Various human body vibration measurements and assessments are defined in the ISO2631, ISO5349, ISO6954 standards and also in the EEC vibration directive 2002/44/EC. These assessments are based on the analysis of acceleration data that has been weighted (filtered) with appropriate vibration weighting values. The data must be in units of m/sec2.
The vibration weighting values are defined in ISO8041:2005. Basically they are a set of filters that must be applied to the acceleration data in order to make evaluations and assessments on the effects of vibration on humans. They are defined as a set of weighting classes. Each class is associated with particular measurement directions, measurement positions and assessment type.
A selection of defined weighting classes are shown in Table 1. The directions are orientated such that the x direction is positive back to chest, the y direction is positive right side to left side and the z direction is positive from feet to head.
|Wb||Weighting for vertical whole body vibration (seat z direction): ISO2631-4|
|Wc||Weighting for vertical whole body vibration (seat back z direction): ISO2631-4|
|Wd||Weighting for horizontal whole body vibration (seat surface x and y directions): ISO2631-1|
|We||Weighting for rotational whole body vibration: ISO2631-1|
|Wf||Weighting for vertical whole body vibration (z direction motion sickness): ISO2631-1|
|Wh||Weighting for hand arm vibration (all directions): ISO5349-1|
|Wk||Weighting for vertical whole body vibration (z direction): ISO2631-1|
|Wj||Weighting for vertical head vibration (x direction recumbent ): ISO2631-1|
|Wm||Weighting for whole body vibration in buildings (all directions): ISO2631-2 and ship vibration measurements (all directions): ISO6954|
The vibration weighting is applied by initially pre-filtering the data with a band pass filter between a lower frequency, f1 and an upper frequency, f2 as defined in Table 2. The appropriate weighting functions are then applied to the resulting band filtered data.
|Vibration Weight Class||Lower Frequency f1 Hz||Upper Frequency f2 Hz||ISO8041 Compliant Minimum Sample Rate (Samples/sec)|
Strict ISO8041 Compliance
ISO8041:2005 specifies that the acceleration data must be sampled at an adequate rate for the appropriate vibration weight class filter to be applied. In order to achieve strict ISO8041:2005 compliance the minimum adequate sample rate must be at least nine times the relevant low pass pre-filter cut off frequency, f2. As shown in Table 2 the minimum ISO8041 compliant sample rate is 900 samples/second (9*f2 = 9*100) for all vibration weight classes with the exception of class Wf where it is 6 samples/second (9*0.63) and class Wh where it is 11331 samples/second (9*1259).
The DATS software provides a particular function to perform the vibration weighting for a specified vibration weight class. In addition, DATS also provides a package to generate vibration assessment reports compliant with ISO2631-1, ISO2631-2, ISO2631-4, ISO5349 and ISO6954.
An example seat surface acceleration is shown below in Figures 1 and 2. Figure 1 shows the raw acceleration data and Figure 2 shows the acceleration after the appropriate ISO8041 compliant vibration weighting has been applied. In this example the data is seat surface data in the X direction so the appropriate vibration weight class is the Wd filter.
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