WBV v1 Technical information

IntroductionLong term exposure to high amplitude whole-body vibration is associated with the subsequent development of back pain1-3. ISO2631.1 provides guidance regarding the measurement of whole-body vibration and interpretation of the results4. However, the cost and complexity of commercially available devices for measuring whole-body vibration is a barrier to the collection of the systematic data required for the management of occupational whole-body vibration exposures in dynamic workplaces. The aim of the WBV application is to allow the use of an iPod Touch to estimate whole-body vibration amplitude as part of a whole-body vibration management program.

Accelerometer specificationsThe 5th generation iPod Touch (Apple Inc., Cupertino, CA) (123 x 59 x 6 mm, 88g) incorporates a factory calibrated LIS331DLH (MEMS type) accelerometer (STMicroelectronics, Geneva, Switzerland) providing three dimensional 16 bit data output configured to a range of +/- 2g. The maximum sampling rate which may be obtained within the iOS operating system is restricted to a nominal 100 Hz (inter-sample interval of 0.01s).

Sampling rate & variability

Selecting a 100 Hz sampling rate results in a bi-modal distribution of inter-sample intervals.

A nominal 99 Hz sampling rate results in an actual sampling rate of the order of 89 Hz. Typical inter-sample interval variability is illustrated below. Sampling rate limitations are likely to be the principal source of any inaccuracies in the estimation of whole body vibration in comparison to gold standard devices which sample at rates of the order of 8 kHz.

Prop

ortio

n (%

)

0

10

20

30

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Inter-sample interval (s)0 0.002 0.004 0.006 0.008 0.010 0.012 0.014 0.016 0.018 0.020 0.022 0.024

WBV v1.0 Technical Specifications

Robin Burgess-‐Limerick, 2014 WBV_tech_spec_v1.1 ergonomics.uq.edu.au/WBV

Calibration dataAn example of Z direction accelerometer data gathered from an iPod Touch placed on the SV111 calibrator is provided below. These data provide an approximation of the 1 m/s2 r.m.s. vibration at 15.9 Hz provided by the calibrator within the limitations of the sampling rate and noise inherent to the device. The average amplitude (r.m.s.) of the acceleration data collected by five iPod touch devices was 0.962 m/s2, an average constant error of -0.038 m/s2.

Acce

lera

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(g)

−0.9

−1.0

−1.1

Time (s)0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6 2.8 3.0

The spectral density of data obtained from an iPod Touch placed on the calibrator at 15.92Hz is also illustrated. While the dominant frequency is faithfully reproduced, there is evidence of aliasing (small peaks at 3Hz and 29Hz) that arises as a consequence of the relatively low sampling rate.

Frequency weighting ISO2631.1 provides frequency weightings Wd and Wk to be applied to horizontal (X & Y) and vertical (Z) accelerations respectively.

WBV applies these frequency weightings to the raw accelerometer data, adapting Matlab code provided by Irvine5.

0.5 Hz resolution

Spec

tral d

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m/s

/s]2

/Hz)

0

0.5

1.0

1.5

2.0

2.5

3.0

Frequency (Hz)0 5 10 15 20 25 30 35 40



ISO2631.1 Frequency weightings

0.125 0.25 0.5 1 2 4 8 16 32 640.0

0.2

0.4

0.6

0.8

1.0

Frequency

Wei

ghtin

g

Wk (Z)Wd (X&Y)

The consequences of this frequency weighting are illustrated below. The lower trace is raw Z direction accelerometer data collected at a nominal 99 Hz sample rate (actual rate 89.5 Hz) while driving a light vehicle. The results of applying the Wk frequency weighting are illustrated in the upper trace. The weighting acts as a high pass filter in that very low frequency components of the accelerations are removed.

RMS & VDV calculationISO2631.1 defines two primary measures of whole body vibration amplitude. The root mean squared amplitude (aw, units m/s2) is defined by:

The Vibration Dose Value (VDV, units m/s1.75) is a fourth power measure which is consequently more sensitive to high amplitude values (jolts/jars). VDV is defined as:

The VDV measure is cumulative and increases with the duration of the measurement. To allow comparison between trials of different duration the VDV is expressed as VDV(n) where n refers to an n hour exposure, eg., VDV(8) normalises the trial to an 8 hour duration. VDV(n) = VDV x (n / trial duration in hours)1/4

Accuracy in comparison with Gold StandardsThe accuracy of the WBV application used in conjunction with an iPod Touch in comparison to gold standard devices is the subject of ongoing investigation. However, preliminary indications are promising6. Two investigations have been completed in which raw data were gathered from iPod Touch devices simultaneously with measurements by gold standard devices and the iPod data were subsequently analysed via Matlab.

