Ball posession: Roller (ball magnet) or vacuum?

[ttldomination]

Originally Posted by Ether

It would be most interesting to hear from teams with effective 2-roller ball magnets whether their bottom bar a) is fixed and does not spin or b) is free to spin or c) is motorized


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Our bottom bar is fixed and it works like a dream. I can officially say that when I have the ball, I can drive it like I stole it. Heck, we can even play defense with a ball in our rollers.

Originally Posted by RRLedford

Do all these lower roller schemes have wheels ahead of them to prevent them being bashed into the humps? Or. perhaps they retract when traversing the humps? Then, if they retract, how can they prevent past 3" incursions when pressure from the ball pushing on them displaces them out of their proper incursion preventing location?
-Dick Ledford

Our front wheels were off-set from the front of the chassis by roughly 2" before we put on the roller system. We toyed around with the idea of having the bottom bar articulate, but in the end, we had to move our entire drive train up 2" to keep the bottom bar between the front two wheels.

[RRLedford]

Originally Posted by ttldomination

Our bottom bar is fixed and it works like a dream. I can officially say that when I have the ball, I can drive it like I stole it. Heck, we can even play defense with a ball in our rollers.

Our front wheels were off-set from the front of the chassis by roughly 2" before we put on the roller system. We toyed around with the idea of having the bottom bar articulate, but in the end, we had to move our entire drive train up 2" to keep the bottom bar between the front two wheels.

Sounds like a lot of work to get good ball possession, but confirms how critical it to get this feature right, that you would re-do the whole drive train to handle the lower roller correctly. With our 8" Plaction wheels this approach is not even an option for us.

-Dick Ledford

[Tom Line]

Originally Posted by Ether

It would be most interesting to hear from teams with effective 2-roller ball magnets whether their bottom bar a) is fixed and does not spin or b) is free to spin or c) is motorized


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We have a very effective one, and the bottom bar does not spin. You really don't want it to - if it spun it would back drive and the ball would simply squirt out when you tried to back up.

[Ether]

Originally Posted by Tom Line

if it spun it would back drive and the ball would simply squirt out when you tried to back up.

Hi Tom,

That is simply not true. With a free-spinning lower roller, the design approach is different: If you design it right, the ball is continuously rotating when being possessed. The backward rotation of the ball makes the ball follow the robot when the robot backs up.

In fact, there was a bot with a very effective ball magnet at GVSU that used a free-spinning lower roller bar and a motorized upper roller.

See this post: http://www.chiefdelphi.com/forums/sh...6&postcount=51


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[lenny8]

Originally Posted by Ether

Hi Tom,

That is simply not true. With a free-spinning lower roller, the design approach is different: If you design it right, the ball is continuously rotating when being possessed. The backward rotation of the ball makes the ball follow the robot when the robot backs up.

In fact, there was a bot with a very effective ball magnet at GVSU that used a free-spinning lower roller bar and a motorized upper roller.

See this post: http://www.chiefdelphi.com/forums/sh...6&postcount=51


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sounds like our set up. just add the "entrapption stars" and your set

[waialua359]

Originally Posted by Ether

Hi Tom,

That is simply not true. With a free-spinning lower roller, the design approach is different: If you design it right, the ball is continuously rotating when being possessed. The backward rotation of the ball makes the ball follow the robot when the robot backs up.

In fact, there was a bot with a very effective ball magnet at GVSU that used a free-spinning lower roller bar and a motorized upper roller.

See this post: http://www.chiefdelphi.com/forums/sh...6&postcount=51


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Ours doesnt work quite like how you describe it. When the robot is idle, the ball doesnt spin since its wedged so tight with the roller. When we full reverse in low gear using an AM supershifter, the ball spins to follow the robot and we never lose possession of the ball. On our practice field, the ball did indeed continue to spin freely when the robot was idle. However, the carpet at the competition is slightly different from ours and doesnt allow our ball to spin at all.
My point is that the ball doesnt have to be continuously spinning to achieve constant possession of the ball moving backwards.

[ttldomination]

Originally Posted by RRLedford

Sounds like a lot of work to get good ball possession, but confirms how critical it to get this feature right, that you would re-do the whole drive train to handle the lower roller correctly. With our 8" Plaction wheels this approach is not even an option for us.

-Dick Ledford

Right. We are using 6" plaction wheels, so we had to slide up all of the modules to make sure the bottom bar cleared.

It was a lot of work, but it was well worth it. The control was *awesome* and it really boosted our performance from our first regional.

[Ether]

Originally Posted by waialua359

Ours doesnt work quite like how you describe it. When the robot is idle, the ball doesnt spin since its wedged so tight with the roller. ... My point is that the ball doesnt have to be continuously spinning to achieve constant possession of the ball moving backwards.

Of course. There is more than one way to skin a cat.

Two-roller ball magnet designs in which the ball does not rotate continuously when possessed have already been described in this thread:

http://www.chiefdelphi.com/forums/sh...0&postcount=63

http://www.chiefdelphi.com/forums/sh...2&postcount=60

http://www.chiefdelphi.com/forums/sh...9&postcount=56

Question for you: is the lower bar in your design fixed or is it free to spin?


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[thefro526]

Here's a picture of our ball possessor: http://www.chiefdelphi.com/media/photos/35345?

Basically, on either side is a set of wheels that spin inward towards the robot. As we contact a ball the wheels attempt to drive the ball inward towards the robot until they physically cannot, at which point the ball is pinched into the mechanism. At that point the ball is pinched in the mechanism and can't escape unless the mechanism is powered in reverse or the kicker is fired. We used a window motor for this so that we wouldn't have to stall the motor to keep the ball pinched in place, but this could be replicated with any motor as long as you come up with some system to prevent the motor from burning out.

There are two down falls to this sort of mechanism though, the first of them being the opening size of the pincher. The effective opening is similar to that of a suction cup so it requires a certain degree of precision to grab a ball. We found that as long as the center line of the ball is within ~4" of the center line of the robot we're fine. The second down fall is that I mounted the possessor rigidly to the frame of our robot. This can and did lead to at least one carrying penalty because the ball can leave the floor as we drive into the Goal, and also as we encounter bumps in the playing field. We plan to revise the design for the Championship to make it both more robust and "Carry-Proof"

[Tom Line]

Originally Posted by Ether

Of course. There is more than one way to skin a cat.

Two-roller ball magnet designs in which the ball does not rotate continuously when possessed have already been described in this thread:

http://www.chiefdelphi.com/forums/sh...0&postcount=63

http://www.chiefdelphi.com/forums/sh...2&postcount=60

http://www.chiefdelphi.com/forums/sh...9&postcount=56

Question for you: is the lower bar in your design fixed or is it free to spin?


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As you said, there's always more than one way to skin a cat (I'm going to have to look up where that euphemism came from: it isn't the more pleasing when you think about it ).

However, "trapping" the ball, as it were, rather than spinning it, makes a much easier system. You don't have to worry about roller speed, you don't have to worry about moderating your back up or turning speed, you don't have to worry about the ball rolling off sideways when you turn, you don't have to worry about differing ball surfaces or differing carpet cof's.

In fact, all you need is a small motor driving the top bar and you're set. That's why we chose that system.

I've seen excellent backward-ball-spinnering systems too, however we found that going that direction took a more powerful motor that we wanted to use elsewhere, as well as all the considerations I listed above.