Hi all,
I am interested in long-distance ocean-going drones.
To date, I am aware of only 2 craft that have made it across the Pacific; both WaveGliders. They are the Papa Mau and the Benjamin. Both released at the same time from San Francisco, it took 1 year for the Papa Mau and another 3 months for the Benjamin. They were both solely under wave-power using the SV2 version.
However, 2 more WaveGliders released at the same time failed to make it from SF to Japan.
Another boat, the Honey Badger, is a sailboat and made it from SF to Hawaii in 34 days. This design is different to a regular sailboat, but clearly it works.
Scout was an attempt to go across the Atlantic using solar power and a propeller but got lost about half way across.
The new SV3 version of the WaveGlider is now more of a boat-hull design than a surfboard and now possesses a solar-powered propeller pushing the underwater glider.
BlueRobotics are beginning to produce the T100 and soon the T200 thrusters, with 5lb and 10lb thrust respectively. Two T100s were used to power their SolarSurfer in an attempt to travel up the Californian coast but some weed fouled one of the thrusters (possibly a prop spur would be the right solution?). Plus, when the sun went down, there appeared to be just as much drift during the night than forward motion during the day.
An underwater torpedo glider like SLOCUM or Seaglider needs very little energy, but working at depths complicates things immensely.
And here is my question:
"If you wanted to go from one side of the ocean to the other, which design would you choose? And which route would you choose?"
If I wanted to go westwards across the Pacific, I could potentially use the equatorial currents and use OSCAR (Ocean Surface Current Analyses - Real time) to help catch the best currents but then a couple of days ago I saw a beautiful map at Global Fishing Watch (something Google are involved in) and saw that those same currents are highly fished, and so probably best avoided due to the danger of nets.
I like the SailDrone idea as there is less underneath to catch upon compared with the SolarSurfer and WaveGliders, but the guidance tech on the SailDrone is probably harder to master. Other sailboats don't appear to have much success.
To date, thrusters and props for this size of boat have not proven themselves of this type of endurance. I would like to see how well the T200s fair out at sea.
A good design might include:
- A hull that cuts through the water like the SV3 rather than gets bumped around like a surfboard.
- A glider with a thruster on the glider like the SV3, but with a tether that can be detached in case of emergencies e.g. fishing net entanglement which seems to be the biggest risk.
- A self-righting hull.
- Two or three T200 thrusters attached to the hull of the boat, but with only one in operation at a time as the other two are redundant until one thruster fails (problem: the glider below should be in front of the boat and pulling it along)
Thoughts?
Replies
That's nice work on the yacht. I don't think two pilots are necessary, airplanes or quadcopters have a lot more processing demands and they fly ok.
I think the code needs to be changed anyway, so the timing of the response can be tuned.
Here is a rough sketch-up of the electrics (for the record - never let me wire a house!).
One of the parts I am not certain about is the communication between Pixhawk and RockBlock. A requirement is that there is two-way communication so that course alterations are possible - maybe a Raspberry Pi/BeagleBone Black board in between them is necessary to handle this. Each message needs to be encoded so that it will fit within x number of characters to be within the limit of satellite messages.
Another option is a camera. If an image can be squeezed down and sent back home, it might be a nice addition to check on sea-state, the condition of the boat and the surroundings.
Hi,
The easyest method of connecting pixhawk with rockblock would be serial connection. That's pretty much wiring, or at most a simple electric adapter circuit betwen them.
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Pixhawk may also command the pomp and monitor for humiditywater level. Yet with more interfacing.
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I assume the the direct wire from battery is for battery level monitoring. Should rather use this module http://store.3drobotics.com/products/apm-power-module-with-xt60-con...
It has two functions - one is to provide a 5V source like a normal BEC. The other is to provide information regarding battery voltage & current usage to the controller.
This way it could also know how much current the actuators are using and detect certain malfunctions (e.g. a blocked actuator will manifest a high stalling current)
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For the sake of some added reliability I would use a pair of solar panels, each with it's own charger and battery
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The actuators also need some kind of power interfacing, (e.g. H-bridge and/or relays)
They also need a position traductor, to feed back actual position info back to the pixhawk
Unless they-re RC servos, then they can be powered through a 5V BEC, and their position is encoded in the RC servo signal.
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Now I see I've missed some of your messages, I'll folow them in a minute..
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PS I think the satellite short message system have too small messages to be usefull for image transmission. Or at least hard to aford, a 100kbyte jpeg image would require 500 messages
here-s another hull sketch, made from one rectangular 250x1000x3mm XPS foam sheet for its body, a pair of 5x10x1000mm wood (pine) spars for gunwales, and three small pieces of 6mm thick XPS for bulkheads.
The first photo is without bulkheads, so the hull has a sharp V shape. The bulkheads round a bit the V shape. I has no deck since it is only another building/shaping sketch.
Did not used a spar for the keel so it is not quite straight nor very rigid.
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It has a much finer length/beam ratio, of 9:1, more useful for a multihull.
