WISAN

Dec. 2005 Field test on RT11 bridge in Potsdam, NY

Current test is one of the many tests of WISAN sensor nodes performed in the field conditions.

The tasks for this test included:

1. Testing of the chosen MEMS acceleration sensors and signal conditioning circuitry
2. Establishing vibration energy levels at various locations
3. Establishing natural frequencies of a girder on RT 11 bridge
4. Testing of the time synchronization between sensor nodes

The WISAN project is funded by ,New York State Energy Research and Development Authority Transportation Research Board of the National Academies and industry partners, who's support we gratefully acknowledge. 

Test setup and preparation of WISAN nodes

Each sensor node consisted of a low-noise MEMS accelerometer, gain and offset correction circuitry, 4th order anti-aliasing filter with cutoff frequency of 30Hz and a WISAN sensor node (Figure 1-2). The whole assembly was packaged into an ABS plastic box. The battery pack was attached to the lid. A key switch allows turning the power on/off without opening the box. The box is mounted to the bridge by the means of Neodymium Iron Boron (NdFeB) magnets.

Figure 1. Sensor box                                       Figure 2. Keyed power switch and magnetic mounting to the bridge

Data from the sensors were wirelessly sent to the coordinator modules attached to a notebook PC via USB interface (Figure 3). The Labview interface allowed visualization of the sensor data and storage on the hard drive. Responsibilities of the coordinator node include control over the network, including ensuring reliable data delivery, issuing command to the sensor nodes and receiving data from the sensor nodes, and maintaining global time synchronization in the nodes. The coordinator node was powered off the USB interface.

Figure 3. Coordinator attached to a notebook

Acquisition of vibration data was performed in two basic configurations:

Test 1. Four sensors were placed on girder with approximately 12 ft between the sensors (Figure 4). The closest to the support is identified by number 1000 in the following figures. The sensor closest to the mid span of the girder had the number 1003.

Test 2. Two sensors were placed side-by-side at two different locations on the girder. Sensors 1003 and 1002 were placed together at the previous location of sensor 1002, sensors 1000 and 1001 were placed at the previous location of sensor 1001.

Figure 4. Location of sensors on the bridge

Test 1. Acquisition of vibration data

Test 1 pursued the first three goals of the experiment.

First, verify the performance of the MEMS acceleration sensor and related signal conditioning circuitry on the data acquired from the real bridge. The MEMS accelerometer sensor has been tested in the laboratory conditions but field performance has not been evaluated so far. The sensitivity of the accelerometer was set to be ±35mg by the adjustable gain circuitry. The constant offset generated by gravity was compensated the same circuit.

Second, establish vibration levels available at different locations of the bridge, and specifically look at the difference between vibration levels at supports vs. vibration levels close to the mid span. The vibration levels are important indicator of how much energy can potentially be harvested from the bridge at various locations.

Third, establish natural frequencies of the bridge which is important for future damage detection experiments as well as for design of energy harvesting devices that will target the frequencies with the highest energy content.

After initial installation of sensors the data collection procedure consisted of the following steps:

- the coordinator node and the sensors were powered up
- the offset compensation circuit on the sensor was adjusted to produce a steady state reading around the midrange, compensating for slanted surface of the girder's flange
- the sensors were configured via the wireless interface to acquired data at 100Hz, ADC turned on, sensor turned on, time synchronized between the nodes
- vibration data was acquired several times for the period of time between 1 and 5 minutes

The following Figures illustrate the data from one of the experiments. Figure 5 shows the time series from each of the sensors. As it can be seen from the Figure, the location at support is experiencing little excitation by passing traffic and amplitude of vibration decays approaching the supports. Figure 6 shows frequency spectra of the girder vibration. All sensors except the support-mounted device show the same major harmonics reflecting the natural frequencies of the bridge.

Figure 5. Time series data from 4 sensors.

Figure 6. Frequency spectra of the sensor data.

Test 2. Verification of sensor repeatability and time synchronization

The goal of test 2 was to prove repeatability of sensor readings acquired by multiple sensors at the same location. A part of test was verification of global time synchronization between sensors. Each sensor is essentially an independent device sending data to the coordinator module. Time synchronization is essential for reliable operation of many damage detection algorithms. Figure 7 shows the time series data acquired for around 90 seconds from sensors 1002 and 1003 placed at the same location. Visually the both sensors are providing virtually identical data. Figure 8 provides an overplot of one waveform on top of another, showing that the two series are virtually identical except for some random noise. To check the synchronization of data in time Figure 9 a short piece of the waveform is zoomed in on Figure 8. Except for a small offset which is created by minor difference in sensor inclination relative to gravity, two waveforms are identical and track one another sample-by-sample. Figure 10 illustrates identical frequency spectra produced by both sensors.

Figure 7. Data from sensors installed at the same location

Figure 8. Overplot of the sensor data

Figure 9. Time series data from sensors 1003 and 1002.

Figure 10. Overplot of the frequency spectra from sensors 1003 and 1002.

Conclusions

The current test has shown reliable data delivery by multiple networked WISAN sensors in field environment, established some baseline values for the energy levels available for harvesting from the bridge and proved time synchronization between sensor nodes in a WISAN network.