Measurement 137 (2019) 302–311

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Measurement

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Visualization of pyroshock wave reduction by insulator using a lasershock based simulation method

https://doi.org/10.1016/j.measurement.2019.01.0460263-2241/� 2019 Elsevier Ltd. All rights reserved.

⇑ Corresponding author.E-mail address: leejrr@kaist.ac.kr (J.-R. Lee).

Byung-Hak Jung a, Yong-Woon Kimb, Jung-Ryul Lee a,⇑, Dong Seong Kim c

aDepartment of Aerospace Engineering, KAIST, 291 Daehak-ro, Yuseong-gu, Daejeon 34141, Republic of Koreab The 6th R&D Institute, Agency for Defense Development, Jinhae P.O. Box 18, Gyeongsangnam-do 51678, Republic of Koreac The 4th R&D Institute, Agency for Defense Development, Yuseong P.O. Box 35, Daejeon 34186, Republic of Korea

a r t i c l e i n f o

Article history:Received 9 October 2017Received in revised form 13 January 2019Accepted 15 January 2019Available online 22 January 2019

Keywords:Pyroshock visualizationLaser shock simulation methodShock insulatorSpace structure

a b s t r a c t

Various pyro-devices are used in space structures because of their high power-to-weight ratio, high reli-ability, and compact size. However, when a pyro-device fires, it generates a strong, high-frequency shockwave called a pyroshock wave, which causes high-frequency-sensitive electronic equipment to malfunc-tion. Therefore, it is important to analyze and minimize pyroshock waves near electronic equipment toprevent damage. In this paper, we propose a shock wave scanning method by a Q-switched laser to ana-lyze shock reduction and visualize shock wave propagation in a real-world complex structure. This tech-nique was applied to evaluate and visualize the shock reduction by rubber insulators mounted in aconfiguration such that a pyroshock wave is transferred and propagated via a butyl rubber mount toan electronic component mounting panel. An average of 20% in the mean acceleration differences inshock response spectra between an actual pyroshock wave and a laser-simulated pyroshock wave wasachieved. The use of the butyl rubber mount reduced the shock wave by 75%, and the reduced pyroshockpropagation was quantitatively visualized.

� 2019 Elsevier Ltd. All rights reserved.

1. Introduction the mechanical excitation test and an actual explosion. Mechanical

More pyro-devices have recently been used in space structuresto actuate more complex and effective separation mechanisms.Pyro-devices provide a high power-to-weight ratio and high relia-bility and have a compact size [1]. Pyro-devices are separators inthe forms of explosive bolts, nuts and clamps [2]. Typical applica-tions are missile payload deployment, space launcher stage separa-tion, spacecraft separation and pilot ejection systems. However, astructurally guided shock wave is generated in such structureswhen a pyro-device explodes. This shock wave is called a pyr-oshock wave and is classified as near-field, mid-field, or far-fielddepending on its frequency and/or amplitude [3–5]. A high-frequency pyroshock wave may damage a space structure. Specif-ically, a shock wave above 3 kHz causes electronic equipment,relays and switches mounted on a space structure to malfunction[6]. Various methods have been used to numerically and experi-mentally simulate pyroshock waves. Numerical simulation meth-ods include the finite element method (FEM) [7], the statisticalenergy analysis method (SEA) [8] and the hydrocode method [9].However, numerical simulation methods are less reliable forcomplex structures. Experimental simulation methods include

excitation tests include a drop test machine [10], an electrodynamic shaker and an impact device [11], and actual pyro-devices have been used in explosion tests [12,13]. However,experimental simulations are case-dependent, structures can bepermanently damaged, and many sensing channels, much timeand money are required.

A pyroshock wave simulation method using a laser shock wavehas been recently introduced [14]. A shock wave can be induced bythe impingement of a laser beam pulse without damaging a struc-ture, and a laser shock-based simulation of an actual pyroshock canbe performed regardless of the complexity and geometry of astructure. The laser shock simulation method can mimic an actualpyroshock wave, and its propagation can be visualized in both thetime and frequency domains by pyroshock wave propagationimaging (PWPI) and shock response spectrum imaging (SRSI). Ingeneral, a shock response spectrum (SRS) is used to quantitativelyrepresent a pyroshock wave [15]. An SRS is a graphical representa-tion of the peak acceleration response with a single degree offreedom that has a different natural frequency.

