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 bigkahuna,
Bought the HDPE here
The only problem with this HDPE is it is only 12" (11 3/4" actual size) in width which, as I have said before, will limit you to only four Thrusters if you use the two WTC configuration.
Regards,
Tom C AVD
Hi All,
Today I completed the fabrication and installation of the additional cross braces on the ROV chassis. The additional cross braces are mounted front and rear at the top of the chassis and front and rear at the bottom of the chassis. The cross braces were fabricated from leftover HDPE during the construction of the ROV chassis side plates. The additional cross braces will provide hard points to mount skids at the bottom of the chassis and additional flotation material at the top of the chassis if necessary. The two pictures below show where the additional cross braces are located:
Cross Braces Front View
Tomorrow I plan to begin the fabrication of the WTC mounting structure. I plan to cut strips of HDPE and then bevel one edge that can be wedged against each side of the WTC outer circumference to locate the WTC laterally on the main cross brace/WTC support. I will then see about the availability of stainless steel straps to provide a vertical clamping force of the WTC against the main cross brace/WTC support surface.
More to come.
Regards,
Tom C AVD
Hi All,
The temperatures here in Ft. Lauderdale have been approaching close to 100 deg F during the day and I must be drinking a couple of quarts of water each day when working in my workshop which is not, unfortunately, air conditioned.
Be that as it may, I was able to fabricate all four of the additional cross braces so that they will be ready for placement and attachment to the chassis this weekend.
Once the additional cross braces are attached to the ROV chassis I will begin placing and attaching the horizontal and vertical Thrusters to their respective mounting points. I will then begin the process of fabricating the holding fixtures, to be placed on the main cross brace/WTC support, to anchor the WTCs to the main cross brace.
I should have some pictures of what I accomplish this weekend by late Sunday or early Monday.
Regards,
Tom C AVD
Tom,
If anyone has a T100 just use Archimedes principle, with either salt or fresh water.
Ken McMillan
Hi All,
I got a lot accomplished today!
I completed the installation of the chassis right side cantilevered horizontal Thruster support and then moved on to complete the attachment of the chassis right side panel to the cross brace/WTC support.
After attaching the chassis right side panel to the cross brace/WTC support, I mocked up the ROV chassis with a horizontal and vertical Thruster and positioned one of the 4" dia WTCs on the center of the cross brace. Pictures of the mockup are below:
Chassis Right Side
As can be seen in the top photo, the WTC will not be centered horizontally on the cross brace. The reason for this positioning is to help offset the horizontal Thruster weight aft of the horizontal center of mass and to provide clearance for the cables between the rear of the WTC and the rear horizontal Thrusters. There will be an identical WTC mounted under the upper WTC to hold the LiPo batteries.
Tomorrow I will start on constructing and installing the upper and lower cross braces. They will provide additional lateral stiffness and serve as hard points for mounting skids on the chassis bottom and for mounting additional buoyancy tubes on the top of the chassis if necessary. The cross braces will be made up of leftover HDPE from the chassis construction and will consist of two 1/2" x 1 3/4" x 7" HDPE plates butted together and connected with a 1/2" x 1 3/4" x 3" bridge plate. I plan to test the JB Weld epoxy on the cross braces to see how well it adheres to the HDPE.
More to come.
Regards,
Tom C AVD
Hi Tom,
This is looking great. Excellent progress!
The only thing I want to point out is that unless you add a lot of weight inside, the enclosure will be positively buoyant in water. That means that you actually have to move it backwards to balance the ROV instead of forwards. You want the center of buoyancy over the center of gravity. Adding the thruster weight to the back means that the CG moves back and therefore center of buoyancy must move back.
The best thing to do would be to put heavier items like batteries as far forward inside the tube as possible but leave the tube itself a bit further back.
I'm really impressed with how clean the side panel cutouts turned out. Very nice.
Best,
Rusty
Hi Rusty,
Here is an interesting take on the relationship between the center of gravity and the center of buoyancy in a vessel taken from the engineeringtoolbox.com website:
When the hull is upright the center of gravity and center of buoyancy is on the same vertical line.
For most hulls the center of buoyancy is below the center of gravity and the hull is said to be metastable.
When the hull tilts the center of gravity remains in the same position related to the hull (the hull is not changed and/or the cargo is not moved). The center of buoyancy moves to fit the new center of gravity of the volume of water replaced by the hull. At first the gravity force and the buoyancy force creates a
that tries to move the hull back to upright position. If the hull is tilted to much the center of buoyancy moves to a position where the buoyancy and gravitation force starts to create a moment that will
the hull.
I find it interesting that the center of buoyancy is below the center of gravity in a surface ship. The question that begs explanation is what is the relationship between the center of buoyancy and the center of gravity in a submersible? Is it the same or is a modification to the theory required?
However, the WikipediA, in its Buoyancy description, indicates that for a surface vessel it is best to have the center of buoyancy above the center of gravity for stability which make intuitive sense to me, but is not always the case.
Obviously the two most buoyant components of this ROV are the two WTCs. As a baseline I can weigh the two tubes and then calculate their volumes to determine their unloaded specific gravity. Then I can add the weight of the control module to the upper WTC and the weight of the batteries to the lower WTC and recalculate their specific gravities to get an idea of their buoyancies.
The HDPE has a specific gravity of .95gm/cm^3 which is very close to neutral buoyancy. I believe I can calculate the volume of the side panels, sans the cutouts, to determine the buoyancy of the panels.
Have you determined the buoyancy of the T100 Thrusters?
Comments please?
Regards,
Tom C AVD
Tom,
Looks great. The Blue Robotics specs say that the T 100 thrusters weigh 120 grams with the cord in water. I assume fresh water.
Regards,
David R. Boulanger
David,
Thanks for the kudos, much appreciated.
But without knowing the volume of water (fresh?) it displaces, we cannot calculate its buoyancy even though we know its weight.
Regards,
Tom C AVD
Tom,
The buoyancy is the weight in air minus the weight in water, which are both provided in the specs. It looks like the weight with BlueESC is missing form the docs so I'll make sure that get fixed.
Also, if you're interested, we have a buoyancy calculator that can find the buoyancy based on volume, density, and/or weight. Might be helpful: http://docs.bluerobotics.com/calc/buoyancy/
-Rusty
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