Structural Health Monitoring Using Smart Sensors - ideals ...

percent noise. The flexibility matrix is estimated both before and after damage is
introduced, and then the DLV method is applied to these matrices. The accumulated stress
calculated in the DLV method is an indication of damage. The stress in a damaged
member should be zero. The accumulated stress in the damaged element is plotted on
Figure 5.9, together with the flexibility matrix estimation error. The stress is indeed zero
for 0 percent data loss. As the amount of data loss increases, the accumulated stress
becomes larger. A stress value around 0.1 is considered small, indicating that the element
is damaged. The estimation error for these quantities due to noise is shown in Figure 5.10
for comparison. From these figures, a loss of 0.5 percent data is considered to be
equivalent to 5 to 10 percent of measurement noise.
From this analysis, data loss is found to introduce error into the modal analysis; the
error affects the subsequent analysis. As data loss increase, the quality of data degrades.
Because of the statistical approach (i.e., averaging) associated with the PSD and the CSD
estimation, a certain amount of data loss can be accommodated. A loss of 0.5 percent data
might be acceptable, considering that a corresponding 5 to 10 percent observation noise is
unexceptional in the monitoring of civil infrastructure.
5.2.2 Packet loss estimation in RF communication
The packet loss rate is experimentally estimated in this section. Together with the
results from the previous section, experimental data are examined to see whether the
packet loss rate is acceptable for SHM applications.
1. Experimental setup
Eight Imote2s, one programming board, and a PC were configured to perform this
experiment. One of the Imote2s is programmed as the base station. Another Imote2 works
as the sender. The other six Imote2s are configured to be receivers. A java program
running on the PC reads parameters from an input file and sends commands to the base
station node. The base station receives information about the number of receivers, node
||F-F0||F/||F0||F
0.11
0.1
0.09
0.08
0.07
mean
mean ± RMS
0.06
0 2 4 6
Noise Level (%)
8 10
(a)
61
Normalized accumulated stress
0.2
0.15
0.1
0.05
0
mean
mean ± RMS
0 2 4 6 8 10
Noise Level (%)
Figure 5.10. Noise level dependency of (a) flexibility matrix estimation error; and (b)
normalized accumulated stress in the damaged element.
(b)