Establishing Primary Shock Sensitivity Capabilities at NMISA€¦ · •A shock exciter based on a rigid body motion of an anvil. •A shock exciter based on wave propagation inside

Establishing Primary Shock

Sensitivity Capabilities at NMISA

Presented by:

Ian Veldman

Test & Measurement Conference

October 2018

NMISA Primary Shock Sensitivity Capabilities

• International SI Development

• Shock Sensitivity as a Measurand

• NMISA Primary Shock Calibration System• Shock Exciter

• Shock Control

• Laser Interferometer

• Data Capturing

• Signal Processing

• Signal Conditioner

• System Performance & Validation

• Conclusion

International SI Development• In an ever-changing world, the International System of Units (SI) is

evolving as well.

• Over the past years, metrologists all over the globe have been collaborating on the re-definition of SI.

• The focus of this work is the re-definition of the kilogram in electrical terms (Watt) by means of the Kibble balance or by means of a well-defined Silicon (Si) sphere.

• This research in an improved realisation of the SI aims at providing industry with access to higher accuracy measurement units that support manufacturing to:• tighter tolerances,

• less waste of valuable resources,

• an improved bottom line,

• an improved environment.

NIST Lego Kibble Balance

NIST Kibble Balance

International SI Development (2)

• Higher accuracy on the shop floor can also be achieved by reducing the number of calibration or measurement steps between the relevant NMS and manufacturing.

• To shorten the traceability chain, NMISA endeavours to realise NMS for Southern Africa using primary methods.

• Recently added to the collection of NMISA primary NMSs is an accelerometer shock sensitivity calibration system, using laser interferometry.

• The system was developed in response to the demand from industry for low-, medium- and high shock traceability.• Predominantly used in safety gear evaluation,

• crash testing

• and explosion monitoring.

• To ensure international equivalence, NMISA developed its primary shock system to conform to ISO 16063 part 13 “Primary shock calibration using laser interferometry”.

Shock Sensitivity as a Measurand

• SANAS/ISO 2041 “Mechanical vibration, shock and condition monitoring Vocabulary”, defines a shock as• “a sudden change of force, position, velocity or acceleration that excites

transient disturbances in a system.”

• It is evident that the term “shock” refers to a mechanical “event”.

• The unit for measuring acceleration of an object subjected to a shock is the same as for a vibrating object:• metres per second square (m/s²).

• No special transducer is required to measure acceleration resulting from a shock.

• The shock sensitivity of an accelerometer could be very similar to the recti-linear sensitivity of an accelerometer.

Shock Sensitivity as a Measurand (3)

• Why do we need a special method or procedure for calibrating accelerometers intended to be used to measure shock as supposed to acceleration?

• The answer this, one requires a detailed understanding of:• How an accelerometer works,

• How does a shock differ from a vibration.

• Piezo-electric accelerometers are mechanical devices with a second order frequency response.


0,180

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10 100 1 000 10 000 100 000

S qa

(pC

/)m

/s²)

)

Frequency (Hz)

ENDEVCO 2270Frequency Response


• When calibrating an accelerometer’s sensitivity, not the shock sensitivity, the metrologist employ a frequency step approach to determine the accelerometer’s sensitivity at each applied vibration frequency.

• This is generally done over the frequency range from 10 Hz to 10 kHz in ⅓ octave steps, performing narrow-band measurements.

• In the frequency domain, each of these measurements will be a single line at the applied vibration frequency.

• The frequency response of the accelerometer has effectively no influence on the measurement accuracy, even for calibrations at frequencies approaching the accelerometer’s resonant frequency where the sensitivity changes are substantial.

• In contrast with rectilinear calibration, calibrating the accelerometer’s shock sensitivity, requires the metrologist to accelerate the accelerometer using a shock pulse.


Shock Frequency Spectrum


• Analysing the shock in the frequency domain shows it contains a range of frequency components.

