Structural Health Monitoring (SHM) board series for Imote2 smart sensor platform
A variety of sensor boards for SHM applications using Imote2 sensor network was developed by Dr. Hongki Jo, as the part of his PhD research at the University of Illinois at Urbana-Champaign; which include 1) High-sensitivity accelerometer board (SHM-H), 2) high-precision Strain sensor board (SHM-S), 3) Data AcQuisition board (SHM-DAQ), 4) wind Pressure sensor board (SHM-P), 5) 24-bit DAQ board (SHM-DAQ24), and 6) structural control board (SHM-Control).
** More than 350 units of these sensor boards are currently being used in 17 institutes in 5 countries in the world. These sensors will be available in the market soon through the Embedor Technologies (http://embedortech.com/).
A variety of sensor boards for SHM applications using Imote2 sensor network was developed by Dr. Hongki Jo, as the part of his PhD research at the University of Illinois at Urbana-Champaign; which include 1) High-sensitivity accelerometer board (SHM-H), 2) high-precision Strain sensor board (SHM-S), 3) Data AcQuisition board (SHM-DAQ), 4) wind Pressure sensor board (SHM-P), 5) 24-bit DAQ board (SHM-DAQ24), and 6) structural control board (SHM-Control).
** More than 350 units of these sensor boards are currently being used in 17 institutes in 5 countries in the world. These sensors will be available in the market soon through the Embedor Technologies (http://embedortech.com/).
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High-sensitivity accelerometer board (SHM-H)
SHM-H board was developed for low-level ambient acceleration monitoring using Imote2 sensor network, as previous sensor boards were not appropriate for low-level vibration (< 1mg) applications. Particularly, long-period structures, such as high-rise buildings and long-span bridges, show very low-frequency and low-amplitude vibration. SHM-H board incorporates the Quick filter's 16bits, 4ch, and programmable ADC (QF4A512) and SiliconDesign's low-noise capacitive-type analog accelerometer (SD1221). To overcome the limitation of 16bits ADC in resolving very low-level signal, a signal designing circuit was carefully designed to amplify and shift the acceleration signal without adding unnecessary noise. Onboard temperature compensation is embedded using the temperature dependent current signal (at pin 8) of the SD1221. Various static and dynamic calibration tests show that it allows up to 0.02mg RMS noise at 2Hz BW, 0.05mg at 20Hz BW, and performs well in very low-frequency range less than 0.2Hz. |
SHM-H board (click picture for datasheet)
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Data AcQuisition board (SHM-DAQ)
SHM-DAQ board (click picture for datasheet)
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SHM-DAQ board was designed to accommodate external sensors into WSSN, which opens all 4ch of ADC to external analog signals. A specially designed signal conditioning circuit allow bipolar inputs (-5V ~ +5V) in addition to singlepole input (0 ~ +5V). Two 10-pins terminal blocks are used for wired connect external sensors (wire to board), and 31-pins and 21-pins Hirose connectors are for stacked connection of external board (board to board). In addition, the SHM-DAQ board provides several digital interfaces to accommodate I2C, SPI, GPIO sensors, and several power supply options of regulated 1.8V, 3.0V, and 5.0V for external sensors.
In the Jindo Bridge project, the SHM-DAQ boards were used to accommodate 3D ultra-sonic anemometers (wire-to-board) into the Jindo sensor network. SHM-S and SHM-P boards, introduced below) were designed to be used with this SHM-DAQ (stacked on it, board-to-board). |
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High-precision Strain sensor board (SHM-S)
SHM-S board was developed for low-level ambient strain measurement using Imote2 sensor network. Considering the ambient strain level of many of civil systems is sub-micron, several hundreds~several thousands of signal amplification is generally required to resolve such low-level signal. However, any possible error in resistance values of the bridge arms or strain gauge that causes bridge unbalance, despite it would be very little, can result in significant drift error in the output and saturation of smart sensor's ADC after such huge amplification; which frequently occurs in field measurements.
Parallel connection of digital potentiometers across bridge arms, adapted in SHM-S, allows the precise control of the bridge balance in autonomous way, allowing up to 2500 signal amplification and 0.2 u-strain RMS noise at 30Hz bandwidth. In addition, 3-wire configuration was considered to reduce the long lead wire effect, and precise shunt resistor was adopted for autonomous onboard calibration. Moreover, optional half-bridge configuration can be used for T-rosette or temperature compensation. The SHM-S board was designed to be used with SHM-DAQ board (stackable), since SHM-S has no ADC. Also software was developed for the autonomous bridge balance and shunt calibration using ISHMP Service Toolsuite. |
SHM-S board (click picture for datasheet)
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Wheatstone bridge circuit of SHM-S board
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Block diagram for autonomous bridge balance and calibration software
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24-bit Data AcQuisition board (SHM-DAQ24)
SHM-DAQ24 board
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Recent advanced Delta-sigma noise shaping technology enabled low-cost and low-power 24-bit Analog-to-Digital conversion, which is appropriate for wireless sensor application. In addition to typical oversampling and low-pass filtering process adapted in general ADCs, Delta-sigma ADCs shape the noise, pushing most of noise toward high-frequency out-of-band area. Hence, much lower quantization noise can be obtained at same oversampling rate, or lower oversampling rate can be used to obtain a required quantization noise level; which require much less power and cost for 24-bits ADC than previous approaches.
SHM-DAQ24 board was developed incorporating TI's 4-channels 24-bits Delta-sigma ADC (ADS1274), which allows up to 10 KHz sampling rate at 7mW/ch power consumption (144 KHz at 70mW/ch). To make the application be more flexible with variable sampling rate and better effective resolution, onboard FIR filtering and decimation process was designed using ISHMP Service Toolsuite; which allows up to 3-bits more effective resolution. |
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Wind-Pressure sensor board (SHM-P)
SHM-P board was developed for wireless wind-pressure monitoring applications. Full-scale wind pressure profile measurement provides important information about the wind-induced structural behavior, however is one of the most challenging monitoring tasks.
SHM-P board incorporate the AMSYS 5812 high-precision pressure sensors (4 units) and a special signal conditioning circuit. Since it has no ADC, should be used with SHM-DAQ board. |
Low-latency DAQ (SHM-SAR)
SHM-SAR board was developed for wireless structural control application. As the time delay issue is the most critical in structural control, successive approximation resistor (SAR) type ADC (Analog's 4ch 16-bits SAR ADC: AD7682) was adapted for minimal latency. As other DAQ boards do described above, the SHM-control board also provides both wire-to-board and board-to-board connection for flexible ways to accommodate external sensors to the wireless network.
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