Structural Health Monitoring Using Smart Sensors - ideals ...

input forces applied at nodes in the local sensor community to stress in elements in the
same community. The structural analysis is, thus, performed as the product of the modest
size matrix and DLVs
The DLV method implementation on the Imote2 is now numerically examined. The
DLV method based on a known mass perturbation is employed in this numerical
validation; a similar procedure was employed to implement and validate the SDLV
method.
Prior to monitoring local sensor communities, mass normalization constants are
estimated. These constants are also estimated on the Imote2. The structural responses of
the truss before and after the addition of the known mass at node 11 are injected to 10
Imote2s corresponding to nodes 7, 9, 11, 13, 15, 17, 19, 21, 23, and 25. The Imote2 at
node 21 works as the cluster head and applies NExT and ERA before and after the mass
perturbation. Using the identified modal parameters, the cluster head estimates the mass
normalization constants using Eq. (6.21). The estimated mass normalization constants are
listed in Table 7.4 together with the difference between the constants estimated on the
Imote2 and the PC. The mass normalization constant estimation on the Imote2 is
numerically identical to that on the PC.
Monitoring of the local sensor community consisting of six Imote2s at nodes 2, 3, 4,
5, 6, and 7 is now examined. The injection of data, correlation function estimates by
NExT, and modal identification by ERA are repeated for the truss acceleration response
data measured before and after element 9 is replaced with a thinner element. Identified
modal frequencies and mode shapes before and after damage, as well as the estimated
mass normalization constants, are used to construct flexibility matrices. As intermediate
results, the singular values in Eq. (6.24) estimated on the Imote2 and on the PC are
compared in Table 7.5. The DLVs calculated on the Imote2 are also compared with those
estimated on the PC. The singular values and DLVs are considered numerically identical.
The DLVs are then applied to the truss model to estimate the normalized accumulated
stress. As shown in Table 7.6, the stress in element 9 is smaller than the threshold value of
0.3. Because the normalized accumulated stress is only compared with the threshold
value, the stress does not need to have many effective bits. Therefore, the stress estimation
on the Imote2 is performed in a single-precision float data type having approximately
seven effective digits. The normalized accumulated stress estimated on the Imote2 and on
the PC is identical with respect to the precision of the float data type.
7.2.5 DCS logic
The DCS logic to find inconsistency in damage localization results is implemented on
the Imote2. Sensor communities close to each other exchange information about damaged
elements. Because the topology of sensor communities is formulated to have overlaps in
monitored elements, at least two sensor communities monitor each element of the
structure. If some communities find contradicting damage detection results, these
communities repeat sensing, data processing, and the DCS logic. Also, if sensor
communities detect damaged elements in a consistent manner, these communities report
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