ROV Died after a couple minutes in the water


#1

Hi All,

We just came beck from our first in water test. The ROV was working fine Initially, no problems, and then all of a sudden after a couple minutes swimming around in the water it died, all power was lost, and when we tried to boot it here on the bench we got no response / communication between the TENDAS. we can't find any water in any of the tubes that would have shorted the system.

We are in the process of trying to troubleshoot this now, but i though I would ask if anyone had any suggestions of what may have gone wrong?

Nathan


#2

Eric is putting together a troubleshooting guide right now, which should be helpful for situations like this. Simple question, and sorry if it's so basic, but how were the battery levels?


#3

Battery Levels are fine, we've narrowed it down to this. It dies when we use the thrusters, and will not start up out of water until we open one battery tube and then close it (like all we do is pop the cap and put it right back on). prior to doing this the Controller board gives a power light, but no blinking brx light, we cannot connect and the entire system is down. its as if it needs a reset for the batteries. any suggestions?

Nathan


#4

To Give some information, when we put it in water and don't use the thrusters it works, if we us the thrusters intermittently it works, but after some thruster use (a couple of minutes) of heavy thrusters or anything like that it suddenly shuts off. if we then take it out of the water it will not boot up until we open a battery tube ( either one) and close it again. once that is done its starts no problem, and the process repeats itself. There is no water in the battery tubes, or in the e can, and it seems rather strange.

the biggest thing I guess is the arduino gets power when it shuts down, but nothing else does. the BRX light also doesn't blink as usual, but as soon as the batteries are re connected it starts.

Any help would be appreciated. we are on a slight time crunch to get it finished up by Wednesday next week if possible.

Nathan


#5

Hi Nathan:

It sounds like what's happening is that you're tripping the battery overcurrent protection that is built into the individual lithium cells. Once that happens, the only way to reset the protection is to remove all current draw from the cell, which you can do by pulling off an battery tube endcap.

You can watch all this happen in the telemetry if you're driving the ROV hard. In the telemetry field there are voltage measurements for the two battery packs (I think they're called BatV1 and BatV2, or something like that). Normally each batter pack will read something like 11 volts. But under hard driving conditions, you will occasionally see the voltage on one pack go to zero. At this point you're basically only driving on one of the battery packs, and further hard driving can kill that, too, leaving one with a dead ROV. Until you open up a battery tube, that is.

We have also noted that if you get a leak in a battery tube, it can short out the battery protection circuit and cause a shutdown as well.

At some point we'll probably investigate the characteristics of the battery protection circuit, to see if some fix (maybe adding some capacitance on the battery bus?) would reduce the incidence of nuisance trips. One could try using unprotected cells, but we definitely don't recommend that- the protection circuit is there for a reason. What might end up happening is that we just limit the max throttle of the vehicle to a level that won't ever trip the battery protection circuits. But I would prefer not to do that, as there might be a situation where you actually need that power. Perhaps a better solution is some kind of limit checking on the telemetry values that warns the user that he has popped the overcurrent limit on one of the packs.

-W


#6

Oops. Correction to what I wrote above. There is only one measurement of battery voltage, but the current being drawn from each battery pack is measured separately. In the telemetry field on the left side of Cockpit, the measurands are BT1I and BT2I. If both battery packs are working, the currents should be fairly close to one another. If one of the battery packs has shut off due to an overcurrent, the current on that tube will be zero.

-W


#7

Hey Nathan,

I had a similar issue one of the first times I started. It turned out that I had done a pretty crappy job soldering on one of the battery tubes. When I set it up the particular way I do for the bench test it would work because the solder would come in contact, but when It was put in the water only one of the battery tubes would supply power. When I went to scoot around with the thrusters (with any amount of aggressiveness) it would kill.

I don't know if this is your problem. If it is, its an easy fix!

-Cannon


#8

Thanks Walt,

That's really all the information I needed! thanks a bunch!

Nathan


#9

Did you program the ESCs?


#10

I did, I suspect we were just driving the vehicle to hard. lessons learned i suppose!

Nathan