Quote:
Originally Posted by JABot67
I believe our idea for a bottom bar was developed independently of the other teams. We knew during the build season that we could add a bottom bar if we found that our top roller was not effective enough at grabbing the balls. Seeing that our first competition was during week 1, we didn't get the chance to see how well the bottom bar worked on other machines before we added ours.
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We did not develop our "pinching roller", independent of other teams. Our ball-magnet was inspired by the incredible design and prototype team of 148 & 217. JVN and I have been discussing this since week 2 at Cass Tech....he's been waiting for me to give them props.
Our original design concept and prototype had a lower bar incorporated, but somewhere in the design finalization and build process the lower bar was discarded. Why? Not sure, it wasn't my sub-system to design.
We ran driver tryouts with the practice robot, after the build season was completed (yes, we are always way behind schedule), and determined that the top roller only design was ineffective for the way we wanted to the game.
Around the same time, the 148 video was released. Looking at how they were able to grip the ball, drive, spin, and kick...it was exaclty how I wanted our robot to perform. I started disecting the video to determine how they were able to do it. After reviewing the video many, many times....I determined 148 and 217 were using a similar setup to our roller, but with a lower bar. I then attempted to determine how their setup worked....it wasn't until Paul Copioli posted this statement:
"If the normal force between the ball and the floor is less than the normal force generated between the ball and the "magnets", then the ball will stay possessed by the robot. This doesn't mean the normal force between the ball and the floor is zero."
That the light turned on for me. I cut a peice of 0.5" AL rod, drilled two holes, bolted it to the frame and our practice robot had a "ball magnet".
We went to Kettering with a plan to incorporate the lower bar onto the competition robot. The frame was slightly different where the lower bar bolted on, so we had to use aluminum angle bolted to the frame, then bolt the bar too the angle. As IKE eluded too, it took us about 2-3 matches, before we finally tuned in the "ball magnet". After that we just had to keep adjusting it each match to make sure it would work.
(After Kettering we changed the lower bar from AL-6061 to Heat treated Stainless Steel, since the AL bar kept getting bent in during matches, to increase the stiffness.)
Friday evening, we were called to the field the robot, for the refs to give us the paper test. We passed everywhere, except when the front of the robot was on the 1/2" plywood bumps near the large bumps on the field. We were told to adjust our style of play to avoid carrying a ball. We adjusted by bumping the ball and allowing it to roll back to us before we grabbed it. I don't think we were called for carrying at Kettering (I could be wrong).
After Kettering, I contacted Karthik about how they avoided carrying near the plywood....he send me pics of their spring loaded design. We decided against modifying our design before seeing how it would be called at other regionals. In the end, I think the calls were less and less strick the later the season got.
Eventually, we started grabbing the balls near the bumps and backing away without worrying about penalties. Were the balls off the ground? Theoretically, probably a paper thickness. For how long? Whatever it takes for a robot to cover 6" @ 6-8ft/s. Our ball grabber wasn't really good enough to hold a ball across those bumps anyways...we usually lost it, then reaquired it quickly afterwards.
The final specs on our ball magnet were top roller, powered by a Window Motor with a 5" pulley to a 1" roller, connected by round belting that slipped on the window motor pulley to keep constant tension on the roller and ball. Top roller was mounted to the frame, with bearing blocks slotted to allow for vertical adjustment of its position. Lower Stainless Steel bar, basically mounted to the frame and adjusted via washers.
So in the end the HOT team once again owes its success to the IFI guys. Without their videos, we would not know what level we need to perform up too before the season starts. Thank you 148/217!