With the BlueRobotics team significantly lowering the cost of admission for marine robotic applications and 3DR manufacturing the highly successful autopilot, Pixhawk and Pixhawk 2, I'm of the opinion it might be time to do some research into an affordable AUV that is more than a "one-off" and doesn't cost $50,000.
I'm in the process of finishing up my OpenROV Mid-Sized Observation Class ROV and turning my sights toward other projects to meet requirements in the marine sector. Rusty and I have had intensive discussions on the future of area, and its very bright. Although I'm starting to gather parts for an ASV project, an AUV is definitely on my list for something that I can drop off the side of a boat and quickly run a transect scan on a designated target and return with photos of the seabed.
Looking into the problem, the biggest hurdle is a vessel knowing its position underwater, or at least a decent estimation to guide it. From reviewing the wiki on the APM Extended Kalman Filter, it looks like this would fit the bill in "plane" mode. The EKF takes measurements from all available sensors and can work out position and velocity, even without GPS for short periods. Obviously, GPS corrections will be required periodically to prevent the propagation of error from becoming too large.
Although we can use GPS, the INS and magnetic compass, possibly the optical flow sensor, we would still need a depth sensor, sounder and associated interfaces to Pixhawk. I'm not quite sure how to develop and interface those. Maybe substitute the depth sensor for the barometer? And an acoustic sounder for the LiDAR?
In the very least, I am confident I can get a hull put together using the BlueRobotics 4" enclosure at maybe 36" long, and then 3D print a bow and tail section with a GPS and communications mast. I was looking at using either the MARES or COTSBot for the basic design layout. I favor using thrusters over control planes because 1) we don't have a reliable deep actuator 2) the less moving parts, the better.
Just some thoughts, as I'm not very far along designing this one yet, but I thought others might be interested or able to give their input. This intended to be a simple design that I place over the side, get it's bearings, dive, run a straight line about 10ft off the bottom, surface, get a fix, turn around and dive again to "mow the lawn". I've been working with others on imaging the seabed with Structure from Motion software to make up 3D models.
A very nice design to say the least.
I assume that since you are designating it an AUV that it will be used for search, survey or surveillance purposes?
I have no experience with developing marine robots or AUV, ROV but let me know your opinion if any such vehicle cluld be developed as Lego Kit,
made of evacuated, sealed, stainless stell tube and have thrusters, battery unit, controller, sensors, radio, wiring simply attached to the tube (waterproof sealed)\
to let such AUV, ROV, marine robot Lego Kit to go for tests in swimming pool.
We risk nothing since thrusters can come with individual LiIon/LiPo batteries,
just attached to the tube, camera can be set on/off remotely.
5 thrusters, in theory, can be controlled remotely, individually from a ground/surface station,
calculating geolocation based on telemetry callbacks (heartbeat clocked).
So autonomous thrusters are attached to the evacuated tube at selected places,
autocalibration procedure is run to get correct control parameters
and let's go.
Ground station can be 1 channel protocol sending prefixed signatures/ messages, read by individual thruster ( thruster name, command (engine speed: 1-100), on/off, time on/time off, turn left/turn right Boolean variable: yes/no).
Do you need 5-6 or more channels, ok, just select the required number of virtual channels towork for you.
Slow transmission protocol should work fine in underwater environment.
Any idea ?
If you are going to use an extended BR 4" Series Acrylic Tube for the AUV hull, how do you propose to attach the Thrusters to the Hull Tube?
I guess that you could use modular extensions added to the front and rear of the Hull Tube to mount the vertical and horizontal thrusters?
You might want to take a look at this older AUV discussion thread.
Upon reviewing your OpenROV Mid-Sized Observation Class ROV I would recommend that you forgo the metal frame and brackets for either ABS or HDPE sheets and stainless steel screws especially if the ROV is going to used in salt water.
You might want to take a look at my ROV Project here.
Thanks for your input, having an expert like you coming in is definitely appreciated.
I was gearing this first one more towards search and survey rather than surveillance. All the data and imagery it collects will have to be post-processed after the vehicle is retrieved. I'm quite sure others can come up with more advanced payloads, but my programming skills are pretty lacking.
Yes, I was planning on using 3D printed rounded extensions to the bow and stern with holes for the vertical thrusters and integrate side mounts for the horizontal ones. I'll make up a drawing here soon. Where I could, I was going to fill it with marine epoxy to make it more rigid.
