Hi All,
I am sure that many of you have heard of the OpenROV Project which involves building your own ROV from a kit. I am an experienced model builder with plenty of experience and a reasonably equipped workshop, but I was a little taken aback at the complexity of assembling the OpenROV from their kit of parts. It seemed to me to be a high school to college level project requiring a small team of experienced modelers to successfully put the OpenROV together though I could be wrong.
It appeared to me that there were a number of critical assembly requirements that had to be performed spot on as there was no going back to realign or reseal once the assembly was completed. Therefore I have been looking around to see if there might be an easier way to design and build a ROV similar to the OpenROV without having to perform some of the critical assemblies required to complete the OpenROV.
One area of the OpenROV design that I took exception to was the thrusters. They were not going to last very long when immersed in seawater due to the exposed bearings and stator/armature. To me this was a kind of deal breaker as to the questionable lifespan of these thrusters. I know that commercial thrusters are not cheap (think Seabotics or CrustCrawler) and the OpenROV project was just trying to overcome the high price of commercial thrusters with their homebrew design. Still the unknown lifespan of the OpenROV design did not leave me with a warm feeling to say the least.
Then came along Blue Robotics and their T100 Thruster KickStarter Project Link. Their design approach looks good and they have actually tested and characterized prototypes of the thrusters they will deliver to the pledgers. With the addition of the in-thruster water cooled ESC this design becomes very attractive in that it reduces the number of wire penetrations in the Water Tight Compartment (WTC).
Great! Now we have a source of reasonably priced thrusters so what is next? Well, that is where you, the ArduBoat members, come in.
Let's start thinking about the WTC, navigation controller, communication, power, ballast, buoyancy, etc. and attempt to come up with a reasonably priced ROV that the average ArduBoat member might want to consider building.
Regards,
TCIII AVD
Replies
@bigkahuna,
I believe that one atmosphere (14.7 lbs/in^2) is equivalent to around ~30 in Hg (760 mm Hg).
According to the Pressure at Depth Calculator, the pressure at 33 ft is ~2 atmospheres (62 in Hg) because you have not only the weight of the water affecting the pressure, but the addition of atmospheric pressure (14.7 lbs/in^2) at the water-atmosphere interface.
As long as I can draw a small vacuum that is not stable when the vacuum pump is turned off, then I will still have a vacuum leak at even a lower vacuum in the WTC.
Regards,
Tom C
@bigkahuna,
NiMH batteries have nowhere near the capacity of LiPos when you consider size, weight and recharging time.
I have used NiMH batteries quite a bit in small to medium size surface vessels, but any vessel, surface or subsurface, of large size or of high endurance, I defer to LiPos or the like.
Regards,
Tom C AVD
NiMh are bigger, heavier, and (from what I've been able to find) more expensive than LiPo, but do offer the advantage of being available to fit a 3" cylindrical housing (designed for diving lights) and using the same leads for discharge and recharge. Weight isn't really as critical in a ROV as it would be in a UAV. I'll have to look at LiFePo4 a bit more. I've not found many suppliers for packs that would fit a 3" housing.
Here's one that would fit: http://www.batteryspace.com/Customize-LiFePO4-26650-Battery-12.8V-9...
Hi All,
Some thoughts on the vacuum testing of WTCs:
There was a period during my engineering career at Raytheon when I was involved with the construction of a large vacuum chamber and later on the vacuum testing of space satellite payloads so I learned a lot about the ins and outs of vacuum testing.
During the pump down of any vacuum chamber you will get outgassing from whatever components are in the chamber to be vacuum tested. Once the outgassing is complete the vacuum pumps only have to contend with chamber leaks as there is really no way to totally eliminate them.
In the case of the Blue Robotics WTCs I feel that pumping them down to a vacuum of around 18 - 22 inches of Hg will be good enough to detect and mitigate any potential leaks. However I would like to stress that it may not be a good idea to pump down a WTC that has LiPo batteries in it as this might cause outgassing issues that could damage the batteries.
Comments?
Regards,
Tom C AVD
Wow, rough luck with the hand pump. Mine is 4 years old and still going strong. I personally would stick with a simple hand pump. They make one out of plastic for another $10 more that looks identical to what SeaBotix and VideoRay ship with their ROVs.
I've used electric vacuum pumps for vacuum bagging composites and that would be way over kill IMO. You don't need that much of a vacuum to make sure it's sufficiently sealed. Google for a VideoRay user's manual and see what they recommend.
I'm afraid your experience with potting a cable penetrator just confirms what I've been saying all along. I don't know of any way of fixing a leaking fitting that's been potted. Perhaps some "penetrating epoxy" might work? I've been on the fence what to use on my build. I was going to give the BR penetrators a try but now I'm leaning more towards using some stainless steel cable glands. That's what I've used in the past and they've worked well.
@bigkahuna,
You can buy the same HF $40 Mityvac from Amazon for $29.
The 12 vdc oil less vacuum pump will pull about the same vacuum (18 - 22 in Hg) as the either the $25 or the $40 HF vacuum pump without the sweat of doing it by hand.
Regards,
Tom C AVD
Hey Tom,
Years ago I did a lot of Scuba diving. Deep, over 100 ft. I had a dive light that had Acme threads and an o ring sealing the water out. It never leaked more than a drop or two. And I think that water came in through the switch that was sealed with two o rings on a shaft. In other words the Acme threads and o ring never leaked. I'm just wondering if a WTC that is going to be accessed frequently would not be better off designed with a threaded end on it. Maybe Rusty will also give his opinion on this. It just seems like a functional and easy solution.
Regards,
David R. Boulanger
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