Sensor Development for the iMote2 Smart Sensor...

Sensor Development for the iMote2 Smart Sensor Platform

Jennifer RiceMarch 7, 2008

SPIE Smart Structures/NDE 2008

http://www.rollanet.org/~conorw/cwome/de_bridge_corrosion4.jpg

Jennifer RiceSPIE Smart Structures/NDE

March 13, 2008

IntroductionAging infrastructure requires cost effective and timely inspection and maintenance practicesThe condition of a structure following an extreme loading event must be quickly assessedStructural health monitoring systems must provide relevant information without data inundationThe 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

http://www.rollanet.org/~conorw/cwome/de_bridge_corrosion4.jpg


March 13, 2008

Advances in microprocessor and MEMS technology have led to the potential for highly adaptable, densely distributed smart sensor networksFeatures of a smart sensor:

Smart Sensors

Wireless Radio

Data StorageProcessor

Wireless sensorsWireless sensors Smart SensorsSmart SensorsWired sensors Wired sensors

Self-powered

Low Cost

ScalabilityScalability

Utilize the computational power of the smart sensor nodes to realize distributed computingLimit the amount of raw data that is shared amongst sensors

The evolution research on sensor networks for structural health monitoring


March 13, 2008

Intel’s iMote2Second generation of Intel moteGeared toward higher bit-rate applicationsLow-power 32-bit XScale processor (PXA271)Scalable processor speed to improve power consumption802.15.4 radio (ChipCon 2420)Data storage

256KB SRAM32MB External SDRAM32MB Flash

Mica2 iMote2

Microprocessor ATmega128L XScalePXA271

Clock speed (MHz) 7.373 13-416

Active Power (mW) 24 @ 3V44 @ 13 MHz,

570 @ 416 MHz

Data rate (kbps) 38.4 250

Program flash (bytes) 128 K 32 M

RAM (bytes) 4 K 256 K +32 M external

Nonvolatile storage (bytes)

512 K 32 M(Program flash)

Size (mm) 58 x 32 x 7 48 x 36 x 7


March 13, 2008

Sensing with the iMote2The iMote2 has all of the desired characteristics…except the sensor

Sensor InterfaceConnect to main board via two connectorsInterface allows flexibility in sensor applicationsDigital I/O options only

Two options…Utilize the Basic Sensor Board created by IntelUser/developer must provide the sensor board(s)

Advanced Connector

Basic Connector

Main Board

Sensor Board

Battery Board


March 13, 2008

Intel’s Basic Sensor BoardOriginal Basic Sensor Board features

Light SensorTemperature/Relative Humidity SensorST Microelectronics Digital Accelerometer

3-axes± 2g measurement range12-bit resolutionAnti-aliasing filters with selectable cutoff frequencies

ITS400 Sensor Board (Crossbow) additional features

Additional temperature sensorGeneral purpose ADC

4-channel12-bit resolution

Top

Bottom


March 13, 2008

Sample rate fluctuation on a single sensor

Intel’s Basic Sensor BoardLimitations:

Low ADC resolutionLimited signal processing capabilitiesInaccurate sampling rateClock jitter (sample rate fluctuation)

Variation in sample rates amongst sensors

Decimation factor

Cutoff frequency(Hz)

Sampling rate (Hz)

128 70 280

64 140 560

32 280 1120

8 1120 4480


March 13, 2008

iMote2 SHM-A BoardDesigned specifically for vibration based structural health monitoring applications

MotivationTypical civil structures exhibit low frequency response but damage is often evident in higher modes

Flexible measurement bandwidthVibration based SHM is often limited to ambient excitation

High resolution, low-noise data is requiredDistributed SHM requires synchronization of signals from each sensor node to a high degree of accuracy

Accurate clock and timing components are required

Primary design goalsProvide flexible, user-selectable anti-aliasing filters and sampling rate optionsUtilize commercially available, low-cost MEMS components3 axes accelerationHigh ADC resolutionHigh sensitivity


March 13, 2008

3-axis Analog Accelerometer

Single-pole RC Low-pass AA

Filter fc = 500 Hz

Single-pole RC Low-pass AA

Filter fc = 50 Hz Gain

Difference Amplifier

16-bit Analog-to-Digital Converter

20 MHz Crystal

iMote2 SHM-A Board Components

SPI Interface Imote2

Top Bottom


March 13, 2008

iMote2 SHM-A Board Components

ST Microelectronics Analog Accelerometer3-axes acceleration0.66 mV/g resolutionLow-powerModerate noise (50μg/√Hz)Cost: ~$12 per chip

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


March 13, 2008

Filter ConsiderationsAccelerometer internal resistor + external capacitor create a single-pole low-pass RC filter

