Today

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Today

• Accelerometers
• Reference model and siblings
• Calibration
• Types and models
• Common usages
• Fixing and precautions

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Reference scheme and siblings
Vibration to be measured
Vibration of the inertial mass
Vibration of the inertial massrelative to the
shell
Imposing the dynamic equilibrium
Second order model
z(t) is the output signal, which can be sensed by
a suitable relative transducer.
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Reference scheme and siblings
Substituting complex expressions in (1)
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Reference scheme and siblings

• Different quantities could be the input signal
• Displacement
• Velocity
• Acceleration

Seismometer
Velocimeter
Accelerometer
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Reference scheme and siblings
Dividing both numerator and denominator by kel
and
Thus
Seismometer
Velocimeter
Accelerometer
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Seismometer
The measurand is the displacement of the
vibrating element.
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Accelerometer
The measured displacement of the inertial mass is
the output signal ? deflection
accelerometer.
Km/ks static sensitivity
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Reference scheme and siblings

• Deflection accelerometers can be realized by way
  of different transducing principles, adopted to
  sense the inertial mass displacement
• relative vibrometer accelerometers capacitive,
  inductive, potentiometric
• strain gauge accelerometers
• piezoelectric accelerometers
• MEMS accelerometers capacitive or
  piezoresistive
• For example let us take into account the strain
  gauge accelerometer.

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Reference scheme and siblings
Deflection accelerometer properties
DEFLECTION NULL MODE
RELATIVE ABSOLUTE
CONTACT CONTACTLESS
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Reference scheme and siblings

• Accelerometers LOAD EFFECT
• on mass-load relationship
• on the resonant frequency of the measurand system

m
Therefore accelerometer mass should be small
compared to the vibrating mass being measured
M
F
M
F
K
C
K
C
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Calibration of accelerometers
Accelerometers are calibrated at 1000 rad/s
159.2 Hz The applied acceleration is 10 m/s2.
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Calibration of accelerometers

• The aforementioned interferometric technique is
  used for high level calibration (primary
  standards). Other techniques are commonly used
  for lower levels instruments
• Back to back

Vu
amp.
Accelerometer to be calibrated
meter
Vr
amp
Reference accelerometer
Sensitivity
Aosin?t
Vibrating table
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Calibration of accelerometers
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Accelerometers types and models

• Capacitive accelerometers sense a change in
  electrical capacitance, with respect to
  acceleration.The accelerometer senses the
  capacitance change between a static condition and
  the dynamic state.
• Piezoelectric accelerometers use materials such
  as crystals, which generate electric potential
  from an applied stress. This is known as the
  piezoelectric effect. As stress is applied, such
  as acceleration, an electrical charge is created.
• Piezoresistive accelerometers (strain gauge
  accelerometers) work by measuring the electrical
  resistance of a material when mechanical stress
  is applied
• Hall Effect accelerometers measure voltage
  variations stemming from a change in the magnetic
  field around the accelerometer

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Accelerometers types and models

• Magnetoresistive accelerometers work by measuring
  changes in resistance due to a magnetic field.
  The structure and function is similar to a Hall
  Effect accelerometer except that instead of
  measuring voltage, the magnetoresistive
  accelerometer measures resistance.
• Heat transfer accelerometers measure internal
  changes in heat transfer due to acceleration. A
  single heat source is centered in a substrate and
  suspended across a cavity. Thermoresistors are
  spaced equally on all four sides of the suspended
  heat source. Under zero acceleration the heat
  gradient will be symmetrical. Acceleration in any
  direction causes the heat gradient to become
  asymmetrical due to convection heat transfer.
• Micro-Electro Mechanical System technology is
  based on a number of tools and methodologies,
  which are used to form small structures with
  dimensions in the micrometer scale (one millionth
  of a meter). This technology is now being
  utilized to manufacture state of the art
  MEMS-Based Accelerometers.

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Null mode accelerometers
The acceleration is compensated by a retroaction
that keeps the mass into position null mode

• high sensitivity (gt 1000 mV/g)
• low frequencies (0?500 Hz)

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Piezoelectric accelerometer

• The spring-damper parallel is given by a
  piezoelectric prism, having a small mass and a
  high stiffness.
• Normally the crystal is pre-loaded.
• No damping devices are inserted the damping
  ratio is very low.

High frequency accelerometer
High sensitivity accelerometer
Mass 43 g Sens.10 pC/(ms-2) Freq. 1-4800 Hz
Mass 0.63 g Sens. 0.15 pC/(ms-2) Freq. 1-26000 Hz
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Piezoelectric accelerometer
Once an acceleration is applied and a charge
distribution is realized on the crystal, this has
to be measured. The crystal has high impedance
and so the measurement should be undertaken by a
high input impedance voltmeter in this way a
discharge of the crystal would be prevented.
Since this is often impossible, a pre-amplifier
is connected to the transducer before passing the
signal to a common meter. Pre-amplification can
be either charge or voltage amplification. Typical
ly charge pre-amplifiers are used or built-in
tension amplifiers (located within the transducer
or immediately at its output connection). Tension
amplification far from the transducer is to be
avoided since it is sensitive to the length of
the cables connecting the transducer to the
pre-amplifier.
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Piezoelectric accelerometer
These are available on the market with different
names ICP (trade mark of PCB piezotronics), IEPE
(Bruel Kjaer) and many others. The commonly
used acronym is ICP Integrated Circuit
Piezoelectric.
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Piezoelectric accelerometer
ICP/IEPE amplification needs a power supply.
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Piezoelectric accelerometer
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Accelerometers types and models
Please notice piezoelectric transducer are
insensible to static and quasi-static excitation
due to the crystal self adjustment in time
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Common usages

• Vibration measurements
• human comfort evaluations
• industrial diagnostics on machinery
• civil diagnostics on buildings
• Acceleration measurement
• inertial navigation systems
• impact detection and measurement
• dynamic force/displacement characterization

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Fixing and precautions
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Fixing and precautions
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Fixing and precautions
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Fixing and precautions