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

SPEKTRA Schwingungstechnik und Akustik GmbH Dresden APMP Conference 2013 1

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


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


Shock excitersdand

Reference Standardsfor Calibration


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


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


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


High-Shock CalibrationHigh-Shock CalibrationAppropriate Reference Standards?

Displacement u(x,t)

FF

tt

F

t

aa

tt

a

t

LaserVibrometer

( )= measured velocityv t


( )= measured velocityv t

Strain Gauges


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


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?


Strain GaugesStrain GaugesMeasured velocity sensitivity with a Laser Vibrometer


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


laser vibrometer at output endstrain gauge at the force input end

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


Laser Vibrometer


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


− Digital Velocity Decoder VD-09s


Relative Deviation between AmplitudesRelative Deviation between Amplitudes measured with two Vibrometers




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?


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


,


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



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


Time [μs]


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


Time [μs]


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



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 %


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


Calibration of a High-Shock Sensor


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


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%


‐2,5%a[m/s²]

Conclusion


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


Thank You!


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