This paper presents a novel quantitative and visible analysismethod of shock reduction due to a passive insulator. In thismethod, a laser shock wave is generated in the entire region ofinterest with a Q-switched laser and a laser mirror scanner, anda simulation algorithm [14] is applied to convert the laser shock

B.-H. Jung et al. /Measurement 137 (2019) 302–311 303

to an actual pyroshock. An experimental model was made by sim-plifying an actual structure consisting of a vehicle equipment bay(VEB) and an electronic equipment mount on a space launcher. Apyroshock wave was generated using an initiator as a pyro-device and the propagated wave was measured by four laser Dop-pler vibrometers (LDVs).

In case of shock with high amplitude and frequency componentsuch as pyroshock, if a contact type accelerometer is used, it is likelyto be detached due to resonance. Due to these problems, there is aprecedent in 1998, which proposed ameasurement technique usingthe characteristics of a strain gauge that does not cause resonance athigh frequency due to its light mass [16]. However, in order to use astrain gauge, it has to be attached, and once attached, it cannot bemoved. In this experiment, the limitations mentioned are solvedthrough the noncontact measurement technique using the laser.

However, in the noncontact measurement technique usinglaser, the noise changes sensitively according to the laser incidentangle [17]. Unfortunately, it is practically impossible to place allLDVs at exactly same angle (90 degree) with specimen surface.Therefore, retroreflective tape that reflects the laser beam at thesame path as the incident path are attached to the measurementpoint, which eliminated the laser incidence angle problem.

Three experiments were carried out to compare the shock wavereduction in each one. First, a control group with no shock wavereduction was set up by fastening steel mounts between the sub-structure and the main-structure of the VEB to provide a propaga-tion path for the pyroshock wave to the electronic equipment, andbetween the pyro-device (i.e., the initiator) and the main-structure,so the shock wave generated by the explosion was propagated tothe main structure. Second, an insulator was placed in the pathof the pyroshock wave propagation to the sub-structure, and asteel mount was placed between the pyro-device and the mainstructure. Third, a steel mount was placed in the pyroshock wavepropagation path to the sub-structure, and an insulator was placedbetween the pyro-device and the main structure. To observe theshock reduction in the entire region of interest, laser shock signalswere acquired by laser beam scanning with a laser mirror scanner.Two arbitrary points of the four measurement points of the actualpyroshock wave were used to train the laser shock signal. The lasershock signal at the two points was trained by the simulation algo-rithm in Ref. [14] and the gain obtained during the training processwas applied to all laser shock signals in the scanned region (i.e., theregion of interest). The shock wave propagation was visualized byusing algorithms of PWPI and SRSI in the region of interest usingthe laser simulated pyroshock wave. This experimental methoddemonstrates that a laser-simulated pyroshock is useful for evalu-ating shock wave reduction.

Fig. 1. (a) Actual satellite-equipped electronic equipment (Courtesy to KARI) (b) The expmounting panel.

2. Pyroshock wave measurement in a shock wave reductionstructure with insulators

Fig. 1(a) shows the electronic devices installed on the VEB of theexpandable launcher and Fig. 1(b) shows a simplified experimentalmode for studying pyroshock wave propagation. The main struc-ture was made of AL6061 material and it had a width of1000 mm, a height of 500 mm and a thickness of 10 mm. Thesub-structure corresponding to the electronics mounting panelwas also made of AL6061 and it had a width of 250 mm, a heightof 150 mm and a thickness of 1.5 mm.

Fasteners were placed on both the left and right sides of thespecimen during the pyroshock wave measurement test, as shownin Fig. 2(a). The initiator (i.e., the pyrotechnical device) was placedon the right side of the specimen as shown in Fig. 2(b), measure-ment points Point 1 to Point 3 were located 210 mm from the ini-tiator, and Point 4 was located in in the middle of the sub-structure. The initiator explosion generated a pyroshock wave thatwas measured on the top of each bolt and in the middle of the sub-structure where reflective films were attached to minimize thenoise from Point 1 to Point 4 by the four LDVs. A frequency rangebetween 100 Hz and 100 kHz, which has been used in typical pyr-oshock measurements, was set up using the bandpass filter in theLDV controller. The signal acquisition conditions are shown inTable 1.

