Ball posession: Roller (ball magnet) or vacuum?
 

How are you possessing the ball? How well does it work? Any innovations from the original ball magnet or standard vacuum design?

We went with a vacuum and are very pleased with the results. It uses an ungeared FP motor and a 5" shop vac impeller. The end-effector is a thin plastic food container. We can drive at full speed while maintaining control of the ball.

It seems most teams went with roller designs. I'd be interested to see how these compare to vacuum designs.

Re: Ball posession: Roller (ball magnet) or vacuum?
 

We went with vacuum but actually added a ball magnet to grab moving balls and push them into the vacuum. It works very well for us. We can easily score three shots in 20-30 seconds if balls are nearby the bot.

Re: Ball posession: Roller (ball magnet) or vacuum?
 

we tried a ball magnet, but it didn't work, it just sucked the balls under the frame

Re: Ball posession: Roller (ball magnet) or vacuum?
 

AIR Strike is using a vaccum.

You can see it in this video:

Click

Re: Ball posession: Roller (ball magnet) or vacuum?
 

@ people using vacuums with those yellow funnels, where do you buy them? I looked around mcmaster but didnt see them...

Re: Ball posession: Roller (ball magnet) or vacuum?
 

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

Re: Ball posession: Roller (ball magnet) or vacuum?
 

to those using the vacums, how are you stopping the suction when you kick? or are you?

Re: Ball posession: Roller (ball magnet) or vacuum?
 

Home depot is where we bought ours.

Just shutting off the vacuum prior to kicking equalizes the pressure fast enough that the ball will disconnect when kicked.

Re: Ball posession: Roller (ball magnet) or vacuum?
 

I spy, with my little eye, two dog toys....

Re: Ball posession: Roller (ball magnet) or vacuum?
 

how'd you make the vacuum?
we know you replace the old vacuum motors with the FP motors but we cant get it to work

Re: Ball posession: Roller (ball magnet) or vacuum?
 

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.

Re: Ball posession: Roller (ball magnet) or vacuum?
 

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

Re: Ball posession: Roller (ball magnet) or vacuum?
 

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

Re: Ball posession: Roller (ball magnet) or vacuum?
 

We used a dump valve.

It is just a PVC pipe hooked up to the vacuum, with a pneumatic cylinder inside of it, to open the top. It gave us 5 different kicking modes, (Long w/dump,w/out, med with, without, short) with some great consistency. We would open the dump valve about 10-25ms before we kicked.

Re: Ball posession: Roller (ball magnet) or vacuum?
 

Do you happen to have an item # or a description of its real use? I just spent 2 hours in home depot looking, and the employees are no help.

Re: Ball posession: Roller (ball magnet) or vacuum?
 

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?

Re: Ball posession: Roller (ball magnet) or vacuum?
 

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.

Re: Ball posession: Roller (ball magnet) or vacuum?
 

How well does that work? We are currently using one window to a pvc roller with 35 teeth on the window and a 12 tooth on the pvc. It is a bit too slow for us to really grab the ball well.

Re: Ball posession: Roller (ball magnet) or vacuum?
 

As of right now it is fairly effective but we are still maximizing the effectiveness.
The model that you see in the picture there is a 36:16 reduction from the denso motors with 2.5" diameter Vex wheels. We found that while driving backwards at full speed we cannot hold the ball as well as we desire, so we have changed the ratio to 48:10, to give us about 400 rpm. This may change again before our regional (week 5) but after a bit of testing we will know for sure.

Re: Ball posession: Roller (ball magnet) or vacuum?
 

for the florida regional, we had a shop vac that was powered by a fisher price motor. but when we get to palmetto, we are planning on testing our ball magnet to see if that works any better

Re: Ball posession: Roller (ball magnet) or vacuum?
 

We used long rollers with latex flaps at BAE, which did not work well at all. We changed to vacuums for our week 2 regional at WPI and we were unparalleled in our ball possession. However, the ball had to be centered as we drove into it in order for us to gain possession. We were able to go around anybody once we got possession, using our swerve drive at full speed. Now we may be changing to something that's even better :D . I'll hesitate to call it a roller but it's along those lines...

Re: Ball posession: Roller (ball magnet) or vacuum?
 

I'd look at the lack of rigitidy (softness) in addition to gripiness and speed of rollers.
I also saw a lot of FP and gearboxes for the rollers.
We used a CIM with a 2:1 reduction and adjust the maximum output via programming. We used 60% in SD, but for some reason had to step it up to 80% in AZ to achieve the same result.
Go figure.:confused:

Re: Ball posession: Roller (ball magnet) or vacuum?
 

Check out 1501's ball handling abilities. We are using a two wheeled system and it's performance far exceeded our own expectations.

http://www.vimeo.com/10396844

Re: Ball posession: Roller (ball magnet) or vacuum?
 

It looks like you have three wheels :-)

Seriously though, did you mean that only 2 of the 3 wheels are driven? Are they omni or mecanum?


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Re: Ball posession: Roller (ball magnet) or vacuum?
 

Two wheeled ball magnet.

Re: Ball posession: Roller (ball magnet) or vacuum?
 

We have a three wheeled Kiwi drive system for the robot and a two wheeled Dribbler System for holding the balls while we move around the field.

Re: Ball posession: Roller (ball magnet) or vacuum?
 

Right. I got so caught up watching the fascinating video that I lost the context.

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Re: Ball posession: Roller (ball magnet) or vacuum?
 

I'm not familiar with Kiwi drive. Does that use omni wheels?


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Re: Ball posession: Roller (ball magnet) or vacuum?
 

Yes it does. Kiwi is a three wheeled drive utilizing omni wheels. We used the A-M Nano gearboxes direct driving the omni wheels.

