Introduction and Motor power, board brownouts, etc.. questions and maybe ideas


#1

Hi everyone, I'm new here. I'm currently going to school for a Computer and Electrical engineering. I have a good bit of experience with just about everything. Programming in at least 6 languages including assembly. Mechanical--I can fix just about anything that may break on a vehicle. Electronics -- have designed and build a few single board computers and various other electronics, down to designing and prototyping the PCB.

I don't have even my first part ordered yet -- trying to read up on the forums and anticipate possible problems. I'll need to address a couple issues I see with my intended use and would like some input from some of you that have already built yours.

I'm a diver and will want to use the ROV mostly in the Ft. Lauderdale area and in some of the springs and cave systems around the Gainesville/Ocala/High Springs area.

From what I can dig up, the max speed of the ROV as it's currently designed is about 2.2mph. We routinely encounter 1-2kt currents out past the 3rd reef (90-130ft water depth) and Going into a spring it's not uncommon to encounter 3kts+ of current, so for practical use out there I will need more speed.

The motors seem capable of pushing more so I take it the batteries are the limiting factor? I remember reading some posts about brownout issues. The motors are rated at a max of 9 amps..but that's not considering they're completely submerged and can be cooled quite effectively. I would guess they can be pushed to the limits of the ESC with enough voltage. I'm new to the hobby arena of ESCs and hobby motors so I may not be understanding things correctly. The ESCs say 3s lipo...they probably wouldn't be happy with more than 12v eh?

Can someone confirm batteries/brownout issues is one of the limiting factors? If so would a separate buck/boost regulator for the electronics make sense? They sell them pre-packaged on ebay for cheap. http://r.ebay.com/Lk6yqh

I read the tether is handled by a hacked up powerline network adapter. I saw it already being discussed but I didn't see anyone actually report being able to send power down the tether. What size wire is included with the kit? 20awg should be able to handle 200w at 48v. Just rig a charger to it to keep the batteries topped off.

All this extra power would of course generate more heat...from regulators, li-on charger, ESCs. I saw it had already been mentioned to dump the heat onto an aluminum sink that contacts the water. Don't see any reason why that won't work as long as a good method of sealing it can be devised.

Sorry for the long-winded post. After typing it I wondered if it would be better broken into separate posts...hmmm

Daniel


#2

Hi Daniel

The issue with brown outs I think has been addressed by using the LiPo Batteries and changing to the new Graupner props. The new props greatly reduces the current draw on the batteries plus gives more thrust. As far as the Battery Voltage goes this is limited by the ESC's which operate at 12V so S3 LiPo's is max of what you should be using. The idea of sending down 48Vdc to the Rov and using a DC-DC Converted is a good way to help top off the batteries keeping in mind that they require the proper LiPo Chargers. I think the Tether is 22 gauge wire, that is what I am using, but depending on the length of wire you use will determine the voltage out at the ROV end. I have not tried to power the ROV from the tether yet as I have plenty of power in my ROV to handle any dives that I do. There are a few people that have been working on this and they may wish to chime in on this topic. Also keep in mind that 500' of wire on a spool with 48v going through it makes a great magnet and and there may be some heat dissipation as well.

Keep us posted on any work you do so we can all learn from it.

Regards

Dave


#3

One thought on the ESCs bolted to an aluminum sink in contact with water. Is there any voltage potential on the ESC sink? By bolting it to an aluminum sink, especially in salt water may turn up some voltage leakage issues if so.

I think heat will be one of my bigger issues, the water we dive in gets up to 30C(86F) and higher in the summer,

I was afraid of the 12v limit on the ESC as I saw the 3s lipo limit in the specs. Once mine come in I'll probably run one up to 16v just to see what happens. Or higher if the magic smoke manages to stay inside.

do you think I'll have a big electro-magnet in the spool even though it's a twisted pair? I already figured on the spool dissipating about the same power as the rover is using since half the voltage is lost in the wire. Maybe a water shower will keep it cool if heat becomes an issue.

If magnetism becomes an issue we could just switch to a/c and rectify it at the rover.

One of my pipe dreams for this rover was already mentioned in another thread but I haven't seen much activity on it -- hunting and neutralizing lionfish. They really are exploding in population in south FL.


#4

I had success in the past running the ESC at 20V.. sounded like a wind machine with the props on. I eventually did burn one ESC out... but it was fun while it lasted.

-Brian


#5

Hi Daniel:

A couple of quick comments:

- Right now the motor power is limited more by the available battery current than by the voltage. So I don't think increasing the voltage will help anything. A more interesting experiment would be to modify the battery pack design to allow more current to get to the motors, so that the top end of the throttle range could be better utilized.

- The current combination of the lithium batteries and the Graupner props works pretty well. There are still some issues at very high throttle settings, but you don't really need the power under most driving conditions. Small ROVs are tough to use in fast currents, and I will be curious to see how your tests go, whether you modify the motors or not. Similarly, the battery life with the current design seems satisfactory for most applications, without sending power down the tether. There are safety implications with sending power down the tether, especially if there are divers in the water. If you really need the extra power or battery life, and decide to implement a power-over-tether scheme, please pay particular attention to the safety issues.

-Walt


#6

Hmm then maybe with a little bit of cooling they will be fine...although maybe not necessary according to Walt. We shall see.


#7

So if more current is needed then maybe just 2 more strings of LiPo and some extra buoyancy would give more down time and help out with the current issues.

I'm curious as well how it will do. Might have to add a gyro and some autonomous attitude correction ability to deal with eddies and turbulence, especially near reef shelves and fingers. Might be able to take some lessons from the r/c heli/quad guys, they have some very small boards with multiple ESCs and gyros already set up.

As for safety with a powered tether...If the supply is allowed to float, safety issues should be a minimum...the only return available for the current would be the ROV itself or the tether if the insulation were compromised. Voltage would be limited of course. I wouldn't think sending 120v+ down the tether would be a good idea at all. But with some extra batteries the entire concept may be a moot point.