2.1. Measurement of pyroshock wave reduction.

Before confirming the shock wave reduction effect by the insu-lator, the signal-to-noise ratio (SNR) verification of pyroshock mea-sured by LDV was carried out. For this purpose, the measuredsignal is classified into the signal before the trigger (Noise) andthe signal after the trigger (Signal) based on the trigger occurrencetiming as shown in Fig. 3. Then, the standard deviation of the noisepart and the geometric mean of the signal peak-to-peak are substi-tuted into Eq. (1) and quantified as [dB].

SNR dBð Þ ¼ 20 � log GMsignal PtP

STDnoise

� �ð1Þ

SNR ¼ Signal to Noise Ratio

GM ¼ Geometric mean

STD ¼ Standard dev iation

PtP ¼ Peak to Peak

erimental model for the vehicle equipment bay structure and the electronic device

Fig. 2. Measurement setup of an initiator-induced pyroshock on the experimental model using the four LDVs.

Table 1Data acquisition condition for pyroshock measurement.

Shock source CF Fs (MHz) Filter (kHz) Number of samples CFmr (m/s/V) AdB (dB)

Initiator 1.42 1 0.1�100 60,000 50 0

CF: Calibration factor of LDV sensor.AdB: Attenuation factor of attenuator.CFmr: Measurement unit of LDV sensor.

Fig. 3. Signal-to-noise ratio of pyroshock at point 1, (a) Close-up of the noise section.

304 B.-H. Jung et al. /Measurement 137 (2019) 302–311

Table 2 shows the SNR of the measured signals at all sensingpoints in each case. The average SNR is 150.4 dB (Standard devia-tion, STD: 7.57) for steel mount - steel mount case, 128.5 dB(STD: 4.04) for rubber mount - steel mount case, and 116.2 dB(STD: 3.96) for steel mount – rubber mount case. From theseresults, it can be seen that the pyroshock was measured at a signif-icantly high SNR.

In addition, the precision of the measurement [18] system canbe confirmed through the SNR distribution at each point in a speci-

fic experimental environment. In other words, the precision of thepyroshock measurement system is verified through the STD of theSNR at each measurement point in each experimental environ-ment. Table 2 shows that SNR (low STD) is similar for each exper-imental environment. This indicates that the pyroshockmeasurement system has high precision.

To study the effect of the mounting materials to reduce theeffects of the pyroshock wave, the three specimens were preparedas shown in Fig. 4. As shown in Fig. 4(a), the first experiment was

Table 2Signal-to-noise ratio of pyroshock at each points.

SNR [dB] Point 1 Point 2 Point 3 Point 4

Steel mount – steel mount 151.9 142.7 146.8 160.3Rubber mount – steel mount 126.0 129.3 124.8 133.8Steel mount – rubber mount 121.0 116.6 115.8 111.3

Fig. 4. Specimen configurations for studying pyroshock reduction with noncontact laser pyroshock measurement: (a) Steel mount – steel mount (b) Rubber mount – steelmount (c) Steel mount – rubber mount.

B.-H. Jung et al. /Measurement 137 (2019) 302–311 305

configured with steel mounts alone at four different points: twosteel mounts supported the Al-alloy panel for mounting the elec-tronic devices, a single steel mount alone, and a steel mount forthe initiator. As shown in Fig. 4(b), three butyl rubber mounts thatsupported the sub-structure replaced the steel mounts: the panelfor electronic device had two mounts, and one mount was by itself.Butyl rubber has been reported to reduce the effect of an impactwave, for example, in Ref. [19]. As shown in Fig. 4(c), a butyl rubbermount replaced the steel mount for the initiator, and the mountsfor the sub-structure were still steel mounts.

2.1.1. Pyroshock wave measurement results for the insulationfasteners

Fig. 5(a)–(c) shows the actual measurement configurations ofthe schematics depicted in Fig. 4(a)–(c), respectively. The blacksignals in Figs. 6(a)–8(a) represent an actual pyroshock signal over

time measured at each point on the propagation path, and the backsignals in Figs. 6(b)–8(b) represents SRSs from Point 1 to Point 4.The measured signals indicated that the amplitude of the shockwaves in configurations that contained a rubber mount were moreattenuated than with a steel mount – steel mount configuration.The blue signals are laser-simulated pyroshock signals, which arediscussed in Section 3.

