spektra schwingungstechnik und akustik gmbh dresden

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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