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Unread 12-10-2002, 03:29 PM
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Strategies to prevent overcurrent/breaker trip

Please describe in this thread your team's method of preventing motor overcurrent/overload (and subsequent breaker tripping) with an optimized drivetrain.

What are the different methods to prevent breaker trips, and what are the pros and cons of each? How did you implement it with a multimotor gearbox? Can your method adapt and still protect the breakers in case of sudden huge load changes like ramp inclines, goal towing under all load conditions, your robot being pinned, or tug-of-wars?

Suggestions I've seen so far, each with their own strengths and weaknesses:

1) Design the drivetrain to simply tire slip at any torque greater than 30A will provide. (Paul Copioli, Thunderchickens)

2) Software limiting PWM at "full stick" unless a "turbo" button is hit (Al Skierkiewicz, 111)

3) Current sense and throttle back (various teams), including Tim Skloss' "Brown Toast Breaker" method (team 930): Current sense & integrate, modeling the breaker's heating curve. Throttle back whenever the "toast starts to get brown"...

Current sensing is "currently" being discussed at: http://www.chiefdelphi.com/forums/sh...?threadid=2205

Has anyone implemented any of the following?
- Breakaway/autoslip clutch of some kind
- Automatic transmissions (NOT operator controlled)
- *TORQUE* sensor feedback to the RC (i.e. strain gauge)
- How to throttle back multimotor drives CORRECTLY to prevent backdriving any of the motors
- Automatic load shedding / load prioritizing
If so, please describe the system.

Anyone have another system??

<edit>
If you've already discussed your system in another thread, please just cite the thread (add a link) and summarize it here. I'd love to see this thread become a nice summary / collection of all the methods in one place! Thanks!
</edit>

- Keith
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Last edited by kmcclary : 12-10-2002 at 07:59 PM.
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Unread 12-10-2002, 04:42 PM
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Re: Strategies to prevent overcurrent/breaker trip

Our current limiting sounds similar to 930's, though maybe not as sophisticated as you describe. I've already described our system in this thread, but i'll summarize it here too:

We didn't do any integration in an attempt to predict and preempt a circuit breaker tripping. We simply sense the current being drawn by our drive motors, and programmatically limit the power requested of the drive system.

Each time through the control loop, if the current being drawn by any one of the drive motors is above a certain threshold, the program scales back the power being requested of all the drive motors. The scaling factor starts at 100%, and every time through the control loop that the current is above the threshold, the factor is cut back by 5 percentage points (but not below 10%.) Once the current gets back below the threshold, each time through the control loop the scaling factor is increased by 1 percentage point until it gets back up to 100%.

I was able to experimentally determine a threshold which would prevent the breakers from tripping even with the wheels stalled when pushing against an immovable object.
Quote:
Originally posted by kmcclary

Has anyone implemented any of the following?
...
- How to throttle back multimotor drives CORRECTLY to prevent backdriving any of the motors
As I was tuning our system, it appeared to me that the only thing required to keep our drive from being back-driven was to not allow the scaled back pwm value to fall into the "dead zone" between 117 and 137.
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Last edited by Greg Ross : 12-10-2002 at 04:59 PM.
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Unread 12-10-2002, 05:14 PM
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Spray all the breakers with cold spray right before the match. Canned air will do the same thing if it's held up side down and sprayed.

Use lots of fans around your motors and breakers

Wrap the main breaker in some foam rubber to isolate it from shock when you get hit by another robot. A sudden jolt can cause it to trip.

You may also want to mount the breaker panel on something soft like a stack of O-rings where the mounting screws are.

If you use 4wd skid steer, design 2 or all of your tires to have traction only in the forward and reverse directions of the robot. Team 599 knows about this. Too much side traction will cause breakers to trip when turning.
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Last edited by sanddrag : 12-10-2002 at 05:17 PM.
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Unread 12-10-2002, 06:04 PM
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Quote:
Originally posted by sanddrag
Spray all the breakers with cold spray right before the match. Canned air will do the same thing if it's held up side down and sprayed.
Bingo.

We used that at the Midwest Regional last year after our several trips at the Buckeye Regional. After that we had minimal problems, compared to the problems we had before.
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Unread 12-10-2002, 08:18 PM
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My team is working on a current-sensing circuit similar to Team #111's. Basically it'll display the current level of each motor and total current on a display mounted on controls. Not sure if it'll be programmed to affect the 'bots behavior yet. Thanks to team Wildstang for their help.


