Civil Engineering Reference
In-Depth Information
power-less. Once the event is detected by the component, it will wake up the node
to which it is connected, generally by generating an interruption.
One type of event is external radio transmission. Waking up sensor nodes using
this strategy is generally called “radio-triggered wakeup”. When the wakeup
message is received by a node, its radio-triggered circuit collects enough energy
to trigger the interrupt to wake up the node. Note that this is significantly different
from the previously mentioned
SnoozeAlarm
mode or the WOR functionality.
A sensor node with radio-triggered wakeup unit does not need to wake up and
listen to the wireless channel periodically and, therefore, can be more energy
efficient. However, radio-triggered wakeup still has drawbacks. The time it takes for
a node to wake up another is mainly determined by the number of hops between
them. Although this time is generally shorter than in the
SnoozeAlarm
approach, it
can still be significant for a large WSN.
Change in vibration is a special event for SHM, since it is often quite desirable to
have the data sampled when the structure is under relatively large vibration
amplitude. Examples include a bridge with a train passing by or a high building
under strong winds or earthquake. The data sampled during this period are
important, not only because these data have higher signal-to-noise ratio than
those sampled in the normal condition, but also because this is a critical period
when structures have a much higher risk of being damaged. In this condition, the
vibration energy can be taken as the external energy to wake up sensor nodes.
Compared with the radio-triggered wakeup unit, each sensor node can be triggered
almost immediately in the presence of an event.
A special type of wireless sensor node, TelosW (Lu
et al
., 2010), has this
vibration-triggered functionality. However, the sensitivity of the on-board accel-
erometer and the computational capability of TelosW are not suitable in many
SHM applications. Another important problem of using vibration-triggered
wakeup is associated with the difficulty of determining the threshold for each
sensor node. Different locations of a structure have different vibration amplitudes.
Since SHM requires synchronous sampling, different thresholds should be set for
sensor nodes at different locations so as to realize synchronized wakeup. Consid-
ering the number of sensor nodes, the environmental noise, and the complexity of
structural model, such a task is very difficult, if not impossible.
Sleep and wakeup: a hybrid approach
From the discussion above, using a radio-triggered wakeup unit can have relatively
large delay, while using a vibration-trigger unit may have difficulty in realizing
synchronous wakeup. To realize energy efficient, fast, synchronous wakeup, one
promising method is to integrate these two approaches. In this hybrid approach,
the wireless sensor nodes deployed on a structure are divided into a number of
clusters, with each cluster containing ordinary sensor nodes with radio-triggered
units and a few sentry nodes with an additional vibration trigger component.
Sensor nodes in each cluster have direct communication with one of the sentry
nodes in that cluster. The initial threshold value for waking up each sentry node
can be determined by simulation using a structure model or by several rounds of
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