The styrofoam could be glassed for toughness, but now I fancy with just making it from a harder pice of plastic sheet. Something in the weight/bending capabilities of plywood but still wanting to avoid plywood.
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Here are some random ideas on the subject:
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Actuators - there are at least two, one for the rudder and another one for wing tail. The easyest option is to use rc servos, but these have two problems. One is that it's hard to say how reliable are for several months or a year of use.
A RC servo failure points might be its intricate gears, its potentiometer used for position encoding, the motor brushes.
The second problem is they are full time powered, even if they use 100-150mA to hold position in long term that could be significant, 2 servos x 5Volt x 100mA is a continous 1watt draw, or 24Wh / day. A 1 watt of solar panel, in a dark cloudy day may give as low as 0.2 - 0.5 wh, which means that for the 24Wh will require 50-100 watt of solar cells installed.
What I woud use - Linear (screw-and-nut) actuators http://en.wikipedia.org/wiki/Linear_actuator#mediaviewer/File:Linea... powered by stepper mottors. These would be heavier and more expensive but have few advantages:
- holding strenght of the actuator position, without consumption of power. That means if rudder/tail need to be adjusted every 10 minutes on average, and the motor draws 1A for 2 seconds to get into the new position, that means 3-4 mA equivalent continous use. even at 10-15mA average, will need 1/10 the energy drawn by a small, less powerfull RC servo.
- high reliability of the stepper motor and the screw actuator vs rc servo
- easy to water proof with a.. rubber condom :P or finger cut from a rubber glove.
Another random idea, that complicates the robot, is
Using a water propeller powered by a brushless motor.
That could have two important roles,
one is the obvious - to propel the boat a few miles. Like when some delicate manouvering in a tight place is needed, or something got wrong with the wing.
another one is it could be used as a secondary power source, the propeller being dragged in the water would spin the motor that could generate some power. A couple watts could be enough to keep the pilot and actuators going. Or if batteries went down after a couple weeks of heavy clouds.
when it is not used as either motor or generator, the assumption is the propeller will spin freely, that it will have less drag than when the motor generates power.
Just an idea. A brushless, not having brushes can work submerged, yet it needs to be protected against corrosion. The rotor and stator can be coated with resin, continous spinning would prevent fouling in the stator gap, the problem is with the bearings.
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PS in heavy winds, too strong to sail, the spinning propeller, in generator mode, would some drag force, like a small drogue. Just enough to keep the boat aligned with the wind.
Very good effort on your model!
I have been reading this thread here:
http://proafile.com/multihull-boats/discussion-forum/viewthread/365/
Be warned, there is a LOT to read but I did pick up some good tips, plus Peter Worsley posted a few times too. My main takeaways are to make the sail taller and thinner than my original plan - and with a taper. Fat "Hershey Bar" wings do fly, but they are not fast, and I would like to go at least a moderate speed. Also, to place the tail a little further back and make it a little smaller - this gives sufficient turning force but because it is smaller the drag caused by the tail is reduced.
This is interesting reading:
http://www.duckworksmagazine.com/13/projects/rigid/index.htm#.VJ3Kh...
Note that it wasn't a great success. He was using ropes to put the sail in the right position which is not ideal. I also suspect that the wing profile he used (NACA 0012) was not optimum. I do like the wing construction. It is very simple. For the model stretched should be fine. For the real thing, the front of the wing will need reinforcement but I want to see how the stretched film does on the model. I'd want some heavy duty stuff.
I've been trying to source everything that I will need. The local model shop wasn't so useful (no balsa or nichrome wire) but there are some more stores here that I have to check out.
I've also been learning AutoCAD. I am currently terrible at it, but it is helping with picturing the superstructure and placement of the parts I can get here.
I have looked up water absorption rates for EPS and they look to be OK. Even with total immersion in water, it is only about 3% of the weight of the foam. Leaks in the hull caused by smashing into something are more important although shark bites might be more common. I will build in sections within the hull, so that water can't go sloshing about everywhere if there is a hole. I can always paint the fiberglass to help limit the effects of UV radiation - and paint the hull a special pattern to ward off curious sharks.
Anti-fouling is probably not needed as it will (should) be going too fast. There was a fair bit of wildlife on the Lquid Robotics craft, but nothing on the saildrone.
You are right about the stepper motor as it will hold its position. I need to decide on if the motor is directly attached to the cam, or if I use Bowden cables to push and pull (like in Worsley's model). The thinking behind this is to reduce the possible knocking/vibration by the mast getting transferred to the motor. Perhaps some rubber mounts and connectors will work to cancel out that vibration instead. A product like this could work:
http://www.trossenrobotics.com/store/p/5183-4-Inch-Stroke-110-LB-Li...
The same type could be used for the rudder although the movements are a lot more frequent. I read somewhere that they cannot be active the whole time and require about 90% rest time. That means that you can run it for 6 minutes of every hour.
