The afterthought bridge manipulator

[Agentarrow]

One of the big reasons that teams have such trouble lowering the bridge is that many assumed that bridge dynamics of the plywood version would be the same as the real bridge and paid dearly for this oversight. Even a weighted plywood bridge that passes the "battery test" for comp-type dynamics is really different from the actual bridge at the events. Take the time before any future events to prep an overpowered bridge tipper.

[BrendanB]

Originally Posted by SenorZ

Our regional is next week. After watching weeks 1 & 2 we're scrambling to make a bridge manipulator:
Andymark gearmotor (KOP) on a length of c-channel with a small wheel on the end.
This is untested and will not get a chance until Thursday.
Fingers crossed!

We are using the gearmotor to lower our ball pickup/bridge manipulator with a 2 to 1 ratio and we sometimes need to get the bridge rocking to get it down. I'd recommend a 3 to 1 ratio and you'll get that sucker down no problem.

A simple solution I saw at GSR was 1519's. It is a simple triangle driven by a window motor that comes down and they just drive into the bridge and it comes down. Can easily be down with a limited weight budget! You can see it on the side of there robot here!

[quinxorin]

At the beginning of the season, we created a list of priorities for game tasks. We determined that, since the coopertition bridge was just as important as winning, manipulating the bridge was our second priority (first priority went to moving the robot). This allowed us to put a lot of thought into making a simple, elegant manipulator that was finished by the end of week 3. And entirely student-designed and built too!

Here's some links:
3D Model of robot (no special software required)
Robot technical description
Video of bridge manipulator (we're 862, the robot farthest from the camera at the start)

[ksafin]

The problem a lot of teams had was the approach.

Many teams used the approach of ramming the bridge with an angle to bring it down.
Although this works fine with a lightweight easily-turnable bridge that most of us built, it won't work with the real bridge.

The reason being, of the torque you had when you rammed the bridge, only a fraction of it is actually used because you're not applying a direct vertical linear push.

Like someone said, having a 2" bore 12" stroke cylinder will do wonders; if you push vertically on the bridge from above, it'll have no issue pushing it down (consider 60 psi with 2" bore - lots of force!).

[Akash Rastogi]

By the end of our event, we had modified our bridge arm to use a wedge that worked well, should have used that design from the start. If you have a van door motor, use that.

[rcmolloy]

If any teams are thinking of revising their bridge manipulator, I would highly suggest looking at 67's intake, bridge manipulator, stinger combo. After viewing the awesome structure flawlessly pull down the co-op bridge in auto and balance 3 robots in elims, I think there would be a great benefit if a team could mimic what HOT has produced.

[quinxorin]

Originally Posted by ksafin

The problem a lot of teams had was the approach.

Many teams used the approach of ramming the bridge with an angle to bring it down.
Although this works fine with a lightweight easily-turnable bridge that most of us built, it won't work with the real bridge.

The reason being, of the torque you had when you rammed the bridge, only a fraction of it is actually used because you're not applying a direct vertical linear push.

We use the wedge approach, but we made sure it worked right by making sure we built our bridge right. It takes 17.1lbs of force to push an official bridge down at the ends, and on one end of our practice bridge it takes exactly that amount of force. (On the other, it takes a little more, just so we could make sure our wedge performed better than it had to). The wedge begins at 37deg above horizontal at the uppermost end, but is slightly concave, so it ends at 39.75deg above horizontal. It extends 13 7/8 from the frame of the robot, is 12 1/2 inches off the ground, and when the bridge is pushed down it clears the bumpers by about half a millimeter. Very close tolerances! It has some slippery plastic (delrin) covering the sliding surface, so there is very little friction. The wedge is pneumatically driven with two 1 1/16, 10" stroke cylinders.

Want to see it work?

[Sean Raia]

Originally Posted by BrendanB

We are using the gearmotor to lower our ball pickup/bridge manipulator with a 2 to 1 ratio and we sometimes need to get the bridge rocking to get it down. I'd recommend a 3 to 1 ratio and you'll get that sucker down no problem.

A simple solution I saw at GSR was 1519's. It is a simple triangle driven by a window motor that comes down and they just drive into the bridge and it comes down. Can easily be down with a limited weight budget! You can see it on the side of there robot here!

This design interests me, I wish I could get a better pic of that manipulator. I'l go hunting...

@Akash, You mentioned using a van door motor. Does that have enough torque to push the bridge down on its own, or is it better off being used for a wedge?

