Forced needed to get on Bridge
 

Hello, I am just wondering if anyone has found out the force needed to tilt the Bridge so that a robot can get on it. Thanks.

Re: Forced needed to get on Bridge
 

This was posted in another thread i saw a while back as 16 to 18 lbs of force on the most outer part of the bridge. I am not sure of these measurements however, Could someone confirm this?

Re: Forced needed to get on Bridge
 

The weight of 2 batteries at halfway is needed to tip, so ~1 battery's worth is needed at the edge. That's about 13 pounds, so design for 20 lbs to be on the safe side.

Re: Forced needed to get on Bridge
 

That's about right. To clarify, it takes ~59 lbf-ft of torque to tip the bridge.

Re: Forced needed to get on Bridge
 

for teams that have matched their bridge to the specs of competition. how did you adjust the balance of your bridge? did anyone recreate the 16-18 lbs on both sides while keeping the bridge balanced? any info would be greatly appreciated.

Re: Forced needed to get on Bridge
 

With an extra 160 pounds of concrete blocks mounted underneath the deck, our wooden bridge now passes the "two battery" test.

Re: Forced needed to get on Bridge
 

instead of putting aton of uneeded weight on the underside of the bridge we just put surgical tubing


and it is right around 18lbs of force perpendicular to the bridge to push it down

Re: Forced needed to get on Bridge
 

How did you attach the tubing?

Re: Forced needed to get on Bridge
 

We took eye hooks and screwed them into the bottom of the bridge. Then we adjusted the tubing till it met the specs.

Re: Forced needed to get on Bridge
 

alright thanks. i will try that on monday

Re: Forced needed to get on Bridge
 

This will not actually yield the competition bridge dynamics. How much it is off will be situation-dependent. In short though, you're now using spring force (F=-kx) rather than distributed bass to balance the moments. This means that, while you may pass the battery test, as your bridge moves, the moment caused by the spring force will not fully mimic what the simply heavier competition bridge would do. It probably won't affect your bridge lowering tests, but if you're practicing (much less autonomously programming) balancing on it, don't count on the results being the same unless you've tested it.

Re: Forced needed to get on Bridge
 

could anyone who has done this post a picture of the bridge with the surgical tubing set-up?

Re: Forced needed to get on Bridge
 

Our bridge now meets the balance test with a bunch of old dead FRC batteries. It took ten batteries , five per side, to meet the 2012 battery test. See the attached photo. The center of the first 2x4 is 16" back from the edge of the bridge. There's plywood over the top of all the batteries when it's put together.

Re: Forced needed to get on Bridge
 

I'm confused... some people are saying 16-19lbs of force at the end of the bridge and then someonw say the ~59 lbf-ft of torque to tip the brige.. so say if using a window van motor with 25.8 ft-lbf applied to the end of the bridge? Would this tip the bridge or not?

Thanks,
Dan

Re: Forced needed to get on Bridge
 

Where's the confusion? The end of the bridge is about 40-48 inches from the hinge point, depending on which end, so 59 lb-ft of torque would be between 15 and 17.5 pounds.

How much force are you applying to the end of the bridge? A one-foot-long direct-driven rod perpendicular to the motor shaft could work. A two-foot-long rod would not.

Re: Forced needed to get on Bridge
 

My opinion. Don't even think of trying to drive up to the bridge and then use an arm of some kind to tip it down. You have to have extremely good depth perception to make that work unless you have some way of letting the robot do it by itself. Remember, most cases you are on the other side if the bridge at some distance and partially concealed by the bridge.
Better, lower a ramping device with an over center latch and just drive up to the bridge. Let the weight of your bot push the bridge down.
see here
http://s1121.photobucket.com/albums/...t=100_0013.mp4

Hope that helps