Drive motors drawing too much power

[kitare102]

3042 got their robot fully operational last week and has since found that, even under fairly little strain, the drive motors get extremely hot. Also, the left side draws more power than the right. Even after we checked the gearboxes for binds and replaced the motors, it draws consistently about 4 amps more than the other side. Attached are photos to give a view of the chassis. Preview: West coast drive, versablocks, direct driven back wheels, 1/8 inch drop center. Anyone have any ideas on how to fix these problems? I recognize that the length of the drive train gives it a lot of scrub, but we will likely have some omnis to try out on the front and back before the first tournament.

[jeremylee]

Is it possible the center axles aren't parallel with the other axles effectively loosing your center drop? Also, what ratio option do you have in the toughboxes?

[Ether]

Originally Posted by jeremylee

what ratio option do you have in the toughboxes?

^^^^ This.

... and what diameter wheels?

[kitare102]

The drop center appears to be behaving as expected.

I don't know about current on each motor w/o load, as two of the motors currently connected we didn't test last time. I'll see if I'll be able to take apart the boxes again tonight.

4amps higher is consistent both on the ground and on blocks.

Hadn't thought of checking for balanced drivers, thank you.

Thanks for pointing the battery safety issue out, I'll see what we can afford to put on weight-wise.

Gearboxes are toughbox minis geared to 8.45:1 with 6" wheels.

[jeremylee]

Originally Posted by kitare102

The drop center appears to be behaving as expected.

In the one pic, it looked like all wheels where in contact with the ground. Do you notice some rock back and forth on concrete?

10.71 or 12.75 (which is what we are using with 6 in wheels) would help, but shouldn't be nessesary with drop...

Omnis on ends would help if it is a scrub issue, but shouldn't be nessesary with drop...

Do you notice the battery voltage drop much when turning?

[Jeffrafa]

Unrelated, but something else to note is R67B & R76 - I don't see a pressure relief valve connected to the compressor.

For testing drivetrain, I would recommend similar to one of Dale's suggestions - electrically disconnect the motors and test one at a time, making note that speed/direction is consistent, and listening for any unusual noises.

For PWM I also suggest to CAL the controllers together, but I don't think this is a problem over CAN. I would check that the 2 talon's status lights have the same color/blink rate throughout the output range, as a sanity check that they are being commanded with the same output.

Most mechanical issues can be heard, or felt by turning the wheels manually. With the motors electrically disconnected, the two sides should be relatively easy to back-drive on blocks, and should feel/sound consistent, no periodic difficult spots.

[Nathan Streeter]

Originally Posted by kitare102

Gearboxes are toughbox minis geared to 8.45:1 with 6" wheels.

While probably not the leading cause of high motor temps, this way too fast for this year's game (imho). Seems like teams will usually be driving a few feet, almost never going farther than 15ft without stopping and starting. While 8.45:1 with 6" wheels (adjusted top speed of 13.3fps on JVN's spreadsheet) was very good last year, you'd probably be better off having an adjusted speed closer to 7fps.

Perhaps you have a dead motor or one that's fighting the others?

Cantilevering your axles the extra bit to put the chains on the outside probably doesn't help your efficiency, but it hardly seems like that should contribute that dramatically by itself.

[Dale(294engr]]

Assuming you've eliminated mechanical drag: bearings/bushings/chain drive

what is Full Speed current draw of ea motor isolated alone, not driving gears?
is it the same at FS reverse?

Is 4A higher with all wheels off floor and full throttle?

If not repeat test off floor.

start removing things until problem goes away. (4A decreases to equal OK side) remove chains one at time test,...

electrically disconnect ea motor, test, (swap, test)
physically remove ea motor (leaving one of two or three)

make sure 2 motor H-bridge drivers driving same gearbox are balanced. (sharing load -- not contributing to it)

photo from bottom may be instructive.. Good Luck..

Safety issue.. your battery is vulnerable to being pierced by robot collision!!
consider welding/mounting a thick aluminium barrier protection!!

[Ken Streeter]

Originally Posted by kitare102

3042 got their robot fully operational last week and has since found that, even under fairly little strain, the drive motors get extremely hot. ... 1/8 inch drop center. ... I recognize that the length of the drive train gives it a lot of scrub...

Do all 4 drive motors get hot when practicing? If so, I would temporarily ignore the 4-amp difference between right and left drive current.

My first guess at high current draw on that robot from looking at it would be that the amount of force required to get it to turn, being a 6WD skid-steer, is way too high. I would suggest a "powered-off robot spin-in-place test." With the robot powered off and sitting on carpeted floor, put your finger down low on a side corner of the robot frame, (for example, at the front left corner, pushing sideways) and push on the robot to try to rotate it. Can you do this with one finger? two fingers? do you have to push real hard to get it to rotate? However much push you are needing to exert is an indication of how hard the robot needs to push in order to turn. I speculate that drive base is extremely difficult to turn in place.

We had similar issues with our 2011 6WD robot, which was a very similar configuration. It looks like your frame is relatively flexible. The idea with a 6WD drop-center wheel configuration is that the robot weight is primarily supported by the center axle, with less weight on the front/back corners. However, with only 1/8" of drop (that really isn't very much) and what looks like a fairly flexible frame, I wouldn't be surprised if the corner wheels are still fairly heavily weighted. The amount of drop needed depends a lot upon the rigidity of the frame. 1/8" might be enough for a very rigid frame, but you might need a lot more with a more flexible frame. Does the robot seem that it is able to "rock" back and forth from front to back at all? If not, there is likely still too much weight on the corner wheels.

Anyway, with our 2011 robot, our fix was to switch to omni wheels in all 4 corners. Problem solved. Other alternatives are to increase the amount of drop, make the frame much more rigid, or switch to low-traction wheels in the corners.

By the way, to help diganose if the problem is related to turning, you could use the SmartDashboard (or regular Dashboard, if using LabView) to continuously display the PDP current readings for each motor. While the robot is being driven straight forwards and backwards, look at the current readings on the display for all the motors and record those values. Then, repeat the experiment while the robot is spinning in place. If there is a big difference in those values, you have confirmed your problem is related to wheel traction resisting turns. If the current readings are very high for both front/back and spinning in place, then the problem is likely mechanical resistance in the gearbox and drive system. However, my guess is for difficulty turning.

[JamesCH95]

Calibrate your motor controllers if you have not already.

Otherwise, lots of good advice has been given already.

Edit: also, lubricate your chains and gearboxes, almost excessively. I can supply recommendations if you'd like.