[Sb28000]

With the help of 2614 (M.A.R.S. - Thanks again, and congrats on the Regional win!), we built a bridge lowerer on Friday in Pittsburgh - we mounted a van door motor to a length of aluminum angle, and screwed that into a wooden upright already on our robot. We then sandwiched a bar of 80-20 with a 6" KOP wheel on the end of it on one side of the flat part of the shaft, with another piece of aluminum angle on the other side of the motor output shaft. Although functional, beware of the extreme torque and force that pushing down the bridge can cause, and the damage it can do to your 'bot. During a qualification match, we found that our arm did not have enough force to push down the bridge. Having tested it just minutes before that match, we were puzzled until we saw that a nut that had been sitting flush on the 80-20 bar was now pushed into the bar, with the top surface of the nut even with the surface of the bar. One of two bolts holding the whole thing together had now shifted and was not holding tension against the motor shaft. We swapped it out Saturday morning for steel on the surfaces touching the output shaft.

[dsalvucci]

To build on what Dom had posted on regards to the cannon. Team 23 had built a plywood bridge and then put 10 lbs on the opposite side of the bridge for testing so that we knew our air cannon design could push it down. We originally had belts driving the cannon up and down via window motor power, but quickly realized it was not strong enough and switched to a timing belt idea (black plastic belt with teeth on one side).

Opposed to the OP's title, however. Team 23 quickly recognized that the importance of the bridge was way more than shooting and focused largely on a design that could do everything asked. ie the cannon was a ball pickup, shooter, and bridge manipulator

[Brandon_L]

If I saw this thread earlier, I would have posted it as a reply here. Sorry that I missed it. We had the same problem, and this is how we fixed it.

[commodoredl]

Our team, while designing our bridge manipulator, wanted it to be as simple and intuitive to use as possible. We knew that an arm could have multiple uses, but would also need a driver to judge distances to work properly. Instead, we built a wedge that allows the driver to drive straight into the bridge and lower it.
Here's a picture of the wedge in its down position:

It's not perfect. The wedge had to be fit into a very small section of our robot, and because of that it unfolds a little awkwardly sometimes in autonomous (but our human operation doesn't have a problem with it). We also ran into an issue where even though the wedge was supposed to be supported by our bumpers, it still rode up when hitting the bridge (actually pushing back into our main breaker in a practice match and shutting us off).
The solution we came up with to transfer maximum force and reduce the backdriving was a simple bar of angle iron stretching across the wedge near the bottom part of it. This pushes straight into the bumpers and keeps it a tight fit as the robot lowers the bridge.
Altogether it probably weighs 5-10 pounds and uses one window motor. I don't have the exact weight now since we added parts to the subsystem in the competition. But it allowed us to lower the bridge perfectly all weekend.

[mathking]

We built our first prototype with a window motor. Worked great until we weighted the bridge. Then we went to the gear motor. Better but still the same result. So we did a 3.5 to 1 reduction (via chain) and it seems to work now. There is a roller made of delrin on the end to allow it to roll on the ramp surface as we drive forward. We have not been to a regional to test it yet, but since our manipulator will lift the front of the robot off the ground, if it does not push down the bridge making it stronger won't help.

We too waited longer than I would like to get a working prototype, but we did plan for how and where to mount it. We are using a couple of pieces of 1x2 tubing (with slotting to reduce weight). One as a the arm and one as a mount for the arm and motor. One thing we did decide on early was that our design should be able to both push down and push up the bridge.

In any event, the whole assembly weighs about 4.8 pounds, and is one unit. So it just takes four bolts and quick connecting the wires to mount it.

[Brandon_L]

Originally Posted by commodoredl

Our team, while designing our bridge manipulator, wanted it to be as simple and intuitive to use as possible. We knew that an arm could have multiple uses, but would also need a driver to judge distances to work properly. Instead, we built a wedge that allows the driver to drive straight into the bridge and lower it.
Here's a picture of the wedge in its down position:

It's not perfect. The wedge had to be fit into a very small section of our robot, and because of that it unfolds a little awkwardly sometimes in autonomous (but our human operation doesn't have a problem with it). We also ran into an issue where even though the wedge was supposed to be supported by our bumpers, it still rode up when hitting the bridge (actually pushing back into our main breaker in a practice match and shutting us off).
The solution we came up with to transfer maximum force and reduce the backdriving was a simple bar of angle iron stretching across the wedge near the bottom part of it. This pushes straight into the bumpers and keeps it a tight fit as the robot lowers the bridge.
Altogether it probably weighs 5-10 pounds and uses one window motor. I don't have the exact weight now since we added parts to the subsystem in the competition. But it allowed us to lower the bridge perfectly all weekend.

This is exactly what we did, but our wedge was closer to the ground. It seems that the way to go would be wedges with the bridge. Their not too difficult to make, either.

[matteo2171]

We have a wedge powered up and down by a window motor the wedge is pushed back into the bumper when we hit the bridge thus holding it rigid as we lower the bridge, the advatage of it is that A you don't have to retract it to keep moving and B you have the full force of your drivetrain lowering the bridge