Re: Ball posession: Roller (ball magnet) or vacuum?
 

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


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Re: Ball posession: Roller (ball magnet) or vacuum?
 

I didn't have much to do with the programming, but I think our programmer found some code to start from and made it perform better with tweaks of his own.

Re: Ball posession: Roller (ball magnet) or vacuum?
 

Kiwi is a nickname for something properly called a "Killough" drive platform, named for its inventor. I think it was first described in the literature in the late 1990s.

An early application in FRC essentially didn't have any fancy computer software making it work. The driver used three joysticks, arranged in the same orientation as the wheels, and connected with a Y-shaped yoke, to directly control motor power.

Re: Ball posession: Roller (ball magnet) or vacuum?
 

Along with this thread... I have to wonder how many teams have two bars incorporated into their design? We do, but I have seen many other robots use the concept with only one.

Re: Ball posession: Roller (ball magnet) or vacuum?
 

If 148, 217, 1114, 1868, etc. are any indication, two well placed bars beat one bar any day of the week. I've seen a few 1551 type "underrollers" or "split rollers" (3125 has one like it) that work alright, but nothing beats an IFI magnet.

Re: Ball posession: Roller (ball magnet) or vacuum?
 

For those of us who don't know the jargon, what's an "IFI magnet" ?


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Re: Ball posession: Roller (ball magnet) or vacuum?
 

Double roller (or roller and stop) that 1114 / 217 / 148 / 254 seem to be using. I guess I started calling it that since they're all part of Team IFI.

Re: Ball posession: Roller (ball magnet) or vacuum?
 

Team 818 uses carpet tape. Yes there are some legal brands still!

Re: Ball posession: Roller (ball magnet) or vacuum?
 

by "bar" do you mean "roller"?

What's the conventional wisdom on what constitutes "well placed"?

Should the upper bar be forward or aft of the lower bar? And should one bar be above the center of the ball and the other below it?


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Re: Ball posession: Roller (ball magnet) or vacuum?
 

I dont know what the perfect placement would be, but we have two bars that are parallel. The bottom bar is free spinning and there for the purpose of keeping the ball from being sucked under the robot by the top roller. The top roller (we have a fisher-price powering ours) is powered and has some kind of friction producing material on it. I saw many robots at the NASA/VCU with modified pool noodles, while we use friction tape... the same stuff on stairways.

The combination of the two almost pinches the ball, while the free-spinning bottom roller keeps it from picking the ball up.

Re: Ball posession: Roller (ball magnet) or vacuum?
 

1717 uses a sort of a claw to pinch the ball.

Re: Ball posession: Roller (ball magnet) or vacuum?
 

We used a single horizontal roller. It wasn't very effective (lost ball if driving more than half speed backward, and slowed our forward and turning speed) and we're looking to improve on it during off season.

Re: Ball posession: Roller (ball magnet) or vacuum?
 

You can add team 33 to this list.
All during Troy we were tweaking ours so it would stop picking up the ball super slight amounts off the ground. Penelties resulting from this over the course of the quals actully resulted in us just missing 1st seed and breaking up the 469 / 217 combo (we came in a very close second).

By elims we had it working very well. However, by states this weekend you can be sure we will be working even better!

Re: Ball posession: Roller (ball magnet) or vacuum?
 

Is the bottom bar directly below the top one, or is it set back a bit?

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Re: Ball posession: Roller (ball magnet) or vacuum?
 

The ball can only penetrate the vertical plane of the frame perimeter by 3 inches (Rule G46), and no part of the robot may extend beyond the frame perimeter for more than 2 seconds (Rule G30). It's hard to visualize a "claw" that could grasp the 3 inches of the ball that is inside the frame perimeter... unless the "claw" has powered rollers or a vacuum or some adhesive.


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Re: Ball posession: Roller (ball magnet) or vacuum?
 

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

Re: Ball posession: Roller (ball magnet) or vacuum?
 

That's not a soccer ball. And it's not being dragged backward on the ground.

BIG difference.


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Re: Ball posession: Roller (ball magnet) or vacuum?
 

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!

Re: Ball posession: Roller (ball magnet) or vacuum?
 

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.

Re: Ball posession: Roller (ball magnet) or vacuum?
 

We are geared to the fisher prices at I think up by 3:1. We get around 15,000 rpms at the vacuum impeller.

Re: Ball posession: Roller (ball magnet) or vacuum?
 

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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Re: Ball posession: Roller (ball magnet) or vacuum?
 

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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Re: Ball posession: Roller (ball magnet) or vacuum?
 

Are you sure you weren't at VCU? Because that sounds an awful lot like us!:D

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?
-Dick Ledford

Re: Ball posession: Roller (ball magnet) or vacuum?
 

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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Re: Ball posession: Roller (ball magnet) or vacuum?
 

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.

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.

Re: Ball posession: Roller (ball magnet) or vacuum?
 

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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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. ;)

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.

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. :)

Re: Ball posession: Roller (ball magnet) or vacuum?
 

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.

Re: Ball posession: Roller (ball magnet) or vacuum?
 

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

Re: Ball posession: Roller (ball magnet) or vacuum?
 

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.

Re: Ball posession: Roller (ball magnet) or vacuum?
 

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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Re: Ball posession: Roller (ball magnet) or vacuum?
 

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

Re: Ball posession: Roller (ball magnet) or vacuum?
 

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.

Re: Ball posession: Roller (ball magnet) or vacuum?
 

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.

Re: Ball posession: Roller (ball magnet) or vacuum?
 

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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Re: Ball posession: Roller (ball magnet) or vacuum?
 

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"

Re: Ball posession: Roller (ball magnet) or vacuum?
 

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 :D ).

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.