Re: Drop-center drivetrains: Why?
 

And when drive motors rapidly go to stall current, that means...

1. Robot doesn't turn
2. Breakers pop
3. or Worse

We've been using 6WD with dropped center wheels for years (with the exception of last year).

We like using pneumatic center wheels. Makes adjusting the offset very easy.

Re: Drop-center drivetrains: Why?
 

1276 built a launcher in 2008 with a 6 wheel drive with omnis in the corners. I was sure we would get knocked all over the place trying to launch, but it was never an issue. We would've done it again in 2009 were it not for the slippery floor, and the team disbanding in 2010.

That said, in 2008 there wasn't a lot of incentive to knock robots silly, so we may have just gotten lucky.

As a counterpoint to my own counterpoint, I can't think of many situations where if someone is trying to rotate you, you'd be much better off with the stickier wheels. In 2010 if someone was messing with you, you'd drive off before kicking the ball. In 2007, you were probably entangled in the Rack, so the squirrelyness (technical term) might help you swing around to another Spider. In 2006 if someone was pushing on you, it was probably enough to throw off your aim. Additionally, it cut way down on our distance (though our conveyor to our shooter and shooter wheels were all on one CIM, so this may have been less of an issue for others). In 2005, perhaps it could help you stick your ground for just an extra second, but this came turned out so offensively, I'd rather not be rocking back and forth with a 10 pound tetra 10 feet up in the air.

Re: Drop-center drivetrains: Why?
 

Let's look at this a bit more. It's not always as simple as you have stated here. There are situations, or game elements, that do not lend themselves very well to a robot with omni's on the four corners.
If you are playing on a flat surface, then you bet, omni's on the corners is a simple solution. Now, if you want to climb a ramp, or go over a bump, then omni's on the corners add a whole new level of difficulty. I'm not talking so much about traction, but the need to hit the incline squarely. If you are out of alignment by only a few degrees then the robot will tend to slide out, or fall back to the level surface. The steeper the incline, the greater this effect will be.

The bottom line is, make the design decisions based on what you want to achieve in the specific game you will be playing with the robot.

Re: Drop-center drivetrains: Why?
 

aspiring to be engineers :)

I really think "just use omnis" can be a pretty poor solution depending on the game challenge. If you have a long arm, a gentle tap moves your game piece several feet away and there's not much resistance to speak of. Really, it's not that hard to do "all of the analysis". I can tell you what the analysis will say right now: The more rigid your chassis, the better your 6WD can perform. Building a rigid chassis doesn't take tons of time and energy. Even if you do want to do careful and thorough analysis for such a drivetrain, it's not exactly a huge resource drain, and depending on the challenge it could result in a much, much better chassis for your needs.

While some games you probably could get away with a chassis with four omnis, in the past I think more often than not you couldn't. Being spun off target in Aim High, knocking you away from your spider leg in Rack 'n Roll, or taking a mid to long range shot in Breakaway are all situations when I'd rather have a bit more resistance to being spun.

Re: Drop-center drivetrains: Why?
 

I don't know about your reasoning here. It seems in every instance you brought up that someone trying to rotate you is prohibiting you from completing your initial task. In your 2010 example instead of kicking the ball you drive away because someones "messing with you". In 2007 you are swinging from the initial spider your trying to cap to a different one because your being rotated. In 2006 being lined up and not moving was key for 90% of the shooters out there, obviously. In 2005 getting to where you needed to get to was absolutely key.

Not meaning to nitpick here, but the suggestions you make are all alternative maneuvers your team can make to offset being spun easily. In all those instances, I would much rather not be spun and complete the task my team wants to, than come up with a new game plan because someone is playing D on me.

-Brando

Re: Drop-center drivetrains: Why?
 

Make sure when discussing "performance" to use key words like traction, ability to hold ground, ease of turning, manuverability...
Saying "XYZ will perform better" in a thread like this can lead to misconceptions. Many of the parameters for manuverabilty and the ability to hold ones ground are in direct conflict.

Re: Drop-center drivetrains: Why?
 

