© NMISA 2010 Implementing a Shock Calibration System Using a Vibration Exciter and Pendulum Presented by Ian 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 for the shock calibration of accelerometers by comparison. This standard describes calibration techniques, covering shock calibration of transducers for shock levels up to km/s 2.

© NMISA 2010 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/s 2, pendulum systems are used. For peak shock levels up to 100 km/s 2 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.

© NMISA 2010 System description (continue)

© NMISA 2010 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.

© NMISA 2010 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 (û Ref ) and the unit under test (û UUT ) are recorded using a dual channel analogue to digital converter. Knowing the two peak amplitudes, the shock sensitivity of the accelerometer is determined using:

© NMISA 2010 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 4 th order Butterworth low pass filter is applied to the captured time series as a smoothing filter to improve the signal to noise ratio.

© NMISA 2010 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.

© NMISA 2010 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.

© NMISA 2010 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 With û Ref and û UUT known, S UUT is calculated. The sensitivity results are recorded in a result sheet using a Microsoft® Excel spread sheet.

© NMISA 2010 Uncertainty contributors Mathematical model : S S = Shock sensitivity S Ref = Reference accelerometer shock sensitivity û Ref = Reference peak voltage û UUT = UUT peak voltage G Ref = Reference conditioning amplifier gain G UUT = UUT conditioning amplifier gain

© NMISA 2010 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.

© NMISA 2010 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 û Ref and û UUT. –The maximum transverse motion (a T ) was measured using a tri- axial accelerometer and calculated using

© NMISA 2010 Uncertainty contributors –The error due to transverse motion in the presence of transverse sensitivity was calculated using e* T = S T â T cosβ T cosφ T e* T = transverse error S T = transverse sensitivity â T = transverse acceleration amplitude cosβ T = angle between the transverse motion direction and direction of transverse sensitivity cosφ T = phase shift between disturbing voltage and measuring voltage –A worse case scenario is assumed: the transverse motion and accelerometer transverse sensitivity is in phase, cosβ T = 1 the disturbing voltage and measuring voltage is in phase, cosφ T = 1

© NMISA 2010 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.

© NMISA 2010 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.

© NMISA 2010 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.

© NMISA 2010 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 , method 3. UUTReference S S (pC/g) U s (%) VS-WStd-09VS-WStd-012,0373,5 VS-WStd-09VS-WStd-012,0113,5 VS-WStd-01VS-WStd-091,2343,5 VS-WStd-01Laser1,2183,5 VS-01VS-WStd-0167,293,5 VS-01VS-WStd-0166,943,5 VS-01VS-WStd-0966,683,5 VS-01Laser65,803,5

© NMISA 2010 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

© NMISA 2010 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.

© NMISA 2010 Thank you for your attention!