2.1.2. Quantitative shock wave reduction with the rubber mountTable 3 compares the peak-to-peak (PTP) acceleration values at

each point in each experiment. The average PTP reduction rate was69% when the device was mounted with rubber mounts and theinitiator with a steel mount, and 79% when the device wasmounted with steel mounts and the initiator with a rubber mount,compared to mounting both with steel mounts.

Fig. 5. Pyroshock wave measurement setup with one initiator and four LDVs, (a) Steel mount – steel mount, (b) Rubber mount – steel mount, (c) Steel mount – rubber mount.

Fig. 6. Comparison of an actual pyroshock and a laser-simulated pyroshock for the steel mount – steel mount configuration: (a) Time domain signal (b) Shock responsespectrum (SRS).

306 B.-H. Jung et al. /Measurement 137 (2019) 302–311

3. Laser-induced shock wave measurement of the shock wavereduction structure using insulators

3.1. Laser shock scanning measurements

Fig. 9(a) shows the laser shock scanning measurement setupusing the same specimen configurations. As shown in Figs. 9(b)and 10, the differences from measuring an actual pyroshock arethat the initiator is replaced with a PZT sensor and the laser shockscanning is done in the area of interest. The PZT location will workas the shock source in this reciprocal setup and the shock propaga-tion video is generated with laser shock scanning instead of mea-surements at the four points. A Q-switched laser was used togenerate the laser shock wave at a point and this shock generationmechanism was repeated during the laser beam scanning by alaser mirror scanner. The laser scanning data acquired by the PZT

sensor was filtered through a programmable filter. The detailedsignal acquisition conditions are shown in Table 4.

3.1.1. Pyroshock wave simulation algorithmThree different laser shock wave based simulation were carried

out to visualize the pyroshock wave reduction by the insulators inthe same region of interest as that used for the actual pyroshockwave, as shown in Fig. 10. The actual pyroshock in the region ofinterest could be simulated by the laser shock wave simulationalgorithm. Various laser shock simulation algorithms have beendeveloped in Refs. 14, 20, and 21. Only the maximum response oflaser shock wave and the pyroshock wave is matched in the algo-rithm in Ref. 20, so simulation is possible in the local frequencydomain and limited over the entire frequency domain. The pyr-oshock wave is assumed to be attenuated only by distance in thealgorithm in Ref. 21, but a pyroshock wave is greatly affected not

Fig. 7. Comparison of an actual pyroshock and a laser-simulated pyroshock in the rubber mount – steel mount configuration (a) Time domain signals (b) SRSs.

Fig. 8. Comparison of an actual pyroshock and a laser-simulated pyroshock in the steel mount – rubber mount configuration: (a) Time domain signals (b) SRSs.

B.-H. Jung et al. /Measurement 137 (2019) 302–311 307

only by distance but also by geometric shape, so a highly similarsimulation is difficult to obtain in a complex structure. The similar-ity of the simulations in these experiments that were simulated bythe algorithms in Refs. 14, 20, and 21 by the peak-to-peak acceler-ation difference (PAD) and the mean acceleration difference (MAD)are shown in Table 5 [22].

PAD %ð Þ ¼ PTPPyro � PTPSimul

PTPPyro

��������� 100 ð2Þ

MAD %ð Þ ¼ 1z

Xz

i¼1

SRSPyro f ni

� �� SRSSimulðf ni

ÞSRSPyroðf ni

Þ

������������� 100 ð3Þ

where z is number of the natural SRS frequency.The algorithm in Ref. 14 showed the best simulation results, so

we simulated the pyroshock with that algorithm in this study.Fig. 11 is a flow chart of the laser shock based pyroshock simula-tion algorithm of Ref. 14. The algorithm comprises 5 steps. In the

Fig. 9. Laser shock scanning measurement setup to simulate an initiator induced pyroshock on an AL 6061 plate with sub-structure.

Table 3Peak-to-peak acceleration results for each experiment.

Experiment Location Peak-to-peak acceleration [g] Shock reduction rate [%]

AL6061 plate specimen & initiator Steel mount – steel mount Point 1 404.48 –Point 2 317.54 –Point 3 315.72 –Point 4 508.72 –

Rubber mount – steel mount Point 1 113.11 72.04Point 2 124.28 60.86Point 3 102.16 67.64Point 4 119.58 76.49

Steel mount – rubber mount Point 1 95.07 76.50Point 2 62.87 80.20Point 3 72.29 77.10Point 4 82.85 83.71

Fig. 10. Specimen configurations in the laser shock scanning measurement; (a) Steel mount – steel mount (b) Rubber mount – steel mount (c) Steel mount – rubber mount.

