draw, thrust, and components

Anything to do with Propulsion.
a_shorething
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Re: draw, thrust, and components

Post by a_shorething »

That is impressive, but I have to ask about power draw and battery life.

I think the 4s is in the 4 Amp (4000-5000mAh) family of batteries, right?

So that means it can deliver 4000mA of power for an hour and then it's done. If you use more mA then you get less than an hour and if you use less mA you can go more than an hour.

So just like any vehicle your ability to continue to move (and how fast) is going to be a relationship of a LOT of factors with power to weight being one of the main ones. A Lotus is a fast car even though it's got a tiny engine because the car itself doesn't weigh much while a Corvette has to have LOTS more power to get the same performance.

I'm wondering where is the 'sweet spot' for thrusters and for ROVs in general when it comes to performance for a given weight of thruster and cost.

I'd say the average weight of a homebuilt ROV is just a few lbs to up to maybe 10-15 at the most (just guessing) and the ability to get where you're going is limited by lots of factors like visibility and your tether payout/drag data. No one (that I've seen) is cranking along very fast down there, so really the ability to hold position against a current or move just fast enough to keep up with what you're seeing seem to be 'enough' and long battery life is more important than the maximum amount of power you can generate at the top of the power band.

I guess what I'm saying is that I'd like more information from experienced ROV pilots/owners/builders about their experience with thrusters. Is there a performance hit to having thrusters that go zero to max in too short a time? Is there a case where you've felt 'overpowered' or 'underpowered' down there where control was just not fine enough? At what point does control become jerky and unmanageable where the main thing you are thinking of is control rather than just looking around/doing what you want to do?

Is there such a thing as too much power?
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ROVER3D
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Re: draw, thrust, and components

Post by ROVER3D »

Is there such a thing as too much power?
This is an interesting question... maybe advanced

First, you're right. "that means it can deliver 4000mA of power for an hour"
i've "calculated" the power management of ROVER3D for a normal use of 4 hours (nightdive).
So i've checked the systems in use (don't estimate, MESS!).

Four sections of power:
  • System = Controlunit, fibertransmitter, all sensors on, "connected" - 0,7Ah
    Actors = (all)Cameras, all ESC powered on(!), all possible consumers except motors and lights, - 1,6Ah
    Lights = All lights powered 90% - 4,3Ah
    Thrust = 20% forward(4,8Ah), 20% up(3,9Ah) - 7,7Ah
=14,3Ah x 4h = 57,2A
So i use 10x 4S Lipoly 5400mA (25A) = 54A Pack

I select forward and up/down(vertical) because forward is calculated all the time and up/down (two big thruster) are used normally in change with turn or roll/nick. up/down are the maximium of my thruster-pairs execept forward so i used this value.
Lights are 6,9Ah at 100%(!) Dimm your LEDs a little bit and you will have enough light for less power.

The question of thrust...

With ROVER3D i normally use only 10% forward-thrust. It is not easy to control because of the acceleration. I choose this really powerful solution (in oil) with 95mm 4-Blade Propeller to get the thrust (i haven't checked it) for my biggest Problem at ROVERII. The Cable! 50m network cable curls up to 20m in water. Else it get stiff. Think about your cable-solution and what youre plan to use.
But if you ask me i would say its not the question of thrust, but for direction.
For vertical movement i use the big 16cm thruster. These pump alot of water only by turning a flat, diagonally sharpened alu-strip. i've tested the same stripes slightly bent but it was to much thrust on lowest speed-level. To much for minimum vertical movement.
So if it is the question of thrust, its on the minimum-speed level.

The sweet spot - the aquadynamic. In water its just displacement. Because it almost does not matter if it weighs 10 or 30kg. With a poor displacement or wrong thruster solution thrust is only current lost in the water.
I've cut out something for you - full video is still in work, but it shows diving with 20% forward and 30% vertical thrust.


side note: ROVER3D weight 34,8kg.

Is there a performance hit to having thrusters that go zero to max in too short a time?
Of course. "full" power in short time to slow down in front of an object...
a_shorething
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Joined: Sep 10th, 2013, 5:26 pm
Location: New Jersey Shore

Re: draw, thrust, and components

Post by a_shorething »

@ROVER3D,
Thanks for the detailed reply! I didn't want to hijack this thread any more with this discussion so I moved my response to a new thread:
viewtopic.php?f=3&t=1555

Please check in there as I have some further questions and some discussion about power to weight, drag, and some discussion about ROV 'classes'.
a_shorething
Posts: 289
Joined: Sep 10th, 2013, 5:26 pm
Location: New Jersey Shore

Re: draw, thrust, and components

Post by a_shorething »

just got over 100g of thrust on the experimental mag coupler... encouraging progress at last.

Will post more detail soon...
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sthone
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Re: draw, thrust, and components

Post by sthone »

T-100.jpg
T-100.jpg (64.32 KiB) Viewed 12427 times
The Bluerobotics T-100 Thruster

Thrust vs Amps
1 Lbs @ .95 Amps
2 Lbs @ 2.17 Amps
3 Lbs @ 3.8 Amps
4 Lbs @ 6.1 Amps
5 Lbs @ 8.2 Amps
6 Lbs @ 13.2 Amps
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