Sensor Development for the iMote2 Smart Sensor Platform

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Sensor Development for the iMote2 Smart Sensor Platform Powered By Docstoc
					Sensor Development for the
iMote2 Smart Sensor Platform


   Jennifer Rice
   March 7, 2008


   SPIE Smart Structures/NDE 2008
Introduction
 Aging infrastructure requires cost effective and timely
 inspection and maintenance practices
 The condition of a structure following an extreme loading
 event must be quickly assessed
 Structural health monitoring systems must provide
 relevant information without data inundation
 The local nature of damage requires a dense array of
 sensors to adequately assess the structural condition


  Networks of densely distributed smart sensors with
   the ability to process data in a distributed manner
    have the potential to improve SHM dramatically


                                                       Jennifer Rice
                                           SPIE Smart Structures/NDE
                                                      March 13, 2008
Smart Sensors
The evolution research on sensor networks for
structural health monitoring
      Wired sensors   Wireless sensors   Smart Sensors

   Advances in microprocessor and MEMS technology have
   led to the potential for highly adaptable, densely
   distributed smart sensor networks
   Features of a smart sensor:                  Wireless
                                                     Radio
   Utilize the computational
   power of the smart sensor             Processor
                                                              Data
                                                             Storage
   nodes to realize distributed
   computing                          Self-    Scalability         Low
   Limit the amount of raw data     powered                        Cost
   that is shared amongst
   sensors
                                                             Jennifer Rice
                                                 SPIE Smart Structures/NDE
                                                            March 13, 2008
Intel’s iMote2
 Second generation of Intel                                Mica2           iMote2
 mote                               Microprocessor       ATmega128L     XScalePXA271
 Geared toward higher bit-rate
 applications                     Clock speed (MHz)        7.373           13-416

 Low-power 32-bit XScale          Active Power (mW)       24 @ 3V
                                                                        44 @ 13 MHz,
                                                                       570 @ 416 MHz
 processor (PXA271)
                                   Data rate (kbps)         38.4             250
 Scalable processor speed to
                                 Program flash (bytes)     128 K             32 M
 improve power consumption
                                                                          256 K +
 802.15.4 radio (ChipCon 2420)       RAM (bytes)             4K
                                                                        32 M external
 Data storage                     Nonvolatile storage                       32 M
    256KB SRAM                         (bytes)
                                                           512 K
                                                                       (Program flash)
    32MB External SDRAM
                                      Size (mm)          58 x 32 x 7     48 x 36 x 7
    32MB Flash




                                                                     Jennifer Rice
                                                         SPIE Smart Structures/NDE
                                                                    March 13, 2008
Sensing with the iMote2
 The iMote2 has all of the desired               Sensor        Basic
 characteristics…except the sensor               Board       Connector


 Sensor Interface                     Main
   Connect to main board via two      Board
   connectors
   Interface allows flexibility in sensor
   applications
   Digital I/O options only
                                              Advanced            Battery
                                                                  Board
 Two options…                                 Connector

   Utilize the Basic Sensor Board
   created by Intel
   User/developer must provide the
   sensor board(s)


                                                          Jennifer Rice
                                              SPIE Smart Structures/NDE
                                                         March 13, 2008
Intel’s Basic Sensor Board
 Original Basic Sensor Board features
   Light Sensor
   Temperature/Relative Humidity Sensor
   ST Microelectronics Digital Accelerometer
      3-axes                                                 Top
      ± 2g measurement range
      12-bit resolution
      Anti-aliasing filters with selectable cutoff
      frequencies

 ITS400 Sensor Board (Crossbow)                             Bottom

 additional features
   Additional temperature sensor
   General purpose ADC
     4-channel
     12-bit resolution
                                                           Jennifer Rice
                                               SPIE Smart Structures/NDE
                                                          March 13, 2008
Intel’s Basic Sensor Board
 Limitations:
                                             Decimation   Cutoff frequency   Sampling rate
      Low ADC resolution                       factor           (Hz)            (Hz)
      Limited signal processing
      capabilities                              128              70               280

      Inaccurate sampling rate                  64               140              560
      Clock jitter                              32               280              1120
      (sample rate fluctuation)
                                                 8              1120              4480




Sample rate fluctuation on a single sensor     Variation in sample rates amongst sensors

