kubark42

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About kubark42

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  1. kubark42

    Ultralight windvane selfsteer project

    As a mechanical engineer (learned SolidWorks back in the 90s), that you just did that beautiful and professional assembly on an iPad blows my mind. Next thing I know, we'll be doing symbolic math on iPhones.
  2. kubark42

    Ultralight windvane selfsteer project

    Cool. I'm intensely interested in the results of bike cables after years of exposure to marine environment. Anything which is a consumer product and still survives on a boat is awesome. I wonder if hydraulic bike lines would also function as easily, or perhaps even better, than cables. You get the nice fact that at the low speeds you'd see in a self-steering vane, they are practically frictionless around tight bends. Of course, you might also get some real nastiness around the seals.
  3. kubark42

    Ultralight windvane selfsteer project

    Bike cables aren't made with the marine environment in mind. They might or might not withstand it very well. I'm betting @SySunday isn't the first person to use them, so there's probably a body of evidence as to how well they perform.
  4. kubark42

    Ultralight windvane selfsteer project

    Is there a conclusion as to whether sleeved cables are lower in friction than rope and pulleys? I would naively have thought that dyneema through LFRs would have been the ultimate in robustness as it can't clog and won't decay with time. P.S. I love this windvane, it's absolutely awesome.
  5. kubark42

    Switlik CORD vs Lifesling

    Lifesling used to have an inflatable version, but recalled them all and stopped production. The reason that I understand is that that there is a certain failure rate for the inflatable cartridges. If you're wearing the life vest, you can still manually inflate it after failure. But if it's attached to a throwable, it just sinks and becomes useless. Inflation failure is certainly a solvable problem, maybe Switlik has found the right approach. And it's certainly better to have a compact inflatable which might fail to one which isn't installed at all because it's too large, so the possibility of inflation failure isn't a death sentence.
  6. kubark42

    Anyone measuring tidal current?

    14m, wow. That'll definitely cause issues! A couple questions so that if we repeat the experiment we know what's already been done: Are these the Mediatek receivers? Were the receivers at least 2m off the ground? During drone landing/takeoff, we've noticed that ground-plane reflections can cause the GPS to deviate 1-2m horizontally when the GPS antenna is less than 2m off the ground. Were there any buildings nearby which could have caused multipath? Really cool of you to do these tests in the middle of the winter. It's going to drop below 0 tonight in Boston. I'm kind of loathe to go see my boat, floating off the Mass coast and certainly under a foot of snow and ice. I had planned to keep sailing all winter, but the past two weeks have been so cold my crew has shown discretion to my valor!
  7. kubark42

    Anyone measuring tidal current?

    You can (more or less) force the GPS receivers to lock to the same constellations by configuring the minimum elevation a satellite has to have before it is used. This isn't a sure-fire way, but it'll do you well enough in practice if you want to keep prices down. Sounds like the mast and boom could also be the culprit, not necessarily the sail fabric. I don't think I can mention the specific RTK receivers. Just that they were above $10k/ea. The uBlox M8P, their RTK model, is ~$100/ea., or if you want cheap L1/L2 RTK, Swift Nav just released a $600 Piksi Multi (although their first Piksi was underwhelming in real-world conditions, so caveat emptor). I don't have any experience with the uBlox RTK modules, so I can't comment as to whether both modules can be on a roving platform or one must be immobile. That's exactly it, but tactical grade, ! That's a way to spend more money on your instrumentation than your boat. I've had great results with an Invensense, they're about $2-3. They get used extensively in cell phones-- which is why they're cheap--, and in hobby/FPV drones-- which is why I know them quite well. The complete autopilots we designed for Tau Labs ranged from $12 to $100, depending on how much extra stuff there was on top. I think the latest $5 Invensense chips can even fly a drone with no other onboard processing. Somethings you want to think about when you have this kind of precision is the GPS location relative to the center of rotation of the boat. You will absolutely see the pitching and rolling behavior as you sailed along. This has to be accounted for, but it's just an offset. And since it all averages out, the final result isn't that sensitive to the accuracy of the offset.
  8. kubark42

    Anyone measuring tidal current?

