wizdum:
Hello,
First, a little background on the project. I'm part of a team making an underwater ROV for a club project. The ROV has 4 motors that will be wired to 8 relays using this method: http://homebuiltrovs.com/howtorelays.html (except we will be using an Arduino with transistors to operate the relays). We plan to use an old PlayStation 1 controller with the PSX library (in the playground here) for control. The max depth of this ROV wont exceed 150ft, and will most likely not be below 100ft. We can get a substantial amount of cat5 cable from the school, so we plan on using that for the tether. The ROV will be powered by on board batteries, with the PlayStation controller connected to the tether on the surface.
First question, with potentially 150ft of cable, will the PSX library still work? There is going to be a substantial voltage drop over that distance, right? Would we be better off having the Arduino on the surface, and using the cat5 to connect the digital pins to the transistors?
Second question, We would also like to be able to control lighting on the ROV, and read a few different sensors (various water level sensors, temperature sensors, an accelerometer, and maybe pressure sensors). Would it be possible to have a second Arduino on the surface that communicates with the one on the ROV via the tx/rx pins? That way we could output the data from the sensors to an LCD, or to a computer.
Yes to the second question; in fact, if you can instead use a 4 line cable (cat-3 - used for telephones, usually), you'll end up with a lighter weight solution (less for the robot to pull around in the water, less drag means longer run time).
You should also look for (whatever cable you use) something with stranded conductors, rather than solid conductors. Note that such cable will be more expensive than solid-core conductors, but since this cable is going to bend and twist and such, you'll want the flexibility so it doesn't break. If you can find such cable that is designed for pulling, it will also have embedded in it a strong piece of monofilament or similar that can be pulled on (because it is designed for pulling thru spaces, walls and conduits in its normal usage); this will be useful if you need to "rescue" your ROV by reeling it in via the cable.
Also - if you can find it (and afford it), try to get the cable with a silicone outer jacket (and if possible, silicone insulation on the core wires); most cabling only has PVC outer jackets; the problem here is that after you use it enough, it will be twisted and flexed so much that the jacket/insulation will crack, and water will seep inside the cable, eventually running down into and inside your ROV, weighing it down (and possibly shorting things out). Plus, depending on where you use the ROV (fresh vs. sea water), the water and such will break down the PVC over time, causing it to wear even quicker.
Finally - there's always the issue of cable management; with 100 to 150 feet of cable, this isn't an easy issue. You need to be able to reel and unreel it, without it getting tangled or in the way. On professional rigs, this is done with a large spool of cable. You can do something similar - see the following page:
Note - check out the rest of that site, too; it's really worth it for anybody planning on building an ROV:
