TOP HEAVY!!!!
 

We are using 6wd and an off set center wheel. we have our electronics mounted vertical because we had no where else to put them! And we are using 2Cims for our shooter which is on top. When we went to test it last night in the hallway we went half speed and it flipped nothing broke THANK GOD. :eek: Today we are going to try to fix this problem. ANY SUGGESTIONS?

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Are the bumpers on it? If the bumpers are mounted low to the ground, they act a bit as wheelie bars. When fully extended, our Logomotion robot would easily tip without bumpers; with bumpers, we had no difficulties whatsoever.

Disclaimer: Low bumpers may interfere with your bump-traversing abilities.

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How heavy is your robot currently? If you have weight to spare, try adding a big steel plate to the bottom of your robot. If your problem is falling backwards, add it near the front.

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we fall in both directions and no we dont have bumpers on but they are sitting right above the c channel so that we can go over the bump.

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Our robot showed signs last night that it too was a bit top heavy. It has enough torque to flip over backward just by punching the throttle.
Our first option is to move the battery to the front of the robot. That should be enough to give us the stability we need.

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If you could lower the CIMs that you have for the shooter by going to a chain drive, that would help.

Basically, you have too much stuff, too high up.

You could put ballast in the base, but that will only fix one problem (the high center of gravity). It will not address your other (mostly overlooked) problem of moment of inertia. Moment of inertia is the tendency of an object to want to keep rotating once it has started. Or in the case of your robot, keep falling over once it has started.

The best solution would be to lower the weight that you have up high.

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sounds like the dropped wheel was dropped too far... usually 1/16" is a good spot.

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I suggest you post some pictures of your robot, so we can see what you might be able to change to help fix the problem.

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It's really hard to lower the CG of a full-weight, already complete robot...

If your CG isn't centered front-to-back, move weight (the battery, perhaps?) around to accommodate.

As has been suggested, if you have any weight to spare, add it as low as you can... I prefer adding "functional weight" if possible... (i.e. beef up the bottom of the frame, rather than just strapping on dead weight)

Move any heavy objects lower, where possible... the battery, an electronics board, that gearbox/motor for your elevator, etc. Consider using two lighter motors up top on your shooter... Most people are using BB550s, BB775s, or FPs for the shooter wheels - two CIMs is more power than you need.

Take off any weight you can above ~8"... Reduce fasteners, consider rivets instead of bolts, tap rather than using a nut if you can, replace heavy upper frame members with lighter ones. A lot of upper-frames are over-engineered... 80-20 or 1x1x.125" is .5 pounds per foot, twice as much as 1x1x.0625" box. PVC rollers can also be heavy... consider removing excess ones, going to smaller diameters, or at least speed-holing them in acceptable areas. Also, if you have belting for your harvester, powering only one side, putting a thin sheet of lexan on the other side.

Add weight to your bumpers, if their center is below 8" (which it has to be this year). You have 20 pounds for bumpers... consider using steel angle instead of aluminum, using more fasteners, or some such thing.

Consider shortening your entire upper assembly. Given the high-trajectory arcs, you could likely shoot from lower (~40") without worrying about your shots being blocked.

If you really can't lower it sufficiently, and can't use a retractable wheelie bar to help you, consider removing a motor/actuator from up top entirely. It's better to have limited functionality then to spend half of every match on your side!

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You could always add some sort of passive appendage that acts as a wheelie bar of sorts. It'd essentially act as a pendulum when your robot tips any noticeable amount. It should help keep you from tipping.

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Do you have your battery mounted as low as possible? That's the heaviest single component of your robot (almost 14 pounds, about 10% of your complete robot), so it has a big contribution to CoG. We mount ours flat on a board that is mounted to the bottom of the frame. Although, we may change ours to vertical this year, to improve accessibility, since it's all the way at the back and next to the bumper.

Another option is to train your drivers not to hit the throttle too hard. This can be enforced in your software. But this is not an ideal solution; it still leaves your robot susceptible to sudden stops, impact by other robots, tilting while going over the bump or bridge, etc.

Good luck!

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Shaun,

This is a rookie year for you guys, right?

OK, this kind kind of thing is what someone needs to write down so that next year's team knows it. One lesson is that the electronics guys need some space, the mechanical guys need to understand that (because high-up electronics have a tendency to get hurt, and high-up weight is always bad).

Please see if you can take a photo and post it so we can offer reasonable suggestions that will work in the time we have left.

One last thing: Congratulations! Not many rookie teams have a working robot already.

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Now that I have some more time...

We saw in testing our robot a two nights ago that we have a similar (although not as extreme) issue - we can pop a wheelie transitioning from fill reverse to full forward. Essentially, we have too much weight in the back of the robot, and not enough in the front. We did a pretty good job of managing the height of our CG, though, so we don't actually tip over!

