Homebuilt ROVs Forum

UPDATE!
I have had the flame reignited, and work is continuing on my ROV... beginning to think Molasses me be an appropriate name for this thing!

Major updates since my last post:
* 4 - Blue Robotics T100 + BLUE ESC thursters have been purchased!
* Hull (re)design is underway. First test dive scheduled for this weekend! (Memorial Day weekend)
* Tether Design underway
*TopSide Controls may be upgraded to either 2 Arduino units, or one Mega... Added GPS and an additional LCD display screen...
* Target completion date... July 4th, 2015!

I will be adding some notes from the process in the coming days!

Happy Diving!

Ryan
"KR2_Diving"

*** EDIT***
It has come to my attention that some of my numbers are WAY off... gotta love that pesky decimal point... stay tuned... changes coming soon!

Changes added 5/21/15!

Tether Design Update!

So today I received an Amazon box full of 144 ping pong balls! I have decided to attach the ping pong balls to my tether to aid with buoyancy.

To calculate how many ping pong balls I ACTUALLY need, I need to understand a few key concepts:

• 1. Neutral buoyancy is a condition in which a physical body's average density is equal to the density of the fluid in which it is immersed.
  2. If the object weights less than an equivalent volume of water, then the object will float; if it is heavier, it will sink
  3. A submerged object displaces a volume of liquid equal to the volume of the object.
  4. Density is calculated by dividing the mass by the volume.

I measured out my tether and found I have 222ft (67.6m) of cable, which weighs in at 9.84lbs. (4.46kg)

It was a little bit tricky to find the volume of my tether... so I performed, what I am going to call, the "Double Bucket" test.

I proceeded to place a 5 gal bucket inside an empty storage tub. I then filled a 5gal bucket to the brim with water. Once full, I carefully submerged my tether in the 5gal bucket, allowing all the displaced water to pour over into the storage tub. Once my cable was fully submerged, I carefully removed the 5 gallon bucket containing the water and my cable from the storage tub. Next I weighed the storage tub containing the overflow water and subtracted the dry weight of the tub. Based on these numbers, the volume of water that my tether displaced is 202.8 in^3 (3310 cc).

With all the data i was collecting and generating, I thought it was best to arrange it all in a handy spreadsheet!
Now that I know the weight and displacement of my tether, I can determine the additional buoyancy required to make it neutrally buoyant. (NET BUOYANCY)

My tether displaces 202.8 in^3 (3310 cc) of water, which weighs 7.3 lbs (3.31 kg). So the buoyant force already acting on the tether is 7.3 lbs (3.31 kg).

Now it is simply a matter of balancing opposing forces: 9.84 lbs (4.46 kg) of downward force (otherwise known as gravity or dry weight in this case) opposed by 7.3 lbs (3.31 kg) of upward force (buoyancy). For the tether to become neutrally buoyant, the remaining negative buoyancy must be offset: 9.84 lbs (4.46 kg) - 7.3 lbs (3.31 kg) = 2.54 lbs (1.15 kg) of additional buoyancy needed to make my tether neutrally buoyant.

In my tests I found that 1 Ping Pong Ball provides 0.06112 lbs (0.0278 kg) of positive buoyancy. So for my last step, I took 2.54 lbs (1.15 kg) / 0.06112 lbs (0.0278 kg) to find that I need approximately 41 Ping Pong Balls to make my tether neutrally buoyant!

I also found another formula online to try out which uses basically the same numbers, but slightly different math (and a few different unit conversions) to come up with a similar figure. I won't break it down step by step... but here is the math below:
In this case, there is different rounding and more/less decimal places, which would explain why the results are CLOSE but not exact. (39.7 Balls VS. 41 Balls)

So in conclusion:

Conclusion wrote: My 222ft tether (5 sets of twisted pair, shielded) will require around 40 ping pong balls to make it neutrally buoyant.

Hey,

nice experience.
11.8 ping pong balls are correct

But there are two things i don't understand:
How you want to mount the ping-pong balls to the cable?
And how deep you think you can go before the balls collapse?

ROVER3D

ROVER3D wrote: 11.8 ping pong balls

Hey Rover3D,
Not sure 11.8 was based on the right figures! I dug out my old SCUBA diving Manuals and found some better math to work with...

I updated the post above and i think these numbers seem more accurate.

As for the performance of the ping pong balls... i hope to run some tests this weekend!