• It is important to measure the pulse width to determine the frequency content of the shock pulse.

• One need to evaluate the accelerometer’s frequency response to determine the effect of shock pulse width on the shock sensitivity, for the applied shock pulse width.

• High acceleration shocks are generated with short duration pulses.

• They contain frequency components.

• At high frequencies, the accelerometer’s sensitivity increase exponentially, resulting in a shock sensitivity that will be higher than the rectilinear sensitivity.

NMISA Primary Shock Calibration System

• NMISA implemented a shock calibration system in conformance with ISO 16063-13, “Primary shock calibration using laser interferometry.”

• By implementing a calibration system based on• the method specified,

• using instrumentation meeting specifications prescribed,

• a laboratory should achieve uncertainty of measurement (UoM) indicated in the standard.

• The NMISA system for primary shock calibration of accelerometers uses:• a commercial laser interferometer head,

• a commercial pneumatically driven shock exciter,

• in-house developed control software for the exciter,

• signal processing algorithms as described in ISO 16063-13.

Shock Exciter• ISO 16063-13 describes two shock pulse generating methods;

• A shock exciter based on a rigid body motion of an anvil.

• A shock exciter based on wave propagation inside a long thin bar (Hopkinson bar).

• NMISA adapted a SPEKTRA SE-201 PN-LMS, pneumatic shock exciter for its system.

• The exciter is based on a rigid body motion of an anvil.

• The exciter is specified to generate shocks with acceleration ranges of 50 m/s2 up to 100 km/s2.

• Using the different anvils, shocks durations of 0,1 ms up to 5 ms are generated.

• The current NMISA system was developed to cover an acceleration range of 500 m/s2 to 5 000 m/s2 with pulse duration of 0,3 ms to 5 ms.

Shock Exciter (2)

Shock Exciter (3)

Shock Control• The SPEKTRA Shock Box allows control of the exciter using a PC via an

Ethernet interface.

• This includes, but is not limited to:• Changing the anvil.

• Controlling the pressure for launching the projectile.

• Controlling the projectile to anvil distance.

• Moving the projectile to safe positions.

• Generating a shock (launching the projectile).

• The shock profile to be followed during the calibration is captured in an Excel spread sheet.• This allows for versatility and ease to modify various shock profiles.

• The desired peak acceleration level is set and controlled by the software to within a pre-set tolerance.

• The peak acceleration level is set by adjusting the projectile distance and/or air pressure.

Laser Interferometer

• A commercial laser head, containing the interferometer is used as reference to measure the generated shock.

• The laser head provides the “I” and “Q” electrical signals used to calculate the peak displacement signal by demodulating the time signal.

• The laser head is mounted vertically on a 30 mm thick granite top using self-levelling air suspension isolation units.

• This forms a vibration isolated mount for the interferometer.

• The granite table is supported on a free standing aluminium frame that fits over the exciter.

• The frame is constructed of 40 mm aluminium strut profiles.

Data Capturing

• Three signals are synchronised sampled using two National Instruments data acquisition units:• Interferometer “I” signal, 14 bit resolution,

• Interferometer “Q” signal, 14 bit resolution,

• the accelerometer output signal, 16 to 24 bit resolution, sampling rate dependent.

• Each shock event is sampled for a period of 15 ms.• This is at least 3 times the shock duration.

• The captured time series is then reduced to a time signal with a duration of 2·t• where (t) is the shock duration.

Signal Processing

• Post processing of the sock signals are performed in compliance with ISO 16063-13.

• The standard offers three distinct methods to determine the shock sensitivity:• Version without Discrete Fourier Transform (DFT),

• Version with DFT of the velocity values,

• Version with DFT of the displacement values.

Signal Processing (2)

• In order to obtain the shock sensitivity, the following DSP steps are completed:• Low pass filtering (LPF) the signals ensure that they are contained within

the bandwidth (BW) of interest.

