I’m attempting to determine speed through the water, digitally, with intent to provide a dead reckoning position of the unit. A simple flowmeter like this one from Mouser would do the trick – sufficient accuracy, sufficient pressure to withstand the depths I’m looking at, etc. The wiring is fantastically simple, and the digital side of things is easy. But I have no idea how to make the plumbing work.
Is there a pre-existing solution to determine speed through the water? Or, failing that, is there a way to pipe water through the housing to a threaded fixture? A standard fitting that others have used for connecting high pressure tubing from the end caps, maybe?
I’m a bit confused here. The product you’ve linked to looks to be for detecting flow within a hose, and the electronics and cabling don’t seem to be waterproof. I also couldn’t find any kind of depth rating on the Mouser page, or the Seeed Studio (the manufacturer) product page.
Assuming you’re somehow able to waterproof the electronics, I’m not sure how representative the flow measured through the fitting would be of general water speed, because the water going through is in an open system (depending on relative pressure, it can either go through the fitting, or around it), rather than the closed system that the product is designed for (all water must go through).
Beyond that, speed through the water can’t be used to directly provide a dead-reckoning position, because water can have currents and waves, which are ‘flow’ that’s independent of the vehicle motion. That may be possible to correct for somewhat with an accelerometer and gyroscope in an inertial measurement unit (IMU), but they tend to have quite noisy signals, which makes them poor candidates for longer term dead-reckoning.
Generally Doppler Velocity Logs (DVLs) and/or underwater (acoustic) GPS are used, although there are a few other alternatives.
Unfortunately most existing solutions are quite a bit more expensive and/or complex than your flowmeter idea.
So: The flowmeter is not designed for mounting on an ROV, but my thinking is that there ought to be a way to run a hose through a housing to the flowmeter. It won’t give me a number in knots, but the flow should correlate linearly to speed through the water. The pressure rating is the internal rating of the flowmeter, which should get me something like 200 feet deep, which … isn’t great, but it should be good enough for a proof of concept.
The concept I’m interested in proving is that it’s possible to obtain a usable DR using low cost electronics and off-the-shelf stuff. It won’t be accurate enough to position an unmanned vehicle within a foot of a goal after 12 hours of navigating, but it might be close enough that I could come within 50 feet of a target after a 20 min run, in a lake with low set and drift. For my application, surface support (acoustic GPS et al) aren’t really options at all.
Ahh right, understood, that makes a bit more sense
My “open system vs closed system” comments are still relevant here (since water can go around the vehicle), although I’m unsure how significant that would be in practice (it would need to be tested, and would likely vary depending on vehicle shape, rigidity, surface materials, and speed).
My “currents and waves” comment is relevant to this (e.g. speed relative to water may not be the same as speed relative to earth), as well as noting that a single pipe can only measure velocity in a single direction, which may not be the direction of travel (although that can perhaps be worked around by using multiple setups with pipe ends oriented in different directions).
If your lake has minimal currents, and is large enough that vehicle movements will have effectively negligible effects on the water body, that does definitely improve the likelihood that an inertial approach could work. If the vehicle is close enough to the bottom to see it, but far enough not to stir it up then a visual approach (e.g. optical flow) could also be worth considering (cameras are cheap, although the software side may be challenging).
DVLs can be purchased ‘off the shelf’, but at this stage don’t really meet your “low cost” requirement, at least relative to something like an IMU, a few flow meters, and/or a camera or two.
Re: open vs closed system, you’re absolutely right. My hope (unproven) is that the rate of flow through a tube would be related to the speed of the device. There’s a lot of water in the lake, and it doesn’t all need to flow through the flowmeter to tell me how fast I’m moving; I just need a representative quantity of the stuff … I think. Haha, this is a gigantic experiment.
Currents are definitely an interesting problem. Officially, there are no currents in The Lake. But according to anyone who’s done it, there are absolutely currents, some days worse than others. If I can make the rest of it work, I have some ideas for how to include flowing water into the algorithm.
In this particular system, I’m going to assume that the unit is only traveling in the direction it’s pointing. That should be close enough to true that it’ll get a good approximation. Any lateral movement will be driven by current, rather than propulsion, so I’m discounting it (for now). Any vertical movement can be measured with a depth sensor – that gives me an absolute reading, rather than a guess. Alignment between the orientation of the compass and the vehicle will be an interesting challenge – even a single degree off will throw the whole thing out.
Cameras on the bottom are a nice idea. I’d also considered some kind of sonar-based velocity measurement, like bouncing a ping off the bottom and measuring where it hits. But the bottom is mostly a mix of ash and silt, radically uniform in color and texture, and shockingly non-reflective. I’ve actually had days where I only knew I had hit bottom because I couldn’t see anything anymore. “Oh, I’m blind. Or … I’ve arrived at the bottom of the lake and need to go up a couple of feet to see anything.” So a visual system might work in some cases, but the test case I have available would stymie a world class computer vision team, for sure lol.
I think I’m going to end up with just standard bulkhead fittings, and drilling some holes in standard plexi endcaps to fit the fittings correctly. O-rings, silicon tubing, and the like… I think it’ll work. I’ll post pics when I get it together.
Fair enough. Sometimes experiments are worth running, because they can lead to significant progress. Just good to make sure it’s treated as an experiment rather than expected to be a guaranteed success
Haha, such is life sometimes. Underwater environments certainly like to throw out their fair share of challenges. Given that context, inertial (and “surface support”) approaches seem the most likely to yield usable results
