The afterthought bridge manipulator

[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).

[Ken Streeter]

Originally Posted by Sean Raia

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

Below is a somewhat better photo showing the 1519 bridge manipulator on the robot at the Granite State Regional. One of the keys to the design is that the motor alone does not hold the arm in the lowered position -- in addition to the motor, there is a servo-actuated latch which retains the arm in the lowered position so that the motor housing does not bear the load of the bridge when the robot drives into the bridge to press it down.

[Phyrxes]

Depending on your design and use of "appendages" you could also use a "passive" latch using that is manually released after the match. But that would mean you are driving around with it permanently deployed if you have to manipulate a bridge early in a match.

At Chesapeake I noticed a good number of teams redesigning their bridge manipulators to either generate more force to be able to push the competition bridge down or to prevent the reaction force from damaging the deployment method.

[Steve Compton]

Here's alink to 1391 using the wheelie approach;

http://www.youtube.com/watch?v=1Rlp1...f-Pn6P2yG7VPE=

Originally Posted by Kim Masi

I watched a lot of teams (both veterans and rookies) struggle with this over the weekend. I'm not sure if the issue was more geometry or providing enough torque to the bridge to lower it.

One of the most clever methods I've seen is team 103, who has two curved pieces of tubing that flip down and so when they drive up to the bridge the natural motion of their forward momentum causes the bridge to tilt and allowed for them to get up pretty easily. It was a pretty effective, light weight and passive way of lowering the ramp without having to stop and wait for some other mechanism to push the bridge down. If I can find a pic I'll post it.

Or you could try the 25 method: bump the bridge, pop a wheelie and then get on. Seemed to work well for them

[Brian_R]

I'll throw my support behind the designs from 103, 1519, and 2134 mentioned in this thread...

Us at 3710 have had some good results with a ramp-mechanism. Ours is deployed with a 4 bar mechanism to save a bit of space, but also to let mechanical stops and the chassis to bear the forces of impact with the bridge.

This has worked pretty well for a plywood test bridge (built by our buddies at 2809) and will see competition this week at Montreal - we can take the bridge at about half of our maximum speed.

We've whitepapered the design and made our CAD available - it's mostly aluminum angle and ours is bolted right on to a Kitbot on Steroids. Available here:

http://www.cyberfalcons.com/whitepapers/

[BrFrederick]

My team decided tipping the bridge was important, however the weight allocated to doing so was reduced to roughly five pounds for tipping the bridge, a ball scraper to get balls of walls and from under bridges, as well as a moveable ramp on the back of our elevator that allows balls to enter from the inbounder station.

This left me with around 2-3 pounds to tip the bridge as fast and as passive as possible. 10 designs later with the help of many free body diagrams and some tests our 2 pound tipper was born. The two pounds includes the weight of the air cylinders and everything else needed.

The result was a bar of 1by1 AL that is rotated down using a 3/4'' bore, 4 inch stroke air cylinder. Then a over center clamp is rotated over it by a 3/4'' bore , 2 inch air cylinder. Once we fixed some problems with our swerve drive software and could go straight on to the bridge it worked perfectly. However the only problem we have had with this design is currently software has a long delay before the over center will come down. So sometimes we will ram the bridge without the over center clamp on and it will not tip the bridge. However this is an easy fix.

[joek]

Originally Posted by Sean Raia

After our first regional this year, our team has learned a lot about this game.
One thing we (and quite a few other teams) didn't see coming was the difficulty of lowering those bridges.
Our bridge arm was the last system to be designed, and was not given enough weight/space allotment. For that reason, we ended up having a "you lower it and we'l follow you up there" robot. We tried at least two different designs both powered by a window motor, to no avail. Then again, with 5 pounds of weight to spare for a manipulator, what can you expect?

I'm curious as to who else was in this boat, and if they found a way to overcome this problem. It would be interesting to discuss last ditch bridge manipulator implementations, and their effectiveness.

we re-designed our manipulator twice, our final design is capable of holding a bridge level with three other robots pushing down with weaker devices on the other side (happened once at lake superior)

[Teamcodeorange]

You can see our bridge manipulator in our robot demo video:

http://www.youtube.com/watch?v=Fpi9XnwSrpQ

Not to brag but it worked beautifully.

Team 399 and a few others in san diego also had competitive manipulators.

[philso]

We just finished assembling our replacement arm. It uses a van door motor driving an 8 inch diameter pulley to increase the torque and slow down the motion so that the limit switches have time to act. In the video, the limit switches are actually wired in series with the motor so that they cut off the power to the motor at the appropriate time. We used some 3mm Dyneema cord wrapped around the motor shaft and the pulley. We found some "non-traditional" material to make the body of the arm out of. It has more than adequate stiffness, was easy to work and has a low coefficient of friction.

http://www.youtube.com/watch?v=auKJTV7fTj4

We will see how well it works tomorrow at Lone Star.

[Akash Rastogi]

Originally Posted by 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.

^ This was our third iteration of a bridge arm and it worked okay.

V This is the design we had when we won the Mt. Olive MAR District event.

https://fbcdn-sphotos-a.akamaihd.net...9 69416_n.jpg

It is directly driven off a FP motor and gearbox with a coupling. The arm itself is welded tube. It is also our intake extension.

This is the bridge arm/drop down intake we will have attached to the same FP gearbox for the MAR Championship.

https://fbcdn-sphotos-a.akamaihd.net...547560 _n.jpg

https://fbcdn-sphotos-a.akamaihd.net...691814 _n.jpg

Worked out really well for us and I'm extremely proud of our rookie students for striving for continuous improvement.

[aldowyn]

We had this problem. Our arm was actually built in between competitions. The one we had at KC was a simple window-motor powered arm with a wheel on the end. It wasn't nearly strong enough, so we scrapped it.

Then we used two Fisher Price (I think Fisher Price...) motors (and those huge plastic gearboxes...), built an arm for each, and connected them with a bar. THAT worked.