SPEKTRA Schwingungstechnik und Akustik GmbH Dresden
Calibration Systems • Special Equipment • DAkkS Laboratory • Environmental Testing
Reference Sensors for High-g ShockAccelerometer Calibration Systems
Martin Brucke1, Georg Siegmund2, Christian Ehrmann2, Uwe Bühn1, Frank Schulz1
1 SPEKTRA Schwingungstechnik und Akustik GmbH Dresden Germany
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SPEKTRA Schwingungstechnik und Akustik GmbH, Dresden, Germany2 Polytec GmbH, Waldbronn, Germany
High-Shock ApplicationsHigh-Shock ApplicationsWhy do we need High-Shock Calibration?
Calibration of accelerometers in the following applications:AerospaceMilitaryMilitary
Typical acceleration 200 000 gn
E i t l t tiEnvironmental testing:Endurance / Failure investigations of MEMS structures
Typical accelerations 300 000 gn
Research work:Research work:Medical / Biological / Physical / Chemical Research
Typical accelerations >500 000 gn
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yp gn
High-Shock ApplicationsExample Medical / Chemical Research
High-Shock Applications
Investigation of contact forces between small particles (1 ... 50 µm) and surfaces
Typical accelerations >500 000 gn
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Shock excitersdand
Reference Standardsfor Calibration
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Hopkinson-Bar Shock ExciterHopkinson-Bar Shock ExciterTechnical Data – SE-221/222 HOP-HS/VHS
Type of Excitation Sinusoidal ShockShock Amplitude HS 10.000 m/s² to 1.000.000 m/s²
VHS 50 000 / ² t 2 000 000 / ²VHS 50.000 m/s² to 2.000.000 m/s²Pulse Width HS 50 µs PWFS
VHS 20 µs PWFS
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DUT mass Up to 30 gram
Hopkinson-Bar Shock ExciterHopkinson-Bar Shock ExciterWorking Principle
F
t
a
tMP1 MP2MP2 MPnMPn-1
Displacement u(x,t)
c0 = speed of sound E = Young’s modulusA = cross sectional area of bar
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High-Shock CalibrationHigh-Shock CalibrationAppropriate Reference Standards?
Displacement u(x,t)
FF
tt
F
t
aa
tt
a
t
Strain GaugeStrain Gauge
c0 = speed of soundε = strain due to wave
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High-Shock CalibrationHigh-Shock CalibrationAppropriate Reference Standards?
Displacement u(x,t)
FF
tt
F
t
aa
tt
a
t
LaserVibrometer
( )= measured velocityv t
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( )= measured velocityv t
Strain Gauges
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Strain GaugesStrain GaugesWhich Parameters Influence the Measurement Uncertainty?
• c0 = speed of sound in the bar material • KDMS = sensitivity of the strain gauge• US = voltage applied to the strain gauge half bridge
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Strain GaugesStrain GaugesWhich Parameters Influence the Measurement Uncertainty?
• US can be measured precisely
• KDMS sensitivity value from Static Calibration !!−dynamic behavior of strain gauges?− influence of mounting?
• c0 speed of sound in the bar material?0 p
• Dispersion of waves?
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Strain GaugesStrain GaugesMeasured velocity sensitivity with a Laser Vibrometer
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Strain GaugesStrain GaugesDispersion
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Strain GaugesStrain GaugesDispersion
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Strain GaugesStrain GaugesDispersion
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Strain GaugesStrain GaugesDispersion
60 s shock d ration 30 µs shock duration60 µs shock duration 30 µs shock duration
strain gauge at the force input end
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laser vibrometer at output endstrain gauge at the force input end
Strain GaugesStrain GaugesTransfer Calibration with a Laser Vibrometer
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Strain GaugesStrain GaugesTransfer Calibration with a Laser Vibrometer
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Strain GaugesStrain GaugesTransfer Calibration with a Laser Vibrometer
• a transfer calibration with a laser vibrometer can improve the measurement uncertainty due to ameasurement uncertainty due to a strain gauge reference sensor significantly
• using a strain gauge as reference sensor a measurement uncertainty of 3 % to 6 % in the amplitude range 200 000 m/s2 to 2 000 000 m/s2 is possible
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Laser Vibrometer
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Laser VibrometerLaser VibrometerComparison of two Laser Vibrometers
• Laser Vibrometer A−OFV-5000 with OFV-505 optic (10 m/s)p ( )−Digital Velocity Decoder VD-09
• Laser Vibrometer B−OFV-5000-S with an OFV-552 fiber optic (20 m/s)
Di it l V l it D d VD 09
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− Digital Velocity Decoder VD-09s
Laser VibrometerLaser VibrometerComparison of two Laser Vibrometers
Relative Deviation between AmplitudesRelative Deviation between Amplitudes measured with two Vibrometers
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Laser VibrometerLaser VibrometerComparison of two Laser Vibrometers
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Laser VibrometerLaser VibrometerOFV 5000 S High Speed Vibrometer
• heterodyne laser vibrometer• laser wave length λ = 632 nm• center frequency 80 MHz• frequency range (full scale)
DC 1 5 MHzDC … 1.5 MHz
peak velocity 20 m/speak velocity 20 m/speak acceleration 1.88 * 108 m/s2
How can we prove that the laser vibrometer works accurately in the acceleration range up to 2 000 000 m/s2?
