pic: 1683 30-Point Climber

[Whateveryousay]

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[sdcantrell56]

Solid frame! Are those center omni wheels just place holders or actually what you will be running?

[AllenGregoryIV]

I'm a little confused by the omni wheels as well. Are they not powered? It looks like the power take off/Super shifter doesn't have a direct drive output to those wheels.

I'm interested in seeing this climbing mechanism, not really sure how the string works.

[Kevin Sevcik]

I think the string is being used more like a belt. You can see what's going on on the far side. The string is run through an eyelet, then tied off onto the pivot for that big cylinder. Said pivot is set up to run in a slot along the frame. So I think this is a belly climber that's using air and a PTO. That's my read on the arrangement of things anyways. I'm not entirely sure how they get onto the first rung and ratchet up, however.

[Whateveryousay]

Originally Posted by Kevin Sevcik

I think the string is being used more like a belt. You can see what's going on on the far side. The string is run through an eyelet, then tied off onto the pivot for that big cylinder. Said pivot is set up to run in a slot along the frame. So I think this is a belly climber that's using air and a PTO. That's my read on the arrangement of things anyways. I'm not entirely sure how they get onto the first rung and ratchet up, however.

Sir, you are correct. As for the first rung, we will let the pistons assume a straight up 90% angle first, the cylinders will extend, just enough to hook on to the first rung, and we will fire the cylinders, pull the robot half way up. We also have a kickstand mechanism in the middle of the robot, which will make our robot parallel with the 68 degrees angle. And at this point, the wrench will start pulling and that would get our front wheels past the first rung, engaging our passive hook.

[Whateveryousay]

Originally Posted by AllenGregoryIV

I'm a little confused by the omni wheels as well. Are they not powered? It looks like the power take off/Super shifter doesn't have a direct drive output to those wheels.

I'm interested in seeing this climbing mechanism, not really sure how the string works.

Indeed they are not powered, the string does work like a belt, sliding the end of the two 10'' pistons along the side of the robot

[Whateveryousay]

Originally Posted by sdcantrell56

Solid frame! Are those center omni wheels just place holders or actually what you will be running?

We are using those omni wheels for the competition.

[Toa Circuit]

Originally Posted by Whateveryousay

We are using those omni wheels for the competition.

...Why? It really doesn't help you out any.

[John Sabath]

Originally Posted by Toa Circuit

...Why? It really doesn't help you out any.

I've seen similar configurations before, so I'm assuming this one is no different.

The omni-wheels in the center are slightly lower than the traction wheels in the front and the back. The bot doesn't use all 6 wheels simultaneously. Personally, I don't see the advantage over a basic 2 omni 2 traction drive, because they'll both end up handling the same way.

[Nuttyman54]

Originally Posted by John Sabath

I've seen similar configurations before, so I'm assuming this one is no different.

The omni-wheels in the center are slightly lower than the traction wheels in the front and the back. The bot doesn't use all 6 wheels simultaneously. Personally, I don't see the advantage over a basic 2 omni 2 traction drive, because they'll both end up handling the same way.

The configuration you're describing is typically done with all traction wheels, colloquially known as dropped-center 6wd. The purpose of lowering the middle wheel is so you can use all traction wheels while still being able to turn.

The issue with putting all the wheels in the same plane is that it becomes very difficult to turn, because you are trying to drag the corner traction wheels sideways along the ground while you turn. The traditional method of doing 6wd with omnis is to put the omni-wheels in the corners, so that they roll side to side when turning.

Putting the omni-wheels in the center only hurts you. If you don't drop the center wheels, they would never scrub sideways so you get no benefit, and only lose traction. If the center pair of omnis is dropped, the turning center of the robot will change drastically depending on which set of traction wheels are in contact with the ground at the time. This would probably be very hard to control.

I would strongly suggest that 1683 test this on their carpet at the full robot weight to see how it performs. If the center omni is not dropped, I suspect you will find it very hard, if not impossible, to turn. If the center omni is dropped, I suspect it will drive pretty erratically.

