6WD Pros and Cons

[Taylor]

Has anybody toyed with putting omnis in the center of the robot, and treads at the corners? I would be curious to find out how that setup handles, both coplanar and rocker.
Please only reply if you've used this setup, even if it's prototyping. Thoughts or inferences won't help - I'm looking for anectdotal evidence.

[Andrew Bates]

Sorry if this is the wrong thread for this question but seems like it would fit. My team is wondering if going with a 6wd traction wheels all on the same level would be better than a 4wd traction wheels all on the same level.

[Kevin Sevcik]

Originally Posted by corpralchee

Sorry if this is the wrong thread for this question but seems like it would fit. My team is wondering if going with a 6wd traction wheels all on the same level would be better than a 4wd traction wheels all on the same level.

It would be, but not by a whole lot. Assuming a dead center weight distribution on both bots, you'd still be trying to skid the outside wheels. The only advantage you have is that your center wheels don't need to side-slip (approximately), so they don't add any resistance to your robot turning. Since they support one-third the weight of your robot, your outer wheels now support two-thirds the weight and effectively resist turning a third less than they used to. It's sort of like magically reducing your weight by a third while keeping the turning torque approximately the same. It's probably a lot more like reducing the side-to-side friction of your wheels by a third while keeping the forwards-backwards friction the same.

Point being, those wheels and your robot will still resist turning, they'll just resist it somewhat less.

[AdamHeard]

Originally Posted by corpralchee

Sorry if this is the wrong thread for this question but seems like it would fit. My team is wondering if going with a 6wd traction wheels all on the same level would be better than a 4wd traction wheels all on the same level.

Kevin is right.

It'd be better to go with 6WD, or make two of the four wheels on a 4WD omniwheels. 70/494 had a cool way of doing it in 2006 where the opposite corners were omni's so they weren't as easily rotated by opponents.

[caraddicted101]

I would recommend 6 wheel with the center wheel lowered just a bit. I have found that the amount at which you should lower the center wheel depends on the type of wheels that you are running and the type of tread that you are using. When you lower the center wheel you want to be careful not to lower it too much because the robot may bounce back and fourth when turning hard. I have found that the optimal height to lower the center wheel is between 1/16 to 1/8 of an inch. This Has worked for me, but if you use thicker tread, or tread that has a lot of grip you might want to change it a bit. My suggestion would be, if you have time, make the center wheel adjustable and let your drivers test out variable heights. Make sure to allow for the extra weight if you are only running a test chassis, as the extra 80+ pounds that you put on top of the chassis will affect the way the robot reacts.

--hope this will come in handy--

[Frinkahedron]

Originally Posted by caraddicted101

I would recommend 6 wheel with the center wheel lowered just a bit. I have found that the amount at which you should lower the center wheel depends on the type of wheels that you are running and the type of tread that you are using. When you lower the center wheel you want to be careful not to lower it too much because the robot may bounce back and fourth when turning hard. I have found that the optimal height to lower the center wheel is between 1/16 to 1/8 of an inch. This Has worked for me, but if you use thicker tread, or tread that has a lot of grip you might want to change it a bit. My suggestion would be, if you have time, make the center wheel adjustable and let your drivers test out variable heights. Make sure to allow for the extra weight if you are only running a test chassis, as the extra 80+ pounds that you put on top of the chassis will affect the way the robot reacts.

--hope this will come in handy--

One thing to watch out for, don't try and save weight by using too light of a material on the center axle. Last year, we drove the robot around all day during Space Day at the National Air and Space Museum and the axle and wheel ended up shearing off completely at the end. We replaced them with steel instead of aluminum ones after that.

[Cory]

Originally Posted by Frinkahedron

One thing to watch out for, don't try and save weight by using too light of a material on the center axle. Last year, we drove the robot around all day during Space Day at the National Air and Space Museum and the axle and wheel ended up shearing off completely at the end. We replaced them with steel instead of aluminum ones after that.

What alloy Aluminum were you using?

[lukevanoort]

Originally Posted by Cory

What alloy Aluminum were you using?

And what diameter?

[Frinkahedron]

Originally Posted by lukevanoort

And what diameter?

Sorry for the late reply. I do not remember, as I was a measly programmer/drive team member.