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Laser VibrometerLaser VibrometerLaser Vibrometer - Electrical Measurements
Matlab Waveform Simulation
Vibrometer Controller
Communications Software
Signal Generator Tektronix AFG3252 2 GS/s, 240 MHz
OFV-5000-S
TF HP LP OFF FM Signal
Oscilloscope Tektronix DPO 4032 2.5 GS/s, 350 MHz
Trigger Velocity
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,
Laser VibrometerLaser VibrometerLaser Vibrometer - Electrical Measurements
Trapezoid tRise=5 μs, a=4E6 m/s2x 10
6
• Simulated Doppler input signal (trapezoid )
20
2
4
]
• 20 m/s peak velocity(operating range limit )
• Variation of rise time
10
Vel
ocity
[m
/s]
0
ccel
erat
ion
[m/s
2 ]Variation of rise time• Acceleration calculated by offline
differentiation
0-4
-2
Ac
0 2 4 6 8 10 12 14 16 18 20
0
Time [μs]0 2 4 6 8 10 12 14 16 18 20
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Laser VibrometerLaser VibrometerLaser Vibrometer - Electrical Measurements
20
Trapezoid tRise=1.25 μs, a=16E6 m/s2
1.6
x 107
0.8
s2 ]
10
eloc
ity [
m/s
]
0
eler
atio
n [m
/s
Ve
-0.8
Acc
e
0 2 4 6 8 10 12 14 16 18 20
0
0 2 4 6 8 10 12 14 16 18 20
-1.6
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Time [μs]
Laser VibrometerLaser VibrometerLaser Vibrometer - Electrical Measurements
2
20
Trapezoid tRise=0.3125 μs, a=64E6 m/s2
6.4
x 107
Rise time is related
]
3.2
/s2 ]
Ringing at very short rise time
to an acceleration level higher than required
10
Vel
ocity
[m
/s
0
cele
ratio
n [m
/
V
-3.2
Acc
0 2 4 6 8 10 12 14 16 18 20
0
Ti [ ]0 2 4 6 8 10 12 14 16 18 20
-6.4
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Time [μs]
Laser VibrometerLaser VibrometerLaser Vibrometer - Electrical Measurements
Gauss tPulse=5 μs, a=1.44E7 m/s2
1.5x 10
7• Gaussian velocity input signals similar to Hopkinson Bar
20
0.75
2 ]
similar to Hopkinson Bar• 20 m/s peak velocity signal
(operating range limit )
10
Vel
ocity
[m
/s]
0
Acc
eler
atio
n [m
/s2
• Variation of pulse width• Comparison of
d l ti k
0
-1.5
-0.75measured acceleration peak values with peak values calculated from input curve
0 2 4 6 8 10 12 14 16 18 20Time [μs]
0 2 4 6 8 10 12 14 16 18 20
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Laser VibrometerLaser VibrometerLaser Vibrometer - Electrical Measurements
Results Gaussian input signals
Pulse Width
(full cycle)
Pulse Width@ 0.606 vpeak
Source Acceleration
(peak)
Output Acceleration
(peak)
RelativeError( y ) (p ) (p )
80 µs 26.4 µs 0.886·106 m/s2 0.90·106 m/s2 1.6 %20 µs 6.6 µs 3.54·106 m/s2 3.59·106 m/s2 1.4 %5 µs 1.65 µs 1.42·107 m/s2 1.44·107 m/s2 1.4 %
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Laser VibrometerLaser VibrometerLaser Vibrometer - Results
• comparison measurements showed very low deviations in the measured shock amplitudes (< 0 4 %)shock amplitudes (< 0.4 %)
• the relative error between outputthe relative error between output values and simulated pulse parameters is also low (<1.6%) at high acceleration levelshigh acceleration levels
• a measurement uncertainty of < 3% y %in the amplitude range above 2 000 000 m/s2 is possible
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Calibration of a High-Shock Sensor
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Calibration of a High-Shock SensorCalibration of a High-Shock SensorTechnical Data
Amplitude Range: 2 000 000 m/s2
Resonance Frequency: > 1 MHzWeight: 1 5 gramsWeight: 1.5 grams
Good amplitude linearity up toGood amplitude linearity up to 500 000 m/s2 assumed
Sensitivity independence fromSensitivity independence fromshock duration assumed
Comparison of high-shockComparison of high-shockcalibration results with areference calibration on a traceable
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low shock pendulum makes sense
Calibration of a High-Shock SensorCalibration of a High-Shock SensorResults
1,0%
1,5% Deviation of accelerometer sensitivity compared to reference value determined on a traceable shock pendulum
0,0%
0,5%
‐0,5%
,0 100 000 200 000 300 000 400 000 500 000 600 000
laser vibrometer as reference sensor
‐1,5%
‐1,0%laser vibrometer as reference sensor
strain gauge as reference sensor
‐2,0%
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‐2,5%a[m/s²]
Conclusion
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ConclusionConclusion
• strain gauges as well as laser vibrometers are i t f f h k lib ti tappropriate reference sensors for shock calibration up to
amplitudes of 2 000 000 m/s2
• a transfer calibration of a strain gauge reference sensor with a laser vibrometer decreases the measurement uncertainty to a sufficiently low amount (< 6%)
• a measurement uncertainty of < 3% in the amplitude t 2 000 000 / 2 ith hi h d lrange up to 2 000 000 m/s2 with a high-speed laser
vibrometer as reference sensor is possible
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Thank You!
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