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
Hi All,
A couple of homebuitrovs.com members have asked some questions about the purpose of the WTC vacuum test so here is my answer:
Since I want to determine the possible leak rate (effectiveness) of the End Cap double O ring seals, the cable penetrator O ring seals, and the effectiveness of the cable potting without putting the WTC into the water and letting it sink to 100 ft and sit a while before bringing it to the surface, pulling a vacuum on the inside of the WTC with the End Caps in place is the next best thing.
By pulling a high vacuum in the WTC I can simulate the pressure of the water pressing against the End Cap double O ring seals, the cable penetrator O ring seals and the cable potting to help me determine if the O rings are providing a good seal and that there are no leaks through the cable penetrator potting.
The reason that I am using an empty WTC for the test is to avoid the outgassing of the tray materials and batteries that will occur when I draw a vacuum on the WTC. The only items that will outgas will be the ends of the ESC power cable wires/connectors that protrude into the WTC interior which, when completely outgassed, will no longer contribute to the vacuum within the WTC. A this time the only contribution to the vacuum within the WTC will be any leaks around the various WTC seals and the cable penetrator epoxy potting.
Regards,
Tom C AVD
It's always a good idea to give your water tight housings a vacuum test anytime they've been opened. It's the only real way of knowing that you've sealed everything up correctly and is S.O.P. with almost every commercially available ROV out there.
T
Hi All,
Today was fairly productive for me as I fabricated the WTC vacuum pump adapter fitting and cut out the navigation controller tray for the navigation controller WTC.
To fabricate the WTC vacuum pump adapter I started with a blank cable penetrator and machined one through hole and two counter bores into the body of the penetrator that mimicked the standard Blue Robotics 6 mm cable penetrator. I drilled the through hole and the two counter bores with my cheapie Skill drill press to see how well the holes would align and they came out pretty concentric:
Blank Cable Penetrator machined to accept Vacuum Tubing Plastic Splice
The machining of the blank cable penetrator was then followed by CAing one of the Vacuum Pump plastic tubing plastic splice into the lowest counter bore. I then mixed up some Loctite Marine grade epoxy and filled in the area of the upper counter bore between the body of the cable penetrator and the plastic tubing splice.
Plastic tubing splice potted into machined Cable Penetrator
The following two pictures show the Harbor Freight Vacuum Pump and the complete WTC Vacuum Testing Components:
Hand Vacuum Pump
While I was waiting for the epoxy to cure I cut a piece of 1/4" thick HDPE to the same dimensions (3" x 10") as the battery compartment tray to fabricate the navigation controller compartment tray. Like I did with the battery compartment tray, I beveled the edges of the tray so that the edges of the tray are tangent to the inner circumference of the navigation controller WTC.
Tomorrow I will proceed to finish the fabrication of the navigation controller compartment tray and attach the Signal Junction Board to the tray.
More to come.
Regards,
Tom C AVD
Hi All,
Some thoughts concerning using the T100 with and without the built-in ESC.
If I had to do this ROV Project over again, I think that I would go with the T100 Thrusters without the built-in ESC. Since I have gone to a two WTC configuration with a separate battery compartment WTC, there is plenty of room in the navigation controller compartment for 6 or more Thruster ESCs and the navigation controller and associated components since the batteries are no longer contained in the navigation controller WTC.
The addition of the T100 built-in ESC controller/signal cable just adds an additional layer of complexity to the ROV configuration that many ROV builders may want to do without. I have been trying to decide if I should put connectors on the signal control cables between the Signal Junction Board and their respective cable penetrator cables. I have actually made the Signal Junction Board signal cables long enough that I can solder them to their respective cable penetrator cables and have plenty of cable length to remove the navigation controller tray out the front of the navigation controller WTC. So I will probably solder the Signal Junction Board signal cables to their respective cable penetrator cables and be done with it.
Comments?
Regards,
Tom C AVD
Yes, I've discussed this with Rusty: using thrusters with BlueESC's means two cables per thruster and twice as many cable penetrations. I think running all leads (power and control) through a single cable would be a better arrangement. For now my first build will use T200 thrusters and standard ESC's. The main housing will hold the electronics and ESCs and a second housing just for batteries.
Just fyi, we've had a few people successfully pass both the power and signal cable through the same penetrator. I can't speak to how well it works or if it introduces any noise on the I2C lines, but it's possible.
-Rusty
Hi Rusty,
Having the 12 vdc power cable right next to the signal cable in the cable penetrator might induce unwanted noise on the I2C signal lines due to current fluctuations in the power cable. The I2C signals are very susceptible to noise as it is.
Regards,
Tom C AVD
Tom,
Yes, I agree. That's why we separated the cables in the first place. However, they could probably be used side by side if you are using PWM signals instead of I2C. I think that applies to quite a few people, most likely.
Best,
Rusty
Hi Rusty,
Yes, I plan to start with the PWM signal in the Signal Cable to control the T100 Thruster ESCs, but I might eventually try the I2C control signals to be able to monitor the brushless motor performance while under load and over a long period of operation.
Regards,
Tom C AVD
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