[Shane 2429]

the 6 wheel drop drive is also commonly refereed to as west coast drive or wcd

[JohnSchneider]

Originally Posted by Shane 2429

the 6 wheel drop drive is also commonly refereed to as west coast drive or wcd

Being a 6wheel-drop-drive and WCD are two different things (but not mutually exclusive). You might read one of the 800 threads on it to understand the differences, but there are differences between a standard 6WD and WCD(besides the fact WCD can have 8 wheels...)

[sgreco]

Originally Posted by Shane 2429

the 6 wheel drop drive is also commonly refereed to as west coast drive or wcd

6 wheel drive dropped center is exactly what it says, 6 wheels, drop the middle on each side.

A 6 wheel drive can be a West Coast Drive, but WCD is more specific; it is typically classified by live axles, cantilevered wheels, dropped center on a 6 wheel (or 2 center wheels dropped on an 8). Of course there is no strict definition, but if you look at a 254 bot, you'll be able to see a little more clearly.

Here's pic, it's 968's practice chassis from what feels like eons ago http://www.chiefdelphi.com/media/photos/31597


Now to the topic of the thread. The theory of a dropped center 6 wheel drive is that only 4 wheels touch the ground at a time. That should decrease the frictional drag, since you're effectively cutting the wheel base in half on a given turn. Shorter wheel base means shorter lever arm for the frictional force opposing your slip turn, and thus an easier turn. The problem with an omni wheel in the middle is that you're omni wheels are linearly aligned with your axis of rotation, so a push to the middle could cause you're robot to lose control. Omni wheel are made to decrease/(ideally all but eliminate) kinetic friction of the sliding wheel. If in fact your center of rotation is about your two center wheel, there shouldn't (ideally) be any kinetic friction in the sideways direction, so your ability to turn will be the same omni wheel or with a traction wheel. The problem is, now with omni wheel, when you're being pushed, you want that friction for stability, but you no longer have it. I endorse switching the omni wheel out for a traction wheel.

[Whateveryousay]

Originally Posted by Nuttyman54

The configuration you're describing is typically done with all traction wheels, colloquially known as dropped-center 6wd. The purpose of lowering the middle wheel is so you can use all traction wheels while still being able to turn.

The issue with putting all the wheels in the same plane is that it becomes very difficult to turn, because you are trying to drag the corner traction wheels sideways along the ground while you turn. The traditional method of doing 6wd with omnis is to put the omni-wheels in the corners, so that they roll side to side when turning.

Putting the omni-wheels in the center only hurts you. If you don't drop the center wheels, they would never scrub sideways so you get no benefit, and only lose traction. If the center pair of omnis is dropped, the turning center of the robot will change drastically depending on which set of traction wheels are in contact with the ground at the time. This would probably be very hard to control.

I would strongly suggest that 1683 test this on their carpet at the full robot weight to see how it performs. If the center omni is not dropped, I suspect you will find it very hard, if not impossible, to turn. If the center omni is dropped, I suspect it will drive pretty erratically.

Thank you, we will do the testing really soon. The center wheels are dropped by 1/8'', last year we had trouble turning with 6 wheels all pneumatics and center dropped. We only solved the problem when we center-dropped more than 1/4'', so this year we are going to try this configuration.

[Nuttyman54]

Originally Posted by Whateveryousay

Thank you, we will do the testing really soon. The center wheels are dropped by 1/8'', last year we had trouble turning with 6 wheels all pneumatics and center dropped. We only solved the problem when we center-dropped more than 1/4'', so this year we are going to try this configuration.

Pneumatics wheels can be very tricky to get to work in a dropped center configuration (just ask 233 about their 2006 robot!) because they squish. This is good for traction, but it can effectively eliminate the "drop" that enables you to turn. Your issue wasn't that all-traction wheels didn't work, but that your center wheel squished down and took away an 1/8" of drop.

You're on the right track to solving the issue by placing a non-pneumatic wheel in the center so it won't squish down, but you don't need an omni there. I think you'll find much better performance if you swap it out for a treaded traction wheel, like the Plaction or Performance wheels from AndyMark or a Traction Wheel from VexPro.

They won't squish down, so you'll still get all the maneuverability of the drivetrain you have with the omni, but they'll have better traction and your robot won't try to turn about the outer edges of the frame.

Hope this helps you guys some, best of luck this year!