308 B.-H. Jung et al. /Measurement 137 (2019) 302–311

Table 4Data acquisition conditions for laser shock scanning measurement.

Shock source Sampling frequency (MHz) Filter (kHz) Number of samples Pulse repetition rate (Hz) Scan (mm)

Laser beam pulse 1 0.1�100 30,000 20 Width = 480Height = 248Interval = 2

Table 5Comparison of the results processed by the simulation algorithms in Refs. 14, 20, and 21 for the experiments in this study.

Simulation Difference (average) Steel mount – steel mount Rubber mount – steel mount Steel mount – rubber mount

AL6061 plate specimen & initiator Ref. [14] MAD (%) 20.51 24.74 22.04PAD (%) 11.47 23.13 8.69

Ref. [20] MAD (%) 56.04 50.94 46.16PAD (%) 10.56 13.87 20.82

Ref. [21] MAD (%) Could not be simulatedPAD (%)

Fig. 11. Flow chart of the laser shock-based pyroshock simulation [14].

B.-H. Jung et al. /Measurement 137 (2019) 302–311 309

first step, the pyroshock signal is measured using LDV sensors. Inthe second step, a laser shock signal generated by a Q-switchedlaser is acquired via a PZT sensor. In the third step, the shockresponse spectrum (SRS) is predicted by adjusting the characteris-tic difference between the laser shock and the pyroshock. In thefourth step, a signal decomposition and synthesis method is usedto simulate the time domain signal of the pyroshock by using thedata for the predicted SRS to adjust the laser shock data. In the fifthstep, the simulated pyroshock data is used to generate the pyr-oshock wave propagation imaging (PWPI) and shock responsespectral imaging (SRSI) videos [14].

For this simulation experiment, the gain from step 3 for SRSprediction was obtained with the actual pyroshock wave signalsat Points 2, and 3 and the laser shock wave signals at the samepoints. The gain was applied to the laser shock wave signals in

the scanning area including Point 1 and Point 4, so that the pyr-oshock wave propagation in the region of interest could be simu-lated. The simulation similarity was verified at Point 1 and Point4 of each experiment via PAD and MAD as shown in Figs. 6–8

3.1.2. Laser simulation of the pyroshock wave reductionFig. 10 shows the laser shock wave measurement environment,

which is same as in Fig. 5 except that the pyro-device was replacedwith a PZT sensor. Because of the reciprocal setup of the lasershock wave propagation imager in Fig. 9(a), the sensor locationacts as a shock source. Each blue signal in Figs. 6(a)–8(a) representsa time domain signal of a laser-simulated pyroshock wave and inFigs. 6(b)–8(b) represent an SRS of simulated pyroshock wavesignals.

Fig. 12. Pyroshock reduction study using shock insulation materials based on the pyroshock simulation video results: Freeze frames of PWPI at 930 ls & SRSI at 10 kHz, (a)Steel mount – steel mount (Electronic annex 1: PWPI and Electronic annex 2: SRSI), (b) Rubber mount – steel mount (Electronic annex 3: PWPI and Electronic annex 4: SRSI),(c) Steel mount – rubber mount (Electronic annex 5: PWPI and Electronic annex 6: SRSI).

Table 6Similarity between an actual pyroshock and a laser-simulated pyroshock for eachexperiment.

Experiment Simulation results(%)

AL6061 plate specimen &Initiator

Steel mount – steelmount

MADaverage 20.51PADaverage 11.47

Rubber mount – steelmount

MADaverage 24.74PADaverage 23.13

Steel mount – rubbermount

MADaverage 22.04PADaverage 8.69

310 B.-H. Jung et al. /Measurement 137 (2019) 302–311

In the first simulation in the configuration described in Fig. 10(a), the laser simulation at Point 2 shows 6.67% PAD and 16.44%MAD, and the laser simulation at Point 3 shows 18.11% PAD and17.65% MAD. Fig. 12(a) is a snapshot of PWPI and SRSI for theentire region of interest generated by applying the gain acquiredin the training process to the residual laser shock waves. The lasersimulation at the verification points was 15.71% PAD and 20.80%MAD at Point 1, and 5.40% PAD and 27.13% MAD at Point 4.