• Demodulating the interferometer signals using the arctangent method to obtain the shock displacement time series.

• LPF the displacement time series to suppress high frequency noise.

• Obtain the acceleration signal by double differentiating the displacement signal, with suitable LPF after each differentiation.

• Finally, LPF is applied to both the acceleration- and accelerometer output signals.

• All “irrelevant” data is removed by selecting only the relevant 2·t time duration of the signals.

• The pulse width is determined by measuring the time difference of the accelerometer output signal at 10 % of its peak value.

• Finally, the shock sensitivity is calculated as the ratio of the two peak values of interest.


• The three time series signals, u1 (ti), u2 (ti) and uU (ti) are obtained using equidistant sampling.

• The interferometer “I” & “Q” signals u1 and u2, while uU is the accelerometer output signal.

• The modulation phase values, φMod, is calculated using:

𝜑𝑀𝑜𝑑 𝑡𝑖 = tan−1𝑢2(𝑡𝑖)

𝑢1(𝑡𝑖)+ 𝑛𝜋

Where n = 0, 1, 2, …

• Having demodulated the interferometer signal a series of displacement values can be calculated from:

𝑠D 𝑡𝑖 =𝜆

4𝜑𝑀𝑜𝑑 𝑡𝑖

• The subscript D indicates that the displacement values, s, are distorted by high-frequency noise.

• This high-frequency noise is removed using a suitable LPF to obtain s(ti).


• From the filtered series, s (ti), the filtered acceleration series, a (ti) is determined by double differentiation and filtering of s (ti) using the differentiation formula

𝑣𝐷 =1

2Δ𝑡𝑠 𝑡𝑖 + 1 − 𝑠 𝑡𝑖 − 1

𝑎 =1

2Δ𝑡𝑣 𝑡𝑖 + 1 − 𝑠 𝑡𝑖 − 1

• Finally, the shock sensitivity Ssh is calculated using

𝑆𝑠ℎ =𝑢𝑝𝑒𝑎𝑘

𝑎𝑝𝑒𝑎𝑘

Signal Conditioner

• ISO 16063-13 recommends that the accelerometer and conditioning amplifier be calibrated as a unit.

• In circumstances where the shock sensitivity of the accelerometer alone is required, the conditioning amplifier high pass- and low pass filter settings are set in such a manner as to ensure that the frequency content of the shock pulse falls within a linear portion of the amplifier frequency response.

• Currently, the mean sensitivity (gain) of the amplifier over the frequency range equal to the frequency content of the shock pulse, is used as the gain correction applied for the amplifier.

• The standard deviation of the mean calculation of the gain, as well as the effect of utilising an averaged gain estimation for the correction, is added to the UoM estimation.

System Performance & Validation

• The performance of the primary shock calibration system was validated by comparing sensitivity values of two different laboratory standard charge accelerometers.

• The sensitivities of the accelerometers were compared using:• the NMISA primary shock system,

• the NMISA’s primary recti-linear calibration system,

• The NMISA secondary shock calibration system.

• It shall be noted that at the time, the reference accelerometer used for secondary shock calibrations was traceable to NMISA primary vibration calibration system.

• The different sensitivity values are referred to as:• Sqa11: Accelerometer charge sensitivity obtained as per ISO 16063 part 11

• Sqa13: Accelerometer charge shock sensitivity obtained as per ISO 16063 part 13

• Sqa22: Accelerometer charge shock sensitivity obtained as per ISO 16063 part 22

System Performance & Validation (2)

• The system was validated over the acceleration range 500 m/s2 up to 5 kms2 using two different anvils.

