# Tracking Sensor Location by Video Analysis in Double-Shell Tank Inspections
## 3 Results
\begin{table}
\begin{tabular}{c c c c c c c c c} \hline  & **run1** & **run2** & **run3** & **run4** & **run5** & **Average** & **std dev** & **\% Error** & **Target** \\ \hline FS & 156.24 & 157.39 & 158.74 & 153.33 & 152.64 & 155.67 & 1.25 & 2.14 & 152.40 \\ FS & 168.24 & 164.11 & 149.27 & 133.57 & 161.88 & 155.41 & 9.97 & 1.98 & 152.40 \\ FS & 138.26 & 148.56 & 132.07 & 146.21 & 158.72 & 144.76 & 8.33 & \(-\)5.01 & 152.40 \\ FS & 166.71 & 159.31 & 137.48 & 143.69 & 146.13 & 150.66 & 15.20 & \(-\)1.14 & 152.40 \\ BS & 141.70 & 152.52 & 154.82 & 143.27 & 129.57 & 144.38 & 7.01 & \(-\)5.26 & 152.40 \\ BS & 153.99 & 144.12 & 173.71 & 166.41 & 139.52 & 155.55 & 15.07 & 2.07 & 152.40 \\ BS & 142.88 & 153.81 & 177.22 & 181.05 & 163.22 & 163.64 & 17.54 & 7.37 & 152.40 \\ FT & 154.42 & 149.22 & 131.95 & 143.16 & 151.91 & 146.13 & 11.77 & \(-\)4.11 & 152.40 \\ FT & 194.58 & 156.18 & 174.24 & 167.56 & 182.45 & 175.00 & 19.21 & 14.83 & 152.40 \\ FT & 88.43 & 79.78 & 76.93 & 99.94 & 77.02 & 84.42 & 5.99 & 3.86 & 81.28 \\ BT & 126.13 & 148.89 & 92.18 & 100.77 & 130.03 & 119.60 & 28.54 & \(-\)21.52 & 152.40 \\ BT & 144.41 & 160.74 & 267.46 & 241.84 & 140.80 & 191.05 & 66.83 & 25.36 & 152.40 \\ FW & 164.80 & 152.61 & 149.62 & 167.20 & 144.89 & 155.82 & 8.04 & 2.25 & 152.40 \\ FW & 144.85 & 135.50 & 137.12 & 156.97 & 139.76 & 142.84 & 5.00 & \(-\)6.27 & 152.40 \\ FW & 163.80 & 154.26 & 156.55 & 156.66 & 176.80 & 161.61 & 4.98 & 6.05 & 152.40 \\ FW & 153.66 & 144.39 & 192.53 & 239.20 & 201.25 & 186.21 & 25.54 & 22.18 & 152.40 \\ Total & 2403.11 & 2361.39 & 2461.88 & 2540.83 & 2396.58 & 2432.76 & 50.49 & 2.77 & 2367.28 \\ \hline \end{tabular}
\end{table} 
Table 2: Test of predictions of offline video analysis in a simulation of data collection in nondestructive double-shell tank evaluation. Figure 7: Video analysis of a 6-inch run starting from the entrance of air slot mockup. Data points show accumulation of short-distance measurements based on matching features at 5-frame intervals. Line shows a linear fit to data. exceeds 20% probably due to sluggish motion as the back crawler navigates a turn. Error in the prediction of the accumulated distance traveled is small (see last row of Table 2) due to the cancellation of errors occurring with opposite signs. To test the accuracy of real-time sensor tracking, we added motor controls to stop the crawlers when the video analysis predicted the sensor had moved a specified distance. We performed ten experiments to move the sensor forward 6 inches from a starting position with back crawler at the entrance to the air slot mockup in the lab. After motor control stopped the sensor movement, we measured the actual distance moved and calculated the percentage difference from the 6-inch target for sensor movement. The mean and standard deviation of the percentage difference between the target and the actual distance moved was 5.4 \(\pm\) 3.9%. In all but one of the 10 real-time tracking experiments, the actual distance moved exceeded the target distance. To illustrate the impact of this type of error distribution in real-time sensor tracking, we simulated data collection at ten sites if the actual distance traveled between sites was taken from our ten real-time 6-inch tracking experiments. Table 3 shows the position error at each simulated data collection site. The position error on sites 5-10 exceeds the tolerance of 1 inch (25.4 mm). Since the expected error in real-time tracking over a 6-inch distance is only 8.2 mm, this simulated failure of real-time sensor tracking is clearly due to the accumulation of errors from inadequate compensation for motor-control delay. This problem will be addressed in future research.