So I’ve been trying to figure out a good way to keep an ROV stationed around a given location without requiring active station keeping (such as with a boat controlled by an attentive captain or some sort of dynamic positioning system).
The general problem is that with any kind of wind or water current, a vessel on the surface (which presumably the ROV is attached to) will get pushed away from where the ROV was initially deployed, and even if the ROV itself has the ability to fight that current, the vessel it is attached to will eventually tug it away. Vehicles (manned or unmanned) that have to actively stay above a site are necessarily complex, so any way to keep an ROV on station passively is likely to be easier to implement, more robust and lower cost.
My thought has been that a simple way to keep station would be to employ a sort of clump weight that would sit on the bottom and would be heavy enough to stay put but light enough to be transported to a site easily. Presuming that the ROV would be examining things on the bottom, the tether would have to go this distance anyway, and the tether could be made thicker and stronger along this length since that portion would not need to be pulled by the ROV. This clump weight could be as simple as a hunk of metal that a tether goes through, or as advanced as a lander that houses multiple ROVs (similar to the one I drew in this post several years ago)
Of course, a drifting boat on the surface would still be likely to drag the weight, so instead, a cable running up from the weight could go to a floating radio buoy that is not big enough to drag the clump weight. If the buoy could be made to relay a wireless signal to a the shore or a nearby boat (which would no longer have to keep station precisely), I believe this system would allow ROV operations to happen with much less effort- especially in rough seas.
Some means for isolating the movement of the buoy from the clump weight would be needed to remove impulsive vertical loading from surge which could snap the tether and also to remove lateral loading which could cause the clump weight to drag). To address this, a float that is suspended from the tether several meters below the surface (which I call a “load stabilizer”) could be added to the system.
-----How it works----
As you can see from the photo at the top of this post, we’ve already built a prototype of this system which we tested briefly at Lake Tahoe a few weeks ago. Obviously a beautiful day on the lake is not exactly the rough conditions this system is designed to handle, but it’s a start. I thought I’d post a little bit more about how the system works…
Although the system could be made to work with simple WiFi, we chose to use a long range “Ethernet Bridge” system that is capable of communicating high bandwidth signals over several kilometers. We chose to use the RocketM5 system by Ubiquiti. For the buoy, we used an omni directional antenna, and on shore we used a 90 degree “sector” antenna. Here’s a list of our BOM for the radio system which ended up costing around $500 total.
1 RocketM5-USA 802.11N MIMO 5 GHz Rocket AP US
2 AMO-5G10 5GHz AirMax Dual Omni, 10dBi
3 RocketM5-USA 802.11N MIMO 5 GHz Rocket AP US
4 AM-5G20-90 90 Degree 5GHz MIMO 20dBi w/cables
This system works great (you can see us using an earlier non-buoy-based setup with these radios here). Effectively, using a set of these radios is like having a REALLY long Ethernet cable stretched between your two points of operation. The signal that goes in on one end comes out on the other. This is called a “bridge” mode, which worked great for us but there are many other configuration options to choose from if that’s your thing. The radios are powered with POE (Power Over Ethernet) and come with a wall-pluggable POE injector. On the boat (or buoy) side, where wall power is not available, we just got a 12v POE injector from Amazon and powered it using a USB/12v battery power supply.
I’ve attached a functional diagram below which I (of course) drew with MSPaint and am quite proud of.
One thing to note is that we chose to use hollow walled pipe instead of normal SCH-40 PVC because the weight of solid PVC was significantly greater. The pipe has a 4" outer diameter. Also, we ended up having the tether come out of the top of the buoy to make the electronics more reparable and reduce leaking risk.
Finally, not shown in this diagram is an additional 1" OD PVC pipe equal in length to the buoy that was made to dangle below the buoy an additional meter and a half or so with a few kg of weights on the end. This helped keep the buoy stable and guided the tether downward. The overall diagram shown as the second image of this posts depets this extension.
Here’s an image of the raw electronics used before they were mounted to the platform which would be slid inside the tube.
Here are those same electronics, ready to be placed in the buoy tube. A piece of hot water heater pipe insulation foam was placed in the bottom of the buoy to keep the electronics platform (containing the antenna) high above the waterline. An optimized design might utilize a longer electronics platform which would put everything other then the antenna as low as possible to help with stabilization.
During our Tahoe deployment we successfully operated ROVs attached to the buoy from our cabin on the shore several kilometers away. There is still a lot of work and testing to do. We must find ways to make the buoy sit more vertically, we want to try using a load stabilizer, and we want to test the system in rough seas. Once that’s done we may also want to add features such as a GPS module so the buoy can report where it is, a flashing beacon that makes the buoy easier to find, and perhaps optimize the buoy to be more easily deployed.
All of this is a work in progress, but I thought it would be good to post where we’re at and get some feedback.
Special shout out to Erika Bergman (shown below) who did most of the building for this prototype and was out with the buoy during our test deployments. You’re the best, Erika!