In the second simulation in the configuration described inFig. 10(b), the laser simulation at Point 2 shows 29.83% PAD and22.12% MAD and the laser simulation at Point 3 shows 13.97%PAD and 14.02% MAD. Fig. 12(b) includes freeze frames of PWPIand SRSI videos for the entire region of interest. Throughout thefreeze frames, the pyroshock wave propagated to the main-structure does not change, but the pyroshock wave propagated tothe sub-structure is significantly attenuated. The simulationresults at the verification points are 23.28% PAD and 30.43% MADat Point 1, and 25.45% PAD and 32.37% MAD at Point 4.

In the third simulation with the configuration described inFig. 10(c), the laser simulation at Point 2 shows 14.35% PAD and28.14% MAD and the laser simulation at Point 3 shows 10.78%PAD and 16.97% MAD. Fig. 12(c) includes freeze frames of PWPIand SRSI videos for the entire region of interest. The freeze framesin the time and spectral domains shows that the pyroshock isattenuated during the transfer to the main-structure, so that thepyroshock wave transmitted to the sub-structure is also attenu-ated. The simulation results at the verification points were14.35% PAD and 28.14% MAD at Point 1, and 12.61% PAD and23.99% MAD at Point 4. Thus, a laser-simulated pyroshock wavecan represent an actual pyroshock wave in the region of interestwith an adequate simulation similarity.

3.1.3. Pyroshock wave visualization and a simulation summaryTable 6 summarizes the average PAD and MAD from Point 1 to

Point 4 for each experiment. Fig. 12(a)-(c) show the PWPI and SRSIsnapshot of the simulated pyroshock wave in each experiment. The

amplitude of the shock wave transmitted to the sub-structure isdamped by the insulator in the pyroshock wave propagation pathor between the pyro-device and the main structure.

4. Conclusion

This paper demonstrated that a laser shock induced by a Q-switched laser is appropriate for an analysis of shock reductionand visualization of the effect of an insulator in a complex struc-ture. Three experiments were carried out. The first experimentwas configured with steel mounts alone. In the second, a steelmount was fastened between the pyro-device and the main struc-ture, and butyl rubber mounts were placed in the path on whichthe pyroshock wave was transferred to the sub-structure. In thethird, a butyl rubber mount was placed between the pyro-deviceand the main structure, and steel mounts were placed in the pathof the pyroshock wave to the sub-structure. The pyroshock wavegenerated by initiator was measured by four LDVs on the tops ofthree bolts fastened at 210 mm intervals from the initiator and atthe in the middle of the sub-structure. Three laser shock basedsimulations were also conducted to evaluate and visualize the pyr-oshock wave reduction. In the simulations, a laser shock wave wasinduced in the region of interest by a Q-switched laser with a laser

B.-H. Jung et al. /Measurement 137 (2019) 302–311 311

mirror scanner. The laser shock wave data was acquired by attach-ing a PZT sensor to the location where the pyro-device was fas-tened in the actual pyroshock wave measurement test. Theacquired laser shock signals predicted the actual pyroshock waveby means of a simulation algorithm that used iterative decomposi-tion and synthesis and considered stop band effects. The pyroshockwave reduction could be quantitated through the PWPI and SRSIvideos by the laser shock based simulation in each experiment.The average pyroshock wave reduction was 69% for the rubbermount – steel mount experiment and 79% in the steel mount – rub-ber mount experiment. The PAD and MAD at the verification points(i.e., Point 1 and Point 4) in each experiment indicated that thelaser-simulated pyroshock waves showed excellent similarity tothe actual pyroshock waves over the entire region of interest.

Acknowledgments

This research was financially supported by the Ministry ofTrade, Industry, and Energy (MOTIE), Republic of Korea (SourthKorea), under the ‘‘Commercializing fuel cell electric vehicle com-ponent industry and R&D Support Program” (reference numberR0006462) supervised by the Korea institute for Advancement ofTechnology (KIAT) and by the research grant (PMD) of the Agencyfor Defense Development and Defense Acquisition ProgramAdministration of the Korea government.

Appendix A. Supplementary data

Supplementary data to this article can be found online athttps://doi.org/10.1016/j.measurement.2019.01.046.

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