Structural Health Monitoring Using Smart Sensors - ideals ...

1.8715 x10 5 #ofdatablocks
1.871
meas urement 1
measurement 2
Time (sec)
1.8705
1.87
1.8695
1.869
0 50 100 150 200 250
Figure 5.23. Variation in the sampling frequency over time.
the sampling frequency. The difference in two consecutive timestamps fluctuates by about
0.1 percent. Though imperfect time stamping on Imote2 is a possible source of the
apparent nonconstant sampling rate, fluctuation with a nonzero average values suggests
that the variable sampling frequency as a credible cause of the phenomenon. With this
fluctuation, measurement signals may suffer from a large synchronization error.
5.3.4 Realization of synchronized sensing
The observed problems discussed previously are addressed using the smart sensor’s
computational capabilities. Failure during sensing is detected and sensing is repeated until
success is achieved. The other three issues are dealt with concurrently by resampling the
measured time histories based on time stamps at the end of each block of data. These two
approaches realize synchronized sensing and are discussed in this section.
1. Detection of sensing failure and repetition of sensing
To detect sensing failure, a timer on the Imote2 is utilized. This timer is set when
sensing is initiated; the timer is scheduled to be fired after the planned total measurement
time has passed. If sensing is successfully completed, this timer is stopped at the end of
sensing. Therefore, the timer never fires unless sensing fails during measurement. If the
timer fires, a flag is used to mark sensing failure. After a predetermined time has passed,
the manager sensor inquires of the other nodes and itself whether sensing has failed. If a
flag indicating sensing failure is found, the memory space storing acquired data is
reinitialized, and a task to restart sensing is posted at all of the nodes.
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