Structural Health Monitoring Using Smart Sensors - ideals ...

Correlation function estimate (m 2 /s 4 )
2
4
0 0.1 0.2 0.3 0.4 0.5
Time (sec)
–
Figure 5.4. A correlation function estimate.
4
2
0
associated quantization error. Because the difference between these two signals is smaller
than , only the estimate on the Imote2 is shown on Figure 5.4.
Data transferred in a network is reduced as explained in section 5.1.2. In this example,
data transmission is reduced by a factor of 5, as compared to centralized correlation
function estimation implementation. This reduction factor can be larger depending on the
number of sensors and the associated averaging. This reduction shows the advantage of
the distributed correlation function estimation.
Further consideration is necessary to accurately assess the efficacy of the distributed
implementation. Power consumption of smart sensor networks is not simply proportional
to the amount of data transmitted. Acknowledgment messages are also involved. The
radio listening mode consumes power, even when no data is received. However, the size
of the measured data is usually much larger than the size of the other messages to be sent
and should be considered the primary factor in determining power consumption. Small
data transfer requirements realized by the proposed model-based data aggregation
algorithm will lead to reduced power consumption.
5.2 Reliable communication
RF communication is not reliable unless lost packets are specifically addressed.
Packets may not be transmitted properly. When the distance between nodes is too long,
packets may not reach the destination. Multiple nodes trying to send packets at the same
time can cause packet collisions. If packets carrying commands are lost, destination nodes
fail to perform certain tasks. The sender is unsure whether the destination nodes have
received commands. If packets carrying measurement data are lost, destination nodes
cannot fully reconstruct the sender's data. Therefore, packet loss may cause a system to be
in an unknown state and may degrade measurement signals. SHM applications employing
smart sensors must address packet loss.
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