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.
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.
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.
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.
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.
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.
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.
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
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:D .
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.
Like most, are team cam to GTR with 2 window motors on a arm to lower the bridge, Did nothing. We then took a Cim with a tough box and attached that to the arm, again did nothing.
We are now using a 1 1/2" bore cylinder attached to a arm as a 3rd class lever. We get about 50lbs on the end of are arm. It also has the ability to raise the bridge and lock it in a balanced position. It’s working great so far in the Waterloo regional.
We finally got to use our bridge manipulator today and found that you don’t actually need all that shallow of an angle to get the bridge down. While I would certainly advocate a design like this if you have the room, we have a wedge that extends out around 7" but down 12" and still works (something like a 50 or 60 degree angle). You need a bit of a running start to make sure the bridge gets down well but we’ve balanced once in practice today and numerous times on the practice field with this so far.
Of course it may not hold up for an entire regional of beating, but we’ll know that at the end of Saturday.
Also for teams throwing something like this on last minute, you don’t have to use a motor to drive it if you have no other appendage outside the robot. We have a hinge and bungee cord to flip out the wedge as soon as the robot moves and it hangs out the entire match. Saved us the weight of a motor and the time of getting it hooked up.
We came up with this around week 2. Only mistake was direct driving it with a PG71. Worked fine on the practice bridge, but the real thing would’ve laughed at it. So we’re running on a 256:1 P60. After shearing pins, replacing them with PG71 pins and then shearing sun gears, we put a latch on it. Otherwise it works good. An interesting point is that the angle has relatively little effect on the holding torque necessary, unless your pivot is well above 12".
Our team had that in our first regional, but we still weren’t able to lower the bridge. I assume it’s lack of torque in our drive train, but I want to get other opinions.
We’re using 6-wheel drive, with KOP Traction wheels in the center and omni-wheels for the other four. Each side is driven by one CIM (which I think is our problem) into a CIMple box, and the chains are all 1:1 ratios. Another problem was that a last-minute shooter design change forced us to move the arm to the side, and we couldn’t afford the weight of a second arm (connected so as to not violate the one appendage rule) on the other side. We plan to fix both of those problems for our next regional, and I hope that will allow us to easily lower the bridge.
Ours is a two cim drive, I cant say for sure but I think that could be our problem. A lower angle on the triangle would probably be best, its less resistance.
Are the chains really 1:1 from the CIMple box? With 6" wheels that’s geared for about 25 ft/sec, that is very fast and with only 1 CIM per side that is a very small amount or torque. The kit drive train reduces 12 to 26 out of the CIMple box and that provides a bit more torque. My team goes 12 to 36 with two CIMs per side to make sure we have enough torque.
Our team had a similar device mounted off to one side of a wide robot. It was not very effective. The drive train has 4 CIM motors each driving a Mechanum wheel. The robot would pivot toward the other side when the bridge pusher contacted the bridge. We concluded that only the two motors on the side with the bridge pusher were contributing thrust towards pushing the bridge down. We are in the process of replacing it with a sturdier device that is mounted in the center and powered by a geared-down van door motor. Our weight estimates show that we should be able to install a second pusher arm on the shaft of our mechanism.
One of the reasons for gearing down the van door motor is to get greater torque. This may allow us to help balancing by holding the bridge up and lowering it slowly, damping the sometimes violent oscillations. The other reason is to slow down the motion of the mechanism so that the limit switches have time to work. The first one that our team built was directly powered by a window motor. It would come down with a loud “whack”. It eventually destroyed the limit switch. The motor only had to rotate about 90 degrees to deploy the mechanism and at 84 rpm, that takes only a fraction of a second. It is quite possible that the damage to the old arm was self-inflicted when it smashed into the end stops.