Although we did not use omni wheels this year, we did us mecanums, and we noticed that the robot wouldn't glance off inclines, but rather tend to align with them. There probably is some angle where the robot would tend to glance off rather than self-align, but my experience tells me that this angle is sufficiently wide to easily drive over a bump or up a ramp.

Perhaps you did not understand that I was referring to a 6wd chassis with two traction wheels in the center and four omni wheels on the corners. In my experience these have been fairly resistant to being spun and do not rock much, if at all; a good compromise in my opinion.

I was not referring to a chassis stiffness analysis, but rather keeping track of CG location for the purposes of reducing rocking while manipulating a game piece. While these are both straight-forward analyses to perform, not every team has a lab full of CAD-capable computers and/or trained operators ready to do them.

Re: Drop-center drivetrains: Why?
 

I caught that. 6 wheel 4 omni isn't terrible or anything but it is more easily spun and negates a lot of the advantages of a rocking drivetrain. The CG issue, while relevant, really isn't something you need computer precision to analyze. If you don't want to rock with an arm too much, you can adjust the drop a little, put the CG a bit more to one side fore-aft, or even run an 8 wheel drive if your CG is centered. You should be able to get a good enough estimate with just some napkin sketches of where the "heavy stuff" goes. CAD isn't required to figure it out.

Even if you do find you need more precision than "I think it'd be best if we put the battery... here", CG analysis is always beneficial on a competition robot.

Re: Drop-center drivetrains: Why?
 

People refer to "rocking" all the time and cite it as a problem with manipulators.

Think about the math here. With a 33" wheelbase (37" long chassis, with 4" wheels), that means you have 16.5" between wheels. With a .1875" drop on the center, you can rock 1/100 of a degree. Obviously that's exaggerated when you have a long arm sticking out, but come on. .010 degrees of rock is nothing. It's hardly noticeable. It doesn't affect precise positioning whatsoever.

Re: Drop-center drivetrains: Why?
 

An interesting idea inspired by this thread would be to (gasp!) raise the center wheel on the fly in order to reduce turnability similar to the nonadrive concept.

Hmm.

Re: Drop-center drivetrains: Why?
 

Agreed.

Even with a 10' arm on there, if your manipulator needs to be accurate in placement to 2" or less, you really have a poorly designed manipulator that would be difficult to score with anyway.

Re: Drop-center drivetrains: Why?
 

For what "benefit"?

Before people start going off and design new for the sake of new, they should evaluate what a well designed full treaded drop center 6wd can do. There is a reason 60 started doing it, 254/968 always does, and we always love to copy it.

Re: Drop-center drivetrains: Why?
 

I am aware that CG/tipping moments can be roughly estimated with napkin calculations. Chassis stiffness is not analyzed so easily.

We've also done a drive-train with two omni wheels in the middle and traction wheels on four corners :eek: these omni+traction 6wd robots, in either configuration, have all been successful.

I'm not trying to invalidate your points, Chris, they are indeed accurate. While the does trade off a little hit-and-spin stability, it also gains turning speed and virtually eliminates jitters from scrubbing the outside wheels and rocking. As with any robot design choice it has to be made in context of the teams strategy in that year's game. I feel that this disclaimer need not be made with every post though.

Re: Drop-center drivetrains: Why?
 

Perhaps you were blessed with better drivers than us. It just seemed that the vast majority of the time we were being defended, it was a better use of our time to leave and sneak to a different location rather than wiggle back and forth with someone slamming on us. :)

Cory, we had issues with rocking and the dropped center wheel, but we may have dropped more than .1875, it's been a while. In 2006 when shooting from the floor, rocked one way we would make shots, rocked the other way they went high (or low). We also had some tracking issues with our dead reckoning autonomous.

Re: Drop-center drivetrains: Why?
 

We have (on more than one occasion) used a dropped a foot under the front or back of the robot to assist in turning. This raises one set of wheels off the ground.
For some of the other discussion, there is no substitute for practice.

A common joke among music professionals goes like this...
A person on a street in New York is asked "How do you get to Carnegie Hall?". The response is "practice, practice, practice!"