Drones offer a new and revolutionary solution to unmanned sewer inspections. Where boats and other robots are unable to overcome obstacles, drones can overfly them, extending the range of inspectable areas, with the only limitation being the sewer size. These inspections are one of the use cases where ASIO’s long flight time really proves useful!
Indeed, the distances between access points (manholes) are sometimes large (over 150m) by conception or some of the access points may be blocked (due to age, by parked cars or other obstacles) or obstructed on the inside (for example by ladders or structural damage). In such cases, long range flight is a necessity to be able to carry out the inspection of the condition of the sewers over a maximal length.
From the pilot’s perspective, the first step to any sewer inspection is the definition of the areas of interest within the sewer, which is achieved through discussion with the engineers in charge of the infrastructure. These areas of interest can be for example a short section that passes underneath a road, where structural integrity must be assessed, or the whole length of the sewer to look for structural sinking. In most cases, the industrial services of the cities will have access to maps where the sewer and the access points are mapped out (although in some cases maps may not correspond to what we observed during the inspection), which are used as the planning base.
It is during this planning phase that the limitations of the system must be considered. The main limitation in such missions is the communication range, that will be affected by the environment’s properties (material and path curvature), and the distance. As an example, in a concrete sewer, the range in a straight line can exceed 200 meters, whereas a 90° turn will lower range to only tens of meters after the curve, limiting the inspectable area, depending on the access points locations with respect to curves.
Once the theoretical plan is defined, the preparations are put in place. For sewers with manholes in the middle of roads, a section of the road must be appropriately barred to allow the pilot to fly directly down the manhole while staying just next to it. At this point, it is not uncommon that some access points are either impossible to open, due to lack of maintenance or simply to age. These unplannable events and are often discovered only on the day of the inspection, since some of these access points can only be opened once proper road blockade has been put in place. In these cases, missions must be readapted to take this into account and the flight path is modified through discussions with the on-site team consisting of engineers (inspectors), city services and flight assistants.
Once the new plan has been defined taking into account these conditions, the next step is placing the antenna in the right position. Due to the very low penetration of the communication waves, the antenna must be placed within the flight environment. To do so, a range extender, a 10 meter (or 20 meter) long cable that “extends” the antenna is required. This allows the pilot to sit comfortably outside the sewer while the antenna maintains line of sight with the drone. For access points directly above the sewer, the antenna can be simply lowered in the sewer pipe. When the access point is not directly above the sewer, the pilot and eventual assistants have shown great creativity by designing tools to place the antenna properly.
With the antenna properly placed and an access point, the mission is a go. The herelink ground station (controller) is powered up and the screen is shared to a computer on which the inspectors can visualise the image feed in real time.
A freshly charged battery is inserted in the drone and after a thorough inspection of the drone, especially the sensors cleanliness and the propulsion unit’s unobstructed functionality (the pilot will rotate the propellers each 2-3 turns to check for smoothness), the drone is powered up.
Once it has successfully booted, in order to minimise the workload and to focus on the actual flying rather than fiddling with parameters, the pilot then prepares the drone for the mission. This implies selecting appropriate parameters before entering the sewer. The trims are properly set, the controller sensitivity is set, most often in “standard”, to make sure drafts can be compensated while maintaining control of the drone, the LED power is set to 100%, the visual lock stabilisation is activated and depending on the mission, it is decided whether photogrammetry mode should be active for the flight, capturing high-resolution pictures at the set frequency.
From there, with LEDs at full power, the drone is either flown down through the manhole or in some cases the pilot decides to tether it down with the assistance of the team to gain time and decrease chances of collision during the descent.
In some instances, a first reconnaissance flight may prove very informative to understand the environment and decide how to fly (which assistances to use etc…).
The drone is now in the sewer and ready to carry out the inspection. The pilot can take a seat while looking at the herelink controller screen, allowing him to see what the drone can see. Away goes the drone. The inspectors see a point of interest and ask the pilot to get closer. Using the shutter button on the controller, the pilot takes 12 MP images of the point of interest, and carries on with the mission after consulting the inspectors.
Once all the data has been captured, the pilot can bring the drone back, while being cautious not to lose focus. The drone arrives under the manhole and is ready to ascend to the street. The ascent is initiated and the drone pops out of the manhole.
The mission is a success, the drone is landed and powered down. The antenna and range extender are recovered from the manhole and stored in the transport case. The SD card is removed from the drone and data is shown to the inspectors, who can then immediately download all the captured footage.
Using the ASIO Explore software, the pilot can then provide a more detailed report, showing all discovered points of interest, and may even build a 3D point cloud model showing the location of these points of interest with respect to the starting point.
Based on all the data, the engineers start working on finding solutions to fix the points of interest if damage was discovered and the ASIO is off to a new mission!
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