pic: Concept 8wd Drivetrain

[B.Johnston]

Originally Posted by JVN

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.

1. No wheels are dropped, all 4 are in-line.
2. The 3 of the wheels are dropped so only one pair is raised (front or back)
3. The middle 2 wheels are dropped, and the robot drives on these most of the time.
4. For fun... let's also talk about a "standard" 6WD with middle wheel drop.
5. aaaaand... we'll show a 6WD with NO wheel drop.

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.

Not to stir the waters here, but, there are a few benefits to having a few more parts and or a little more weight via an 8 wheel drive that havent been discussed yet.

The 2008 1680 design was 8 wheel drive with a 4 in line (no drop) profile.

Each side comprised 2 separate modules of:
1 - 2.5inch cim motors with a 3.57 reduction (14:50 gearing)
1 - 6inch "First Wheel" direct driven in the middle front or rear position and
1 - 6inch AM Omni Wheel at the outside corners driven by 25 chain from the directly connected center wheels.

This gave us.

1. A long support polygon.
2. Short overhangs for obstacles.
3. No wheels dropped, all 4 in-line traction.
4. A short effective wheel base for tank pivot turns.
5. A moderate increase in resisting being spun vs the (6 Omni-Wheel-Omni combo.

And new to this disscussion ... Fault Tolerance.

Each side could maintain some motive force with the following failures

1. 1 chain = 75% wheels driving (2 Wheels - 1 Omni).
2. 2 chains = 50% wheels driving (2 Wheels).
3. 1 Transmission = 50% wheels driving (1 Wheel - 1 Omni).
4. 2 Transmissions = 0% wheels driving (Okay Dead in the water here).

Granted Multi speed transmissions may have some mechanical advantage here.
But there's no reason you couldn't have different mechanical ratio's and electronically gear the drive pairs.

Imagine...

the rear module geared down for pushing,

the front module geared for speed,

under load (pushing/acceleration)the rear pair could receive max current and the front would operate at a reduced level...
until the rear has reached its "slip" limit...
the current drive would then be redirected to the front (still at a loaded condition)...
this would pull the now unloaded rears to an rpm above that which they have torque at.
Presto.. two separate velocity profiles, no gear changes.

Something to consider.

PS 3.57:1 as a final drive ratio was a little fast last year (34ft/sec)