Light vehicle accuracy data

Three dimensional accelerometer data were gathered simultaneously from 5th generation iPod touch devices placed under the seat pad accelerometer of a gold standard vibration measurement device placed on the driver’s seat while four different light vehicles were driven in a range of environments (highway, suburban streets, gravel roads & off road). Two gold standard measurement devices were employed, SV106 (Svantek Sp., Warsaw, Poland) and a Type 4447 Human Vibration Analyzer (Brüel & Kjær Sound & Vibration Measurement A/S, Nærum, Denmark). Forty-two trials ranging in duration from 10 to 55 minutes (median = 15 minutes) were recorded.



IS0 2631=1:1997(E) @ IS0

The weighted r.m.s. acceleration is expressed in metres per second squared (m/s*) for translational vibration and radians per second squared (rad/s*) for rotational vibration. The weighted r.m.s. acceleration shall be calculated in accordance with the following equation or its equivalents in the frequency domain

1

1

where

a,,&) is the weighted acceleration (translational or rotational) as a function of time second squared (m/s*) or radians per second squared (r-ad/s*), respectively;

(time history ,), in metres per

T is the duration of the measurement, in seconds.

Frequency-weighting curves recommended and/or used for the various directions and their applications are listed in tables 1 and 2 and discussed in the following subclauses and in annexes B, C and D. Numerical values of the weighting curves are given in tables 3 and 4 and exact definitions are given in annex A.

6.2 Applicability of the basic evaluation method

6.2.1 Definition of crest factor

For the purposes of this part of IS0 2631 the crest factor is defined as the modulus of the ratio of the maximum instantaneous peak value of the frequency-weighted acceleration signal to its r.m.s. value. The peak value shall be determined over the duration of measurement (see 5.5) i.e. the time period T used for the integration of the r.m.s. value (see 6.1).

NOTE - The crest factor does not necessarily indicate the severity of vibration (see 6.3).

6.2.2 Applicability of the basic evaluation method for vibration with high crest factors

The crest factor may be used to investigate if the basic evaluation method is suitable for describing the severity of the vibration in relation to its effects on human beings For vibration with crest factors below or equal to 9, the basic evaluation method is normally sufficient. Subclause 6.3 defines methods applicable when the basic evaluation method is not sufficient.

NOTE -. For certain types of vibrations, especially those containing occasional shocks, the basic evaluation method may underestimate the severity with respect to discomfort even when the crest factor is not greater than 9. In cases of doubt it is therefore recommended to use and report additional evaluations also for crest factors less than or equal to 9 according to 6.3.

Subclause 6.3.3 indicates ratios between magnitudes evaluated by the additional methods and the basic method, above which it is recommended to use one of the additional methods, as a further basis for judgement of the influence on human beings.

6.3 Additional evaluation of vibration when the basic evaluation method is not sufficient

In cases where the basic evaluation method may underestimate the effects of vibration (high crest factors occasional shocks, transient vibration), one of the alternative measures described below should also be determined - the running r.m.s. or the fourth power vibration dose value.

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6.3.1 The running r.m.s. method

The running r.m.s. evaluation method takes into account occasional shocks and transient vibration by use of a short integration time constant. The vibration magnitude is defined as a maximum transient vibration value (MTW9, given as the maximum in time of a&o), defined by:

aw(to) = 2

01 2 GEvv t dt

fo - a

where

aw(t) is the instantaneous frequency-weighted acceleration;

z is the integration time for running averaging;

t

t0

is the time (integration variable);

is the time of observation (instantaneous time).

. . 0 (29

This formula defining a linear integration can be approximated by an exponential integration as defined in IS0 8041:

The d ifference in result is very smal I for application to shocks of a short duration compared to z, and somewhat larger (up to 30 %) when a pplied to s hocks and trans ents of Ion ger d uratio n.

+ 7 [a,(r)]* exp[y] & ---oo

2

. . .

The maximum transient vibration value, MTW, is defined as

MlW = max [a,&)]

(39

. . . (49

i.e. the highest magnitude of a,,&) read during the measurement period (T in 6.1).

It is recommended to use z= 1 s in measuring MTW (corresponding to an integration time constant, “slow”, in sound level meters).

