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B.3.2.
B.3.2.1.
Measurement Process Overview
Procedure
The force measurement system was switched on 15 min prior to the measurement process to reach
thermal stability and was zeroed at the begin of the procedure. By the help of a millimetre grid
that was placed on the top plate of the force platform, the distance between ξ = 11 consecutive
point was fixed at 10 mm. Therefore, the measurement range was set at 100 mm. Furthermore,
all the points were laying on a line which is parallel to the X-axis4 and crosses the geometrical
center of the top plate of the force platform. Moreover, all the points were contained within the
area ±50 mm from the geometrical center of the top plate of the force platform in both the X- and
Y-axis.
A dead load (M) was displaced on the top plate of the force platform from the point P0 to the
point P10 gradually in 10 consecutive stages that corresponded to ξ fixed points and then was
returned back from the point P10 to the point P0 in a similar manner. This operation was repeated
two times. During the displacements of the dead load M from the point P0 to the point P10 and
back to the P0 , at each one of the intermediate points where the load M was placed gradually, the
COP was measured for 10 repeated times in an interval of 30 sec between each repetition. Each
repeated COP data was collected for a time period of 5 sec at sampling rate of 30 Hz under a set
of repeatability conditions of measurement. There are k = 10 repeated COP samples comprised of
150 data each, for each one of the ξ fixed points P0→10 , replicated r = 4 times. For each point the
mean value and standard deviation for each repeated COP sample have been computed, as well
as the overall mean value comprised of all the data of the k = 10 repeated samples. In total four
overall means have been calculated at each point, two in the P0 → P10 direction and two in the
P10 → P0 . The distance of each point from P0 was calculated (Fig. B.5a).
To ensure that the dead load was placed accurately on the coordinates indicated by the millimetre
grid a point loader was used. The dead load M was placed on a metallic platen that in turns was
placed above two parallel adjustable metal bars sustained on the ground away from the force
platform being evaluated. A stylus of 5 mm diameter was stuck to the surface of the platen at
its geometrical center in order to transmit the load on the top plate of the force platform. This
procedure was repeated for the four different reference loads (M1 = 98 N, M2 = 196 N, M3 =
294 N) (Telju, Spain) for the two force platforms used in the study. The same operator conducted
the whole procedure.
B.3.3.
Definition of the Measurement Model
The measured distance between the two fixed points is
DCOP = ∆COP + "COP
where "COP is the total error associated with the measurement.
Supposing that we have m output signals (measured distances) that are linearly proportional
to the input signal (nominal distances), a regression line can be fitted to the data (Fig. B.5b).
4
and Y-axis
150