Hey KR2_Diving,

Congrats on getting to the point you are...I'm so far behind on my project! I WISH July 4th would be my completion date! I'm finally back to daily work on it though and I'm currently just finishing my design on a hydrostatic chamber and will begin building that next week. I'm shooting for depths of 300+' and the chamber should come in very handy. Maybe I'll get to test some ping pong balls too!

Re: your tether - you mentioned a 1 core twisted pair, but then another place 5 pair? Is that right? So a souped up ethernet with an extra pair?

I've been reading through your calculations as I need to do the same soon for a tether. I am designing the guts of my rov to work with either cat5 or fiber. I have cat5 five currently. My question is, as detailed as your calculations are with the ping pong balls, are you concerned with the potential air within the sheath of your tether? Or perhaps you have a solid core cable?!

I know I've got plenty of air in my cat5 and am considering striping the shealth and stringing it inside of nylon braided rope. With a 75m tether I'm sure you want to explore some good depths and I think the tether may get heavier on you as you submerse. Perhaps I missed something on your project page...I've just read the most recent posts.

Looking forward to learning how your tests turn out.

Cheers

Today I did some more trial and error testing to find out the best way to attach my ping pong balls to the tether... and I think I have a final plan.

Here were the two version I tried. Option 1 used small zip ties and the polyester sleeving.
Option 2 used heat shrink and the polyester sleeving.

After trying them both, I decided to go with the zip tie method.

When working with the polyester sleeving, I did make one additional observation that is going to require me to get a new toy. I noticed that the end cut in the factory with a HOT KNIFE had significantly less fraying then the side I cut with my scissors. So with this in mind, I am going to hunt down a hot knife that i can use to cut my poly sleeving!

For those of you that are curious, here is a part breakdown of the components I am using: A.) McMaster Carr part #7568K11
Nylon ID tag Cable Tie $6.75/Pack (50pcs)
(Great for marking length on tether so I know how much tether is in the water!)

B.) McMaster Carr part #71295K62
Colored Nylon Cable Tie $2.20/Pack (100pcs)

C.) McMaster Carr part #9284K6
Expandable Polyester Sleeving $10.16/10ft roll

D.) Ping Pong Balls from AMAZON.
$10.45 for 144pcs (See Note Below)

Note: These balls are CRAP. This particular listing is full of lies. I have complained about it to Amazon, and in turn, they did provide a refund. For the moment, I am proceeding with these inferior balls, only because I have already begun the assembly process. I might change my mind before i am finished... I will let you know...

My plan B for balls is to buy from my local Dick's Sporting Goods.
These balls work out to be $0.53 / each, but are a FAR superior quality to those I got from Amazon at $0.07/ball. (And I can pick them up on a whim!)

I have learned a lot about ping pong balls in the past week!
Key things to look for:
40mm is the standard size
2.7g is the standard weight
The higher the number of stars, the better the quality.
Only 3 star or higher are permitted for tournament play!
Even the World Beer Pong Championship has strict rules on ball quality (40mm 3 Star!!)
Most "bulk listings" are for practice or hobby balls, not for good quality balls.
Best price I have found for GOOD quality balls is HERE. ($0.27/ball)

Hope this helps someone out!

Happy Diving!
Ryan
"KR2_Diving"

At present, my concern in the cheap balls are likely to deform under pressure... they are quite "soft."

Excellent way of securing them to your tether!

Ryan,

I was looking at your code on page 5 of this thread,

I was wondering why you are using a low/med/high output and not fully proportional?

I'm writing the code for my next project which will feature on here soon, but i've come up with a formula (i say formula, 4 lines of code in arduino) that will give fully proportional thruster control, for vectored thrust if you are interested.

My intention is to have fully proportional output for vectored thrust, with possibly a pot to control max thruster power.

Bindo

Bindo wrote:Ryan,
I was wondering why you are using a low/med/high output and not fully proportional?
Bindo

Hey Bindo,
I know at the time, I was struggling to find a good way to run this proportional while only using one joy stick!

i would be very interested in seeing what you came up with!

BR,
Ryan
"KR2_Diving"

Haha i feel your pain.

I actually used an excel spreadsheet to work out my calculations, it took me about 4 hours to come up with this. (though to look at, you wouldn't think so! it was more of a EUREKA! moment )

rxdata.surg = right stick forward/backwards = fore/aft
rxdata.sway = right stick left/right = left/right
rxdata.head = left stick left/right = rotate left/right


I'll see if i can somehow share the spreadsheet once i get to work, left my pen drive in my works computer!


Bindo