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Piezoelectric Accelerometers Theory & Application. Piezoelectric Effect. Piezoceramics or Quartz generate a charge output when subjected to force. Piezoelectric Accelerometer. A piezoelectric accelerometer consists of a piezoelectric material and a seismic mass . - PowerPoint PPT Presentation
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Metra Mess- und Frequenztechnik Radebeul / Germany www.MMF.de 1
Piezoelectric AccelerometersTheory & Application
Metra Mess- und Frequenztechnik Radebeul / Germany www.MMF.de 2
Piezoelectric Effect• Piezoceramics or Quartz generate a charge output when
subjected to force.
forcechargepiezo constant
Fqd
piezo disk
F q
F
q = d F.
Metra Mess- und Frequenztechnik Radebeul / Germany www.MMF.de 3
Piezoelectric Accelerometer• A piezoelectric accelerometer consists of
a piezoelectric material and a seismic mass.• The charge output is proportional to acceleration.
F = m a
charge sensitivity:
B = qaqa
Acceleration a
mSeismic mass
Piezoceramics
q .
Metra Mess- und Frequenztechnik Radebeul / Germany www.MMF.de 4
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.
shea
r fo
rce
Cover
Socket
Piezo ceramicsPost
Base
Seismic mass
Ring-shapedpiezo ceramics:
Metra Mess- und Frequenztechnik Radebeul / Germany www.MMF.de 5
Other Types of Vibration SensorsComparison with piezoelectric accelerometers:
Sensor Type Advantages Disadvantages
Piezoresistive(strain gauge)
• Measures static acceleration
• Robust
• Limited resolution• Only up to some kHz• Power supply required
Electrodynamic • Measures static acceleration
• Only for low frequencies
Capacitive • Measures static acceleration
• Cheap manufacturing by semiconductor process
• Low resolution• Fragile
Metra Mess- und Frequenztechnik Radebeul / Germany www.MMF.de 6
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.
Metra Mess- und Frequenztechnik Radebeul / Germany www.MMF.de 7
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.
Metra Mess- und Frequenztechnik Radebeul / Germany www.MMF.de 8
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 www.MMF.de 9
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.
f r0,5fr0,3fr
f
1,00
1,10
1,30
rel.Ampl.
10 % Error
30 % Error
f r
0,7
f l
+/- 3 dB Frequency Range
Metra Mess- und Frequenztechnik Radebeul / Germany www.MMF.de 10
Selecting an Accelerometer Model (1)Acceleration to be measured:
• Very low Seismic Accelerometers(...µm/s² to 1 m/s²
• Medium Standard Accelerometers(1 to 10 000 m/s²)
• Very high Shock Accelerometers(> 10 000 m/s²)
KB12
KS76
KD93
Examples:
Metra Mess- und Frequenztechnik Radebeul / Germany www.MMF.de 11
Selecting an Accelerometer Model (2)Weight:The weight of an accelerometer should not exceed 10 % of the test object weight.
• Very light Miniature Accelerometers (weight < 5 grams)
Examples:
KS91
KS94
Metra Mess- und Frequenztechnik Radebeul / Germany www.MMF.de 12
Selecting an Accelerometer Model (3)Environmental Conditions:
• Humidity and Dust Industrial Accelerometers(with protection grade IP67 and higher)
• Grounding Problems / Ground Loops Industrial Accelerometers(with insulated case)
Example:
KS80
Metra Mess- und Frequenztechnik Radebeul / Germany www.MMF.de 13
Accelerometer Mounting Methods
StudMounting
MagneticBase
InsulatingFlange
DirectAdhesive
ProbeAdhesivePad
Performance:
Max. Frequency:
good poor
> 20 kHz < 1 kHz< 5 kHz
Metra Mess- und Frequenztechnik Radebeul / Germany www.MMF.de 14
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.
Unevensurface
Roughsurface
Flexiblecoupling
F
Metra Mess- und Frequenztechnik Radebeul / Germany www.MMF.de 15
Instrumentation for Charge Mode Sensors• Charge mode accelerometers require a charge
amplifier input.• A charge amplifier is a differential amplifier with
capacitive feedback. R
C
-
+
GND
Sensor
q
vout
inf
f
qf
Metra Mess- und Frequenztechnik Radebeul / Germany www.MMF.de 16
Instrumentation for ICPAccelerometers• 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.
Integrated Charge Converter
Instrument
I
Q UPiezoSystem
C
coaxial cable,over 100 m long
ICP® Compatible Transducer
c
const
Constant current source
Decoupling
Metra Mess- und Frequenztechnik Radebeul / Germany www.MMF.de 17
Signal Conditioners from Metra
M68 series charge & ICP conditioners
M108/116 multichannel ICP conditioners
M32 ICP conditioner
M28 ICP supply
Metra Mess- und Frequenztechnik Radebeul / Germany www.MMF.de 18
Accelerometers with TEDS to IEEE1451.4TEDS = Transducer Electronic Data SheetNew feature for ICPcompatible 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
Metra Mess- und Frequenztechnik Radebeul / Germany www.MMF.de 19
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.
Metra Mess- und Frequenztechnik Radebeul / Germany www.MMF.de 20
Accelerometer Calibration Vibration Calibrators from Metra
Model VC10
Model VC100
• Fixed frequency: 159,2 Hz• Vibration level: 10 m/s²• Max. sensor weight: 500 gr.• Battery powered
• Frequency range: 70 - 10000 Hz• Vibration level: 1 m/s²• Max. sensor weight: 400 gr.• Sensitivity display• PC controlled• Frequency sweeps