The one design challenge I am going to have is I haven't decided how I want to mount the nose and tail sections to the endcaps. I think I have one of two options:
1) Mount it to the bolts that secure the end flanges
2) Create a bit of an extension to the cones and band them to the cylinder
I really like your ROV and I think it will do great, but I lack the heavy machining tools you have to cut ABS or HDPE. I went with the Makerbeam (6063-T5 aluminum) because it was pre-cut and easy for me to assemble and make modifications to. I also wanted to make something repeatable for someone else instead of a "one-off". Back when it only had 3 thrusters and was fully functional, I had it in the open ocean for about an hour, rinsed it off and had no corrosion or pitting either on the frame, brackets or bolts. I'm pretty happy with it so far. It should be finished by the end of the month.
I'm not up on the current Lego offerings these days, but if you have a control system in mind, go for it. I picked the OpenROV control for my ROV because it met my requirements for direct control with an HD video feed. I'm choosing to go with Pixhawk for the AUV because it handles autonomous control better. It's all a matter of control, not necessarily motors or pressure housings, I've seen pretty much everything used.
To attach the nose and tail cones to the sealing flanges, I would recommend printing them so that it inserts slightly into the inner diameter of the flange and you can glue it in place. You won't need a face-seal O-ring at all. No matter the printing method, I wouldn't trust a 3d printed surface against an o-ring.
Are you planning to have the 3d printed surfaces be a pressure hull? If not, then I would use a normal flat end-cap on the flange and find some way to attach the nose and tail cone. This might be best because you won't have to worry about the permeability of the material or the strength of the walls. Just leave some vent holes for water to flood and drain.
Do you know the displacement (density) of the 4"Series Acrylic Tubes per foot of tube, offhand?
Tom C AVD
Just the tube is about 190 in^3 per foot and weighs 1.67 lb per foot. You have to remember to add in the weight of the flanges and everything as well to get net buoyancy.
You can enter the total displacement and weight on the "Buoyancy Calculator" here to get the net buoyancy. It's about 5.2 lb net buoyancy for one foot of tube but that doesn't include flanges, end-caps, etc.
Sorry Rusty, my descriptions are garbage sometimes. Below is how things would get sandwiched together:
The nose/tail cones would bolt on over the top of the endcaps using recesses (not pictured) in the cone itself.
The printed pieces are not designed to be pressure resitant, only the enclosure. I would fill as much as possible the inside to add some rigidity, but other than that it would be free flooding.
Looking dead on, the nose could probably go all the way around, but the tail would have to have a section missing to access some of the penetrator locations for the vent valve, on/off switch, and a charge port so I don't have to take the thing apart each time.
Thanks for the info, much appreciated.
I am trying to get an idea of the buoyancy of Kevin's design using a 36 in 4" Series Acrylic Tube.
Even with batteries in place he may have too much buoyancy?
Tom C AVD
Okay, great! I think this is simplest and best route to go. It'll be nice that you can have penetrations on both ends of the enclosure. Maybe you can find a way to quickly remove the tail cone so that you don't have to leave exposed access points for the vent and such.
Okay. The tube will be about 570 in^3. Let's guess that the rest of the parts like the nose cone, tail cone, thrusters, etc, have about 150 in^3 of displacement (they won't be positive, of course, but they do contribute to displacement). That's 720 in^3 total resulting in about 26 lb of buoyancy so the total weight of the vehicle would have to be about 24-25 lb as well to reach slightly positive buoyancy.
With the calculator, you can put in that volume and look at the "buoyancy" section to see the total buoyancy. Then put in the vehicle weight to the other box to get net buoyancy.
24 lb seems like a pretty reasonable weight for a vehicle like this with a big enclosure, lots of batteries, and a few thrusters. I think it will work out nicely.
Spent the better part of today working on designs in 3DS Max. I'm still pretty terrible in AutoCAD, but I'm working through the tutorials when I can.
So from the first picture, you can see that my original idea of putting the vertical thrusters inside the nose and tail cones won't work. Already the thruster shroud pokes through a bit of the mesh on either side.
So on to the next revision. For this, I simply placed the thrusters on each end. I'm wondering how rigid this will be.
Below is the 36" version.
Below this is the 24" version.
I'm not going for super maneuverability on this one as it's mostly going to be used for long transects, but I'm thinking the 24" might be a better start and expand into the 36" if need be. Thoughts?
Also, any recommendations between the T-100/200 for something this size and power? I'll get into what I'm thinking for batteries in my next post.