Robot refuses to turn!

[Joe Johnson]

You are not getting full torque to the wheels, simple as that.

Either you are not powering both motors (all 4 really) or you are do not have the gearboxes build right or your chain is eating up the torque or something.

Traction is not the problem.

Hunt down the torque loss.

Joe J.

[Edoc'sil]

The whole wheel is gone, nothing is touching. The carpet has concrete beneath it.

Yes I am directly powering the cims on both sides from the battery, with no breakers involved.

Can you please confirm that you're running dual cims on each side, and not singles? ~ This is true as well.

I will check that the voltage isn't dropping too much, but if we are drawing stall current (130A?) from 4 motors the voltage will decrease a ton no matter how charged it is.


As you said, something is wrong. The output shaft spins without applying to much force, and you can easily spin the wheels by hand. You can tell there is some efficiency loss but it seams to be working more of less in proper order.

I will see if I can find a way to make it all spin better, but I am not hopeful.
I will also see if i can get a battery reading 13 volts.

[Al Skierkiewicz]

Ed,
You have demonstrated a classic issue with high friction wheels in a tank turn. The motor wires heating up is a sign that should not be ignored. If you were feeding the motors through circuit breakers, they would be tripping. As others have pointed out above, teams have discovered that raising/offsetting the height of the wheels prevents the incredible side load of all wheels on the floor. This cause each of the CIM motors into near stall current draws. At 129 amps each that is a total of 516 amps. You might have also noticed that the Crio is inhibiting output for a short period of time (if it is not rebooting). When the battery voltage falls to 5.5 volts, the Crio inhibits output in anticipation of a voltage fault. This causes a somewhat jerky motion during the fault. With a 516 amp draw, the battery will not be 12 volts no matter how well it is charged. You will drop voltage across the internal resistance of the battery that may take it down 5 volts or more.
Dr. Joe will give you some insight on design and as other have pointed out, wheel choice can have an effect as well.
If your motor wires were hot, the internals of the motor were hot as well. A "nose" check around the motors will tell you if you have stressed the motors.

[Joe Johnson]

See if you can measure the current when the wheels are off the ground. You can also infer current by measuring the speed and backing out the motor speed then measure the motor current and doing some simple calcs to get motor current, but I prefer to know motor current directly.

Hey, I just had a brainstorm. How are you managing thrust on your wheels? Scrubbing the tires puts in a very high side load on the tires. If this side load is not managed well (for example if you let the tires rub against the frame) then that adds to the turning resistance that your motors have to overcome.

I bet that there is something going on there. Take some pictures and let us know.

Joe J.

[Edoc'sil]

Originally Posted by Al Skierkiewicz

Ed,
As others have pointed out above, teams have discovered that raising/offsetting the height of the wheels prevents the incredible side load of all wheels on the floor.

This is whats driving me crazy, we have in effect done this by removing the front wheels.

@ Joe
Sounds like a good idea to test the motor draw.
There is about .75 in clearance on either side of the wheel that is set by a spacer that doesn't rub the wrong way against the wheels in..... wait a sec.

I have an idea that could potentially cause the bearings to seize up. I'll get back this evening.

[Al Skierkiewicz]

Four sticky wheels on the floor are enough to stall the motors. If the robot runs OK with the wheels off the floor then that is like waving a red flag.

[Chuck Glick]

This may sound like a dumb question, but by any chance are your supershifters in high gear when doing these tests? It may sound silly, but that might alleviate your problem with turning in the mean time.

As for the AM omnis you posted, they are the same diameter.

[billbo911]

On a team I mentored a few years back during Rack n' Roll, we had a 4 wheel tank drive with high traction wheels. It would not turn either. We overcame the issue by adding zip ties in an "X" pattern around the front wheels.

With six wheel drive, you would need this on both front and rear wheel sets. If this works for you, it won't cost much and is easily modifiable by adding more or fewer ties across the tread.

Alternatively, run one more layer of tread around the center wheels only, thus giving a "drop center" effect. Make sure to secure this second layer very well.

[Joe Johnson]

With just the middle and rear wheels in play the aspect ratio is 24 Wide by 12.5 between wheels, this is enough that the thing should turn, even with very grippy wheels (given 2 CIMS each side plus 24:1 to 8" wheels).

