OpenROV #1790 with Modifications

[Kevin_K]

Attached are a few pictures of my completed v2.7 #1790.

The modifications include:

1. BlueRobotics OpenROV Conversion Kit containing 2x T-100’s and 1x M-100 and associated mounting hardware.
2. Sea Con Micro Wet-Con 2 pin connector for a detachable tether connection.
3. Re-positioned IMU to the old starboard motor mount.

I still have to add flotation (will be General Plastics R-3300) as the thrusters make the ROV fairly back heavy.

To date, #1790 has satisfactorily completed its functions and watertight integrity tests.

[Stretch]

Nicely done! Did you use Blue Robotic’s ESC’s or the ones that came with the OpenROV kit? Any idea how many hours you can operate on a single charge?

We’ll have to get together sometime to fly our ROV’s together and compare notes.

[Kevin_K]

The ESC’s were the stock ones on the v2.7 control board, which I think are the Afro 30’s. The thrusters were simply “dropped-in” to the current set-up.

No idea on the battery duration , but probably way less than a stock set-up. I’ll know more once I toss it in the pool and drive it until it dies.

Yeah we’ll definitely have to do a get together once I get it balanced, I really want to see how well it handles saltwater and current.

[Tom_Vidar_Salangli]

great job

I’m looking forward to the robustness due to exposition to saltwater for the Thrusters.
Will they corrode?, what to do for maintenance?

-Tom-Vidar

[Kevin_K]

First dive report:

The ROV performed very well even with the makeshift floats I made. Power Setting 2 seems to work best. Any higher and the ROV is pretty quick and wants to nose down while driving forward. The vertical thruster works very well for diving but is a little slow for surfacing.

Sorry it was getting a bit dark, I had to turn on the lights

@Tom_Vidar_Salangli The thrusters are specifically designed not to corrode in saltwater, which is my primary water medium I intend to use it in. The only maintenance one needs to do is rinse them with freshwater after use.

[Tom_Vidar_Salangli]

Looks great

Just wonder, will these T-100 thrusters be run by the ESC’s that came with my V2.6 or do I have to replace my ESC’s?

-Tom-Vidar

[forttusken2001]

Where do you plan to get the R-3300 foam and how much does it cost? I need the 1200’ capable stuff but of course not a whole sheet. My hardware store insulation foam crushed at 65’.

I have an ROV that is a bit bigger but runs about the same current draw with two 5000mA LiPoly batteries so I am interested to hear your run times also.

Thanks,

Chris
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[Paul_S]

Awesome! I run the Sea Con 2 pin connector tether disconnect too! I added a ziptie to ensure that it doesn’t get yanked off if I need to pull 'er back up for some reason. I’m adding the blue robotics thrusters tomorrow!

[Scott_W]

Looks good Kevin

Have you found a cheaper source for the Seacon connectors or are they still silly prices?

I agree with @Paul_S and look for some sort of simplistic tether tension relief system across the plug (fine as is in the pool but if you need to retrieve it in the open ocean via the tether well mmmmm?)

With trying to prevent the unit from nose diving on power settings above setting 2 (above and beyond getting the unit neutrally balanced) since your adding foam to balance the unit up play with its location of the foam for changing centre of gravity V centre of thrust. Also can the thrusters be lowered a few mm (eg if the centre line of the thrusters is above the centre of gravity it will push the unit down)

Just a thought
Scott

[Stretch]

I’m on the road so am not able to post graphics so I’ll do my best to explain what’s causing your ROV to pitch when thrusting and some ideas to fix it:

(Warning, over simplified explanation follows.)

First a little background: Any object that floats in water has two main forces acting on it, a buoyancy vector which pushes up on the object and a gravity vector pulling the object down.

So long as the buoyancy force is greater than the gravity force it will float. When the buoyancy vector is directly above the gravity vector then the object is considered “stable”. The greater the distance between the center of gravity and center of buoyancy, the more stable that object is (ie. the more force that is required to tilt the object).

The thrusters in your ROV create a third force, and the further that center of force is from the center of the line between your CofB and CofG, the greater the effect it can have on tilting the ROV.

So how to fix it? A couple ways: First, you can move the thrusters forward so it is roughly in line with the CofB / CofG, that way any force added will not have as much of a tilting effect on the ROV. Unfortunately the OpenROV design doesn’t really allow for this so this leads to option number two, increase the ROV’s stability. The easiest way to increase the ROV’s stability is to add more flotation at the top and some additional ballast as low as you can until the ROV is neutrally buoyant. It will take some experimentation to get it right, but I think this will be the easiest option.

[Kevin_K]

@Tom_Vidar_Salangli You should be fine with the v2.6 ESC’s,

@forttusken2001 I was originally planning on buying straight from General Plastics because I also need a larger piece for my next ROV. I’ve custom ordered parts before so once I do some more R&D on buoyancy placement, I’ll put in a request.

Sorry I don’t have good data yet on the run times, but during my trials, they have all been over an hour and that was with the “auto depth” function on pretty much the whole time.

@Paul_S Love to see your ziptie strain relief when you get a chance. I added two when I attached the female end to the frame.

@Scott_W The only source for the Seacon connectors “off the shelf” has been from Amron. I don’t mind the extra cost because I know what I am getting and I know they will work at depth.

I am up for suggestions on the plug. I currently have a piece of thick black heatshrink over the previously heatshrunk connection so not all the tension is on the solder connection. It’s sturdy, but I won’t be lifting it out of the water by the tether anytime soon. A fair trade off for having the tether detachable.

