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Bart - Argonaut Jr's
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1) RoboRealm
-- Software suite that processes the video, and has easy to use
interface modules for the game
controller and the Pololu Micro Maestro controller that runs the servos and motor
speed controllers. The game controller will use the
joystick module
and the Pololu servo controller has a custom
Pololu
Maestro module. RoboRealm also has a web server to make the video available to the Internet
when a connection is available.
2) Pololu,
Micro Maestro 6-channel USB Servo Controller and
Documentation
and Forum
-- A very small card that controls the RC motors Electronic Speed
Controllers (ESC) and servos.
3)
Lowrance StructureScan, sonar imaging is unique in that it use
Ethernet network to transfer the signal between the the supplied
network box and the display console making to possible transmit the
transducers information up the ROVs umbilical. By slowly
rotating the transducers two side scan transducers 180 degrees with
a servo; we will be able to capture an 360 degree view of the area
around the ROV out to and beyond 100ft even in muddy water.
4) Putting a compass in the field of view of a camera works nicely,
but we'll go a step further and add a
digital
compass that provides the heading over an RS232 serial interface
with the Pololu, Micro Maestro.
5) Microseven IP camera has
an SD card on it where it can record D1 video as well as a built-in
web server enables video live anywhere over the internet.
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Brushless Motor Thrusters
We are using RC (radio controlled) hobby type motors with props added on for Bart's thrusters. These are widely available and relatively inexpensive, small, but powerful motors than can be run even in salt water with the motor completely exposed to the water. Read more here: DC Motors and Props.
We tested a 1000 Kv Outrunner Brushless RC Motor and 30 amp and 30 amp BEC ESC (Electronic Speed Controller) $8.50 for both, plus another $8 for shipping from China. The results were mixed. One of the three motors caused the ESC to shutdown after a short time of running underwater. The cause is likely due to a bad battery we were using. Also when running at higher seeds, the prop which is actual a fan blade taken from an PC's power supply would cause the motor to run too slow and fall too far out of sync from that the ESC's motor speed sensor would be expecting.
We also learned that we need and ESC for RC Cars that have the ability to run the motor in reverse. Just because the ESC says "Reversing" does not mean it can run the motor in both forward and reverse. What the Chinese translator means is that you can wire the motor to run in forward of reverse.
Mounting a prop to this motor was fairly easy. We just turned a PVC plumbing fitting in the lathe to fit over the motor and into the fans hub. It would take more time, but you could do this with a drill, small drum sander attachment to fit inside the PVC pipe so it spins the pipe, and sand paper to cut down the outside dimension of the PCV.
For our second test we purchased 5 - 45 amp ESCs for cars because they can run the brushless motors in forward and reverse. Add we went with 5, even more powerful motor that runs at 800Kv or slower which is better for turning bigger props and getting more power. Finally we got a couple of 1/4 scale servos for tilting the camera and rotating the sonar transducer.
Power
Battery On Board
Commercial ROV's are powered from the surface with very high AC voltage much like large power transmission lines that must transfer electricity over long distances. However there is obviously a lot of inherent danger that goes with high voltage AC around water. In comparison DC voltage at or below 12 volts or less is safe even when directly exposed to salt water. The problem is that 12 volts will not travel through a long wire without a significant loss of voltage. We could use a larger wire, but then the motors on the ROV would be dragging a huge cable along and any amount of current would in turn drag the ROV around. We also want to go really deep eventually, like 3000 feet and do it without a large ship to carry around the cable, so putting the batteries on board the ROV is our only real option.
Lead Acid or AGM Battery?
AGM Batteries like the Optima brand batteries are tightly coiled sheet of lead and matting that have successuflly been used at great depths as in 12,600 feet when doing work on the wreck of the Titantic. However, much less expensive lead acid batteries can make that trip too provided they are compensated with oil that floats on the surface of the liquid electrolyte in place of the air normally found inside these batteries. Lead acid batteries are also less prone to premature death due to deep discharge.
Inside or Outside the Hull?
Both AGM and especially lead acid batteries produce hydrogen gas and oxygen which is explosive. So placing them with all of the electronic stuff is best avoided. It would also be convenient if we could quickly and easily swap the batteries out for a pair of batteries that are fully charged. So outside the hull means we don't need to open up the hull.
How Big A Battery?
There are lots of good sources that explain batteries in detail but one I found very useful is: http://www.gizmology.net/batteries.htm because it has a simple explanation of Peukert's Number and a calculator that will show you how much power you really have.
Here is an comparison between an Optima AGM and a deep cycle lead acid battery with an ROV constantly using 14 amps at 12 volts. Lead acid batteries generally have a Peukert number that is higher that AGM's meaning that the don't like being discharged at a high rate, so it takes a 75 ah lead acid battery to match a 55 ah AGM battery. The lead acid will provide 29 minutes when 20% discharged, but unlike the AGM, the lead acid battery can easily survive being discharged to 60%. So if you think you'll constantly use 14 amps, then you will need 1 of these batteries for every 30 minutes of run time. In practice, it is unlikely you will actually need 14 amps constantly.
| Battery | Amp Hours (ah) |
Peukert Number |
Rate of Discharge |
Actual Capacity |
100% Discharge (Harsh) |
80% Discharge (Ok) |
20% Discharge (Good) |
| Lead Acid | 75 | 1.3 | 14 amps | 33.9 ah | 4 hr. 25 min. | 1 hr. 56 min. | 29 min. |
| Optima AGM | 55 | 1.1 | 14 amps | 42.2 ah | 3 hr. | 2 hr 24 min. | 36 min. |
The depth of discharge (DOD) has a major effect on the life
expectancy of a battery. Discharging only 80% of the total capacity
of the battery will typically get you 25% more cycles than total
discharges, and discharging to only 20% will make the battery last
essentially forever. Car batteries, however, have to be treated
differently - they're not designed to discharge even 20%, and will
be damaged if they're deeply discharged. A "deep cycle" battery, on
the other hand, can typically survive 400 full discharges. And
as a rule of thumb, AGM batteries are more susceptible to damage
from DOD than lead acid batteries. I have run lead acid
batteries completely dead and was able to recharge them. I've
done the same to a couple of AGM's and had to go buy two very
expensive new batteries.
Main Power Switch
A really easy way to put add a water proof switch is to put a reed switch inside the hull and a magnet outside the hull. The just move a magnet over the location of the reed switch. You'll need to keep the magnet over the reed switch as long as you want it on, and you will need the reed switch to trigger the coil on a relay, because most reed switches can only handle about 1 amp.
If you want to avoid having to rig the magnet so it stays over the reed switch then you can buy an expensive locking relay or you can build Awni Zaidoon's cool little circuit. Then all you need to do is pass a magnet over once to turn on our ROV and again to turn it off.
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Resources
www.hobbyking.com Good
selection of RC Motors and ESC.
http://www.ertyu.org/steven_nikkel/ethernetcables.html How to
wire Ethernet Cables
Battery Resources:
http://www.gizmology.net/batteries.htm Includes discharge
calculators