                                                                            Jennifer Rice
                                                                SPIE Smart Structures/NDE
                                                                           March 13, 2008
iMote2 SHM-A Board
Designed specifically for vibration based structural
health monitoring applications
  Motivation
      Typical civil structures exhibit low frequency response but
      damage is often evident in higher modes
         Flexible measurement bandwidth
      Vibration based SHM is often limited to ambient excitation
        High resolution, low-noise data is required
      Distributed SHM requires synchronization of signals from
      each sensor node to a high degree of accuracy
        Accurate clock and timing components are required
  Primary design goals
      Provide flexible, user-selectable anti-aliasing filters and
      sampling rate options
      Utilize commercially available, low-cost MEMS components
      3 axes acceleration
      High ADC resolution
      High sensitivity
                                                                  Jennifer Rice
                                                      SPIE Smart Structures/NDE
                                                                 March 13, 2008
iMote2 SHM-A Board Components

                                                         20 MHz
                                                         Crystal
                       Single-pole RC
                        Low-pass AA
                            Filter
                         fc = 50 Hz
                                        Gain          16-bit Analog-
 3-axis Analog
 Accelerometer
                                        Difference      to-Digital     SPI Interface   Imote2
                                        Amplifier       Converter
                       Single-pole RC
                        Low-pass AA
                             Filter
                         fc = 500 Hz




                 Top                         Bottom


                                                                                   Jennifer Rice
                                                                       SPIE Smart Structures/NDE
                                                                                  March 13, 2008
iMote2 SHM-A Board Components

 ST Microelectronics Analog Accelerometer
   3-axes acceleration
   0.66 mV/g resolution
   Low-power
   Moderate noise
   (50μg/√Hz)
   Cost: ~$12 per chip


 Design limitations
   High output impedance (110 kΩ)
   High margin or error in output resistor (±20%)

                                                 Jennifer Rice
                                     SPIE Smart Structures/NDE
                                                March 13, 2008
Filter Considerations
   Accelerometer internal resistor + external capacitor
   create a single-pole low-pass RC filter

 Internal to          R
Accelerometer
                                                      1
                Vin       C       Vout   fc =                      φ ( f ) = tan −1 (−2π fRC )
                                                2π Rout Cload

                              External



   Not effective anti-aliasing filter
                                                                 Rolls off at 6dB
         Very slow roll-off                     0dB              per octave after   0°
                                                3dB              fc

         Non-linear phase response                                                  45°


                                                                                    90°
                                                            fc                             fc

   Potentially large error introduced to signal by resistor
   error – different for each channel

                                                                                        Jennifer Rice
                                                                            SPIE Smart Structures/NDE
                                                                                       March 13, 2008
Filter Considerations
 Capacitor chosen sets the “cut-off”
 Consider three potential cut-off frequencies: 50 Hz, 500
 Hz, and 1500 Hz (maximum allowed)
                          Filter Gain                                                      Phase                                                       Maximum Phase Mismatch
       2                                                          20                                                                         12



       0                                                           0
                                                                                                                                             9




                                                                                                                    Phase Difference (deg)
                                                                                                                                                                               fc = 50 Hz
       2                                                          20                                                                                                           fc = 500 Hz



                                                        Degrees
  dB




                                                                                                                                             6                                 fc = 1500 Hz
       4                                                          40


           fc = 50 Hz                                                  fc = 50 Hz                                                            3
       6
           fc = 500 Hz                                            60
                                                                       fc = 500 Hz
           fc = 1500 Hz                                                fc = 1500 Hz
       8
             20           40            60   80   100             80                                                                         0
                                                                         20           40            60   80   100                                 20        40            60        80        100

                               f (Hz)                                                                                                                            f (Hz)
                                                                                           f (Hz)




 Phase mismatch occurs when channels have different
 values of output resistance (due to resistor error)
 To avoid potential phase mismatch and signal distortion,
 use the highest possible cut-off frequency
 Address aliasing in another way…
                                                                                                                                                              Jennifer Rice
                                                                                                                                                  SPIE Smart Structures/NDE
                                                                                                                                                             March 13, 2008
Digital Signal Processing
 Quickfilter Programmable Signal
 Conditioner
    4-channels, single- or double-ended
    16-bit analog-digital-converter (ADC)
    Programmable gains
    Built-in anti-aliasing filters
    Individually programmable digital FIR
    filters
    Digital SPI output
    Cost: ~$18 per chip

 Flexible signal processing
    User-selectable anti-aliasing filters, sampling frequency
    Oversampling-filtering-decimation provides sharp roll-
    off and linear phase response
    Similar to PC-based analyzers

                                                             Jennifer Rice
                                                 SPIE Smart Structures/NDE
                                                            March 13, 2008
Quickfilter
       Block diagram of QF4A512

