The afterthought bridge manipulator

[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

[Jeffy]

An easy way to make one of these work is to use a "winch-like" system. Pull the system down with a small pulley and some tether. Spring it back up. (or vise versa) It's super easy to get a very large reduction using a system like this and It is also very easy to change on the fly by adjusting the point at which the tether attaches to the arm.

[Brian Ha]

I'm sure it has been mentioned, but a passive arm is a very good way of doing it and conserving weight. We did ours with a window motor and it weighed 4 lbs. Although after watching week 1 and 2 it is getting a massive redesign. Haha it might weight a little more than 4 lbs now, but hey lets cheese away!

[flameout]

Team 957 found success with our bridge manipulator as well -- but I wanted to share it because we did not use a wedge.

We had an arm that pivoted off a spot about two inches inside of the frame perimeter. It extended close to 14 inches out of the frame perimeter, and had the Tetrix wheels from last year's minibot kit. Therefore, I'd put the lever arm from the pivot to the contact point with the bridge at about 14 inches.

We originally planned to use a Banebots RS-550 with the AndyMark Gearbox and a Banebots 256:1 P80 (rated at 85 ft-lbs), but those were out of stock when it came time to order them. Instead, after doing the math and determining that this would be acceptable (if marginal), we used a kit Fisher-Price gearbox from a previous year (no gearing between the gearbox output and the arm's input). We strapped on a potentiometer for PID control of the arm.

We used a Jaguar to operate this motor, intending to use current control through CAN to limit our maximum current output. However, we wanted to have PWM control as a backup, and tuned that loop first. After finding out how fast that arm is*, as well as how noisy the data from the potentiometer is (I think we may have had a bad pot, too, which was replaced), I re-structured the code to allow limiting the commanded speed (not just duty cycle command) and to not rely on differentiating the potentiometer's output. We then relied on the Jaguar's dynamic braking to slow the arm down (and also relied on its linearity for our control loop tuning). This removed CAN-based current control from our plans. We limited our maximum duty cycle to 30% for testing with our bridge, but were prepared to raise it for competition.

Come competition, we went out on the practice field and tested with it a bit. At 50% duty cycle, it did not effectively push the bridge down (due to lack of torque). At 70%, the Jaguar's overcurrent protection kicked in well before the bridge even made it halfway down. At 60%, it was successful in operating the bridge (note: this will probably vary based on the motor used... I don't recall off the top of my head which we used). Although this warmed up the motor really quickly, we did not need to use the arm for more than a couple of seconds at a time, so it was not an issue during our matches.

If you do choose to use a Fisher Price gearbox-based arm, I'd recommend that you be ready to spend hours testing it, rewriting code, and tuning it. Also, verify (theoretically or practically) that the exact motor you use is capable of tilting a competition-weighted bridge.

A wedge is probably quicker and more effective at tipping the bridge, but may be harder to design (especially if, like us, only a few members of your team have the math knowledge to compute the required angles to tilt the bridge and handle the generated torque). Also, our arm is capable of lifting the bridge to let balls caught under it roll out. Had our drive team (which included me, our lead driver) known where the balls were getting stuck (right on the end, where they're in the way, invisible to the drive train, and not on the ramp), we could've swept them out from under the bridge, too.

* We have a video of the arm accelerating from a stop 5 inches above our bridge to an impact with our bridge in 3 frames... at 30 frames per second. The commanded rate limitation was implemented to decrease this impact. You can find this on YouTube at http://www.youtube.com/watch?v=nXLTAL7Q3oo -- the fast motion is visible at about the 70-second mark.

[wireties]

We have yet to play - our first regional is this week. Out lowering device is a very stiff arm run by a van door motor plus a 4:1 gear reduction. It easily lowers a weighted practice bridge (with less than 50% PWM setting).

Should it work in competition?

TIA

[Sb28000]

Originally Posted by wireties

We have yet to play - our first regional is this week. Out lowering device is a very stiff arm run by a van door motor plus a 4:1 gear reduction. It easily lowers a weighted practice bridge (with less than 50% PWM setting).

Should it work in competition?

TIA

I would have to say that this will be more than sufficient. We ran a van door motor with no additional reduction and were able to lower the competition bridge relatively easily. See my other post in this thread for details of our set-up - I just caution you to be sure your arm and mounting methods are sturdy enough and not likely to bend/deform in any way while trying to push down a bridge in a match (which we had happen to us twice).