Drop-center drivetrains: Why?

[AdamHeard]

Originally Posted by Chris is me

If we're engineers, I feel we shouldn't be content with the what without the why...

We're not engineers, small correction. Statement is true otherwise.

[Al Skierkiewicz]

Simply put, without dropping the center wheels, drive motors rapidly go to stall current in turns.

[IKE]

Do a little more digging. A common questiont that many 6x6 drop center guys will ask is "How much drop are you using?". While a fair question, the same amount of drop will have various effects on different chassis. CG placement and Chassis stiffness, and overall goals/objectives play a big part into what is considered effective amounts of drop.

[JamesCH95]

Originally Posted by IKE

Do a little more digging. A common questiont that many 6x6 drop center guys will ask is "How much drop are you using?". While a fair question, the same amount of drop will have various effects on different chassis. CG placement and Chassis stiffness, and overall goals/objectives play a big part into what is considered effective amounts of drop.

As an example, 95 has used dropped center designs when the robot did not have precise manipulators mounted on the chassis, where the robot rocking back-and-forth did not adversely affect controllability. On two robots that DID have bigger arms that needed to be controlled with precision we used omni wheels on the four corners so that the robot was more stable and the arm was easier to control.

EDIT: To avoid confusion, I am talking about a 6wd platform with two traction wheels in the middle, and omni wheels at each corner.

[JesseK]

Originally Posted by JamesCH95

... On two robots that DID have bigger arms that needed to be controlled with precision we used omni wheels on the four corners so that the robot was more stable and the arm was easier to control.

Anecdotal evidence would suggest that rocking drive trains still maintain the maximum manipulator control aspect given a proper c.g. As an example from 2005, 254's robot had a rocking 6WD with a very long 2-PID arm. When fully extended the arm would shift the c.g. slightly, yet when it was down the tetras were easy to pick up because the c.g. stayed towards one side or the other when the robot was not accelerating.

Unfortunately it is exceptionally hard to integrate c.g. from scratch ahead of time; rough estimates are the best I've ever seen derived at the early stages of design. Since drive train is one of the three systems that MUST work (drive train, electronics, programming) in order to do anything effective in a game, most teams prototype like crazy on a pre-season prototype drive train. Additionally, finishing the production drive train early in the season is atypical of actuality for most teams.

Ergo a wise suggestion to anyone worried about c.g. shift on a rocking drive train is to have a pre-season prototype ready to go such that manipulators may be directly mounted to it during build season prototyping. This will give earlier estimates on c.g. and should produce constraints on shifting c.g. that may dictate where to put the 'heavy' elements (compressor, battery, drive train gear boxes, etc) on the production robot.

[JamesCH95]

For all of the analysis that could go into a chassis like that it is simpler to use omni wheels on the corners and be done with it. That frees up all the time and energy saved in chassis analysis for use in more unique challenges.

[EricH]

It's simpler, unless the game requires pointing in one direction for any length of time (like 2 seconds to unload a game object). The defensive robots love 6WD robots that have omnis on the corners, something about they're easier to knock out of position rotationally.

A pneumatic center wheel on a drop-center is actually a pretty good idea: you can fine-tune the drop a little bit to get exactly the characteristics you want, even between matches.

[JamesCH95]

Good point about the defense.

Changing pneumatic tire pressure would also change gearing and grip, but those might also be good things.

[Ian Curtis]

Originally Posted by EricH

It's simpler, unless the game requires pointing in one direction for any length of time (like 2 seconds to unload a game object). The defensive robots love 6WD robots that have omnis on the corners, something about they're easier to knock out of position rotationally.

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.

[Brandon Holley]

Originally Posted by iCurtis

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.

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

[IKE]

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.

[Ian Curtis]

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.

Originally Posted by Brandon Holley

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

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.

[Al Skierkiewicz]

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!"

[billbo911]

Originally Posted by JamesCH95

For all of the analysis that could go into a chassis like that it is simpler to use omni wheels on the corners and be done with it. That frees up all the time and energy saved in chassis analysis for use in more unique challenges.

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.

[RyanN]

Originally Posted by Al Skierkiewicz

Simply put, without dropping the center wheels, drive motors rapidly go to stall current in turns.

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.