Bridge device failure fix

[Brandon_L]

Many teams are finding out as they compete in their first event that their bridge manipulators are not strong enough to lower the competition bridges. We fell to this same fate, and spent our spare time at the event redesigning it. It was a simple enough fix that you could make a quick swap if you need it. I thought I'd share it on here, maybe someone else could get some use off of it. Excuse the paint drawing.



The red blob would be your bumper, COR is your center of rotation from the motor that is powering your device, and I drew in a badly drawn basic frame & front wheel. Note none if this is to scale. For best results, your COR should be around 12.5 to 13 inches off the ground. If its not, don't panic, you can modify the shape of the arm so that when the end is at its horizontal position it is ~12" off the ground.

There are three basic parts, labeled A B and C. Part A will extend from your center of rotation out of your frame horizontally to its max of 14" out of your frame. From that point, cut another piece (labeled B) so that the length allows it to go from the end of A to just in front of your frame below the bumper. Then simply make a final part (C) that holds B in place at the proper angle (which should be somewhere around 20 degrees, most likely, from part A ).

You just drop this arm down and drive into the bridge, and the edge of the bridge will follow the angle of part B and slide right into your wheel, and from there you just drive up. Its extremely smooth and reliable. Let me know if this helps anyone, just thought I'd post it up here. Remember to file and round any sharp edges!

EDIT: Works best with a window motor (worm gear) or van door (high torque, wont backdrive). I have no idea how it will react with other motors, you may need to lock it down somehow once its dropped.

[Sean Raia]

I'd love to have your input here so we can keep everything in one place. Im confused as to how the part where b extends past c is supposed to swing down under your bumpers, but nonetheless thanks for sharing.

[Brandon_L]

I had the same worry when we built this, but when you raise the arm up from how its shown in the picture it didn't even touch the bumper. Just remember its not being raised/dropped straight up, its rotating down. I'll take exact measurements of ours when I get the chance to. But this is how we figured it out.

Unrelated, motor torque/power pretty much doesn't effect this. The fact that you have 120lbs of robot pushing against the bridge on an angle is what makes this work. You just need to make sure the arm stays horizontal.

[Ether]

Originally Posted by Brandon_L

I had the same worry when we built this, but when you raise the arm up from how its shown in the picture it didn't even touch the bumper. Just remember its not being raised/dropped straight up, its rotating down.

I think what Sean Raia is saying is that your drawing is not to scale. See the green arrow pointing to where the blue circle intersects the red bumper.

[Brandon_L]

Originally Posted by Ether

I think what Sean Raia is saying is that your drawing is not to scale. See the green arrow pointing to where the blue circle intersects the red bumper.

The drawing is not to scale, as noted in the first post. I will get exact measurements of everything we used when I can. With the addition of the circle you drew in, you can see how it does work if it were to scale.

[Taylor]

Do you know the angle formed between members A and B?
We can obviously figure it out with a bit of trig, but I don't want to rely on a not-to-scale sketch.

[Brandon_L]

Originally Posted by Taylor

Do you know the angle formed between members A and B?
We can obviously figure it out with a bit of trig, but I don't want to rely on a not-to-scale sketch.

We found it by just placing a piece of the metal we were using from the end of A, then down to our frame, making sure when we rotated it it would clear the bumper. It was a on the spot trial and error type of thing. B will not actually touch your frame. Ours floats an inch or so in front of it. As long as the height of it is equal to or a bit below your frame, so that the bridge gets kicked under into your wheels.

Again, I will try go get measurements when we unbag for our 6 hours which would be this saturday.

[Hebrew]

How fast can you get on the bridge with your two front wheels from when the triangle thing first touches the bridge? (I would like to compare it to the speed of our pneumatic arm)

I see a lot of teams with this having to push against the bridge for quite a while before it comes down enough.

[Brandon_L]

with us, its the same speed your driving pretty much. The lower the angle, the better. The closer you get to 90 degrees the more its like hitting a wall, and more force is needed. my guess is ours came out around 20 degrees and its literally almost the same as just driving forward

[E-Stop]

Most teams are using Window or Van door motors but we ended up with an Andy Mark geared motor from the KoP using a 35 chain with a 4:1 gear ration and it had more then enough torque at orlando and we even pushed from the side and can let robots up from the same side to balance themselves if we are doing the cooperation .

[George C]

Looks a whole lot like what we came up with 6-7 weeks ago. We made ours higher (14") off the floor in case the bridge isn't quite level, used a dogleg to get the Centre of Rotation (about 4" off the floor) lower to keep weight low and put adhesive-backed UHMW on the underside of it. We also left a gap in the bumper so it is much closer to the frame. We found that the weight of the robot with a 14" moment arm stripped the gears in the window motor in our first version so we added a pneumatic latch which engages when it's down. It also helps us get over the bump. When it's up, we can use it to tilt the bridge away from us. It doesn't slow us down much so far.