Structural Health Monitoring Using Smart Sensors - ideals ...

Time (sec)
80
60
40
20
0
-20
-40
-60
-80
-100
50 100 150 200 250 300
Repetition
Figure 5.20. Time synchronization error estimation.
of the two above-mentioned factors explaining the error in the time stamps. This figure
indicates that an upper-bound estimate of time synchronization error is 80 s.
The time synchronization error estimated above is considered small for SHM
applications. The delay of 10 sec corresponds to a 0.072-degree phase error for a mode at
20 Hz. Even at 100 Hz, the corresponding phase error is only 0.36 degree.
While a global clock estimates 2 seconds after a beacon signal is found to be accurate,
local clocks may drift over time. Large clock drift necessitates frequent time
synchronization to maintain certain accuracy. The clock drift in the Imote2 is estimated
next.
The same approach is utilized to estimate clock drift. Upon reception of the second
packet, which requests replies, the receivers return to the sender their offsets to estimate
the global time, instead of global time stamps. If clocks on nodes are ticking at exactly the
same rate, the offsets should be constant over a long time. This experiment, however, did
not show constant offsets. Figure 5.21 shows the offsets of nine receiver nodes. One of
them stopped responding around 45 seconds, exhibiting a short line on the figure. This
figure shows that the drift is quite constant in time. The maximum clock drift among this
set of Imote2 nodes is estimated to be around 50 s per second. Note that this estimate
from the nine nodes is not the upper limit of clock drift because of the small sample size.
This drift is small but not negligible if measurement takes a long time. For example, after
200 second measurement, the time synchronization error may become as large as 10 ms.
One solution to address this clock drift problem is frequent time synchronization.
Time synchronization is performed often before the effect of clock drift accumulates and
results in large time synchronization error. However, frequent time synchronization is not
always feasible. When other tasks are running, such as sensing, time synchronization may
not perform well. Time synchronization requires precise time stamping as explained
earlier. Moreover, sensing requires precise timing and needs higher priority in execution.
Scheduling more than one high priority tasks is challenging, especially for an operating
system such as TinyOS which has no support for real-time control. If the time
78