Strategies to prevent overcurrent/breaker trip

[Clark Gilbert]

Here is something i posted about "cold spray" last season sometime. The figures were done by an engineer that wanted to figure out if all the circuit breakers were the same, and if cold spray/freeze spray, etc. had an effect on them.

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Ok....I am not an electrical person and this is basically all copied and pasted from an email i recieved about our breaker problems...this may help some people...


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I've picked two of the five breakers as being much better performers, these two
>go about 45% longer at 120 amps, compared to the worst breakers.
>
>COLD SPRAY WORKS
>Precooling the breaker provides 60% more duration on the first power cycle.
>This rises to 120% more duration on repeated power cycles at 5 minute intervals
>(to simulate the high cycle rate found in finals competition). Bottom-line: a
>10 second blast of cold spray will restore the breaker to its original
>performance even if it has just been overloaded. Where do we get more of this
>stuff. I've used my only can during this test?

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1. Suspend our best circuit breaker by the 6 ga. wire and 2-4 strips of latex
tubing, directly between the battery and the power panel
2. Possibly move the power panel closer to the battery (or the battery closer
to the power panel) so we can cut down the length of 6 ga. wire.

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Ok....it's me again.....i know our final solution to our problem was that we did some electrical work...mounted the breaker onto a flexible/bouncy kind of V shaped lexan strip...and we really use the Cold Spray....

Originally posted by Gope
Im not sure as to the legality of this, although it is quite tricky, and I am rather impressed by whoever created it. Although, In my opinion, it violates the "spirit of the game" and "gracious profesionalism" in that it gives an unfair and unjudgeable and unregulatable advantage to the team who uses it. Although, in any other instance in life, i would say "more power to you, you beat the system." So take my word for what you will, whatever that is.

Unless I missed something, there is no rule that says you cannot do this. Power to you people. Now you know, so use it.

[RebAl]

Cooling off the circuit breaker defeats the purpose of a breaker in the first place, by cooling them you are allowing more current to pass through, the reason its tripping is that there is too much current, it has to be thought of in that respect, otherwise you might as well just circumvent the breaker all together

2 exceptions:

1- the breaker is tripping due to mechanical forces
2- you cool the breaker between matches

[Bduggan04]

Originally posted by RebAl
Cooling off the circuit breaker defeats the purpose of a breaker in the first place, by cooling them you are allowing more current to pass through, the reason its tripping is that there is too much current, it has to be thought of in that respect, otherwise you might as well just circumvent the breaker all together

2 exceptions:

1- the breaker is tripping due to mechanical forces
2- you cool the breaker between matches

Freeze spray never allows significantly more current to pass through the breaker. It should be noted that every time the breaker trips it is easier to trip the next time. Freeze spray can help to get the breaker back to normal, in other words it will trip as if it was new or close to it. Last year, the breaker problems became more apparent than in past years. We had a breaker that took only a slight tap with a screw driver to trip it because it had tripped so many times. While I know this thread was designated for design strategies, there are several tricks you should use around the breaker that we learned last year.
1. The freeze spray does work. We used it a lot last year.
2. Mount it loosely and orient it in a way that the switch is not in line with the most likely collision direction.
3. If a breaker has tripped several times, replace it. Especially do this if you have many matches left to play.
4. Don't use the breaker as a switch. These breakers aren't switch rated, and by using as a switch it subsequently is easier to trip. Turn the robot off by unplugging the battery cable.

[Andrew]

Freeze spray never allows significantly more current to pass through the breaker. It should be noted that every time the breaker trips it is easier to trip the next time. Freeze spray can help to get the breaker back to normal, in other words it will trip as if it was new or close to it. Last year, the breaker problems became more apparent than in past years. We had a breaker that took only a slight tap with a screw driver to trip it because it had tripped so many times. While I know this thread was designated for design strategies, there are several tricks you should use around the breaker that we learned last year.
1. The freeze spray does work. We used it a lot last year.
2. Mount it loosely and orient it in a way that the switch is not in line with the most likely collision direction.
3. If a breaker has tripped several times, replace it. Especially do this if you have many matches left to play.
4. Don't use the breaker as a switch. These breakers aren't switch rated, and by using as a switch it subsequently is easier to trip. Turn the robot off by unplugging the battery cable.