If I am reading the following link correctly, the Saildrone moved the rudder every 2 and a half seconds on average.
http://saildrone.com/index.php/test/78-saildrone/88-100-days-at-sea
That is a lot of movement.
If the boat rocks around, will the wind-vane be better on a gimbal, or is it likely not an issue given that a trimaran or stablised monohull is used?
This is the Saildrone patent, and what I could gather from it:
http://www.google.com/patents/US20140261126
The primary hull has reduced bouyancy to cut though the waves (with half of the hull breaking the surface) and the outriggers raised higher to sit on top of the waves. If there is too much bouyancy, is there too much drag with the hull slapping on the top rather than cutting through the waves? As the hull is not designed to plane (you need a lot of energy to do the with multihulls), then a sharp V-shaped hull should sit OK in the water.
The tail has 2 settings - maximum and neutral (or 3 if you include maximum in the other direction).
In one configuration, the rudder is attached to the keel - although this was not used on Saildrone.
The patent mentions a rudder tab, but I cannot see it in any of the photos.
The keel design in the final design is slightly different to the patent.
Wireless and radio controls communication devices are used. I presume that one of them is used to communicate with the wing so that direct cabling is not required.
There is an accelerometer. This would give the roll, pitch and yaw of the boat - and the method they probably measure the number of waves bumped into.
A wing angle sensor. This would not be needed on ours if the wind-vane works OK, although it's quite a nice thing to have. "the wind direction is estimated based on a compass reading and a rigid wing angle". Is this a better way to do it?
Selectively floodable compartments. This allows the boat to mostly submerge for reasons of stealth or bad weather.
A weapon is one of the payloads, or attached to the keel.
The wing contains at sealed compartment to help ward against turn-turtling.
Yes you can use a smaller tail on a longer boom, But the smaller it is the slower it will move the wing when wind changes direction or speed. That can mean for a short time the wing could give more lift than desired and heel the boat in a capsize. The saildrone I think got it just right. Sorry if I missuse the sailing terms, I try to check them in this glossary I found http://www.schoolofsailing.net/terminology.html
------------For wind direction I think the free rotating wing gives a better aproximation than a wind vane in top of the mast. Yes it has a slightly offset from the tail action, but that can be meassured on ground .. There is a direct relation between tail angle and wing-to-wind angle, if the whole wing-tail assemble is aerodynamically balanced. Yes the boat rolls and moves with the waves but in a multihull it is relatively upright - if it is not racing, the heel is small.
A magnet attached to (inside) the wing close to two linear hall sensors on the mast should be enough for a reasonable estimation of the wing position. Very much the way an electronic compass works.
I think that's what they mean a wing angle sensor - a method to meassure rotation of the wing relative to the mast.
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"If I am reading the following link correctly, the Saildrone moved the rudder every 2 and a half seconds on average."
They say "wing tail cycles" and "rudder cycles". I think they reffer how many times the computer(s) checked they have correct position, most actuator movements within a couple seconds (less for the wing tail) should be either nil or minimal.
I just dismantled the model :) It was too wide and bulky for a multihull and too narrow for a sailing monohull.
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That foamed-ribbs and film wing is a nice example, I've found it too. That's pink XPS foam. It could have a leading and a trailing edge spar. You can cut ribs in foam with a sharp utility knife if the sheet is no thicker than 2cm. Nichrome wire you can find in an (broken)electric air heater or hair dryer.. Its curled there but carefully you can straighten it.. it's like soft steel wire.
What is interesting is the idea of window film for the cover of the wing. That can be quite tough, if it is the "security" kind used to prevent thieves smashing the window. It also has good UV protection, enough to protect the styrofoam ribs. It also heatshrinks. The problem might be the fact it normally uses water-based adhesive for gluing on the glass. So it's also easy to peel off with water.. probably that needs being washed off and use another, waterproof adhesive to glue it on the ribs.
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A long time ago I started a wing like that, 2m long, 40cm wide. Naca 0015 or 0016 I still have its skeleton around. The second picture is a detail of the trailing edge, with a rounded spar and thin aluminium sheet for the steep angle.
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These are the two patents:
1) http://worldwide.espacenet.com/publicationDetails/biblio?DB=EPODOC&...
2) http://worldwide.espacenet.com/publicationDetails/biblio?DB=EPODOC&...
I've posted on another forum to get some feedback on which hull would be optimum. There are pros and cons to all of course:
http://www.boatdesign.net/forums/boat-design/best-hull-autonomous-w...
Multi-hulls are inherently more stable and less prone to capsize, but getting the hull and balance right is crucial. I guess the key is to make the whole boat inherently unstable when capsized. Instead of the floats being the same length as the center hull, maybe they should be shorter. I have no idea what the technical term is...but I'm going to go with 'Leaning Jowler' from the game Pass The Pigs (see link below). When capsized, the boat should at some point roll into this position and then pop back up.
https://c1.staticflickr.com/3/2348/2303153546_9701f655dc_z.jpg?zz=1
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