               AA Filter
                                                     Cascaded             Cascaded          FIR Filter
                                 12-bit ADC      Integrator-Comb         Integrator-
PGA                                                     (CIC)          Halfband (CIH)
                                fs1 = 12.5MHz
             3rd Order Bessel                    Filter/Decimator     Filter/Decimator     User Defined
                fc = 500kHz
                                                   Filtered and         Filtered and        Many filter
 Gain                           Digitized with                        decimated to fs2
              Analog anti-                       decimated to fs2*4                          designs
amplifier                            high
              aliasing filter                                          Compensates for      available
                                oversampling      Sinc filter
x2, x4, x8                                                               CIC droop in
                                     rate        Amplitude droop                          Designed with
                                                                      frequency domain     QF software
                                                   Linear phase
                                                                      Maintains linear
                                                                          phase



       Oversampling-filtering-decimation serves two purposes
         Increases resolution by reducing quantization noise
         In conjunction with simple analog anti-aliasing filters,
         produces un-aliased signal


                                                                                          Jennifer Rice
                                                                              SPIE Smart Structures/NDE
                                                                                         March 13, 2008
FIR Filter Design
                    Built-in digital signal
                    processing
                    Software allows for
                    many filter design
                    options
                    User specifies
                    characteristics such
                    as sampling rate and
                    cut-off frequency
                    Filter design is
                    optimized and header
                    file created
                    Included when
                    sensing application
                    loaded on the iMote2

                                        Jennifer Rice
                            SPIE Smart Structures/NDE
                                       March 13, 2008
Software
 TinyOS is the operating system for many
 smart sensors including the iMote2
   Small memory footprint
   Power efficient
   Large user community

 Driver was developed in TinyOS to control
 the functions of the ADC
   Allocate memory
   Load FIR filter coefficients
   Set sampling rate
   Do timestamping
   Write data

                                              Jennifer Rice
                                  SPIE Smart Structures/NDE
                                             March 13, 2008
Design Validation
 Calibration testing performed                                                        Time Histories

 on bench-scale uniaxial shake
                                                400
                                                                                                                     Ref erence

 table
                                                200                                                                  SHM-A Board




                                 mg
                                                     0
 Signal compared to wired                       -200
 sensor                                         -400
                                                          0       0.5        1            1.5               2        2.5            3
 Excellent agreement in time                                                           Time (sec)

 and frequency domains                          10
                                                     4
                                                                                  Power Spectrum



                                                     2
                                                10




                                 mg2/Hz
                                                     0
                                                10
                                                                                                                      Ref erence
                                                                                                                      SHM-A Board
                                                     -2
                                                10
                                                          0   5         10       15        20      25           30         35       40
                                                                                        f (Hz)
                                                                                  Transfer Function
                                    Magnitude    1.5


                                                     1


                                                 0.5


                                                     0
                                                          0   5         10       15          20        25       30         35       40
                                                                                          f (Hz)




                                                                                                           Jennifer Rice
                                                                                               SPIE Smart Structures/NDE
                                                                                                          March 13, 2008
Clock Accuracy
 Sampling rate accuracy estimation:
   Sample rate set to 1000.32Hz (dt = 9.997e-04sec)
   Record timestamp from processor every 10th sample
   Subtract consecutive timestamps and divide by ten to
   get average timestep over ten data points
         10.000

          9.999
                                                                                            Measured sampling rate:
          9.998

          9.997                                                                             1000.44Hz (0.012% error)
          9.996
  msec
μsec
μsec




          9.995

          9.994
                                                                                            Standard deviation of
          9.993                                                                             sample rate fluctuation:
          9.992

          9.991
                  0     100   200   300   400      500
                                             Timestamp Count
                                                             600   700   800   900   1000   0.11μs (0.011% error)

                      Performance may be better than measured because
                      timestamping itself may interfere with hardware timing
                                                                                                                 Jennifer Rice
                                                                                                     SPIE Smart Structures/NDE
                                                                                                                March 13, 2008
Ongoing Work
 Test Structure
    Historic bridge in Mahomet, Illinois
    Truss bridge built in 1912
 Environmental hardening
 Antenna testing
 Multi-scale sensor board for iMote2
 Network optimization/fault tolerance




                                                       Jennifer Rice
                                           SPIE Smart Structures/NDE
                                                      March 13, 2008
Conclusion
 Smart Sensors, which incorporate wireless
 sensing with computational capability, allow for
 distributed SHM scenarios
 The iMote2 provides data storage and
 computational capability required for SHM
 applications
 A versatile accelerometer board has been
 designed for vibration-based SHM applications




                                                 Jennifer Rice
                                     SPIE Smart Structures/NDE
                                                March 13, 2008
Acknowledgement

This work is sponsored in part by the National
Science Foundation, Dr. Shih-Chi Liu, program
manager

Additional support was provided by the
Vodafone Graduate Fellowship program




                                               Jennifer Rice
                                   SPIE Smart Structures/NDE
                                              March 13, 2008