Although it is balanced, I think there is far too much weight forward of the thrusters, not necessarily that they are up too high. I have to play around with it.

@Stretch Thanks for the physics lesson. It’s the same concept when driving ships and the faster you travel, the farther forward the pivot point moves.

So here’s how I’m thinking of proceeding (actually before you mentioned this):

1. Making up a “cap” of flotation to the top of the ROV so it covers the entire top with cutouts for the intakes. Think like the large scale ROV’s.
2. Remove the forward 2 oz weights and add more weight in the aft section until it is neutrally buoyant. This essentially does what you are talking about. This moves the “moment” (I think that is the correct term?) futher aft and closer to the thrusters.

[Stretch]

Right, it’s the same principle except instead of rolling moment (port / starboard) we’re talking about the pitching moment (fore / aft). The ideal situation is to have the center of buoyancy as far above the center of gravity as you can.

Another thing you can do is to move the thrusters down as far as you can go. Hopefully that will bring the center of thrust in line with the vertical center of gravity.

Finding the center of gravity might be something you can do with the ROV out of the water. Simply lift it and have it balance on another object (like a broom stick). That will show you where the fore / aft center is. To find the vertical center is a little trickier. You might be able to use a couple bits of string and some duct tape and try to suspend the ROV above the ground to find it’s “balance” point.

When I get a chance I will try to describe the strain relief VideoRay and others use to send all strain from the tether to the ROV. One of the things I hope to change on my build will be the tether. Something with at least a cover and preferably with a shield or strengthening element.

[Stretch]

Back on the topic of tether strain relief, I found a photo of a VideoRay tether:

In this photo you can see how they do it using “braided cable sleeving”, heat shrink tubing, and a small stainless “detachable chain link”. This link is connected to the ROV frame with a short stainless steel wire with swaged loops at each end. Simple, easy to disassemble (without tools) and effectively transfers all tension from the tether directly to the ROV frame.

[Kevin_K]

Just a quick screenshot from ocean testing this morning. We went out about 2-3 miles offshore and just drifted with the current. For some reason, the IMU wasn’t working correctly, so I’ll have to do some troubleshooting.

[Paul_S]

Stretch, you are spot on, this is an industry standard for the bigger observation class ROVs. I have a simple stainless steel carabiner and ziptie setup (not on the ROV yet) but once I get a cable swagging tool I’m planning on adding a SS cable.

[Paul_S]

Kevin,
I know there’s been talk about the 2.7 having troubles with the IMU because of the change in EM radiation from the different motors and from RF interference from the tether due to shielding. I’m currently trying to work on a solution. I think mounting it on the top of the unit (on the other side of 2 layers of acrylic) may help. I’m going to test it this week, if the acrylic alone is not enough, I may put a layer of this stuff between the motors and the IMU (somehow) http://solutions.3m.com/wps/portal/3M/en_US/ElectricalOEM/Home/Products/ProductCatalog/~/3M-EMI-Shielding-Fabric-Tape-CN-3490-Copper-Nickel-Non-Woven-Polyester-Fabric-Conductive-Acrylic-9-8-x-10-?N=5561499+3294764031+4294907173&rt=rud

[Kevin_K]

You all knew it was coming…

Change 1: Thanks everyone for the great feedback. I’ve made some adjustments and now #1790 drives perfectly straight even without the auto depth function. Even on Power Setting 5 (which is ridiculously fast), the ROV is straight as an arrow.

1. Moved thrusters to be inline with the old motor mounts.
2. Moved foam from the top to inside the shell. No modifications to the foam blocks have been made.
3. Moved the IMU from its motor mount position to the stock position by the handle.

I’m just glad these designs are forgivable for both products. It also helps to build it knowing you’re going to have to change things around.

@Paul_S Thanks for the insight. My IMU works perfectly in the pool but “freezes” when I drop it in the ocean. Stretch was thinking it might have to do with motor interference and I think he might be right. We know that already messes up the magnetic compass. The other issue is that there is a metal shield around the pressure sensor.It’s the only exposed metal in all the electronics. I know it’s worked well for others in the ocean before, so I’ll just have to keep troubleshooting.

[Stretch]

Amazing what difference an inch or two makes, eh? Glad you’ve got it balanced!

My thought was that perhaps there’s a stray electrical leak somewhere that only appears in the higher conductivity salt water. Kind of a stretch but I guess it could be possible.

[Mike_Sinclair]

Amron

Hey Kevin,

Thanks for documenting your progress and upgrades! Would you mind providing the exact product details for the sea con connectors that you ordered please? I don’t want to order the wrong ones! Did they take long to ship to you? I also got the OpenROV thruster kit, but I will be attempting to install on the 2.5 version.

Thanks!

[Kevin_K]

Hi Mike,

Of course, here are the direct links for the two pin version if you were just making up a detachable tether. You will have to ignore the pictures on the site, they are not correct. Also, there is a third pin that will be on the connectors. This is not a power pin, just an alignment pin.

Seacon Micro Wet Con MCIL2M (Male)
Seacon Micro Wet Con MCIL2F (Female)

If you want the locking rings to go with it:

Seacon Micro Wet Con 2-8 Contacts Locking Sleeve MCDLS-M (Male)
Seacon Micro Wet Con 2-8 Contacts Locking Sleeve MCDLS-F (Female)

They took an average time to get to me, about a week. I plan on buying more for my larger ROV and I already have a 4-pin connector for the IMU, but I need some more 2-pin and 3-pin connectors.