We weighed the robot by putting on end on blocks, and the other on a scale (may not be 100% accurate, but close enough) - we found the back weighed about 65lbs, and the front about 30lbs. We have a little more weight to add still (light weight panels, some small modifications as we see things aren't working correctly, and our bridge manipulator), but we should be able to add some nice steel plates to the front somewhere to help balance out the CG.

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In addition to fixing the core problem, which is a high CG, you can do some software controls to prevent the driver from accelerating too quickly. Ramping up speed changes over a second or two can really help, although make sure your drivers are aware of the impact that might have on balancing. I'm not generally a fan of putting dampeners and filters between the driver and robot, but this is sort of 'free' and can quickly tuned or turned off as needed.

If you have a gyro, you could also use that. If the gyro saw the bot tipping too much, it could either limit any further acceleration or even go in the other direction momentarily. Traversing the bridge or bump would obviously cause some issues with that, so that may just not be practical this year.

Get your CG as low as you can and consider software dampeners to control any remaining issues.

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I would first calculate your center of gravity. Then I would play around and try to move a few things to the front. If you had a picture that would really help us.

When testing I would make a roll Cage just encase the worse happens. We learned that in 2010 when our robot completely flipped over.

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Are you doing a one wheel or a two wheel shooter? If it's a one wheel shooter, the easiest thing you could do is remove one of the CIM motors from the shooter. The shooter's top speed would be the same, although it would take longer to spin up to its top speed.

Edit: if it is a two wheel shooter, you can probably put a sheet of something behind one wheel and convert it to a one wheel shooter.

Depending on what the robot looks like, it might be feasable to lower the entire shooter before you bag. I know we have mentioned it, since our wide configuration robot is tippy and it wouldn't be that hard to shorten our conveyor. That would sacrifice some shooting height to gain stability, which could potentially be a good trade.

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I would also added the bumpers for the simple fact that you are adding mass to the base of the bot, therefore, lowering the CG.

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Get some 775s or 550s and CIM-U-LATOR gearboxes and drop several pounds per motor up top. Add the CIMs back to the drive train down low.

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Absolutely agree here. Just look up the equations for angular momentum and kinetic energy of rotation and you will see why a tall robot may not behave well. Reducing the height (if you can) will pay back large dividends in stability. It is really difficult to increase the weight of your base by 25% late in the build and not go over weight. But decreasing the height the same amount is often quite possible, and may not even be that difficult.

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I guess I should have posted a link about 5 weeks ago...sorry!

http://www.chiefdelphi.com/media/papers/1974

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Lots of good suggestions so far. One thing that hasn't been mentioned, probably because it isn't a great solution, is that a robot with low-friction wheels (e.g. omni wheels) will slide instead of tipping. Not great, but if all else fails, it's something to try.

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(emphasis added)
CIM == 2.8 lb each
775 == 0.75 lb each
CIM-U-LATOR == 0.5 lb each

Total weight savings == 3.1 lb (top only)

If you add the CIM's to the bottom, it's a net gain of 2.5 lb. Is that in your weight budget?

If you can (I know it's late in build season), shortening the robot as others have suggested can be a big improvement. CoG goes down, angular momentum goes down, stability goes up. Our team did this, and we believe the advantages outweigh the disadvantages.

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Get rid of your 80/20 "camera mount" up top including the uprights, leave the shooter support.

Replace with some of that wonderful lexan you got accidentally.:D

Look to relocate the battery in the center behind your lift mechanism, instead of at the back of the robot.

Also, consider moving the transmissions and motors forward, in between the front two wheels (instead of the back).

I'm currently looking at small pictures on my phone . . have you guys actually gotten the scale to weigh it yet?

The bumpers will help your top bottom a bit, but you NEED to fix the fore/aft problem.

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I posted about Team 4's "Tip-proof" robot in CD a few years ago.

It has an educational and entertaining YouTube clip showing what you can do with a very low CG.

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You're right, it's just a few pounds for both motors, clearly.

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Has it occurred to you to not go over the bump?
Use the ramps... or do you have a problem there too..??

Having those big CIMS up high is where to start... see if you can bring them down or use a lighter alternative motor...

But first put your bumpers on and see if you still have the problem

You don't have to go over the bump...
You can play the game by going over the ramps...

There is danger going over the bump... you can be defended and knocked over..(not as strategy...but defense being played....)

On your own ramp you cannot be defended...

just a thought...

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Even with a complete robot, there's still a lot that can be reworked in most cases. First, make sure the heavy components (e.g., battery) are down low and ensure the heavier electronic components are lower on the side pieces.

Also, how offset are your wheels on the drive train? If they are more than 1/8", then you've probably put too much sway in the robot. Normally 1/16" offset is sufficient. This is especially true if your robot has the wide front and a narrower length.

Don't give up until the end of competition--I've watched a team build their entire robot at a competition site.

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MATT look at your inbox!!! And we are not a rookie team for those who are wondering this is our first year with 6WD.

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Wow lucky you, our collaboration robot went up the bridge and took a disastrous spill