    Regarding noise, I think you have such a huge baseline that you'll wind up with much better precision than the worst case scenario I predicted. And you could certainly average on a longer window which would give a better result. And don't neglect the huge impact which comes from tying in to an IMU. I know that's not something you can do without extra effort, but I think you'll be blown away at how much better the result becomes. We use GPSes like that on micro drones and full-scale airplanes (for the drones we use RTK receivers so we have >0.02m precision, which is important because the drones are 10x smaller than a 36 foot boat). I've never done it on a sailboat. Interesting to know about the sail, I'm somewhat surprised it has a noticeable effect. Is it tack dependent, or is it enough that the antenna be near the sail? Have you thought about putting an antenna out on the bowsprit or above the pulpit? I have a trimaran, so not placing under the sail is a lot easier! Thinking of pairing two GPSes together for RTK, there's a uBlox receiver which is RTK, but at much, much cheaper than standard modules. It's not as accurate, but for sailing purposes there's a drop-off once you're calculating heading better than 1deg.
  9. kubark42

    Anyone measuring tidal current?

    I think this is a straightforward state estimation problem. I'll explain it across several posts, as I get time. First let's tackle heading. If you use a few GPS antennas located strategically around the boat (one at the bow and one at the stern, for instance), the distance between the two is so high that they will give very good heading indication. Assuming 10Hz measurements, and a separation of 36 feet (to fit the L-36), the result is quite good. The two graphs show the noised data and the 1s averaged data. +-5degrees is already comparable to compass error The noise model is conservative, as it was given as pure white noise. In reality, GPS noise between receivers tends to be coupled, i.e. if one is misreading to the north and east, so does the other one. The real world scenario is at least 2x better. Instrumentation which integrates a gyroscope and accelerometer would drive that heading accuracy to at least 10x better, and possible 100x better. Conclusion: there is no need to go to heroic lengths to calibrate the compass. Two cheap uBlox GNSS (aka GPS) modules will give comparable performance, and coupling into an IMU will give much better performance. There are several open-source projects which will do this with minimal-to-no effort.
  10. kubark42

    Low Friction Ring Anarchy

    Interesting. Moderators, would you care to comment on whether you censored posts in this threat and if so on what basis? Not saying it was the wrong action, I have no idea what happened behind the scenes. However, having been one of those in charge of an autopilot community, we always felt it was important to be transparent with the community and openly discuss any censorship decisions.
  11. kubark42

    Anyone measuring tidal current?

    We use the NEO-M8Q, but if I were to do it again I'd choose the M8N because it has upgradable flash at the cost of maximum processor performance. You're right that there's no such thing as a free lunch on the analog side, (although there's a caveat below[*]), but a lot of the filter is done in discrete filtering steps which are bounded by processor speed. This processor bottleneck is why the uBlox M8 can only do 10Hz when listening to both GLONASS and GPS. It's just got too much rattling around its little brain to get it all done quickly. (Compare this to some of the L1/L2 GPSes we were messing around with, which could give cm precision on their own, without an IMU for help. They're also $$$...!) Setting aside some pretty big edge cases, one thing that was demonstrated a decade ago was that with quantized systems you got better dynamic state estimation results when sampling 10x faster than sampling with 10x more precision. This is especially true with a moving boat where your filter can converge to >50cm of accuracy, but in a 1s window you might be moving 2-4m. (BTW, the computation delay on the uBlox is around 50ms in the x-y axes, and 70ms in the z-axis.) +1 on using a slow filter for estimating current... it certainly won't change quickly! Saves a lot of work to generate the current measurement with a high speed rate estimator and then just pass that through a super slow first-order low-pass filter. My hunch is that 100 data points would probably make a very stable reading. I follow your reasoning on the GPS accuracy. Once you're sampling at >5Hz, if you wanted to do a bunch better a $1 IMU and $1 magnetic compass is much better bang for the buck. If you're interested in the mechanics, this paper on EKF fusion was written by one of our cofounders. It's brilliant because it's completely model independent, it works as well on a boat as a plane. [*] Allan variance says that you can average across such a big window that your overall result gets continually worse. This makes intuitive sense if we were measure the speed of a car. Most of the day it's parked, and so the average converges to 0 quite nicely. But then if you jump in it and drive away, the longer you keep the average going the worse the average gets.
  12. kubark42

    Anyone measuring tidal current?

