OpenROV motors with different propulsion

propulsion

#1

Dear All,

Me and my colleagues assembled an OpenROV v2.6 last week everything went fine we had some small issues but at the end of the day we did it, and I programmed the ESCs and calibrated the motor.
But when we tested it the swimming pool, one motor was faster than the other ( port and starboard ) so it can't go in straight line it turns left.
We tried many times and I checked the parameters of the ESCs and the calibration of the motor over and over and I didn't find anything wrong, I think that one on the motors is getting less power or just has a weaker propulsion.

Please help me solving this issue.

Thank you.


#2

Hi Mohammad:

Try spinning the props with your finger (when the ROV is off) to see if one motor has more bearing friction than the other). They should feel similar.

Also, see if the ROV goes straight when you try to turn left. This would indicate that the left motor is spinning backwards, and needs to be adjusted in the motor calibration.

Let us know if either of those things help.

-W


#3

do you notice that it 'turns' better one way than another? Mine turns super well to the left, and pretty slowly to the right, still not quite sure what's going on there. Mine also has a tendency to veer left.


#4

If the props have equal friction when you spin them by hand, one other thing you can check is to make sure that there are no nicks on the propeller blades. In our test tank we have noticed a surprising amount of efficiency difference between a propeller that has a nicked-up leading edge and one that has been carefully dressed with a jeweler's file. It's probably not a bad idea to smooth the leading edge of the props even when new, since sometimes there's a little burr on them when they come out of the mold.

-W


#5

my blades are fine, friction feels reasonably equivalent, the right has slightly more friction but it veers left so that means the right is slightly more powerful.


#6

Hmmm. I'm pretty much out of ideas then. You do have the props set to counter-rotate, don't you? That is, of the 3 props supplied with the kit, the ones installed on the horizontal thrusters should be mirror images of each other.

The other thing that might be an issue is drag from the wire bundles inside the open structure of the ROV. If you look at the ROV from the front and the back, does it appear left-right symmetrical? There are obviously some asymmetries due to the way it's designed, but anything you can do to minimize them will help the performance in the water.

-W


#7

#8

Okay I just remembered one more thing, though this is getting into pretty obscure stuff.

If the motors are handled roughly during the installation process, it's possible to break some strands of the leadwires into the motor, usually right where the shrink tubing ends on the motor side. If enough of the wires break, this will create a higher resistance on the motor windings and cause it to lose power.

You can carefully inspect your motors for this, though I wouldn't recommend removing them since that just would compound the problem. The other way to check this would be to get a decent multimeter (one that can measure resistance with a resolution of 0.1 ohm or better), and check the winding resistance for each lead permutation of each motor (A-B, B-C, A-C). They should all be equal and they should be consistent from motor to motor.

To do this you'd have to have the E-tube open and the DB-25 connector detached, so I'd just put this down on the list of things to do the next time you have the thing open for work.

-W


#9

I think I may have figured out the majority of my issue... One of my ESC's was wonky (a pinched and shorting wire inside), so I replaced it and so far in tonights bathtub confined water trials it seems to be responding much better.


#10

Ahh, good. One of the changes in OROV 2.7 is that the ESCs are mounted, programmed, and tested at the factory, hopefully avoiding all the trouble that you've been going through.

-W


#11

It's no real trouble, it just makes me better at fixing and troubleshooting so that i can be a better mentor for the next generation :)