[redbarron]

Go with a 8 wheel drive system

[David]

1. "Rocker Style"
Pros:
Decent Turning (Acts Like 4wd)
Great Traction
Designed Into Kitbot Frame
Good for climbing
Cons:
As the name suggests, if you do not balance it properly, it will rock back and forth constantly.
(I believe Beach Bots 2007 bot are a good example of a well balanced one)

2. Omnis
Pros:
Great Turning (when you want to)
Good traction
Stable (doesn't rock)
Cons:
Easily turned (when opponents want to)
Not as good as "Rocker Style" at climbing

A third type is having 6 narrow traction wheels all touching. This has about the same pros and cons as "rocker style", but it does not rock (Children of the Swamp Designed a nice a year or so ago, and I think Cybersonics use it)

[EricH]

Originally Posted by David

1. "Rocker Style"
Pros:
Decent Turning (Acts Like 4wd)
Great Traction
Designed Into Kitbot Frame
Good for climbing
Cons:
As the name suggests, if you do not balance it properly, it will rock back and forth constantly.
(I believe Beach Bots 2007 bot are a good example of a well balanced one)

2005-2007 all used a similar drive system. Two pneumatic tires as the center wheels; a pair of kit wheels as the front, and either two more kit wheels (2005 and 2007) or another set of pneumatics (2006) in the back.

Actually, it's all about weight placement with the rockers, not the balance. For 2005, we had a lot of the weight even with or behind the center wheels and didn't have a lot of drop. 2006 had a pretty balanced load and you could tell as it rocked a little bit more. 2007 had almost all of the weight in the back. This reduced the rock. Also, because we had pneumatic tires in the middle (I'm not sure where we got them from, sorry), we were able to adjust the rock as desired. Less rock? No problem, just let air out. More rock? Pump them up. We did go through the tires, though. I think I counted three or four sets of center tires that were dead from two robots this year.

As clear a view as you can get of our 2007 drive, aboard the practice robot: http://www.chiefdelphi.com/media/photos/27675
And the 2006, pre-front wheel change: http://www.chiefdelphi.com/media/photos/23376
And both, for comparison (post-wheel change for 2006): http://www.chiefdelphi.com/media/photos/27581

[lukevanoort]

1) Lowered centre
Pros:
Traction - All wheels that are on the ground are powered (unlike 2WD), and you can pretty much use as high traction wheels as you want without turning issues. Also, high traction normal wheels can get a mu of 1.2+, whereas omnis are around 1.0 (assuming AM omnis... I guess with custom one you could get better traction, but your options still are probably not going to be as broad as with normal wheels).

Ramp climbing - It is much harder to bottom out with that wheel in the center than without one, such as in a 4WD. It is also less likely to rotate sideways as it goes up a ramp than a design with omni wheels.

Easy to drive - Since it drives forward on four wheels, but often (depends on how central your weight distribution is) pivots on two, it is not exceedingly difficult to drive in a straight line (if you think this can't be a serious problem, try driving a fast 2WD) yet still turns very easily (In fact, in some of our drivers' early opinions of our 2006 robot, too easily). I have never driven one, but I imagine driving a 6WD with corner omnis takes a light touch (or PID) to avoid driving in unintentional arcs and spinning too fast.

Cons:
It rocks - While this may seem obvious, it can become somewhat problematic at times. For example, when we extend up to score on the top spider our rocking 6WD, combined with our somewhat loose turret, causes our arm to sway a good bit. We can correct pretty easily (the arm has 5 degrees of freedom though, so your mileage may vary), but this can get annoying (especially if this was 2005 with 15+ feet tall arms carring 9 lb tetras instead of 9ish feet tall arms carrying 1lb tubes)

Heavy - Well, this is a characteristic of all 6wds, they will invariably be heavier than if you had made the same system with only four or two wheels.



Something I've always wanted to try is a coplanar 6WD using four traction wheels and two omnis, with the omnis at opposite corners on opposite ends. Kinda like the 2006 494/70 drive system, but with an extra pair of wheels in the middle. I dunno if such a system is necessarily a worthwhile design, but is seems like a good idea to me.

[EricH]

Originally Posted by lukevanoort

1) Lowered centre
Pros:
Traction - All wheels are on the ground, and you can pretty much use as high traction wheels as you want without turning issues. Also, high traction normal wheels can get a mu of 1.2+, whereas omnis are around 1.0 (assuming AM omnis... I guess with custom one you could get better traction, but your options still are probably not going to be as broad as with normal wheels).

Actually, that's not exactly what happens. Really, only four wheels are fully on the ground at any one time. The "extras" just kind of skim along. How much they help is determined by the weight distribution and the drop of the center wheel. In a turn, you might only have two wheels down.