    My airplanes would fly at maximum 10m/s, and I usually liked to keep them at 3-4m/s when possible. They were slow-flight fixed-wing foamies. Great for my kind of research because they didn't need a big field and when one went splat into the side of a steel container nothing ever really broke. The wind speeds were typically higher than the water current, so I don't have experience in your use case, but my upwind ground speeds were def in the 5-7kts range. The MediaTek GPS modules (which the Adafruit uses) are good values for the money, but the consumer/hobby drone industry uses almost exclusively uBlox receivers. They have much better performance across the board. For instance, noise does not increase with the uBlox M8 modules when sampling at higher rates. (Confirmed by measurement and then senior engineering at uBlox). You can choose between 10Hz GPS+GLONASS or 18Hz GPS, although in our tests we got better uptime (we need >99.999% availability for our month-long drone flights) with GPS+GLONASS. I'm not sure what kind of filter you typically use, but a stochastic estimation filter (e.g. a Kalman filter) does not have lag problems. If you tie it into an IMU you will find that estimator performance gives >200Hz results to within 5cm of precision. If you haven't seen it already, you might enjoy reading about Strapdown Navigation state estimators. Another factor in GPS precision is the antenna type. The Adafruit comes with a ceramic patch antennas. These have a very high gain in the normal direction, but it drops off very quickly toward the horizon. The upshot is a 30deg heel will cause many satellites to become invisible to the receiver. For monohull operations a helix antenna is preferable. Heh, hear you on the Edison. I've got several, and even a few Joules and Aeros sitting around. Glad you're putting them to good use, they were a really neat SBC, but Intel was selling each at a loss and couldn't get the price down. Even when we asked them for 10k volume pricing, they couldn't/wouldn't cut prices. Anyway, sounds like you've got something which is already bearing fruit. Let us know how the verification test turn out!
  13. kubark42

    Anyone measuring tidal current?

    This is the genius of Permerlani's method. You don't need an odometer reference, and all the calibration problems it entails. All you need is absolute heading and GPS info. 10Hz GPS is more than sufficient for hovering drones, which is one of the hardest flight modes. Since the boat is always moving, the GPS noise will blend into the background very quickly. 1/10th of a knot is roughly 0.5m/s, which is about five times larger than a GPS's doppler measurement (make sure the GPS gives speed as a result of doppler measurements, not differentiation of position measurements. uBlox is excellent for this). @allene222I gave it some more thought and realized you can try this out really quickly if you'd like to go down this path It will cost about $15 for the hardware, an STM32F3 Discovery board, and then another $30-50 for a uBlox-based GPS. PM me if interested in the nitty gritty.
  14. kubark42

    Anyone measuring tidal current?

    There's a very easy way to get the ego-motion (e.g. boatspeed) for dubins carts (e.g. boats) in moving vector fields (e.g. water currents). Check out IMU Wind Estimation (Theory) by William Premerlani, https://drive.google.com/file/d/0ByvTkVQo3tqXVzBYQUZicUNvbEE/view. I had great success implementing the airspeed estimator part of this with OpenPilot and Tau Labs, two drone autopilots I was involved with before getting into sailing. It was a very reliable estimator, albeit noisy. Here's the source code I wrote if you want an implementation reference: https://github.com/TauLabs/TauLabs/blob/next/flight/Modules/Airspeed/gps_airspeed.c#L99. You'll have to implement the final calcs yourself to get tidal current but those should be straightforward. Sorry I can't get you the graphs from the original pull request, it went away with the dodo when OpenPilot forums were shut down.
  15. kubark42

    Waterproofing neoprene glove seams

    When I looked at my Henri Lloyd gloves (which should be a reputable brand!) I was flabbergasted that none of the seams were sealed. I got one glove wet and watched the water pumping and wicking through. The neoprene is waterproof, but the seams for all the (faux?) leather reinforcing are absolutely not. I found https://www.nrs.com/tech_talk/how_neoprene_is_stitched.asp, and saw that my gloves are stitched with the Single-Needle Stitch, which "This is the simplest type of stitch and is typically only found on thin neoprene gloves. It isn't waterproof, and it makes a bulge on the inside of the garment." I bet the VOR guys are using properly built gloves. The neoprene scuba gloves I have are awesomely warm, they're just too thick and inflexible. If I can seal the stitching I bet it will go a really long way to making my gloves comfortable AND flexible.