Long Rambling post... Beware. There
may be some decent content. No promises.
There are a lot of people throwing out pros and cons for an 8WD that don't make a lot of sense to me. In particular, those people talking about robot turning and traction. Let's talk through this quickly. To me, there are several different configurations that should be discussed.
- No wheels are dropped, all 4 are in-line.
- The 3 of the wheels are dropped so only one pair is raised (front or back)
- The middle 2 wheels are dropped, and the robot drives on these most of the time.
- For fun... let's also talk about a "standard" 6WD with middle wheel drop.
- aaaaand... we'll show a 6WD with NO wheel drop.
Since we're only talking about the effects of the 8WD itself, we don't need to discuss any of the "additional" characteristics of the drivetrain. Examples of some of these characteristics which have
NO bearing on the PRO/CON of an 8WD system are: Motor usage, gearbox/transmission, wheel size, drivetrain configuration (widebody vs narrow), robot CG, overall robot weight, etc.
Though these characteristics are all VERY important to the overall drivetrain performance, let us assume "all other things equal" so we can have a good comparison.
First, we will talk about robot turning.
For reference, everyone should immediately go read this whitepaper and learn all the physics behind it:
http://www.chiefdelphi.com/media/papers/1443
Based on this paper, almost everything "cancels out". (If you don't believe me, read the whitepaper again). The main comparison we need to look at, is the length of the support polygon (as support polygon is the polygon you draw between the points where the robot touches the ground at any given time).
If you took the time to do your HW and read the above whitepaper, and have a good understanding of the physics involved you will understand why.
As you can see in my attached image (excuse the crude drawing). we can compare the cases listed above pretty quickly.
Longer Support Polygon = more turning scrub = higher resistance to being spun, worse robot turning.
Shorter Support Polygon = less turning scrub = less resistance to being spun, better robot turning.
"But wait... I want to change something to affect this"
Well, we could change the traction material on some of the wheels to drastically alter the drivetrain characteristics, but
remember we
promised we would do an "all other things equal" comparison". (We promised, and we need to honor that promise.) Because of this "all other things equal" the above comparisons are pretty straightforward.
Next let's talk quickly about overall traction...
I am one of those people who does not believe "more contact patch" = "better traction". Call me old fashioned, but I think we're pretty darn close to a F= mu * N model for robot traction. (More contact patch DOES however result in reduced tread wear, which is nice.) However, if you're one of those crazy people who REALLY likes more grip on the floor, we just need to make one more comparison. "How many wheels are on the floor?" Take a peek at the attachment one more time and make this comparison, go ahead, I'll wait.
More considerations?
These are the straight-forward ones.
- 8WD makes it easier to build a "climbing robot" while still maintaining a low-ground clearance.
- 8WD has more parts which is by definition more complex to build, heavier, and probably less reliable than an "all other things equal" 6WD system.
- Depending on where the CG is, some of these configurations will "rock less" than others. This may be important to you. You can play with your support polygon to further affect this.
JVN's editorial:
Yes, I would use an 8WD, but probably only in a situation where we needed to climb a step or something and I couldn't make a 6WD climb it elegantly. To me, there just isn't any compelling reason to go this direction for a flat field. The 6WD designs I've played with have a reasonable amount of turning scrub, and turn just fine (I don't need a longer or shorter support polygon, I'm happy with the balance I have).
Honestly, I love my 6WD for a robot which requires "max pushing force". If we ever had a game where pushing wasn't required, I would consider doing a 2 Traction + 2 Omni drivetrain or a 2 Omni + 2 Traction + 2 Omni drivetrain. (These configurations would have great handling with max stability and still reasonable pushing force.) Heck... I might even do 6WD anyways. The important thing, is that I didn't do a swerve drive.
Remember to ALWAYS use physics in engineering discussions. There is really no room for "feelings" in this sort of thing. I don't care how you feel about an 8WD, or what you "think" might happen. I only care about your physical justifications for how and why things happen.
For goodness sakes... READ THIS WHITEPAPER:
http://www.chiefdelphi.com/media/papers/1443
If anyone has any questions about this, feel free to ask.
Good Luck!
-John