• For low shock acceleration of 500 m/s2 up to 2 000 m/s2, an air bearing anvil with a rubber interface was used.• This delivered nominal pulse durations of 2 ms to 3 ms

• For medium shock acceleration of 2 km/s2 up to 5 km/s2 a light-weight aluminium anvil with a felt interface was used.• This delivered nominal pulse durations of 0,3 ms to 0,4 ms

• The accelerometers used were:• ENDEVCO model 2270M8, #16388, single ended accelerometer

• ENDEVCO model 2270, #10370 double ended accelerometer

• En values were calculated for Sqa11 vs Sqa13 and Sqa13 vs Sqa22


ShockLevel

Sq13 USq13 Sq11 USq11 U∆Sq

En

(m/s²) (pC/(m/s²)) (%) (pC/(m/s²)) (%) (%)

500 0,1841 1,1 0,1845 0,3 1,1 0,2

1 000 0,1843 1,1 0,1844 0,3 1,1 0,1

2 000 0,1842 1,1 0,1844 0,5 1,2 0,1

ENDEVCO 2270M8 Low Shock En values: Sqa13 vs Sqa11


ENDEVCO 2270M8 Low Shock En values: Sqa13 vs Sqa22

ShockLevel

Sq13 USq13 Sq22 USq11 U∆SqEn

(m/s²) (pC/(m/s²)) (%) (pC/(m/s²)) (%) (%)

500 0,1841 1,1 0,1845 0,8 1,4 0,2

1 000 0,1843 1,1 0,1846 0,8 1,4 0,1

2 000 0,1842 1,1 0,1846 0,8 1,4 0,1


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E n

S qa

(pC

/(m

/s²)

)

Shock Level (m/s²)

ENDEVCO 2270M8En values: Sqa13 vs Sqa11

Sq13 Sq11

En


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0,0

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100 1 000 10 000

E n

S qa

(pC

/(m

/s²)

)

Shock Level (m/s²)

ENDEVCO 2270M8En values: Sqa13 vs Sqa22

S13 S22 En

System Performance & ValidationENDEVCO 2270 Low Shock En values: Sqa13 vs Sqa11

ShockLevel

Sq13 USq13 Sq11 USq11 U∆Sq

En

(m/s²) (pC/(m/s²)) (%) (pC/(m/s²)) (%) (%)

500 0,2047 1,1 0,2049 0,3 1,1 0,1

1 000 0,2049 1,1 0,2049 0,3 1,1 0,0

2 000 0,2049 1,1 0,2049 0,5 1,2 0,0

System Performance & ValidationENDEVCO 2270 Medium Shock En values: Sqa13 vs Sqa11

Shock

LevelSq13 USq13 Sq11 USq11 U∆Sq

En

(km/s²) (pC/(m/s²)) (%) (pC/(m/s²)) (%) (%)

2 0,2054 1,1 0,2053 0,5 1,6 0,0

3 0,2054 1,1 0,2054 0,5 1,6 0,0

4 0,2055 1,1 0,2057 0,5 1,6 0,1

5 0,2057 1,1 0,2059 0,5 1,6 0,1


-0,5

0,0

0,5

1,0

1,5

2,0

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100 1 000 10 000

E n

S qa

(pC

/(m

/s²)

)

Shock Level (m/s²)

ENDEVCO 2270En values: Sqa13 vs Sqa11

Sq13 Sq11En

Conclusion

• NMISA has successfully developed and implemented a primary accelerometer shock calibration capability over the acceleration range 500 m/s² up to 5 000 m/s².

• Using two different anvils, each with a different interface, shock durations ranging from 0,3 ms up to 5 ms could be generated.

• Although the system was validated over a limited shock range, the system validation demonstrated that the capability conforms to International Standards and can be extended to meet future demands.

• The validation demonstrated excellent agreement obtained between the three different calibration systems and calibration methods:• primary accelerometer calibration,

• primary shock calibration,

• secondary shock calibration.

• The agreement of the results for all three systems were well within the stated UoM for each system.

Documents

Establishing Primary Shock Sensitivity Capabilities at NMISA€¦ · •A shock exciter based on a rigid body motion of an anvil. •A shock exciter based on wave propagation inside