Accelerometers and Their Applications

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					       Accelerometers and Their Applications
   Types of Accelerometers
   Calibration of Accelerometers
   Application: Seismology and Design Accelerations
   Application: True Dynamic Response of Structures
   Application: Health Monitoring
   Application: Research (within NEES Program)
   Tour of UIUC Earthquake Testing Laboratories
Common Accelerometer Types
  – Brassart Seismograph
  – Resistive
       • Strain Gauge
       • Micromachined
  –   Capacitive
  –   Fiber Optic
  –   Servo or Force Balance
  –   Vibrating Quartz
  –   Piezoelectric
Brassart Seismograph
Resistive Operating Principle
     – Voltage output of resistor bridge changes proportionally with applied
         acceleration

                      + Signal   + Power   - Power   - Signal




                                       Fixed Resistors

Sensing Resistor #1
           Flexure                          Mass
Sensing Resistor #2
Capacitive Operating Principle
  – Utilizes frequency modulation technique through varying capacitor bridge



                                    Power   Ground     Signal




             Fixed Capacitors                                   Built-In Electronics
                                ~


                    Insulator
                                                                Sensing Capacitor #1



                     Flexure                    Mass



                                                                Sensing Capacitor #2
                    Insulator
Resistive / Capacitive
  – Typical Characteristics
     •   Measure down to 0 Hz (DC response)
     •   Limited dynamic range (<80 dB = 10,000:1)
     •   Limited high frequency range (<10 kHz)
     •   Often a damped frequency response (0.7% of
         critical)
     •   Sensitivity may vary with input (mV/g/V)
     •   Traditionally fragile (limited shock protection)
     •   Operates multi-conductor cable (at least 3 wires)
     •   Micro-machined versions are small and lightweight
     •   Performance matches cost ($10 to $1000 USD)
Resistive / Capacitive
  – Applications
     • Low frequency and/or long duration events
        – Ride quality
            » Automobile road response
            » Amusement park rides
            » Elevator movement
            » Motion simulators
            » Monitoring vibrations of structures and equipment
        – Aerospace structure modal analysis surveys
        – Crash dummy instrumentation
     • Tilt sensors
     • Airbag or automobile alarm triggering devices
Fiber Optic Operating Principle
  – Amount of light gathered by receivers is proportional to applied
    acceleration
                                  Power   Ground    Signal




                                                              Built-In Electronics
                                                              Transmitter
                   Receiver                                   Receiver


                                                              Reflective Surface


                     Flexure               Mass               Flexure
Fiber Optic
  – Similar characteristics and applications as
    resistive and capacitive sensors
  – Additional features
     • Provision for remotely locating electronics
        – High temperature operation to 1000 F (537 C)
        – Cabling is transmitting only light, which consequently
          eliminates the possibility of RF and EM interference in
          “noisy” environments
        – Traditionally, light loss in long cables and connections
          was a consideration
        – Expensive sensors, cabling and signal conditioning
Servo or Force Balance Operating
 Principle
  – Feedback force required to maintain uniform capacitance is proportional
    to acceleration
                               Power       Ground        Signal




           Sensing Amplifier                                      Feedback Power Amplifier

                                                                  Stationary
                                                                  Support
                                       Capacitance Gap
                   Flexure


                                         Magnetic
                       Coil                                       Insulator
                                          Mass
Vibrating Quartz
  – Resonant frequency difference between elements is proportional to
    applied acceleration
                                  Power     Ground          Signal




            Inverting Amplifier


  Frequency Tracking Amplifiers




                       Flexure       Mass                            Vibrating Crystal #2

           Vibrating Crystal #1                      Mass            Flexure
Force Balance / Vibrating Quartz
  – Typical Characteristics
     •   Measure down to 0 Hz (DC response)
     •   Wide dynamic range (>120 dB = 1,000,000:1)
     •   Extremely stable over time and temperature (ppm)
     •   Limited high frequency range (<1 kHz)
     •   Poor overload survivability (<100 g’s)
     •   Force balance may exhibit large magnetic sensitivity
     •   Very expensive (~$1000 USD)
Piezoelectric
  – Force on self-generating crystal provides charge output proportional to
    acceleration
                                   Signal/Power       Ground




    Voltage or Charge Amplifier


                  Preload Ring
                          Mass
           Piezoelectric Crystal



                           Base
Piezoelectric
  – Typical Characteristics
     • Dynamic events only (>0.2 Hz)
     • Wide dynamic range (>100dB = 100,000:1)
     • Wide frequency bandwidth (<1 Hz to >10 kHz)
     • Solid-state (No moving parts)
     • Self-generating piezoelectric elements require no
       power
     • Operates over two conductors
     • Rugged (5,000 g’s)
     • High temperature charge versions operate to 1000 F
       (537 C)
Summary of Accelerometer Types
  – Many different types of accelerometers are
    available and they often represent an excellent
    choice for making vibration measurements;
    however, accelerometers are not well-suited for
    all applications as no single sensor can meet
    every vibration requirement.
  – Don’t underestimate the sensor selection
    process as it is easy to generate “bad data”
    without the proper transducer.
Calibration: Absolute Method
  – Single channel test where the sensor is subjected to a known,
    accurate and reliable measure of “a”
      • Drop Test                      Fixed
                                                                 Impact Force

