Ground clearance

[tim-tim]

Originally Posted by Adrian Clark

Not necessarily, even if the CG is perfectly centered the robot will always be resting on either set of four wheels. I'm not too sure I understand your logic here, why would the robot not be able to turn in place in your given scenario?

I think what he was trying to say is that when turning we (The RoboBees) were always on the same set of wheels. We knew exactly what axis the robot would turn about.

However, if the CG of the robot is close enough to center that enables the teetering you know longer have this luxury. The acceleration at the beginning of the turn and throughout could switch which axis you rotate about. Meaning it could be between the front and center wheels, or the rear and center wheels. We knew that every turn would be about a central point with almost no variation.

It is like have a 4WD robot with a wheelie bar with powered wheels.

Again, the application should drive the design. This is just another option to keep in mind.

[AdamHeard]

Originally Posted by tim-tim

I think what he was trying to say is that when turning we (The RoboBees) were always on the same set of wheels. We knew exactly what axis the robot would turn about.

However, if the CG of the robot is close enough to center that enables the teetering you know longer have this luxury. The acceleration at the beginning of the turn and throughout could switch which axis you rotate about. Meaning it could be between the front and center wheels, or the rear and center wheels. We knew that every turn would be about a central point with almost no variation.

It is like have a 4WD robot with a wheelie bar with powered wheels.

Again, the application should drive the design. This is just another option to keep in mind.

This different method of raising a wheel results in the exact same setup as a regular 6wd. You still have two 4wd bases, and the cg location (+ accleration) determines rock.

[tim-tim]

Originally Posted by AdamHeard

This different method of raising a wheel results in the exact same setup as a regular 6wd. You still have two 4wd bases, and the cg location (+ accleration) determines rock.

Except that we didn't have the rock due to the location of our CG. The only times we used the front wheels (raised front) were to start the process of crossing the bump and when entering/exiting the ramp.

But I understand what you are saying though. But why would you want a robot with a CG + acceleration that would allow rocking in normal situations? Is there too much variance in when it does or does not teeter to be able to predict what axis the robot will turn about?

We chose this design for stability reasons. We always sit on our rear wheels. Likewise we always turned about the same point.

I guess it is just design intent.

[AdamHeard]

Originally Posted by tim-tim

Except that we didn't have the rock due to the location of our CG. The only times we used the front wheels (raised front) were to start the process of crossing the bump and when entering/exiting the ramp.

But I understand what you are saying though. But why would you want a robot with a CG + acceleration that would allow rocking in normal situations? Is there too much variance in when it does or does not teeter to be able to predict what axis the robot will turn about?

We chose this design for stability reasons. We always sit on our rear wheels. Likewise we always turned about the same point.

I guess it is just design intent.

I'm saying the reason you didn't rock has nothing to do with your different wheel configuration, and everything to do with CG. Your drive is pretty much a drop center 6wd. You still have two 4wd bases, but primarily drive one one. This is true for most 6wd's. We bias weight on all of ours to get a more consistent turning center.

I'm not saying your decision was a bad one, we did the same thing in 2009; Just clarifying for the lesser informed people reading that there isn't some holy grail difference here.

[tim-tim]

Originally Posted by AdamHeard

I'm saying the reason you didn't rock has nothing to do with your different wheel configuration, and everything to do with CG. Your drive is pretty much a drop center 6wd. You still have two 4wd bases, but primarily drive one one. This is true for most 6wd's. We bias weight on all of ours to get a more consistent turning center.

I'm not saying your decision was a bad one, we did the same thing in 2009; Just clarifying for the lesser informed people reading that there isn't some holy grail difference here.

Yes I agree, there is no holy grail answer.

Can I pose the question: if you know are going (significantly) bias the CG to one side of the robot, why drop the center as opposed to raising the front? Not trying to argue the poi t just want a different designers perspective.

[AdamHeard]

Originally Posted by tim-tim

Yes I agree, there is no holy grail answer.

Can I pose the question: if you know are going (significantly) bias the CG to one side of the robot, why drop the center as opposed to raising the front? Not trying to argue the poi t just want a different designers perspective.

Partly because even with a CG bias, you won't ever prevent the robot from rocking onto those other wheels (a 14-15" wheelbase just isn't very stable for any fast robot).

A bigger factor for us is we don't like giving up symmetry on our parts. If we can maintain it, it makes fab and assembly easier/less confusing.

It's a really trivial difference though, between implementations.

[IKE]

I would strongly caution about going less than 0.5" of ground clearance, here is why:
In 2008, it was a flat game in the rules.... except for th 5" plates supporting the poles for the center divider. That a lot of teams got stuf on.

2010 was flat between the bumps, except it wasn't as they added some plywood at teh entrance/exit ramp surface of the bumps so that teams didn't damage the flooring.

2005 was flat, except for the 3/16" plastic triangles on the floor to designate the loading. And the 1.5" tubes you might get shoved over while scoring a Tetra.