Not effective anti-aliasing filterVery slow roll-offNon-linear phase response

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

Vin Vout

R

C

fc

0dB3dB

Rolls off at 6dB per octave after fc

fc

0°

45°

90°

12c

out load

fR Cπ

=

Internal to Accelerometer

External

( )fφ = 1tan ( 2 )fRCπ− −


March 13, 2008

Filter ConsiderationsCapacitor chosen sets the “cut-off”Consider three potential cut-off frequencies: 50 Hz, 500 Hz, and 1500 Hz (maximum allowed)

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 frequencyAddress aliasing in another way…

20 40 60 80 10080

60

40

20

0

20

fc = 50 Hzfc = 500 Hzfc = 1500 Hz

fc = 50 Hzfc = 500 Hzfc = 1500 Hz

Phase

f (Hz)

Deg

rees

20 40 60 80 1008

6

4

2

0

2

fc = 50 Hzfc = 500 Hzfc = 1500 Hz

fc = 50 Hzfc = 500 Hzfc = 1500 Hz

Filter Gain

f (Hz)

dB

20 40 60 80 1000

3

6

9

12Maximum Phase Mismatch

f (Hz)

Phas

e D

iffer

ence

(deg

)

fc = 50 Hzfc = 500 Hzfc = 1500 Hz

fc = 50 Hzfc = 500 Hzfc = 1500 Hz


March 13, 2008

Digital Signal Processing

Flexible signal processingUser-selectable anti-aliasing filters, sampling frequencyOversampling-filtering-decimation provides sharp roll-off and linear phase responseSimilar to PC-based analyzers

Quickfilter Programmable Signal Conditioner

4-channels, single- or double-ended16-bit analog-digital-converter (ADC)Programmable gainsBuilt-in anti-aliasing filtersIndividually programmable digital FIR filters Digital SPI outputCost: ~$18 per chip


March 13, 2008

QuickfilterBlock diagram of QF4A512

Oversampling-filtering-decimation serves two purposesIncreases resolution by reducing quantization noiseIn conjunction with simple analog anti-aliasing filters, produces un-aliased signal

PGA

AA Filter

3rd Order Besselfc = 500kHz

12-bit ADC

fs1 = 12.5MHz

Cascaded Integrator-Comb

(CIC) Filter/Decimator

Cascaded Integrator-

Halfband (CIH) Filter/Decimator

FIR Filter

User Defined

Digitized with high

oversampling rate

Filtered and decimated to fs2*4

Sinc filter Amplitude droop

Linear phase

Filtered and decimated to fs2

Compensates for CIC droop in

frequency domain

Maintains linear phase

Many filter designs available

Designed with QF software

Analog anti-aliasing filter

Gain amplifier

x2, x4, x8


March 13, 2008

FIR Filter Design

Built-in digital signal processingSoftware allows for many filter design optionsUser specifies characteristics such as sampling rate and cut-off frequencyFilter design is optimized and header file createdIncluded when sensing application loaded on the iMote2


March 13, 2008

SoftwareTinyOS is the operating system for many smart sensors including the iMote2

Small memory footprintPower efficientLarge user community

Driver was developed in TinyOS to control the functions of the ADC

Allocate memoryLoad FIR filter coefficientsSet sampling rateDo timestampingWrite data


March 13, 2008

Design ValidationCalibration testing performed on bench-scale uniaxial shake tableSignal compared to wired sensorExcellent agreement in time and frequency domains

0 5 10 15 20 25 30 35 4010

-2

100

102

104

f (Hz)

mg2 /H

z

Power Spectrum

ReferenceSHM-A Board

0 0.5 1 1.5 2 2.5 3-400

-200

0

200

400

Time (sec)

mg

Time Histories

ReferenceSHM-A Board

0 5 10 15 20 25 30 35 400

0.5

1

1.5Transfer Function

Mag

nitu

de

f (Hz)


March 13, 2008

Clock AccuracySampling rate accuracy estimation:

Sample rate set to 1000.32Hz (dt = 9.997e-04sec)Record timestamp from processor every 10th sampleSubtract consecutive timestamps and divide by ten to get average timestep over ten data points

Performance may be better than measured because timestamping itself may interfere with hardware timing

Measured sampling rate:

1000.44Hz (0.012% error)

Standard deviation of sample rate fluctuation:

0.11μs (0.011% error)0 100 200 300 400 500 600 700 800 900 10009.991

9.992

9.993

9.994

9.995

9.996

9.997

9.998

9.999

10.000

Timestamp Count

mse

cμs

ecμs

ec


March 13, 2008

Ongoing WorkTest Structure

Historic bridge in Mahomet, IllinoisTruss bridge built in 1912

Environmental hardeningAntenna testingMulti-scale sensor board for iMote2Network optimization/fault tolerance


March 13, 2008

ConclusionSmart 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


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

Sensor Development for the iMote2 Smart Sensor...

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