                                      Supports
      • Gravity Inversion Test
      • Handheld Shaker                                                         Elastic Suspension
                                                     Flexible                         Cords
                                                 Monofilament Line
               Rotation Fixture

                                                 Mounting Mass
                                                                                          Signal Out
                                                      Accelerometer
        Test Sensor
Calibration: Relative Method
   – Dual channel test where the test sensor and calibrated reference are
     subjected to the identical input acceleration. The ratio of the output
     signals provides the calibration factor.
        • Laser Fringe Counting (Primary Method)
            – Non-contacting measurement principle
            – Structure not affected by measurement device
            – Utilizes “fringe counting” of laser light
            – This method provides primary calibration as it is based on a constant on
               nature…the wavelength of light
            – Expensive
            – Requires relatively large accelerations at high frequencies
            – 25 g’s at 5 kHz; 50 g’s at 10 kHz; 100 g’s at 20 kHz
        • Back-to-Back Calibration (Secondary Method)

                      Test
                 Accelerometer
                                         Vtest
                                          Vref
                Reference
              Accelerometer




                  Controllable
                Acceleration Level
Calibration: Relative: Laser Calibration
  – Non-contacting measurement principle
     • Structure not affected by measurement device
  – Utilizes “fringe counting” of laser light
     • This method provides primary calibration as it is
       based on a constant on nature…the wavelength of
       light
  – Expensive
  – Requires relatively large accelerations at high
    frequencies
     • 25 g’s at 5 kHz; 50 g’s at 10 kHz; 100 g’s at 20 kHz
  – Procedure and set-up is documented in
    approved ISO Standard ISO 5347-1
Calibration: Relative: Back-To-Back
  – Test sensor mounts directly to a reference
    accelerometer which has been previously
    calibrated by primary means or by a recognized
    laboratory
                   Test
              Accelerometer
                                  Vtest
                                   Vref
            Reference
          Accelerometer


       Instrument
      Grade Shaker
               Controllable
             Acceleration Level
Applications: Seismology and Design Accelerations
Applications: Seismology and Design Accelerations
- Response Spectrum (Spectral Response Acceleration)

                               4.5
                                                         NWHL
                                                                             6 ground motions to represent the selected seismic hazard
                               4.0         ELCN                                    (SD1=1.0, SDs=0.636) specified in IBC 2000
                                                                            Average of 6 ground motions
                               3.5
   Pseudo Acceleration / PGA




                                                                              CORR
                               3.0
                                                                                     CENT
                               2.5                                                          IBC2000

                               2.0

                               1.5

                               1.0
                                                  SARA
                               0.5
                                                                          SYLH
                               0.0
                                     0.0                  0.5                 1.0                  1.5             2.0
                                                                      Period (sec)

                                                                                                                 Epicentral              Unscaled   Scale
  Earthquake                                       Date         Magnitude                Station                              Identifier
                                                                                                               Distance* (km)            PGA (g)    Factor
 Imperial Valley                              May 18, 1940       ML=6.3                  El-Centro                   8.3        ELCN      0.319     1.260
    Northridge                              January 17, 1994     ML=6.6          Century city-LACC north            25.7        CENT      0.221     1.802
   Loma Prieta                              October 17, 1989     ML=7.0       Corralitos-Eureka Canyon Rd.           5.1       CORR       0.478     0.836
    Northridge                              January 17, 1994     ML=6.6       Newhall-LA County Fire Station         7.1       NWHL       0.589     0.678
   Loma Prieta                              October 17, 1989     ML=7.0           Saratoga-Aloha Ave.                13         SARA      0.504     0.794
    Northridge                              January 17, 1994     ML=6.6      Sylmar-County Hosp. Parking Lot         6.4        SYLH      0.604     0.662

* The type of epicentral distance is the distance closest to fault rupture.
                                   Mode 1      Mode 2     Mode 3     Mode 4     Mode 5




             i T  M 1             Modes               1          2           3   4 (Vertical)    5
MPF: i   
            i T  M i 
                                       Period (s)         3.05323 0.81950 0.36427         0.32787    0.22872
                                Mass Participation Factor 0.5610     0.2637      0.0729 Neglected 0.0433
i  normalized i-th mode shape          Sum of Mass Participation Factor of listed modes            0.9409
                      4            Left wall original and deformed geometry                                                 4         Right wall original and deformed geometry
               x 10                                                                                                  x 10
         3.5                                                                                                   3.5
                                                                                  Original                                                                                            Original
                                                                                  Deformed                                                                                            Deformed
          3                                                                                                     3


         2.5                                                                                                   2.5


          2                                                                                                     2
Y (mm)




                                                                                                      Y (mm)
         1.5                                                                                                   1.5


          1                                                                                                     1


         0.5                                                                                                   0.5


          0                                                                                                     0
          -1.5            -1     -0.5     0       0.5     1        1.5        2      2.5                        -1.5            -1   -0.5     0      0.5     1        1.5         2      2.5
                                                   X (mm)                                         4                                                   X (mm)                                          4
                                                                                           x 10                                                                                                x 10


                               Left Wall Deformation Plot                                                                            Right Wall Deformation Plot



                                                                                  Whole Frame Deformed Shape                           (Notes: Scale factor 200)
Applications: Model Calibration
Wind Engineering: Burj Dubai

				
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posted:12/3/2011
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