© NMISA 2010

Implementing a Shock Calibration System Using a Vibration Exciter and Pendulum

Presented byIan Veldman

© NMISA 2010

Presentation overview

• Introduction• System description• Measurement procedure• Uncertainty contributors• Measurement results• Conclusions

© NMISA 2010

Introduction

• Mechanical shock (shock) is the measurand of numerous industry-, testing- and scientific research applications.

• In everyday activities, the peak shock is monitored as part of safety measures.

• The NMISA, as the custodian of South Africa’s measurement standards, developed a shock calibration system to provide industry with traceability to the SI for mechanical shock.

• ISO TC108 has developed ISO 16063-22 for the shock calibration of accelerometers by comparison.

• This standard describes calibration techniques, covering shock calibration of transducers for shock levels up to 1 000 km/s2.

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System description

• Depending on the peak shock level, two different shock excitation systems are generally employed.

• For shock signals with pulse durations longer than 3 ms and peak levels below 1,5 km/s2, pendulum systems are used.

• For peak shock levels up to 100 km/s2 with pulse durations as short as 0,05 ms, Hobkinson bar systems are employed.

• The main differences in operation between the two techniques are;– for the pendulum system, the acceleration of the anvil is

measured.– for the Hobkinsonbar system, the shock wave travelling through

the bar is measured.

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System description (continue)

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System description (continue)

• This mechanical shock calibration system was developed around an anvil, suspended with four 2 m long cables to form the pendulum.

• The pendulum is accelerated by an impulse acting on the suspended anvil.

• The impulse is induced using an electro-dynamic vibration exciter, on which a steel ram with a rounded edge is mounted.

• The peak level of the shock is controlled by the peak amplitude of the electrical signal applied to the exciter.

• The shock pulse duration is varied by using rubber contact surfaces of different composites and thickness on the surface being impacted on by the exciter.

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System description (continue)

• The reference transducer with accelerometer to be calibrated is mounted in the “back to back” fashion on the opposite end of the pendulum.

• The peak output amplitudes of both the reference transducer (ûRef) and the unit under test (ûUUT) are recorded using a dual channel analogue to digital converter.

• Knowing the two peak amplitudes, the shock sensitivity of the accelerometer is determined using:

f

UUTfUUT u

uSS

ReRe ˆ

ˆ

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Measurement procedure

• The measurement is initiated by sending a shock pulse to the vibration exciter.

• The AtoD is armed for measurement by a trigger pulse being sent to the AtoD trigger channel some 10 ms prior to the shock pulse being sent to the vibration exciter.

• This “pre-trigger” initiates the 500 ms long data capturing period.• A 4th order Butterworth low pass filter is applied to the captured time

series as a smoothing filter to improve the signal to noise ratio.

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Measurement procedure– captured time series

• The first peak is the shock pulse as the pendulum is accelerated by the exciter.

• The second peak is the pendulum swinging back.

• The third peak is from the pendulum hitting the exciter on its return.

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Measurement procedure– captured time series

• The pulse width of interest is selected by the metrologist with two mouse clicks.

• Using the mouse, the operator can click and drag on an area of interest in order to zoom in on that area.

• The time difference between the two selected points is used to determine the pulse width.

• The difference in amplitude is used to calculate the zero offset.

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Measurement procedure

• Once the signal processing is initiated, the peak determination and curve fitting routines are executed.

• The curve is fitted to the top 5% of the signal only as proposed in ISO 16063-22.

• With ûRef and ûUUT known, SUUT is calculated.

• The sensitivity results are recorded in a result sheet using a Microsoft® Excel spread sheet.

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Uncertainty contributors

Mathematical model:

• SS = Shock sensitivity

• SRef = Reference accelerometer shock sensitivity

• ûRef = Reference peak voltage

• ûUUT = UUT peak voltage

• GRef = Reference conditioning amplifier gain

• GUUT = UUT conditioning amplifier gain

UUT

UUT

f

ffS G

u

u

GSS

ˆ

ˆRe

ReRe

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Uncertainty contributors

• Reference transducer sensitivity– This is the expanded uncertainty reported on the calibration

certificate for the reference accelerometer’s shock calibration.

