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View Poll Results: How does your robot posess the ball?
Roller (Ball magnet) 123 65.78%
Vacuum 35 18.72%
Armature 0 0%
We do not intend to posess the ball 11 5.88%
We did not ship a posession device, and will decide before competition 18 9.63%
Voters: 187. You may not vote on this poll

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  #46   Spotlight this post!  
Unread 27-03-2010, 23:02
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Re: Ball posession: Roller (ball magnet) or vacuum?

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Originally Posted by Chris is me View Post
It's certainly possible for a claw to grip a ball without grabbing more than 50% around a ball. http://www.chiefdelphi.com/media/photos/29535
That's not a soccer ball. And it's not being dragged backward on the ground.

BIG difference.


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Unread 28-03-2010, 01:27
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Re: Ball posession: Roller (ball magnet) or vacuum?

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Originally Posted by Ether View Post
Did you guys home-brew your own kiwi drive algorithm, or is there a good-quality tested library routine available somewhere?


~
We homebrewed all the code from whitepapers... initially we had some code tested on a vex before we went full scale... PM me if you want more info...

Now to get this back on subject, Im interested to see the Techno-Ticks ball magnet working at CT!
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Unread 28-03-2010, 12:18
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Re: Ball posession: Roller (ball magnet) or vacuum?

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Originally Posted by Ether View Post
Is the bottom bar directly below the top one, or is it set back a bit?

~
I believe that it is roughly parallel. Perhaps the bottom one is a smidgeon behind the top one... as its primary purpose is to kep the ball from going back. We have it so the ball goes all 3 inches worth into the frame, so the top bar has the most grip possible.
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Unread 28-03-2010, 12:22
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Re: Ball posession: Roller (ball magnet) or vacuum?

Quote:
Originally Posted by ravichauhan View Post
We are using a vacuum with a mcmaster carr plastic funnel. it worked but it will be more efficient to have a silicone one which we will attach before competition. we are using the fifth CIM for the vacuum because we are using the FP's somewhere else. Its speed is geared up 3:1. That will also be changed before competition to 5:1. What type of rpm are you other non-geared-FP guys getting
We are geared to the fisher prices at I think up by 3:1. We get around 15,000 rpms at the vacuum impeller.
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Unread 28-03-2010, 17:37
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Re: Ball posession: Roller (ball magnet) or vacuum?

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Originally Posted by robself705 View Post
We are geared to the fisher prices at I think up by 3:1. We get around 15,000 rpms at the vacuum impeller.
Are you running the motor at 100% PWM? If so, those numbers don't add up.

The no-load speed of the fisher-price is approx 15,700 rpm.

So if you are geared up 3:1 and you are getting only 15,000 rpm at the impeller, you are running the motor at 5,000 rpm which is less than 1/3 its max speed. This is way outside the acceptable operating region for continuous operation. At 100% PWM (12 volts) and 5,000 rpm, the motor would be drawing almost 48 amps.

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Unread 28-03-2010, 17:43
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Re: Ball posession: Roller (ball magnet) or vacuum?

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Originally Posted by MarkoRamius1086 View Post
I believe that it is roughly parallel. Perhaps the bottom one is a smidgeon behind the top one... as its primary purpose is to kep the ball from going back. We have it so the ball goes all 3 inches worth into the frame, so the top bar has the most grip possible.
I was in the stands at GVSU and noticed a bot with a particularly effective ball magnet. After the match, I walked down to their pit area and talked to them. They had a 2-roller design. The upper roller was powered and the lower was free-wheeling. The lower was set back from the upper about 1 to 2 inches.

As the team explained it to me, the top roller pulls the ball in, squeezing it ever so slightly as it does so, and the lower roller stops it at 3" inside the frame perimeter. Very effective.

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Unread 28-03-2010, 18:19
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Re: Ball posession: Roller (ball magnet) or vacuum?

Are you sure you weren't at VCU? Because that sounds an awful lot like us!
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Unread 28-03-2010, 18:24
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Re: Ball posession: Roller (ball magnet) or vacuum?

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?
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Unread 28-03-2010, 18:32
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Re: Ball posession: Roller (ball magnet) or vacuum?

Quote:
Originally Posted by ravichauhan View Post
we are using the fifth CIM for the vacuum because we are using the FP's somewhere else. Its speed is geared up 3:1. That will also be changed before competition to 5:1
Before going to the trouble of gearing it up (by "up" in this context I assume you mean higher speed at the impeller), you might want to measure your current draw with the 3:1 gearing when operating without a possessed ball. With a centrifugal impeller, the motor will be drawing more current without a possessed ball (more airflow).


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Unread 28-03-2010, 18:59
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Re: Ball posession: Roller (ball magnet) or vacuum?

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Originally Posted by RRLedford View Post
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
Many of these lower rollers are in line with the first wheel's axles, so they will not contact the floor before the wheels do in any circumstance if both wheels are touching the ground.
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  #56   Spotlight this post!  
Unread 28-03-2010, 19:02
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Re: Ball posession: Roller (ball magnet) or vacuum?

You want the two bars nearly as far forward as you can get them. The ball should touch the top one just before the bottom one. The bottom one does not spin. The ball rests on the bottom bar, taking 90% of the normal force. The rest of the normal force is the ball just barely touching the ground. The robot is actually physically carrying the ball, but because the ball doesn't break ground contact it's not a penalty. That how, if you watch 217, 1114, 33, and others, they can whip around, back up, pick balls off the wall, etc because they are quite literally holding the ball.

Watch robowranglers video for a nice closeup of it in action.
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Unread 28-03-2010, 20:31
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Re: Ball posession: Roller (ball magnet) or vacuum?

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Originally Posted by Tom Line View Post
The bottom one does not spin.
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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Unread 28-03-2010, 20:44
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Re: Ball posession: Roller (ball magnet) or vacuum?

368 had a roller-type mechanism which was very effective. It is just as good, if not better, than anything else I have seen. I got a good glimpse of the space cookies 1868 one which I believe uses a clutch. I thought 368's one was better and they can literally move anyway they want using their swerve drive, without ever losing that ball.
I'll let them explain the materials and setup on how they achieved this. It rotates, but I wouldn't really call it a roller.

From what 1086 is describing, it sounds very similar to ours. Ours grabs pretty hard and I'd like to see a team knock ours out once we get it. No one is 3 regionals has done so yet.
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Last edited by waialua359 : 28-03-2010 at 20:47.
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Unread 28-03-2010, 20:51
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Re: Ball posession: Roller (ball magnet) or vacuum?

359... How is your upper mechanism set up? I am interested because we have a Fisher Price with the whole thing on a free-floating, spring tensioned, attachment.
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Unread 28-03-2010, 21:00
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Re: Ball posession: Roller (ball magnet) or vacuum?

The key for us was really wedging that ball in after it gets sucked in.
We dumped the FP because it would stall and have to swap it out after several test runs during build season, even with different levels of reduction.
Once we put in a CIM, it does stall and puts quite a strain on it. However, we swapped out only 1 CIM in 53 matches (3 tournaments) not including practice matches. Its an adjustable-height CIM driven motor driving a pool noodle with a swirl of duct tape with a 2:1 reduction running between 60-80%.
Our lower roller is an idler.
We have a picture of it on our website under regional flyer. Its so simple, a caveman can do it.
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