./ROV:mechanical
From UAFCS
Contents |
UAF ROV Mechanical Design
Ammo cans are very tough, resealable watertight metal boxes. GI Joe Surplus, on Cushman, has them for absurdly low prices. Our large 40mm cans are 5.5" x 17.25" x 9.5" / 14cm x 44cm x 24cm (and cost $10 for 3).
The main things the ROV body has to contain are the main circuit board and onboard batteries.
"Vincent's Choice" ABS Alaskan batteries are 12v 26ah, 6.5x6.88x4.9, 17.8lb, $87 each. There's 4x5.5x9.5 of room left for electronics with two batteries in "large" box. There's 10.5x5.5x9.5 of room left for electronics with one battery in box. In both cases there's a .5 gap to the box wall above terminals. We bought two black Power Patrol SLA1146 batteries, but big batteries seem to come in more-or-less standard sizes and capacities, so they're probably the same type as these. The cases are both black plastic and "flame retardant", both of which sound like ABS plastic to me. It'd sure be handy to be able to just solvent-weld ABS sewer pipe T joints to the battery to use as a handle/mounting bracket!
Another version of these batteries are "approved for transport by air. D.O.T., I.A.T.A., F.A.A. and C.A.B. certified. U.L. recognized under file number MH 20845." We can count on "between 200 and 1000 charge/discharge cycles depending on the average depth of discharge." "The high-impact resistant battery case is made either of non-conductive ABS plastic or styrene." Either of these plastics would be glueable.
Buoyancy
The 40mm cans displace 14*44*24 = 14,784 cc of water, which weighs 14.78kg / 32.5lb. The can and frame together weigh about 20lbs, and the battery weighs 17lbs, so we already need more bouyancy, e.g. via camera pods or just big vertical tubes.
The tether is one piece of cat5 ethernet cable, which tends to sink. To reach neutral buoyancy, we'd need some lift. Last year, we used closed cell foam, which can start to squish at high depth. Another option is some floating plastic rope, which would also help you haul in a dead 'bot; nylon is heavier than water, but polypropylene's density is about 0.9gm/cm^3, or -0.1gm/cm^3 underwater. Copper's density is 8.96 g/cm^3 (7.96 g/cm^3 underwater), and there are 8 24-gauge copper wires in one cat5 cable. Each [24-gauge wire] is 0.51mm across, so there are (pi*(0.051cm/2)^2)*1cm*8 wires = 0.016cm^3 of copper per cm of cable, or 0.13 g immersed weight, which would thus need 1.3cm^3 of polypropylene rope to reach neutral buoyancy. So the rope's diameter should be such that the cross-sectional area is 1.3cm^2, which turns out to be almost exactly 1.3cm diameter (D = 2*r = 2*sqrt(A/pi) = 2*sqrt(1.3cm^2/pi) = 1.28cm = half inch). We'll have to try some 1/2" poly rope, and see if it can float an equal length of cat5 cable.
The "2lb diving weights" in the rescue mission are supposedly 12cm long, 8cm wide, and 3cm thick. They're 2lb in air, but if they're really lead they'd only lose maybe 10% of their weight underwater. Are the competition bags lead filled? 12*8*3 is 288 cubic centimeters, so 2lb weights have an average density (if rectangular) of 3.16 g/cc. Pure lead's density is 11.34 g/cc; lead shot (based on sphere packing) should be 74% of this, or 8.4 g/cc. So the lead should only occupy maybe 40% the volume of the weight's bounding box. That's probably reasonable if the lead is in strips, or padded with thick layer of fabric. Here are some examples.
Bulgin Buccaneer Waterproof Connectors
http://www.bulgin.co.uk/Products/Buccaneer/Buccaneer.html
These might be a possibility for bulkhead connectors. They have a series of IP68 qualified connectors, including ethernet and high-current circular connectors. Mouser stocks them, and they are a bit spendy, but it seems like they would be a very clean solution. The one issue I don't see a simple answer to is how well the cable gland will handle having multiple wires instead of one inserted. They are also a bit spendy. A 7 pin connector can theoretically handle 3 motors, and we would only need one ethernet connector. However, we'd probably need one connector per motor if we wanted the ability to swap single motors without having to pull the connectors apart.
Also, Bulgin offers samples, it's anyones guess if they'd be willing to 'sponser' us though.
I imagine the motor connectors would go something like this:
Motor tail -> bulkhead connector external -> bulkhead connector internal -> header on the circuit board. If we wish to remove the circuit board, we disconnect the header and slide the board out. If we need to swap a motor, we unhook our bulkhead connector and remove the motor. As far as I know, these would meet our requirements.
MUDbot
Tiny 7.62 cans are 3.375" x 10" x 6.5" (6 for $5)
An ISA slot is 5.5" across. My jack-pairs are 1.28" across, so 5.1" can do all 8 jacks (six camera, two external).
Obsoleted Parts
These are parts we've decided not to use, but it's good to keep the specs around for reference.
Ammo cans are available in a variety of sizes:
- Tiny 7.62 cans are 3.375" x 10" x 6.5" (6 for $5)
- Medium 50cal cans are 5.5" x 11" x 6.75" (3 for $5)
- Large 40mm cans are 5.5" x 17.25" x 9.5" (3 for $10)
- Huge 120mm cans are 5.5" x 11" x 31.25" (3 for $20?)
- Dimensions: height x width x depth (opening cover is width x height).
Lead-acid battery sizes:
- Grainger 4RE65 6-volt, 10AH batteries are 2x6x4", 4lb, and $29. We'd need 6 to make an 18V 20AH battery pack, which would kinda fit into a "large" can. Only two would fit in a "medium" can.
- Grainger 4RE67 12-volt, 12AH batteries are 4x6x4", 9lb, and $66. Two would fit across the bottom of a "large" can (for a 4x12x4" 24V 12AH or 12V 24AH battery pack), and you could even do two layers to fit four batteries (a 4x12x8" 24V 24AH pack), leaving a 5.5"x5.25"x9.5" space at the top for the controller board and wiring. 4 batteries would be 36 lbs (and $250), which is a little more than we used for ballast in 2007.
(LA company All Spectrum Electronics has these batteries for $40.)