6.3.2 The fourth power vibration dose method

The fourth power vibration dose method is more sensitive to peaks than the basic evaluation method by using the fourth power instead of the second power of the acceleration time history as the basis for averaging. The fourth power vibration dose value (VDV) in metres per second to the power I,75 (m/sl175), or in radians per second to the power I,75 (rad/s 1,759, is defined as:

VDV= . . . (59

a, (t9 is the instantaneous frequency-weighted acceleration;

is the duration of measurement (see 6.1).

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Outcome measures derived from frequency weighted accelerometer data for vertical direction gathered simultaneously from an ipod touch and gold standard vibration measurement devices are illustrated below. The mean absolute error across the 42 trials was 0.019 m/s2 for the vertical direction and slightly higher for fore-aft and side-to-side accelerations (0.02 m/s2 & 0.015 m/s2 respectively).

Heavy vehicle accuracy data

Three dimensional accelerometer data were gathered simultaneously from 5th generation iPod touch devices placed under the seat pad accelerometer of a gold standard vibration measurement device placed on the driver’s seat of heavy mining equipment whilst operators performed their normal duties.

Fifty-eight pairs of measurements of measurements were obtained. Twenty-six pairs of measurements were simultaneously obtained via Larson Davis Human Vibration Meter 100 (PCB Piezotronics, Inc, Depew, New York, USA) and a 5th generation iPod Touch during operation of a range of heavy equipment (Dozers, Graders, Excavators, Loaders, Haul trucks) at two surface coal mines. A further thirty-two pairs of measurements were obtained from SV106 (Svantek Sp., Warsaw, Poland) and a 5th generation iPod Touch from a range of heavy equipment (Haul trucks, Excavator, Loaders) in operation at a third surface coal mine. Measurement duration ranged from 12 minutes to 54 minutes (median = 29 minutes).

Whole-body vibration amplitudes (r.m.s) are illustrated for X, Y & Z directions derived from frequency weighted iPod Touch accelerometer data as a function of simultaneous measurements taken via the gold standard device. The Bland-Altman plot does not indicate any consistent relationship between constant error and acceleration amplitude.

0.0 0.5 1.0 1.50.0

0.5

1.0

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Gold Standard r.m.s (m/s2)

iPod

r.m

.s (m

/s2 )

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.m.s

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The table below provides the mean constant error (CE) for the fifty-eight vibration measurements made in each direction, the standard deviation (SD) of these measurements, and the resulting 95% confidence limits of agreement. The results suggest that accelerometer data gathered from an iPod Touch are able to be used to measure whole-body vibration amplitude with 95% confidence of +/- 0.09 m/s2 r.m.s. or better, depending on the direction of interest. The accuracy was better in the vertical direction (usually the dominant vibration direction) and the limits of agreement were calculated to be +/- 0.063 m/s2 r.m.s.

X r.m.s. (m/s2) Y r.m.s. (m/s2) Z r.m.s. (m/s2)Mean constant error 0.033 0.021 0.005Standard deviation of constant error 0.038 0.046 0.032Lower 95% limit of agreement -0.107 -0.111 -0.068Upper 95% limit of agreement 0.042 0.068 0.058

Interpretation of results with respect to HGCZ

ISO2631.1 provides the figure below in Appendix B. It is suggested that “health effects are likely” for 8 hour exposures greater than 0.9 m/s2 r.m.s., and that “no health effects have been documented” for 8 hour exposures below 0.5 m/s2 r.m.s. The standard suggests that “caution with respect to potential health risks” is indicated for intermediate values - hence the so called Health Guidance Caution Zone (HGCZ).

IS0 2631=1:1997(E) @ IS0

For exposures below the zone, health effects have not been clearly documented and/or objectively observed; in the zone, caution with respect to potential health risks is indicated and above the zone health risks are likely. This

recommendation is mainly based on exposures in the range of 4 h to 8 h as indicated by the shading in figure B.I.

Shorter durations should be treated with extreme caution.

Other studies indicate a time dependence according to the following relationship:

.T1/4 - awl 1 -a,2

.T'/4 . I . b3.2)

This health guidance caution zone is indicated by dotted lines in figure B.1. (The health guidance caution zones for equations (B.1) and (B.2) are the same for durations from 4 h to 8 h for which most occupational observations

exist .)

