Getting neutral and a prop


#1

I am trying to get my OpenROV neutrally bouyant in water. I have tried small pill bottles wire tied to the side and pieces of wood screwed on the top. Check out the picture. It is a little heavy and I have to keep the vertical thruster on to keep it up. I hate to waste the power keeping the motor on. Any ideas on what to use for "floats"?

Tonight I changed the impellers. I think these work better then the ones in the parts list. The unit jumps out of the water at full power and you need very little movement on the joystick to get the unit to move. I think we need to be able to limit full power, because we have more than we need. I am still having trouble with the unit losing the connection when the battery voltage drops too low. Do you think you could limit the upper range of power by only using 50 percent of the range when you setup the ESC's.?

1842-OpenROV281newprops.jpg (204 KB) 1843-prop3intopview.jpg (163 KB) 1844-prop3insideview.jpg (137 KB)

#2

For buoyancy you can use ping-pong balls. What I would think about doing is adding another pair of tubes next to each battery tube. They can be made out PVC with screw on caps or simple slip on caps. I would make it positive buoyant and add sand or fishing weights/etc. inside the two tubes to reach neutral. You would than be able to reset your buoyant weight when you add payload such as an arm or such.


#3

I like the ping-pong ball idea. I can add as many as I need. The back is the heavy end. How would you attach the ping-pong balls?


#4

The props look great! Where'd you find them?


#5

I am looking for the invoice. I bought them a couple of years ago for my 3rd attemp at under water ROV. I will let you know when I figure it out. I better buy more for my 2nd OpenROV unit.


#6

I found the receipt

Dumas Boats, www.dumasproducts.com

part number 3108, 3 bladed brass propeller, 3 inch right, 2 inch pitch

I wonder if we should use a right and a left pitch for the thursters?

I have 2 rights installed


#7

I don't believe you would need a reverse rotation prop. The boat itself is so small that you are going to over/understeer it 90% of the time anyway. A counter rotating prop would require 2 versions of the code to swap the controls around for one and it IMHO a hassle not needed,

In addition to that I was thinking about the ping pong balls. Hot Glue them to the Rov?


#8

with counter rotating props, you need to swap to of the wires on one of the thrusters, then you change the direction of the motor. so no software change should be needed.

i have thought of this too, to have counter-rotating props. i think this is common on larger boats, but i'm not sure how it is on commercial rov's.


#9

I drew up a ping ball holder yesterday and printed it last night. Need to print a couple more now that I see the ball fits and will replace the intertube this weekend for more testing.

1845-pingballAssemblydrawingv1.jpg (139 KB) 1846-pingpongballholderwithballv1.jpg (124 KB) 1847-pingpongballholderv1.jpg (142 KB)

#10

Won't ping pong balls crush under pressure? You can easily dimple one with your fingers. Yes, the sphere is the strongest shape (I believe), but ping pong balls have a very thing skin.


#11

Ask, wait, and if you don't receive an answer fast enough...google it :)

1. http://physics.stackexchange.com/questions/8846/how-deep-in-the-oce...

Theoretic Answer: "70m The real crush depth will be between a half and a quarter of this value, matching the experimental value of approximately 30 meters."

1.A Answer roughly 30 meters. That link above goes to an annotated Mythbuster article which eventually says this: * Experiment 3: What happens to ping pong balls at 35 ft (ed: I'm assuming this was meant to be written as 35 meters). below? A: At 60psi (~90ft down) ping pong ball pops, so it shouldn't be an issue.

My take on why pops as described in this YouTube Video "Ping Pong Ball Crush"

http://www.youtube.com/watch?v=fh9CxQeN9nk

In that video the instructor states it could crush sooner than its theoretic value because ping pong balls are not an absolute perfect sphere, mainly the seam in them which leads to a weak spot or just a random manufacturing deficiency.

So personally, based on this evidence I wouldn't put too much value into using pingpong balls as a neutral buoyancy compensator at depths greater than 35M because they will fail long before you reach the operating depth (hopefully 100M) of the OpenROV.

Just my 2 cents, however it was cool that Mythbusters actually tested this theory. I wish I could find the original video.