Maintaining Ball Size at Competition


After discussing for a bit and checking around with different teams team 1261 found that the ball diameters varied greatly from the two feet (or 24 inches). Some teams had balls as low as 20 inches and some as high as 28 inches.

Our question is, how is FIRST going to maintain a consistent ball size throughout the regionals?

It seems like a good idea would be to have a sizing cylinder at the events to ensure balls are at 24 inches.

Additionally couldn’t balls be pumped with nitrogen to ensure that their diameter stays about the same throughout the events?

isn’t that what air is? mostly

A better plan might be to make sure your robot will work with balls from 20 to 28 inches diameter…( perhaps less range, to allow for how different teams measure)

Note the precision of this statement from the rules:

The BALLS are approximately 2 ft. diameter exercise BALLs

Our team is assuming that the size of the balls will vary.

The manufacture describes them as 25", not 24" (or two feet).

The cover is really what should be controlling the diameter. Carefully look at the seams and how the zipper functions to verify that you’ve got it inflated to the same approximate size as FIRST is expecting them to be.

The big point, though, is to expect variation and design your mechanisms to tolerate that.

If I remember right, one of the videos FIRST had on after kickoff talked about how to properly inflate the ball within the cover to ensure you were at the proper diameter. If FIRST went to the trouble of making a video with a specific process and acceptance criteria, I think we can trust them to do what the video says to give us as consistent a ball size as possible.

If history is any indication of the future…I kinda doubt it.

While I’m glad that you’re accounting that in the real world millimeters matter, I really don’t think it will mess up your autonomous or tele-op that much. If a ball is visually different than what it’s supposed to be, I’m sure the volunteers will regulate it.

Teams will have to be able to adjust to this. Its just like last year when frisbees became warped throughout competition. I know at least for our autonomous last year we had to choose very carefully which frisbees we pre-loaded because if they were warded they wouldn’t always engage our limit switch.
Teams robots will have to be able to launch and pick up balls with varying diameters. Also the rule says the balls are roughly 2 ft in diameter. Not exactly.

Yes, Derek and Matt were able to get it to specifications easily and effectively. I wouldn’t count on the teenager hanging out at a regional/district whose (one of many) job is to inflate the balls to have such a keen eye for detail.

Depends on your manipulator. If the loader/ejector is relient on the ball having a specific size/surface consistency, the results on Saturday afternoon may be much different than those on Friday morning of competition.

Hence the reason having a ball launcher that is insensitive to inflation was weighted high on our design matrix.

20 to 28 inches does not sound plausible to me. All of the balls that I have seen have been in the 24 to 26 range. I might even buy 23 or 27. Are you sure all of the balls you handled were official FIRST game pieces and not COTS yoga balls or replacement bladders?

I tried to cover that…I guess I was too subtle

Because there will be so few balls out on the field, the Field Supervisor can (and should) monitor this adequately.

2011 was a bit more difficult to monitor due to the number of inner tubes available, but I don’t recall this being a problem that year.

If a team sees a ball that is significantly off from the specified “approximately 2 feet,” they should ask to speak to the field supervisor. The field supervisor should be more than happy to correct the problem.

I haven’t bothered to investigate the ball with.without the cover, but I imagine the cover will do the job of keeping the ball within 2-3 inches of the recorded diameter and within 1oz of estimated weight. However, one little thing teams overlook every year is dealing with changing conditions of gamepieces throughout the event. Don’t rely on the field staff to get every ball to work with your machine (they have more important things to do), engineer a robot that makes the change not matter.

I seem to recall this being quite a bit of a problem in 2011 because of the different shaped tubes. I think in 2011 they had a “device” (2 pieces of plywood a set distance apart) that checked for tube over inflation but don’t quote me on that.

I don’t think the inflation will vary much beyond 24-26 inches but it’s definitely something to account for.


I know quite as few students who have a keen “eye for detail” and a similar number of adults who don’t.

If there was an odd number of disks on the centre line last year, where did the extra disk go? I know the answer.

Where do the ducks go in the winter? Why does Rice play Texas? How many licks does it take to get to the center of a Tootsie-Roll Pop? Somethings we’ll just never know. :rolleyes:


…but we did know. They go on the side of the FMS table.

As Gregor will tell you, on the side of the scoring table. It’s in your manual.

Gregor - Point taken, I should have said “newbie volunteer” or “overworked field supervisor” but my point still stands. Eyeball tests from a distance are not to be trusted, and as the day goes on, I would expect the geometry of the balls to change enough to confound a shape-reliant manipulator, and not be noticed by the field crew.

Exactly, and there are answers for all the others to, if people look hard enough. :wink: