Piezoelectric Accelerometers
Theory & Application
Metra Mess- und Frequenztechnik Radebeul / Germany
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Piezoelectric Effect
•
Piezoceramics or Quartz generate a charge output when
subjected to force.
F
q
q = d.F
F
q
d
piezo disk
force
charge
piezo constant
F
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Piezoelectric Accelerometer
•
•
A piezoelectric accelerometer consists of
a piezoelectric material and a seismic mass.
The charge output is proportional to acceleration.
q
Seismic mass
m
charge sensitivity:
q
Bqa =
a
Piezoceramics
Acceleration a
Metra Mess- und Frequenztechnik Radebeul / Germany
F = m.a
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Accelerometer Design
•
•
Example: Shear type accelerometer with ring-shaped
piezoceramics.
The inertial force of a seismic mass generates an electric
signal at the electrodes of the piezo material.
Ring-shaped
piezo ceramics:
shear force
Cover
Seismic mass
Piezo ceramics
Post
Socket
Base
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Other Types of Vibration Sensors
Comparison with piezoelectric accelerometers:
Sensor Type
Advantages
Disadvantages
Piezoresistive
(strain gauge)
• Measures
static
acceleration
• Robust
• Limited
Electrodynamic
• Measures
• Only
Capacitive
• Measures
• Low
static
acceleration
static
acceleration
• Cheap manufacturing
by semiconductor
process
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resolution
• Only up to some kHz
• Power supply required
for low
frequencies
resolution
• Fragile
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Advantages of Piezoelectric Accelerometers






Extremely wide dynamic range, almost free of noise suitable for shock measurement as well as for almost
imperceptible vibration
Excellent linearity over full dynamic range
Wide frequency range, high frequencies can be
measured
Compact yet highly sensitive
No moving parts – long service life
Self-generating - no external power required
But … they cannot measure static acceleration.
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Charge Mode and ICP Accelerometers
•
Charge mode accelerometers provide the direct
charge output of the piezo material.
Disadvantage of charge output: Maximum cable
length 10 m, special low-noise cables required.
•
ICP compatible accelerometers have a built-in
amplifier.
Advantage of ICP: Cables of several hundred
meters length possible, standard coaxial cables
can be used.
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Characteristics: Sensitivity
• The
sensitivity of charge mode accelerometers is
measured in pC per m/s² or pC per g.
Typical range: 0.1 – 100 pC/ms² or 1 –1000 pC/g
• The sensitivity of ICP compatible accelerometers
is measured in mV per m/s² or mV per g.
Typical range: 1 - 1000 mV/ms² or
10 – 10 000 mV/g
• Sensitivity stated in the data sheet is usually
calibrated at 80 Hz.
Metra Mess- und Frequenztechnik Radebeul / Germany
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Characteristics: Frequency Response
•
•
A typical accelerometer has a resonance at 10 – 30 kHz
For 3 dB accuracy it can be used up to approx. 50 % of
this resonant frequency.
fr
rel.
Ampl.
1,30
1,10
30 % Error
10 % Error
1,00
+/- 3 dB Frequency Range
0,7
fl
0,5fr f r
0,3f r
Metra Mess- und Frequenztechnik Radebeul / Germany
f
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Selecting an Accelerometer Model (1)
Acceleration to be measured:
•
Examples:
Very low  Seismic Accelerometers
(...µm/s² to 1 m/s²
KB12
•
Medium  Standard Accelerometers
(1 to 10 000 m/s²)
KS76
•
Very high  Shock Accelerometers
(> 10 000 m/s²)
KD93
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Selecting an Accelerometer Model (2)
Weight:
The weight of an accelerometer should not exceed
10 % of the test object weight.
Examples:
•
Very light  Miniature Accelerometers
(weight < 5 grams)
KS91
KS94
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Selecting an Accelerometer Model (3)
Environmental Conditions:
•
Humidity and Dust
Example:
 Industrial Accelerometers
(with protection grade IP67 and higher)
•
Grounding Problems / Ground Loops
 Industrial Accelerometers
(with insulated case)
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KS80
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Accelerometer Mounting Methods
Stud
Mounting
Performance:
Max. Frequency:
Direct
Adhesive
Adhesive
Pad
Insulating
Flange
Magnetic
Base
Probe
poor
good
> 20 kHz
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< 5 kHz
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< 1 kHz
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Typical Mounting Errors
•
•
For best performance, particularly at high frequencies,
the accelerometer base and the test object should have
clean, flat, smooth, unscratched and burr-free surfaces.
A stiff mechanical connection between the sensor and
the test object is important. Sheet metal or plastic parts
and other thin and flexible components are unsuited for
accelerometer mounting.
Uneven
surface
Rough
surface
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Flexible
coupling
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F
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Instrumentation for Charge Mode Sensors
•
Charge mode accelerometers require a charge
amplifier input.
• A charge amplifier is a differential amplifier with
Rf
capacitive feedback.
qf
q in
Sensor
Cf
vout
+
GND
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Instrumentation for ICPAccelerometers
Accelerometers with integrated ICP electronics
require a constant current supply of 2 - 20 mA
via the sensor output.
• The constant current source can be part of the
instrument or a separate unit.
•
ICP® Compatible Transducer
Constant
current source
Integrated
Charge Converter
Piezo
System
Q
U
I const
coaxial cable,
over 100 m long
Instrument
Cc
Decoupling
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Signal Conditioners from Metra
M108/116
multichannel
ICP conditioners
M32 ICP conditioner
M68 series charge &
ICP conditioners
Metra Mess- und Frequenztechnik Radebeul / Germany
M28 ICP supply
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Accelerometers with TEDS to IEEE1451.4
TEDS = Transducer Electronic Data Sheet
New feature for ICP compatible accelerometers.
The accelerometer contains a memory chip which
stores:
 Model and version number
 Serial number
 Manufacturer
 Type of transducer, physical quantity
 Sensitivity
 Last calibration date
 Identification of the measuring point
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Advantages of TEDS Transducers
Easier sensor identification, particularly at large
channel number. No cable tracking and marking.
• Automatic calibration of the measuring system.
No manual setup of serial number, measured
quantity, sensitivity etc.
• Sensor replacement without setup ("plug &
play").
• Sensor can be used, even if the printed
calibration data sheet is just not at hand.
•
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Accelerometer Calibration
Vibration Calibrators from Metra
Model VC10
• Fixed frequency: 159,2 Hz
• Vibration level: 10 m/s²
• Max. sensor weight: 500 gr.
• Battery powered
Model VC100
• Frequency range: 70 - 10000 Hz
• Vibration level: 1 m/s²
• Max. sensor weight: 400 gr.
• Sensitivity display
• PC controlled
• Frequency sweeps
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