ROV Little Pig

[Lost Bob]

I have been sitting on the fence, trying to decide what my next hobby is going to be. ROV or Muti-rotor ? Then, I was at Michael's craft store last weekend and saw these 10" dia. white styrofoam balls and it got me thinking about a round ROV. Here is the result.
Basically, it is just some pvc covered with a fibreglass sphere ( using the ball for a mold ). All the wiring will be contained inside the sphere to prevent snags.
I am planning on using a PS2 controller & Arduino mega connected to my laptop for display. My gopro will be hardwired for real time video. ( saw the mod somewhere, have to find it again )
I will use 3 sets of 2 thrusters ( brushless with reversible esc ) to counteract any spinning ( 6 total ).

front

sphere 2a - front.png (151.36 KiB) Viewed 4223 times

Above : 6 brushless thrusters with a couple of led lights and my gopro behind a clear dome ( 4 " dia. main housing )

front left

sphere 2a - front left.png (312.58 KiB) Viewed 4223 times

Above : The 2 vertical thrusters ( T-3 & T-4 ) spin in opposite directions with counter-rotating props.
They are also slightly forward of centre so the nose will ( hopefully ) tip down when I submerge and tip up when I ascend.

bottom left rear

sphere 2a - bottom left.png (159.03 KiB) Viewed 4223 times

Above : The 2 lateral thrusters ( T-5 & T-6 ) both spin in the same direction but with counter-rotating props ( to strafe left/right ).
They are also off-set from centre, one forward and one rearward. Spinning either thruster independently will allow the rov to spin on it's axis, as well as aid in basic turning.

top right rear

sphere 2a - rear right.png (206.83 KiB) Viewed 4223 times

Above : The 2 horizontal thrusters ( T-1 & T-2 ) spin in opposite directions with counter-rotating props. They are parallel to centre so it goes straight forward and reverse.

I am not sure how much I should enclose my thrusters. If you look at the drawings, I drew the left side with the motors exposed down to the mounts.
On the right side, I drew them partially enclosed for better protection. How much should they be enclosed ?

Any comments/thoughts would be appreciated. I will post a parts list asap.
Feel free to PM me if you would like the original sketch-up file.
Thanks

Lost Bob

[sthone]

It looks pretty good, I have some concerns about the lateral thrusters though. You probably don't need two of them (T5 & T6) as you can get the Rov to spin on it's axis with the horizontal thrusters by running one in forward and the other in reverse. Positioning the lateral thrusters at the bottom like that might cause it to sway upwards when used too. (this might depend on the balance of the buoyancy though.)

I see what your trying to do with the position of the vertical thrusters too but if you are constantly using them to maintain depth which is required sometimes (again depending on buoyancy setup) your view is going to be constantly bobbing up and down as it tilts the Rov. You might be better off placing them on the center line and using a tilting camera setup to get the best view.

Overall I like the design it brings new meaning to the term eyeball Rov.

-Steve

[Lost Bob]

@ Steve : thanks for the tips. I appreciate the input from someone with experience, since I have none. I am going to move the thrusters like you suggested. I will move the vertical thrusters in line with centre, same with lateral. I am worried it might just spin in a circle with only 1 lat thruster.
Thanks
Bob

[autonomousROV]

Very nimble.

I second Steve's thoughts on vertical thruster placement. Given your design, things need to be reasonably symmetric. Rectangular designs have a bit more natural resistance to unwanted movements because the mass/buoyancy is spread out. Think of an ice skater spinning...with everything close and circular it will spin easily. Move things out to the ends of a rectangular box and it is less apt to move with every random force.

I disagree with Steve a bit on the lateral thrusters though. One of my biggest issues is getting on a object, but wanting to be a few inches left or right, but still pointed at it. Think of a minor current or tether pulling laterally on you. Do you want to spin left, go forward and spin back right? Na...just a little left "sway" and your back on it. I would keep at least one, but move it to the top (between or in front of vertical thrusters?) so you do not stir up silt and cloud the water. It can be smaller since you will not use it much...just like a plane rarely uses its rudder.

I would also make sure you have enough buoyancy so you can put some serious weight into the bottom. There is mass up high (eg. thrusters) and it needs to be offset. I calculate that you have 18.9 lbs of buoyancy (freshwater) with a 10" diameter ball. It sounds like it should be sufficient, but keep everything as low as possible inside and minimize the hull weight. PVC is heavy.

Since this is your first build, budget double for everything...time and money.

Ironically I've caught the quadcopter bug again and have been on the fence about buying one. I've flown them before at work and have have flown larger RC planes and helicopters for years. Sold the helis and all but 2 planes a few years ago. Not enough time to make it to the flying field. A quad I could use in my back yard.

Keep us updated.

-Greg

[Lost Bob]

@ Gregg : Thanks for your comments, I have already redrawn the rov with the vertical thrusters centered, now I am trying to reposition the laterals. Do you think they will stir up the bottom much since they are not pointed down but are aimed sideways ?
Weight wise, keep in mind the sphere part is just a hollow shell. I kinda figured I would have to play around with the buoyancy since I have so much weight up high. Just hoping I don't have to add weight for balance.
Will post new pics in a few days, getting better at using Sketch-up.
Thanks

-Bob

[autonomousROV]

regarding silting...I work in murky fresh water (5' or less vis typically) so I can never get close enough. If you get close to an interesting item on the bottom you will touch bottom and silt is relatively weightless underwater so it does not take much for it to stir up.

That said, if you know this as a limitation you can deal with it. Horizontal is the least used thruster, and you typically only need a small thrust to get back on the object. Personally my vertical (heave) and lateral (sway) are always as high as possible, but your design might be condusive to a lower, perhaps tunnel design. I would not worry too much about it at this point.

Weight wise, solid or wet hull will not make a difference in performance since the internal water will be trapped. With the wet hull I would get everything built and then fill the top half with closed cell foam or similar material, then put lead in the bottom-most place you can get it. Overall your pig will have the same external resistance and internal inertia characteristics unless you open some very large holes in the walls for water to pass through. Use that to your advantage to make it as stable as possible.

Also, perhaps you should point your camera housing down a few degrees? The intersting stuff is always below. That may allow you to put your lateral thruster on the back, just below the opening, since you could move that up? That will keep silt away from view and weight low.

[sthone]

I think you misunderstood what I was saying, I meant he didn't need two.... but one is definitely a must in my opinion too. Especially if you ever plan on using tools such as a manipulator, the ability to move lateral makes a huge difference.

As for closed cell foam be careful what you use, after watching that Openrov video where they do some pressure tests most readily available foams can't handle much.

-Steve

[Lost Bob]

@ Gregg : I thought about angling the main tube like you suggest but I wasn't sure if I should just build pan/tilt for the camera instead.
Filling the top half with foam is the plan. I was thinking of putting my 4 lipo's in a separate tube under the main hull, moving it fore/aft for balance.

@ Steve : I watched that video, cool the way they expand again after being pressurized. Ever try that 2-part polyurethane foam that you can buy at auto stores ?

-Bob