Ball posession: Roller (ball magnet) or vacuum?

[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


~

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.

[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.

[Dkt01]

We're using a vacuum, but all our driver training was done without a ball possession device. Therefore, we may not end up using anything.

[Bruceb]

As robself said earlier we use both a ball magnet and a sucker. The sucker is 2 shopvac impellers stacked back to back so that the output of the first feeds the input of the second. They are powered by 2 FP motors geared UP 2 to 1. Impeller rpm is about 19,000. Way much suck. We can move anywhere as fast as Robbie can drive and not loose the ball. We have the vac arangement at the back of the bot and the toilet plunger that sucks the ball at the front connected by 1.5 in. pvc with a wye in it. At the wye we have a small piston that holds a flat chunk of rubber over the hole. about 5 ms before we shoot we pull the piston and dump all the vacuum so we kick straight. This is VERY effective. We were the high scoring first zone bot at the Wisconsin regional and ended on the 2nd place alliance.
Bruce

[Austinmead]

sweet... thats cool....ill look into designing that lol

[ernie.]

What I was wondering was, if you were to use a ball magnet/roller would the roller have to spin at the same speed as the wheels? What's the math behind it?

[MrForbes]

Roller needs to spin so that it's surface is turning at least twice as fast (relative to the robot) as the robot is driving, so it can hold the ball as the robot drives backwards.

The actual rotating speed (rpm) depends on the diameters of the roller and the wheels.