Are you talking about the main 60 A breaker or the mini-30A breakers?

BTW, if people are worrying about shock to their breakers, why not use the vibration isolation mounts provided by Small parts to mount them? (Or equivalent product available from this year's legitimate supplier)

Last year, we routinely tripped our 30A breakers on our main drive motor. However, that occurred due to a design flaw in our gear box. The tolerances were too tight and we were machining grooves in our rotating parts. Once diagnosed and fixed (in the off season), we have beaten the heck out of our robot with no breaker trips.

On our first robot (2000), we used a lag algorithm on the joystick to avoid the maximum acceleration at zero speed issue.

In 2001, we picked a gear ratio which seemed to keep us in the "sweet spot" on the torque-current curve. That robot runs cool under all circumstances. Although it is not as fast as we would like.

For future reference, we bought a bunch of 30A breakers from innovation first. Our plan is, when they start popping, replace them. Now that we know about freeze spray, we'll probably incorporate both forced cooling of the breakers (we already do this with our drive motors) and a blast of freeze spray at the start of a match.

As for driver instructions and software over-rides, I think that is a function of your drive team. In general, we prefer to give the driver the ability to push the robot as far out on the performance envelope as he feels necessary. However, we have had operators who needed some software limits to keep from breaking things.

Usually during the course of the competition, we try to customize the controls around the driver's/operator's preferences.

During the off season, we have all kinds of limits which we impose, since we use a lot of untrained drivers.

A question for the breaker gurus...
In 2002, we were running both Chalupas at full blast for the full two minutes + both drill motors at close to their limits + a globe motor + two seat motors + one fischer price motor intermittantly. We never tripped the 60 A breaker. We did mount the breaker switch perpendicular to our direction of travel.

For teams that tripped the 60A breaker, what kind of loads were you putting on your system? Was the breaker trip more shock related than electrical?

Andrew
Team 356

[team222badbrad]

The year when we had to balance our robot kept tripping the 60a breaker. After several trips over the bridge, our robot wouldnt work when the bridge came down. We found out that the circuit breakers would trip when something hard smashed into the robot. SO what we did was mount the breaker with surgical tubing!

[kmcclary]

Let's set the "cooling the breaker" thing aside for a bit...

What ELSE to you do DESIGN wise to keep from overloading the electrical system?

Examples:

1) Do you have a AHL legal slip/breakaway clutch design? Please describe it!

2) Ever done anything funny in drivetrain or software design to avoid stall currents, especially when expecting Tug of War situations? (Ex's: Enable/disable [PWM-like] algorithims to "jerk" your machine? Deliberately adding backlash or an elastic element in the drivetrain so the motor gets a "running start" when reversing?)

3) Four wheel drive users WITHOUT wheel turning: How are you making "holonomic wheels"? (A tire that slips sideways easily to keep turn scrubbing forces from blowing the breakers, while maintaining good forward/backward traction.)

Please, let's hear some DESIGN methods to avoid breaker tripping, in addition to "cooling tricks"...

Thanks!

- Keith

[sanddrag]

I'm not sure if it does a whole lot for current draw but we put a deadband in our drive joysticks. For the first bit of motion in the positive or negative Y axis directions, no signal would be sent to the PWMs. This was more to prevent strain on the drive componenets but it may have done something for current draw.

Other teams I think had a sort of "soft-power" system where power to the motors came on slowly rather than all at once. However, I am not a programmer so I don't know how that works.

[kmcclary]

Originally posted by sanddrag
Other teams I think had a sort of "soft-power" system where power to the motors came on slowly rather than all at once. However, I am not a programmer so I don't know how that works.

That's a "low pass filter" algorithm, an excellent way to prevent sudden spikes in the power. However, the driver deep down must ACCEPT that filtering.