• Peak voltage determination– Taken as the one year accuracy specification of the 12 bit AtoD

card, using the manufacturer’s online accuracy calculator.– A worse case accuracy taking the widest measurement

possibilities into account was assumed.– It was assumed that peak input will be at least 50% of the

selected input range.

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Uncertainty contributors

• Anvil motion/Transverse sensitivity– The anvil’s motion is effectively a circle with a radius equal to the

length of the suspension cables.– This results in non-linearities in the motion of the anvil.– This non-linear motion, combined with the transverse sensitivity

of the reference transducer, results in a systematic error when determining ûRef and ûUUT.

– The maximum transverse motion (aT) was measured using a tri-axial accelerometer and calculated using

yxT aaa

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Uncertainty contributors

– The error due to transverse motion in the presence of transverse sensitivity was calculated using

e*T = ST•âT•cosβT•cosφT

• e*T = transverse error

• ST = transverse sensitivity

• âT = transverse acceleration amplitude

• cosβT = angle between the transverse motion direction and direction of transverse sensitivity

• cosφT = phase shift between disturbing voltage and measuring voltage

– A worse case scenario is assumed:• the transverse motion and accelerometer transverse sensitivity is in

phase, cosβT = 1

• the disturbing voltage and measuring voltage is in phase, cosφT = 1

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Uncertainty contributors

• Pendulum/Exciter Alignment– The influence of miss-alignment between the pendulum and

vibration exciter was determined experimentally.– The angle of impact between the exciter and pendulum was

increased from 0° to 30 ° in 10° steps.– With the exciter impacting at a 30° angle, the transverse motion

did not increase.– A decrease in the peak shock level was noted, equal to the

cosine of the impact angle.

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Uncertainty contributors

• Polynomial fit error– This was calculated as the Root Sum Square (RSS) error

between the fitted values and the peak time series.

• Conditioning amplifier gain– The gain accuracy specified by the manufacturer was used as

the amplifier gain was not calibrated prior to the measurements being performed.

– Calibration of the amplifier can be considered to reduce the over all uncertainty, should it be required.

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Uncertainty contributors

• Repeatability– The calibration system is setup to take five measurements per

calibration.– Three accelerometers were calibrated in a variety of

configurations.– A worst case standard deviation of 0,1 % recorded.– A maximum of 0,5 % was used for the type “A” evaluation in the

uncertainty budget.

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Measurement results

• The system performance was evaluated using a combination of three accelerometers.

– Two laboratory standard accelerometers, VS-WStd-01 and VS-WStd-09, of the back to back type

– One general purpose accelerometer, VS-01

• All three accelerometers have been calibrated using a laser interferometer system in compliance with ISO 16063-11, method 3.

UUT ReferenceSS

(pC/g)Us(%)

VS-WStd-09 VS-WStd-01 2,037 3,5

VS-WStd-09 VS-WStd-01 2,011 3,5

VS-WStd-01 VS-WStd-09 1,234 3,5

VS-WStd-01 Laser 1,218 3,5

VS-01 VS-WStd-01 67,29 3,5

VS-01 VS-WStd-01 66,94 3,5

VS-01 VS-WStd-09 66,68 3,5

VS-01 Laser 65,80 3,5

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Measurement results

• The two laboratory standard accelerometers were calibrated against each other

– using the certified shock sensitivities

– using the sensitivity obtained with sinusoidal calibration.

• VS-01 was calibrated using:1. Using sinusoidal calibration

2. VS-WStd-09 as reference with its shock sensitivity

3. VS-WStd-01 as reference with its sinusoidal sensitivity

4. VS-WStd-01 as reference with its shock sensitivity obtained when calibrated against VS-WStd-09

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Conclusions

• A pendulum shock calibration system was successfully implemented using a vibration exciter and pendulum system with improved repeatability of the shock level.

• The effect of the misalignment of the exciter with the pendulum on the pendulum transverse motion was studied.

• It was found that the pendulum transverse motion is insensitive to the exciter/pendulum alignment.

• Calibration results obtained using three different accelerometers, traceable to shock and linear calibration primary systems, showed agreement within the estimated uncertainty of measurement.

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Thank you for your attention!