Structural Health Monitoring Using Smart Sensors - ideals ...

Supply voltage (V)
4
3.8
3.6
3.4
Node 46
Node 43
Node 51
Node 49
Node 87
3.2
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start
s ens ing1 report s ens ing2 sensing3 s ens ing4 ERA
Figure 8.12. Change in the supply voltage to the Imote2.
sufficient. Although the change in supply voltage during these experiments without
repeated measurements was not recorded, the battery life is estimated by counting the
number of experiments conducted using one set of batteries. The Imote2 utilizes three
AAA batteries. A new set of batteries supplies 4.5 to 4.8 V to the Imote2. The Imote2 can
operate when the supply voltage is above 3.2 V. More than 30 sets of experiments were
performed without changing batteries. Simply dividing the change in the supply voltage
by 30 yields about a 0.05 V decrease per experiment when sensing is not repeated. If
monitoring is performed on a weekly basis, a battery life providing 30 sets of
measurements and damage detection calculations allows the Imote2 to monitor a structure
for about eight months. Note that the Imote2 consumes power even in a sleep mode,
shortening the battery life.
For monitoring of full-scale structures, sensing may continue longer, thereby
consuming more power. For example, 40 seconds of acceleration data sampled at 280 Hz
is utilized in the experiment and damage is successfully detected. However, a full-scale
structure may have lower natural frequencies than the lowest one of the truss (i.e., the
mode at about 20 Hz), necessitating longer acceleration measurements. Though a lower
sampling frequency may be allowed for such structures, the duration of sensing is likely to
be significantly longer than 40 seconds. Battery depletion may become greater than 0.05
V per monitoring event due to the longer sensing time.
8.8 Damage detection results
The damage detection capability of the SHM algorithms implemented on the Imote2
is examined in this section by monitoring the truss with its various elements replaced with
a “damaged” element, i.e., one of reduced cross section. Elements 8, 9, 10, 11, 12, 19, 20,
21, and 22 are replaced one-by-one with the damaged element. For the first five cases,
sensor communities 1, 2, and 3 participate in damage detection, while sensor communities
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