A 3D printed, raspberry-brained ROV project
Posted: Apr 7th, 2021, 6:49 pm
Hello everyone,
Greetings from France. As some reported virus still locks us there, I took advantage of the last few months to immerse myself into a project that had been haunting my mind for a while. I have indeed been sailing across the atlantic coast since my young age, but since I'm a better engineer than a scuba diver, I wanted to build myself a nice-looking drone to be able to explore the depths of these beautiful areas.
I decided to make quite a few unusual design choices regarding the hull and the propulsion system, motivated by the opportunity to broaden the technical aspects of the project and obtain a nice hydrodynamic design in the end. You can judge for yourself. Here is a snapshot of the unfinished CAD assembly, with still a lot of work to be done (I actually did not expect my software to struggle that much on the hull drawing!):
The overall length is about 50cm. As you may see, one big specificity is these two front thrusters mounted on rotating axes which can perform a 150° rotation in both direction (forward/backward, from the vertical position). These are connected to stepper motors that adjust the position based on the feedback of the IMU. The control program is handled by a raspberry Pi, linked to the surface through a CAT6 ethernet cable (which is one of the last items I haven't purchased yet).
This will provide 5 degrees of freedom, which it enough for a natural pilot control.
apart from that, the main specifications are :
- top speed of 4-5 knots (the front brushless motors can provide 2 kg of thrust each, it should be enough)
- rated depth: 30-50 meters (deepest values I could find in the areas I could explore)
- video stream with low latency
- slightly positive buoyancy (as most of ROVs)
- keep price as low as possible
Another specific aspect of this ROV is the hull structure. I am going to use my 3D printer to print the outer skin of the hull (2 mm thick) to replicate the CAD model accurately. This skin will be reinforced and waterproofed on the inner side by a fiber glass + epoxy composite (2-3 mm thick). The hull will eventually be constrained by a few perpendicular frames to provide more rigidity. I hope this will be enough to handle the pressure of 3-5 bars.
For now, I am still working on the hull design (especially splitting it in a lower and and a removable upper part), but the main blocking points I see coming are:
- a waterproofing solution for the thruster axes. I tested simple O-rings surrounded by a 3d printed shaft but it provided too much friction. I am thinking of a "twistable" hose that would be pressed on the hull on one side and the axis on the other side, but it requires a thin, flexible tube and I am not sure if it could handle the pressure
- the printing of the outer hull, that produced retractions on a few tests I made with ASA (similar to ABS in its printing properties) and inaccurate dimensions on the XY plane (of the printing. The Z axis of printing is the forward axis of the ROV). It is gonna be a problem when I subdivide each side of the hull because I can't print each side at once. (because of overhangs)
- the forming of the front glass, which is made of a 4 mm layer of plexiglas that I will try to bend in an oven. I hope this won't lead to high optical distorsions. I don't know either how to do the mold yet.
Greetings from France. As some reported virus still locks us there, I took advantage of the last few months to immerse myself into a project that had been haunting my mind for a while. I have indeed been sailing across the atlantic coast since my young age, but since I'm a better engineer than a scuba diver, I wanted to build myself a nice-looking drone to be able to explore the depths of these beautiful areas.
I decided to make quite a few unusual design choices regarding the hull and the propulsion system, motivated by the opportunity to broaden the technical aspects of the project and obtain a nice hydrodynamic design in the end. You can judge for yourself. Here is a snapshot of the unfinished CAD assembly, with still a lot of work to be done (I actually did not expect my software to struggle that much on the hull drawing!):
The overall length is about 50cm. As you may see, one big specificity is these two front thrusters mounted on rotating axes which can perform a 150° rotation in both direction (forward/backward, from the vertical position). These are connected to stepper motors that adjust the position based on the feedback of the IMU. The control program is handled by a raspberry Pi, linked to the surface through a CAT6 ethernet cable (which is one of the last items I haven't purchased yet).
This will provide 5 degrees of freedom, which it enough for a natural pilot control.
apart from that, the main specifications are :
- top speed of 4-5 knots (the front brushless motors can provide 2 kg of thrust each, it should be enough)
- rated depth: 30-50 meters (deepest values I could find in the areas I could explore)
- video stream with low latency
- slightly positive buoyancy (as most of ROVs)
- keep price as low as possible
Another specific aspect of this ROV is the hull structure. I am going to use my 3D printer to print the outer skin of the hull (2 mm thick) to replicate the CAD model accurately. This skin will be reinforced and waterproofed on the inner side by a fiber glass + epoxy composite (2-3 mm thick). The hull will eventually be constrained by a few perpendicular frames to provide more rigidity. I hope this will be enough to handle the pressure of 3-5 bars.
For now, I am still working on the hull design (especially splitting it in a lower and and a removable upper part), but the main blocking points I see coming are:
- a waterproofing solution for the thruster axes. I tested simple O-rings surrounded by a 3d printed shaft but it provided too much friction. I am thinking of a "twistable" hose that would be pressed on the hull on one side and the axis on the other side, but it requires a thin, flexible tube and I am not sure if it could handle the pressure
- the printing of the outer hull, that produced retractions on a few tests I made with ASA (similar to ABS in its printing properties) and inaccurate dimensions on the XY plane (of the printing. The Z axis of printing is the forward axis of the ROV). It is gonna be a problem when I subdivide each side of the hull because I can't print each side at once. (because of overhangs)
- the forming of the front glass, which is made of a 4 mm layer of plexiglas that I will try to bend in an oven. I hope this won't lead to high optical distorsions. I don't know either how to do the mold yet.