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Unread 12-10-2002, 10:15 PM
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Great Idea!

Quote:
Originally posted by sanddrag
Spray all the breakers with cold spray right before the match. Canned air will do the same thing if it's held up side down and sprayed. [...]
Oh, that's brilliant... (Pouring some liquid N2 on them might be overkill though. )

Anyone have a way to accelerate breaker cooling during the match? Perhaps a clamp-on vertical heat sink fin array (and fan?) to each drivetrain minibreaker with some heat sink grease? Or is the case's thermal insulation value too high for that to be effective?

Too bad Freeze Mist isn't an "electronic part", or I'd suggest buying a can from Future-Active, stick it in the "electronics box" with a kit servo, point its tube at the breaker panel, and have the RC "order a cool down" as needed. <chuckle>

Was there a ruling last year banning "remote sensors" outside of the Electronics Box (EB)? I don't see anyone using remote sensors (other than "wire resistors") outside of the EB. It would be nice to attach detectors to items, such as thermal, torque, or strain gauge sensors.

Is thermal sensing even useful in a two minute match, considering the sensor settling time?

- Keith
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Unread 12-10-2002, 10:20 PM
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I know it's briliant. I'm just sorry I didn't come up with the idea. I read about it in some other thread around this place.
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Unread 12-24-2002, 04:51 PM
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To best way prevent over curent in motors is to have a clutch in the system. this can be the clutch provided in the drill motor transmisions, tire slip, slip in a v belt or round drive drive, or a self made clutch. Stalled motors over heat very quickly, not just because of the high current, because the intergral fan is not moving also. It should be noted that breakers let a much higher curent pass than there rating for a few seconds (this info is out there some where)

For the drive system we fond that tie slip worked best, experment with diferent tires, gearing and the right weight load to find oout what works best. We found that ball pick up systems work best if they can slip also, it pervents excessive jaming as well as over heated motors.

As for other mechisims like lifting arms and such design them to do the normal duty at or less than 20% motor torque rating limit, if you want control with a speed control. Add limit swithces too.

Get a clamp on DC curent meter, learn what is going on. They are great for diagnosis alos, espically if you have two motors powering a mcechinism.
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Unread 12-24-2002, 05:34 PM
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I have a couple of ideas. I was wondering if the following would be legal.

1) An onboard tank with some kind of cold liquid circulating through tubing wrapped around the motors. And/or a fan blowing across the tubes making a cold wind on the breakers.

2)An onboard can of cold spray actuated by a servo.
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Unread 12-24-2002, 07:18 PM
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The old fashioned way...

Sanddrag, in the past FIRST did not allow any fluids that were not lubricants as far as I know. Not being able to regulate the safety factor on the compressed air tanks for FIRST robots would be quite scary to FIRST.

While I am typically a supporter of "drive-by-wire" systems in general, I feel that implementing a drive by wire system may not be as wholly benificial as one may think.

What do I mean by this?

To make a long story short, A FIRST robot is often asked within the course of a match to perform a task that "pushes the envelope of the robot" Placing an indicrimate software restriction on such an action could impede the maximum performance of the robot. Although such a system would certainly allow for consistant, reliable performance, I can think of many many times where a robot "pushed the envelope" and in performing the action could alter the results of a match.

There is still a great deal of credit to be given to an operator who can "feel" how the robot is behaving. While this idea may be a thing of the past and in texts like "The Right Stuff" I have found that to this point, it is difficult to gauge where a software program should draw the line between safety and pushing the envelope.
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Unread 12-24-2002, 07:56 PM
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Sorry... Not on AHL. How about expanding the air supply???

Quote:
Originally posted by sanddrag
I have a couple of ideas. I was wondering if the following would be legal.

1) An onboard tank with some kind of cold liquid circulating through tubing wrapped around the motors. And/or a fan blowing across the tubes making a cold wind on the breakers.

2)An onboard can of cold spray actuated by a servo.
Sorry, BOTH are illegal due to using materials not on the Additional Hardware List (AHL). Remember, All materials on the robot must be either from the kit or the Additional Hardware List (AHL). Cold spray is not on the AHL, and you have no AHL legal refrigerant to circulate. You can use cold spray as a TOOL, but not as a PART on the robot. BTW, I already joked about that before. See above.