Either the torque isn't getting into the gearbox, it isn't getting to the wheels or it is getting to the wheels but something else above and beyond traction to the carpet (including scrubbing loads, aka thrust loads) is using up the torque.

He says that the motors are both wired right. The thing moves at about the right speed going straight. I suppose that going straight it will use the right amount of current.

To me that means something is binding when the robot tries to spin. The wheels could be hitting something, the frame could be racking and causing a bind, the wheel's may have bad thrust bearings or bad thrust load management, the chain may bind up when the wheels see the twisting loads, etc. ... we are looking for something extra that happens only (or most prominently) when the wheels see significant side loading due to being forced to scrub along the carpet.

I'd bet a Mt. Dew on it.

Joe J.

[RyanN]

Hopefully I read everything correctly. You stated that you're running a 6WD bot, which is great!... but you need to consider a few things that have been mentioned already. First off, when everyone says to lower the middle wheel, you really need to do this somehow. We've gotten away with it in the past with pneumatic wheels that we could air up to raise and lower the center of the robot. Very good solution might I say.



That is a quick sketch of a lowered centered wheel. I believe we've used about a 1/8" drop which has worked great for years.

If you can remove the front wheels, and the robot maintains stability, then you have a balance problem. The entire weight of the robot should remain mostly on the center wheels, with very little weight on the outer wheels. You said that you completely removed the front wheels, which leads me to think you have way too much weight in the back. If you haven't already, you should move that roughly 12lb ballast called a battery to the far front, and if that's not enough, the other ballast (cRIO) may work out, as well as most of the other electrical components.... or if you have spare weight, just add some weight to the front.

Like Chuck said, make sure you're not in high gear... although it should be able to turn in high gear, but it would be a very easy programming fix that could shift it into low gear when the joysticks are the inverse of each other. (one is positive, one is negative).

Good luck! I hope you fix it! I know it stinks to be this late in the season when the design is coming together, just to find out that you have a major problem like this. Every team has probably been there at some point, I know ours has. Just think it through, get creative. I do recommend a pneumatic wheel if your design permits. Remember when you're turning, the outer wheels do nothing... except bind the robot.

[Tom Line]

Hrm. Still having problems? Let's confirm a ew more things:

1. Live axle or dead axle, cantalevered or not, and are you using bearings or bushings?

2. How tight are your chain runs, are they nicely alligned, are your transmissions stiffly mounted or flexing?

3. How long are the motor shafts, are the sporckets mounted as close to the tranny as possible to prevent bending moment?

4. What trannies are you using? There are a number of cim pinions that are very close to the same but will not function well interchangeable under load. Have you disassembled the trannies and confirmed, step by step, correct assembly and lubrication?

[joek]

basically, it sounds like you have too much traction on the outside wheels, omni's are probably the best solution, but slick wheels would probably work

[Al Skierkiewicz]

Ed,
I guess we are at that point that we need a real photo of the wheels. As Joe has suggested, there might be an issue with the axle/transmission/wheel interfaces. It is the kind of thing we can look at and have a "I should have had a V8" moment and know exactly what is wrong. For instance, an unsupported shaft can have extreme friction in turns that does not exist in straight driving.

[Joe Johnson]

Originally Posted by Al Skierkiewicz

Ed,
I guess we are at that point that we need a real photo of the wheels. As Joe has suggested, there might be an issue with the axle/transmission/wheel interfaces. It is the kind of thing we can look at and have a "I should have had a V8" moment and know exactly what is wrong. For instance, an unsupported shaft can have extreme friction in turns that does not exist in straight driving.

What ever happened to this poor robot?

I have a sad image in my mind of this little guy going back and forth over the bumps and back again but never turning, never facing the goal or the tower... back... and forth... back... and forth...

Give us an update please.

Joe J.

[Mike Betts]

Originally Posted by Joe Johnson

What ever happened to this poor robot?

I have a sad image in my mind of this little guy going back and forth over the bumps and back again but never turning, never facing the goal or the tower... back... and forth... back... and forth...

Give us an update please.

Joe J.

In order to fall asleep, most of us count sheep. The Good Doctor counts robots!

Who knew?

Mike