A raspberry pi based submersible

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I have always thought that submersibles were cool. This is the 4th one that I have built over the years. Technology keeps getting cheaper, and capabilities more advanced. The ROV runs on two LIPO batteries, brushless DC motors, and a raspberry pi for the brain. A CAT5 cable runs from the ROV to a laptop computer which streams live video. OpenCV is used for the capture. The control of the ROV is a Nintendo nun chuck. I experimented running some closed loop control based off of image processing but found that the lag of the system was too much. I was running an old laptop and one of the orginal raspberry pi's.

When I do my 5th ROV I will incorporate the following lessons learned.

* Streaming good video over CAT5 with a pi was always a sticky point. I ended up using Gstreamer. It seems like I tried so many things and it always lagged. I was able to get a video stream after experimenting. 

* The next ROV will have 5 thrusters and an on-board IMU that can actively level and control. When you have such a small craft that is neutrally buoyant  any forces want to roll it, or cause it pitch. An active system is well within reach.

* Interacing servos to a pi requires that do some magic. I would use an offboard chip to generate the servo pulses for the motor controllers. 

* I ran AFRO ESC's with modified software on them. They worked excellent. You need ESC's that are capable of reverse.

* I was always a little timid sticking LIPO batteries into a sealed container while running them. For batteries I will switch to LiFePO4.

* The tether is always difficult. There are some expensive neutrally buoyant tethers out there, I am not sure how to solve this one yet. 

Here are some explanations of the controls while operating in the bathroom.

Here is the first dive.

Here is an example of closed loop control using openCV.

  • How to Seal Water Tight

    Tom07/30/2019 at 03:12 0 comments

    There was a question on how to create the water tight seal.. here is a piece of aluminum being machined on the Cheap X1 Mill conversion project. The aluminum acts as a flange between the PVC housing and the lexan plate. The aluminum is always attached to the PVC, and the lexan can be bolted and unbolted from it. An Oring sits in the grove just inside of the screws to provide the water tight seal. 

    The O-Ring is not shown the picture below. 

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Maximiliano Rojas wrote 08/11/2019 at 23:00 point

With respects to the IMU and stuff I think that you can use a 9 DOF for full 3D orientation because the rotation in Z axis drifts (like the others angles) because of the error integration in time, so with a 9 does you can use a complementary o Kalman filter to keep data stable and then applied a PID or something.

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Maximiliano Rojas wrote 08/11/2019 at 22:58 point

Nice  Work! I always wonder if I can use ruthless motors like ds2830 in water without any protection against water, and if its applied to salt water too Do you know if it's possible?

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Daren Schwenke wrote 07/30/2019 at 06:39 point

Would be interesting to see if optical fiber is closer to neutrally buoyant.. You can get optical fiber converters for about $25 each, or a pair of BiDi (uses a single fiber for both directions) converters for about $75.  The latter are good for about 12 miles of fiber length.

As for the sealed container concerns, install an air pressure relief valve.  It will never open unless the inside pressure gets up to whatever value you pick.  The common ones are rated around 130-160psi.  Although if your LIPOs are venting you have much bigger issues.

Lastly, you should be putting the majority of your mass (batteries) as low as you can for more stability.  Commercial craft often sling them underneath in separate tubes.

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Tom Nardi wrote 07/17/2019 at 06:06 point

I've been working on a ROV myself in my free time and made some similar decisions. Would love to see more info on this build, might help me fill in some blanks on mine.

Particularly curious about the watertight compartment, I can't quite understand from the pictures how it works. What are the bolts going into?

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Tom wrote 07/30/2019 at 03:07 point

The hull of ROV is PVC, which has an aluminum flange that is permanently attached to the PVC with a bead of silicon and screws. That piece of aluminum has 8 tapped holes to allow the lexan plate to attach to it. The seal between the aluminum and lexan is a large O-ring that gets squeezed down as the bolts are attached. I will attach some pictures as well in the details.

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