2 10

iFi G= g 6,3

G ki m w 4 aJ 2

5 2,s

1.6

1

0,63

0,315

025

0,16

I ! ! I I

\

. . \

----LLy--- LqUdl lull \D. II 1

-0.. ._ \

I

‘-.. -._ \ \_

--._ I I

I

10 min 0,s 1 2 4 8 24

Exposure duration, h

Figure B.l - Health guidance caution zones

The r.m.s. value of the frequency-weighted acceleration can be compared with the zone shown in figure B.l at the

duration of the expected daily exposure.

To characterize daily occupational vibration exposure, the 8 h frequency-weighted acceleration a, can be measured

or calculated according to the formula in 6.1 with 8 h as the time period T.

NOTES

1 When the vibration exposure consists of two or more periods of exposure to different magnitudes and durations, the

energy-equivalent vibration magnitude corresponding to the total duration of exposure can be evaluated according to the

following formula:

1

a* wi .q 2

aw,e = c [ 1 c

, s .

T

63)

where

a,,, is the equivalent vibration magnitude (r.m.s. acceleration in m/s*);

a,i is the vibration magnitude (r.m.s. acceleration in m/s*) for exposure duration Ti.

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No explicit guidance is provided in ISO 2631.1 regarding the evaluation of VDV, although it has been generally inferred that the values referred to for eVDV in note 2 of clause B.3.1 may be utilised, that is a lower value of 8.5 and a higher value of 17 for the VDV HGCZ.



0.9 m/s2

0.5 m/s2

As well as calculating and reporting the RMS and VDV amplitude measures describing each trial, the RMS and VDV(8) values are presented graphically with respect to the HGCZ, by default for an 8 hour exposure.

The application also calculates the the exposure duration required to reach the lower limit of the RMS HGCZ for each direction.

The WBV application allows other exposure durations to be nominated (1, 2, 4, 8, 10 or 12 hours). When a new duration (n) is selected, the VDV(n) is recalculated. The lower and upper bounds of the RMS HGCZ are also adjusted according to ISO2631.1 Appendix B, equation B1 which results in the boundaries listed below.

Exposure duration HGCZ lower bound HGCZ upper bound 1 hr 1.4 m/s2 2.5 m/s2

2 hr 1 m/s2 1.8 m/s2 4 hr 0.71 m/s2 1.2 m/s2 8 hr 0.5 m/s2 0.9 m/s2

10 hr 0.45 m/s2 0.8 m/s2

12 hr 0.41 m/s2 0.73 m/s2

A note about “k”ISO2631.1 refers to “multiplying factors” or “k”, which require the horizontal acceleration values to be increased by 40% (kx & ky = 1.4) when calculating the vector sum (VTV, see clause 7.2.2). However, ambiguity exists in that the multiplying factors are not referred to, nor included in the equations, in Annex B which provides additional guidance regarding the health effects of whole-body vibration. Indeed the only equation in ISO 2631.1 in which the “multiplying factors” appear explicitly in relation to health effects is in the definition of VTV, and “k” is not defined in clause 4 “Symbols and subscripts”.

Given this ambiguity, and the lack of any rationale for the use of such multiplying factors, all acceleration values provided by WBV do not include any additional weighting for X or Y directions.



REFERENCES 1. Bernard, B.P. (Ed): Musculoskeletal disorders and workplace factors: a critical review of epidemiologic evidence for work-related disorders of the neck, upper extremities, and low back. DHHS (NIOSH) Publication No. 97-141. US Department of Health and Human Services, National Institute of Occupational Safety and Health, 1997.

2. Bovenzi, M. and C.T.J. Hulshof: An updated review of epidemiologic studies on the relationship between exposure to whole- body vibration and low back pain. Journal of Sound and vibration, 215: 595–611, 1998.

3. Sandover, J.: Dynamic loading as a potential source of low back disorder. Spine, 8, 652-658, 1983.

4. Burgess-Limerick, R: How on earth moving equipment can ISO2631 be used to evaluate WBV exposure? Journal of Safety and Health Research and Practice. 4(2): 13-21, 2012.

5. Irvine, T. : ISO2631 matlab scripts. http://vibrationdata.wordpress.com/2012/10/21/iso-2631-matlab-scripts/, 2012. (Retrieved 24/7/13).

6. Wolfgang, R. & Burgess-Limerick, R. Using consumer electronic devices to estimate whole-body vibration exposure. Journal of Occupational and Environmental Hygiene. (in press)



http://vibrationdata.wordpress.com/2012/10/21/iso-2631-matlab-scripts/




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WBV v1 Technical information