Last year we tried filtering, but it became an unconscious "war" between our programmer and our driver! <chuckle> Each upped their effects, until the robot was sluggish and the driver started slamming the sticks like buttons. They were BOTH bound and determined to get a specific behavior out of the robot, and they didn't agree on what that was! I even started fearing for the sticks' survival, and we didn't have any spares!

I finally asked for ALL the filters to be ripped out of the code, and pleaded with the driver to "try not to break the robot too often" with the controls... The robot worked fine, AND the code was simpler.

Anyone have better luck with filters?

Anyone have any OTHER good subroutines for breaker protection that work well DESPITE an "insistent driver"?

Has anyone found software "load shedding" to be useful, i.e. locking out the compressor or other "lower priority loads" whenever the drivetrain demands a panic boost? ("Scotty, we need more power for the warp drives!!!" )

- Keith

[Cory]

Originally posted by kmcclary
3) Four wheel drive users WITHOUT wheel turning: How are you making "holonomic wheels"? (A tire that slips sideways easily to keep turn scrubbing forces from blowing the breakers, while maintaining good forward/backward traction.)
- Keith

I believe that you would be talking about Omni-Wheels. There are numerous threads about these. I am not sure exactly what they were called, but they should be fairly easy to find. There are also wuite a few pics in the 2002 robots gallery of teams Omni-Wheels

[Andy A.]

For various reasons, current draw being one of them, our '00 bot used a 6 wheel drive. In this setup, all 6 wheels are powered, where the middle two wheels are set about a quarter of an inch lower then the 4 corner wheels. This allowed us good climibing on the ramp (the primary reason for the 6 wheels) but also reduced scrubbing drasticly when turning.

Of course, having 6 wheels is more expensive (wieght, build time, repairs) then 4, in our case it was a fabulous solution. If you plan on lifiting loads while acending inclines, it's perfect. Also, if you can get away with a two wheel drive, Why not? The bot turns faster and uses less current, the drive line is far more simple to make and repair, and probably most important, is lighter! Naturaly, using 6 or 2 wheel drives creates all sorts of problems, but its all a trade off.


-Andy A.

[kmcclary]

Originally posted by Andy A.
For various reasons, current draw being one of them, our '00 bot used a 6 wheel drive. In this setup, all 6 wheels are powered, where the middle two wheels are set about a quarter of an inch lower then the 4 corner wheels. This allowed us good climbing on the ramp (the primary reason for the 6 wheels) but also reduced scrubbing drastically when turning.

An excellent idea...

OOC, Were the wheels evenly spaced, or was the center pair nearer to one end? How was weight distributed across the wheel pairs? Did your robot rock back and forth between the outer sets of wheels when you reversed, and then run on the "back" (outer) pair each way?

- Keith

[kmcclary]

Originally posted by Cory
I believe that you would be talking about Omni-Wheels. There are numerous threads about these. I am not sure exactly what they were called, but they should be fairly easy to find. There are also quite a few pics in the 2002 robots gallery of teams Omni-Wheels

I'll go check those out. Holonomic drives are pretty cool... Thanks!

BTW... Omni-Wheels are only ONE way of doing a sideslip wheel. It doesn't have to be VERY slippery side to side, only significantly MORE slippery, enough to manage the turn scrubbing forces without sacrificing too much forward and back drive traction.

Another simpler example than homebrew Omni-Wheels might be treads made from repeated slices of inverted U channel bolted to your wheel or tread. If the inverted U can be seen looking at the side of the robot (align from side to side), AND the side to bottom edges of the vertical pieces have a radius on them (rounded, so it looks like a "U" from the front), it makes for good claw traction forward and back, yet acts like a sled runner if shoved sideways. Any slide push causes it to "climb the carpet nap" and slide easily.

Any other SIMPLE methods of making "side slip wheels"?

Remember, this thread is about ANY technique to reduce breaker blowing. Not just wheels, chilling, etc...

- Keith