Earthquake Engineering Research - HKU Libraries - The University ...

553
the laboratory and (2) that imposed during numerical differentiation due to time stepping.
High frequencies introduced during testing occurred because the cameras were mounted on the
reaction floor surround the shake table and the shake table itself induces some motion into the
surrounding floor during earthquake simulation. The magnitude of this noise was consistently observed
to be small and outside of the frequencies of interest (> 50 Hz), thus it could easily be filtered in the
frequency domain using a 4 th Order low-pass Butterworth filter.
Double numerical differentiation also creates noise in acceleration traces. In this case, a 10 th Order
low-pass Butterworth filter was applied after numerical differentiation to remove introduced high
frequency noise. Low pass filters of comparable details were also applied to the conventional
accelerometer measurements.
Figure 4 shows a comparison of
acceleration and velocity time
Iristories (absolute) measured from
the 5 th floor (roof level) of the
structure during the Mexico City
event with the roof isolation system
engaged. Accelerations are
compared between the double
differentiated vision-system
positional data and the conventional
accelerometers. Velocities are
compared between the single
differentiated vision system data
and integrated accelerometer
measurements. These comparisons
are very promising, with differences
in maximum acceleration of 0.06g
and differences in maximum
velocity of 6 cm/second observed.
Analysis of other floor levels and
the different experiments showed
that this was the largest difference
between the two systems. However,
_^
o
to
Time (seconds)
FIGURE 4. Comparison of velocity (a) and acceleration (b) time
histories (absolute) at the roof level - Mexico City input motion
with the roof isolation system engaged.
it should be noted that selection of cut-off frequencies during the data processing was an important step
in the evaluation of each case due to the different frequencies induced by the earthquake motions.
SUMMARY
A vision-based motion tracking system was evaluated for its accuracy in monitoring the seismic
demands induced in a 5-story scale moment resisting frame structure. Reasonable comparison between
time-varying positional responses was observed when comparing the vision system with conventional
LVDT measurements. Although the system was calibrated statically to within a 1 mm displacement,
the average difference in maximum displacements was observed as 1.4 mm. Signal processing and
numerical differentiation of the positional data was required for 'calculating acceleration and velocity
time histories from the vision-based (positional) measurements. Two aspects of high frequency noise