Hey, how about clamping a thermal mass (made out of AHL stuff) onto each breaker and chilling THEM before each round?

--- Air supply expansion as a coolant? ---

Depending on the pneumatics manual rules, you MIGHT be able to create an expansion valve for 120psi air and a heat fin assy to create a pretty lousy cooler out of the compressed air supply... Gad, that "feels" AWFULLY expensive in terms of compressor power usage for a TINY bit of cooling.

Are there any Air Conditioning experts out there that can run the numbers for that one and comment on it? How much cooling can you get by expanding the 120 PSI air supply to atmosphere for two minutes?? Assume you have NO other cylinder loads, have precharged ALL the tanks in this setup before the round, and clamped the heatsink onto ONE breaker:

(compressor)--> (ALL tanks)--> (control valve)--> (expansion_valve & heatsink assy)--> open air

At round start you open the control valve, and run the compressor full tilt. How much cooling could we get, in terms of degrees dropped on that one breaker over the two minutes?


That aside, I'm more interested in seeing DESIGN METHODS to prevent breaker trips listed here, versus simply finding ways to FOOL the breakers with chilling. Anyone have any OTHER ways to prevent breaker trips other than chilling, and the design methods already listed?

- Keith
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Unread 12-24-2002, 08:03 PM
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Re: The old fashioned way...

Quote:
Originally posted by Patrick Wang
Not being able to regulate the safety factor on the compressed air tanks for FIRST robots would be quite scary to FIRST.
The compressor has a safety valve attached, that pops at 120 PSI. Not a problem.

BTW... As a reminder before we get any further into pressures and cooling, this thread is for people to list their general robot design methods to create machines that INHERENTLY prevent breakers trips.

Please list how you design your robots to normally prevent breaker trips, yet still allow you to push the power demand situation whenever needed.

Thanks!
- Keith
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Unread 12-24-2002, 08:19 PM
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How about gearing your robot reasonably, or if you choose to go Extremely fast, step the voltage that is put into your motor so you do not put as large of a strain on the motor.

If you are geared for, say 15 feet per second, you will most likely pop the 30A breakers as you try to accelerate, if you do it with full voltage. But if you start by applying 1V, then 2, then 3... etc, you will not draw as much current. Simple physics, V = I*R.....

Either that or that oboard can of freeze spray


the 60 amp breaker will handle current draws of MUCH more than 60 amps..... these stats were taken from Woody's speach during kickoff in 2002 -

@200 Amps - Trips in 8 seconds

@100 Amps - Trips in 1 minute, 48 seconds

Just know that you cannot pull too much current for too long. If you design within the limits of current draw, your robot will be fine.

Tom
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Unread 12-25-2002, 01:20 AM
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Re: The old fashioned way...

Quote:
Originally posted by Patrick Wang
Sanddrag, in the past FIRST did not allow any fluids that were not lubricants as far as I know. Not being able to regulate the safety factor on the compressed air tanks for FIRST robots would be quite scary to FIRST.

While I am typically a supporter of "drive-by-wire" systems in general, I feel that implementing a drive by wire system may not be as wholly benificial as one may think.

What do I mean by this?

To make a long story short, A FIRST robot is often asked within the course of a match to perform a task that "pushes the envelope of the robot" Placing an indicrimate software restriction on such an action could impede the maximum performance of the robot. Although such a system would certainly allow for consistant, reliable performance, I can think of many many times where a robot "pushed the envelope" and in performing the action could alter the results of a match.

There is still a great deal of credit to be given to an operator who can "feel" how the robot is behaving. While this idea may be a thing of the past and in texts like "The Right Stuff" I have found that to this point, it is difficult to gauge where a software program should draw the line between safety and pushing the envelope.
that is why you have a big red button, and when u push it the program overrides all the little software restriction, kinda like when u overload a cisco router and it defaults to a basic security mode, but also kinda not like a cisco router at all......muhahahahaa
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Unread 12-25-2002, 01:35 AM
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Legality

Quote:
Originally posted by sanddrag
Spray all the breakers with cold spray right before the match. Canned air will do the same thing if it's held up side down and sprayed.
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.
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