Continuous utility pole health monitoring based on finite element analysis and operational modal analysis using DAS
Aspects of the present disclosure describe distributed fiber optic sensing (DFOS) systems, methods, and structures that advantageously enable and/or facilitate the continuous monitoring and identification of damaged utility poles by employing a DFOS distributed acoustic sensing (DAS) methodology in conjunction with a finite element analysis and operational modal analysis. Of particular advantage and in further contrast to the prior art, systems, methods, and structures according to aspects of the present disclosure utilize existing optical fiber supported/suspended by the utility poles as a sensing medium for the DFOS/DAS operation.
1. A method of operating a distributed optical fiber sensing system for continuously monitoring utility pole health and early warning, said system comprising:
a length of optical fiber at least a portion of which is suspended from a utility pole, and
a distributed optical fiber sensing interrogator in optical communication with the length of optical fiber; said distributed optical fiber interrogator configured to generate optical pulses, introduce them into the optical fiber and receive reflected optical signals from the optical fiber;
said method comprising:
operating the distributed optical fiber sensing interrogator and collecting the received, reflected optical signals from the optical fiber, said received, reflected optical signals indicative of vibrational activity of the optical fiber and utility pole in combination (pole-fiber system) at a plurality of points along the length of the optical fiber;
applying, on the received, reflected optical signals from the optical fiber, an operational modal analysis and determining individually, natural frequencies for the utility pole and optical fiber;
comparing, the operational modal analysis determined natural frequencies for the utility pole with a finite element analysis determined natural frequencies for the utility pole; and
generating an alert when a change in the determined natural frequencies of the utility pole exceeds a pre-determined threshold,
wherein the frequencies of the utility pole and the optical fiber cable are determined according to the following relationships:
for the utility pole,
f
p
(
i
)
=
[
∅
(
i
)
)
(
1
-
φ
)
]
2
2
π
L
p
2
(
E
p
I
p
ρ
A
)
and
for the optical fiber cable,
f
c
(
i
)
=
(
i
π
)
2
L
c
E
c
I
c
m
1
+
TL
c
2
E
c
I
c
i
2
π
2
where
ρ is the density of the utility pole;
T is the tension of the optical fiber cable;
m is the optical fiber cable mass per unit length;
Ø(i) is eigenvalue for each mode i;
f p (i) and f c (i) are the natural frequency for each mode i, respectively for the utility pole and optical fiber cable;
L p and L c are the length of the utility pole and optical fiber cable, respectively;
E p I p and E c I c are the stiffness of the utility pole and optical fiber cable, respectively;
A is the cross-sectional area of the utility pole at the butt; and
φ is the amount of taper which is the minimum diameter divided by the maximum diameter of the utility pole.
2. The method of claim 1 further comprising:
continuously monitoring the natural frequency of the utility pole; and
generating an alarm when the natural frequency of the pole is below a predetermined threshold.
3. The method of claim 2 further comprising:
monitoring a cut-off natural frequency for the utility pole due to a pole structural change or a material property change of the pole.
4. The method of claim 3 wherein the alarm is generated when the cut-off natural frequency of the pole reaches a pre-determined threshold.
5. The method of claim 1 wherein a plurality of vibrating structures (pole and cable) are determined and cut-off frequencies associated with each are determined.
6. The method of claim 5 wherein the natural frequency of another vibrating structure is subsequently determined from the cut-off frequencies associated with each of the plurality of vibrating structures.
7. The method of claim 6 further comprising generating a utility pole status report and sending that report to an operator.