You can go less than 0.5" (I have seen many successful bots do this), but I would caution going much below that.

[Nuttyman54]

I concur with IKE

FIRST has a tendency to put plywood and metal plates as field element bases under the carpet, and most of the time that results in about a 1/2" lip in the carpet. I've seen many robots over many years get hung up or spun around when they hit one at the wrong angle, or get pushed over it. Not worth risking when it's easily avoided.

[AdamHeard]

It's worth pointing out that ground clearance between the wheels, and ground clearance in front of the frontmost wheel (ditto on the back) have different needs/affects.

Thanks everyone! I've redesigned my chassis so that there will be 7/8" ground clearance. Makes a lot more sense, and the redesign has allowed me to loose some weight. (I'll try and get a CAD up once the CAD team starts meeting)

[MichaelBick]

Another thing about the inconsistent turning axis is that it can be used to your advantage with the right drivers and driving practice. Watch the 2011 Einstein finals(both matches) and watch 254 very closely. You will see that they do this. One example of the right way to use the inconsistent turning axis is when they come from picking up a tube in the feeder station. They transverse the whole field backwards and then right before they get to the racks, they brake, shifting their turning axis and causing them to use the turning axis closer to the racks, allowing them to turn and immediately score instead of having to turn and then drive closer to the racks.

[AdamHeard]

Originally Posted by MICHAELABICK

Another thing about the inconsistent turning axis is that it can be used to your advantage with the right drivers and driving practice. Watch the 2011 Einstein finals(both matches) and watch 254 very closely. You will see that they do this. One example of the right way to use the inconsistent turning axis is when they come from picking up a tube in the feeder station. They transverse the whole field backwards and then right before they get to the racks, they brake, shifting their turning axis and causing them to use the turning axis closer to the racks, allowing them to turn and immediately score instead of having to turn and then drive closer to the racks.

Very interesting observation. I'm stoked to see what you guys come up with for 2013 btw.

[tim-tim]

Originally Posted by AdamHeard

A bigger factor for us is we don't like giving up symmetry on our parts. If we can maintain it, it makes fab and assembly easier/less confusing.

It's a really trivial difference though, between implementations.

Didn't think about the symmetry of the parts. That is a good point, especially at the manufacturing stage.

And yes, it really is a trivial difference.

Originally Posted by SuperNerd256

I've redesigned my chassis so that there will be 7/8" ground clearance.

This is a good ground clearance. As mentioned earlier, the sub-floors and methods of placing field elements can cause issues at .5". Then there is always random debris from robots that fall off. Yes, you will never be able to completely design for this, but the design should be capable of fasteners, small plates, chain, etc.

Originally Posted by MICHAELABICK

Another thing about the inconsistent turning axis is that it can be used to your advantage with the right drivers and driving practice. Watch the 2011 Einstein finals(both matches) and watch 254 very closely. You will see that they do this. One example of the right way to use the inconsistent turning axis is when they come from picking up a tube in the feeder station. They transverse the whole field backwards and then right before they get to the racks, they brake, shifting their turning axis and causing them to use the turning axis closer to the racks, allowing them to turn and immediately score instead of having to turn and then drive closer to the racks.

Hmmm. So you can control the teeter and use it your advantage. This is something I didn't think could be done. Poofs always teach me something new (in this case with the help of Michael).

[Ekcrbe]

Originally Posted by SuperNerd256

Thanks everyone! I've redesigned my chassis so that there will be 7/8" ground clearance. Makes a lot more sense, and the redesign has allowed me to loose some weight. (I'll try and get a CAD up once the CAD team starts meeting)

Darn, i just got the math ready. But yeah, the low clearance wouldn't have been enough to go over the bump or even get on the bridge. (you could climb over a 2.13" barrier or drive onto a 3.6 degree incline)

[Adrian Clark]

Originally Posted by tim-tim

I think what he was trying to say is that when turning we (The RoboBees) were always on the same set of wheels. We knew exactly what axis the robot would turn about.

However, if the CG of the robot is close enough to center that enables the teetering you know longer have this luxury. The acceleration at the beginning of the turn and throughout could switch which axis you rotate about. Meaning it could be between the front and center wheels, or the rear and center wheels. We knew that every turn would be about a central point with almost no variation.

It is like have a 4WD robot with a wheelie bar with powered wheels.

Again, the application should drive the design. This is just another option to keep in mind.

I might be misunderstanding the conversation, but I'm pretty sure your teammate was talking about supernerds(op) drivetrain. But as for your design, if I understand it correctly, having the CG in the back of the robot is critical since your drivetrain is designed to not teter-totter and be able to interact with the field in ways a standard 6wd drop center can't. I commend you for trying something different, I think your drivetrain has pros and cons when compared to a traditional 6wd drop center, and I love that you were able to boost your ability to traverse field elements with out the standard approach of just making your wheels bigger.