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uncertainty in the values of any kinematic variable depends on the error in the 3D coordinates
that is propagated in any computed value determined from their combination. The propagated
error can be minimized by maximizing the accuracy and precision of the OSS. The accuracy of the
3D coordinates depends on the digitizing procedure and the reconstruction technique, whereas
precision depends on the digitizing procedure (Challis, 2008). A ‘Gage Linearity and Bias’ test was
performed to determine the accuracy of the measurement system throughout the expected range
of the measurements, and a R&R study to determine system’s precision.
By placing the cameras on high tripods it was able to capture the surrounding area of the participants
ensuring good visibility of the tracking markers, since handling box, adjustable platforms, other
equipments, etc., could obstruct the camera’s view of the markers. By increasing the angle between
the cameras the captured reflections are also diminished. The appropiate set-up of the cameras
minimize the dead space in each camera’s field of view and maximize system’s spatial resolution.
Spatial resolution is affected by the capture volume and by camera position in relation to the
capture volume. For an 2.0×2.0×2.0 m3 measurement volume, the diameter of the passive marker
needs to cover about
of the field of view, i.e.,—for a field of view of 2 m the marker diameter
needs to be around 10 mm. In the current study the reflective markers of 20 mm diameter was
Inter-marker distance measurement test was performed where four reflective markers attached
on a bar on known positions and placed at eight different positions throught the established
measurement volume and were recorded for 20 sec at sampling rate of 60 Hz. The motion analysis
system was calibrated once at the beginning of the procedure. The 3D coordinates data were
introduced to the Visual3D (C-Motion) software. The mean value of each sample was computed.
Then, the distance between the markers was computed from their means value coordinate.
Statistical Analysis
The Grubbs’ test was used to check for outliers. A regression line was fitted on the bias against the
reference values and statistics for the linearity and accuracy of the measurement system computed.
It is assumed that biases are corrected by the calibration procedure, therefore, only the uncertainties
of the correction are presented. The range over which the measurements varies due to normal
variation computed by the R&R study. To determine the repeatability of the measurement system
a R&R study carried on. The Minitab 17 statistical software was used for the analysis.
Results Reports
The precision of the measurement system was of 0.0012108 mm. The maximum absolute value of
the bias, the linearity value, and the repeatability standard deviation were used for calculating the
expanded uncertainty of the measurement system. Due to the anisotropic distribution of the error
in the measurement volume, the final measurements took place at the centre of the measurement
volume where the error was minimized. Due to the sensitivity of the error to the calibration