Processing & Feedback

3. Accelerometer Board • Two accelerometers for two axis • Thermometer • 16bit ADC

4. Hardware • Board • Accelerometers Comparison • Noise Floor (Vault Test) • Tilting Calibration • Temperature Calibration • Mote • Antenna • Options and Our Choice • Power • Power Consumption Profile • Power Source Options and Choice • Package • Software Architecture • Overall Structure • High Frequency Sampling • Jitter Test • Jitter Analysis • Multi-hop Communication • Time Synchronization • Reliable Command Dissemination • Reliable Data Collection • Data Analysis • Signal Processing • Hardware Low-pass Filter • Software High-pass Filter • Calibration Process • System Identification • Box-Jenkins Multi-input Multi-output Model Mint Route (Alec Woo, et al) The Flooding Time Synchronization Protocol (Miklos Maroti, et al) Drip (Gilman Tolle)

5. Why temperature calibration? • Accelerometer is sensitive to temperature change • In bridge environment, there exists significant variation in temperature (up to 45F, 40 mG) • We are looking at very subtle signal (down to 0.5 mG) • Signal to noise ratio becomes small under temperature change (down to 1%)

6. Temperature Calibration Test F C 81.1 27.3 67.1 19.5 53.0 11.7 Temperature 3.9 39.0 mG 27.5 0 Thanks to Crossbow -27.5 Acceleration

7. Temperature Calibration • Suggestion to estimate instantaneous regression parameters by windowing the signals • a window of length 199 samples is considered and a linear regression model of the form • accel.count = a + b*temp.count + e • is fit to that windowed segment of the data • The following graphs are the estimated parameters. The parameters include a and b (as defined above) and the standard deviation of e, the error term in the regression model

8. mG 2.75 0.92 Temperature change will not be as dramatic as in the test (with insulation) However, concern for temperature hysteresis drives us to second run

9. Bridge Dimension 2 500 ft 1125 ft 2100 ft 4 8 246 ft 1 * Blue number presents rough ratio Need to cover large bridge, so directional antenna is needed

10. Types of Antenna Dish Yagi Horn Patch Bigger Size Longer Range Smaller Size Shorter Range

11. How much gain do we need? • Golden gate bridge is 6,450 ft including main span and side spans • Assuming 20 hops along the span, each hop is 340 ft • 433MHz Mica2 reached 100ft (84% success) • 916MHz Mica2 will reach 50ft • 916MHz with maximum radio power will reach 225ft (4.5X increase: 10dBm to -3dBm) • 1.5X increase in range is needed • 3.3 dBi antenna is needed

12. Antenna Candidates Bi-directional patches (2.4GHz) From Superpass 4.8” 4.4” 11.5” 4.5” 5.5 dBi 9 dBi

13. Performance of antenna 5.5dBi antenna with Telos Maximum output power (0dBm) 0.5ft above the ground in front of Soda Hall 3ft above the ground, success rate was close to 100% 9dBi antenna showed very similar behavior (0% above 0.5ft, >99% above 3ft) The distance to the ground is more important than the gain of the antenna

14. Power Consumption Data • Board only: 26.7* • Idle: 39.8 • One led on: 42.6 • Erasing flash: 77.5 (with one led on) • Sampling: 42.6 (with one led on) • Transferring data: 46.0 (with one led on) * unit is mA at 9V

15. Node Deployment Plan 10 nodes 30 nodes 10 nodes * Nodes on both sides of span

16. Power Consumption • The most burdened node transfers data one third of time • Tadiran 5930: 3.6V, 19Ah, $17, D size • Usual 9V alkaline battery has 625mAh (12X) • Usual 1.5V D battery has 18Ah (2.5X) • 3 of Tadiran 5930 costs $51, and lasts 23 days • 3 weeks is good enough

17. Power Consumption (cont) • With optimal sleeping, 30 days • Board itself consumes significant amount of energy Power source Power source Switch Switch Sensor Mote Mote ADC Sensor ADC

18. PC Reliable Data Collection

19. Footbridge & Location of Nodes Quarter Half 1/8 Quarter 260ft 7 2 4 6 16ft Marina Berkeley 8 1 3 5 Base Station

20. Data is from single-hop version

21. We will collect data for multi-hop version

22. Data Analysis • Characteristic vibration modes were observed

23. Conclusion • Practical problems to solve • Interesting challenges and research topics (more efficient reliable transfer) • Future Work • Handling temperature hysteresis • Antenna • More efficient data collection (pipelining?)

24. Questions

30. Examples Gain, angle, size, weight, (price), range compared to wire whip antenna

31. Comment on Availability • Availability of horn antenna is limited • Antenna for 433MHz can be found, but with some more work (probably from abroad)

32. Suggested Choice – Yagi, Patch • Dish is overkill • Yagi without shell • Enough gain • Long (15” for 916MHz) and narrow • Robust to strong wind • Horn has limited availability • Patch • Enough gain • With large surface (8.5X8.5” for 916MHz) and thin • Plate part can be a problem under strong wind

33. Bi-directional Patches

34. References (Antenna) • Antenna vender • http://www.hyperlinktech.com • http://www.antennafactory.com • http://www.antennafactor.com • http://www.antennasystems.com • Horn antenna • http://www.phazar.com • Antenna in general • https://ewhdbks.mugu.navy.mil/ANTENNAS.HTM • dBi, dBd • http://www.tmeg.com/tutorials/antennas/antennas.htm • Golden Gate Bridge Facts • http://www.thoma.com/thoma/ggbfactsX.html

35. dBi, dBd • dBi (decibels-isotropic) – a unit of measuring how much better the antenna is compared to an isotropic radiator • dBd (decibels-dipole) – compared to a dipole antenna • Dipole antenna typically has a 2.4dBi gain • Wire whip antenna (used in mica2) would have 1.5dBi gain

40. One Base Station w/o Pipelining • If only one base station is located at kth place from the right, total transfer time is • 2 * { (k-1)k/2 + (16+5-k)(16+5-k+1)/2 + 4(16-k) + 6} * (single one-hop transfer time) • Minimum 248 when k=12 or 13 • When flash is full (6min data at 200Hz, 5 channels = 10min xfer), and with 800bytes/s bandwidth*  2.1 days of data collection * Every data in this file is based on Mica2 Minimum positions

41. One Base Station w/ Pipelining • Assuming communication can occur 3 hops away, lower bound is 3 * 50 times of single one-hop transfer (10min)  25 hours • Bottleneck is speed of data arriving at the base station with space among them • With N base stations, time will become 25/N hours Space preventing interference Base Station

42. More thought on pipelining • As the network gets bigger, we can get more benefit (N versus N2) • However, in small network, path is not long enough for multiple transfers to happen • With 4 sinks, assuming perfect pipelining (can be unrealistic), it takes 4.2 hours (20min * 50 / 4) Base Station

44. Preliminary BOM

46. 1. Radio with Mica2 • 19.2Kbps = 2400bytes/s = 66.7pkts/s • Media access control reduces this to 52pkts/s. • Single mote can achieve 42pkts/s. (probably due to processing overhead?) • 2. UART • 57.6Kbps = 7200bytes/s = 200pkts/s • 3. TOSBase • In TOSBase with Mica2, • ### 34pkts/s max in theory. • 23pkts/s is a reliable upper bound.

47. PC PC PC Overall Process 1. Trigger Sampling 2. Transfer Metadata 3. Transfer Data * PC has most of intelligence. Motes are almost stateless.