|
  
|
||
|
|
||
Measurement modes
The next step is to define the measurement. There are two basic modes of operation. First is the Whole body mode and the second one is Hand arm mode.
Different modes define different Basic filters 
used to simulate human response to the vibrations. Those filters are defined from numerous measurements of natural frequencies of certain parts of the human body.
We can also use the Linear filter to check the measurement chain or the Custom filters. If we use custom filters, the individual Filter 
can be chosen from the list on the right side to do special measurements (for example building vibrations or sea sickness).
I would like to point out that we need to keep in mind the high pass frequency limit of the sensor and the amplifier used. For hand arm mode, the high pass frequency is 6.4 Hz, which is easy for any sensor. For the whole body the frequency limit is 0,4 Hz, where we need to choose the sensor carefully. We can also use higher filters (like 3 Hz), if we know there is no frequency content below this limit. This will help to perform a measurement faster and with less error (lower frequency filters means longer settling times).
The recommended sampling rate of the measurement also depends on the application. For hand arm, the minimum sampling rate is 5 kHz, while for all the others 1 kHz is enough.
Special attention must be paid to the Wf filter for motion sickness (for example on ships) where the frequency limit is only 0,08 Hz. We need a very special sensor to measure this.
With a custom filter, we also need to define a weighting K factor 
. This is a multiplication factor for each axis when calculating the vibration sum.
Modes of operation
Calculated parameters
Next we need to select the calculated parameters on Calculate type section. These parameters can be either Overall values, which mean that we have only one value at the end of the a measurement, and/or interval logged values. If we have interval logged values, the time interval for logging in a contiguous field is defined in sec. For example, if we select to have Interval logging for RMS with 5 second intervals, we will get a new value of RMS after each 5 second interval. After that, the value is reset and the calculation is started again over.
We have several parameters to calculate. In short, the "root means square" (abbreviated RMS) value is a statistical measure of the magnitude of a weighted signal, Peak is the maximum deviation of the signal from the zero line, Crest is the ratio between the peak and rms, VDV is the fourth power vibration dose value, MSDV is the motion sickness dose value while the MTVV is the maximum transient vibration value, calculated in one second intervals.
Parameters to calculate
Each value is calculated for each axis individually while the RMS, MSDV, VDV and MTVV are also calculated for the sum of all three axes. These values are enough to evaluate the human vibration exposure according to ISO 2631 and ISO 8041.
The next output is the weighted raw channel. This is the full speed time signal weighted with a chosen filter. We can use those channels for the calculation of the FFT or CPB spectrums.
al and D are the values based on ISO 2631-5 which describe the calculations and the limits for lumbar spine response to vibrations. The base for this standard is that the professional drivers of buses or trucks are exposed to vibrations when driving on rough roads or over bumps. Multiple shocks cause transient pressure changes at the lumbar vertebral endplates which can causes damage after years of driving.
The al is the lumbar spine response from excitation measured in all three directions. The D value is the acceleration dose, measured from the lumbar spine response. These values are enough to evaluate the human vibration exposure according to ISO 2631-5.
