View Full Version : What is the difference between a 6 wheel swerve and a 4 wheel swerve?
Peyton Yeung
08-08-2011, 13:23
I have been reading some post on CD about 4 and 6 wheel swerve drives and I was wondering if anyone could tell me the pros/cons of both, or even just the difference.
Dillon Carey
08-08-2011, 20:18
well...one has 4 wheels and the other has 6.
The main reason for 6 wheels is to gain the ability to operate in a similar fashion to a 6 wheel tank drive, while still being able to swerve.
4 wheel is lighter and easier to build and less complicated but 6 wheel enables you to have tank drive when your wheels are orientated in the straight position. 1625 used 6 wheel swerve in 2010 and they really used the ability to tank well and it was a big factor in them making it all the way to einstein.
PAR_WIG1350
08-08-2011, 22:31
The least flexible form of swerve has all wheels linked for both steering and drive either physically or in code. This is often referred to as crab drive.
The next level adds to this by separating the drive control to drive the left and right sides separately while steering the front and back wheels separately.
Both of these are often hybridized by adding skid steering to the maneuvers available to the drivers which allows for pivoting. However if swerve drive is pushed to its full potential this becomes unnecessary. By steering and driving each module independently the robot becomes physically capable of rotation about any given point with the robot at any given angle as well as linear translation at any given angle. The only limiting factor with such a layout is in the software used to coordinate the modules.
In other words, swerve drive is an incredibly diverse family of drive trains, but the number of wheels used has less of an effect on the characteristics of the drive train than how they are linked together does.
AdamHeard
09-08-2011, 01:16
well...one has 4 wheels and the other has 6.
The main reason for 6 wheels is to gain the ability to operate in a similar fashion to a 6 wheel tank drive, while still being able to swerve.
I keep seeing this posted, but I don't understand how that is an advantage.
What is a 6wd doing that a swerve can't (aside from possibly climb easier, which wasn't the reason posted in the document...)?
Nick Lawrence
09-08-2011, 02:42
I keep seeing this posted, but I don't understand how that is an advantage.
What is a 6wd doing that a swerve can't (aside from possibly climb easier, which wasn't the reason posted in the document...)?
If the wheels are not steered independently, (which I don't remember them being on 1625's 2010 robot) then it allows the robot to rotate about its center much easier, with far less turning scrub than a 4WD crab setup. It also gets you to Einstein and turns lots of heads and costs a small fortune to make.
-Nick
Andrew Schreiber
09-08-2011, 09:55
One is silly and nonsensical and the other is something 1625 builds. ;)
Adam, the 6wsd turns easier than a 4wsd is the same reason that a 6wd turns better than a 4wd. However, all of this goes out the window when you turn your wheels so they are perpendicular to the long side of your chasis. I'm not sure how it would behave then.
Edit: answered my own question. You can't turn while strafing along the short axis of your bot assuming the the wheels are L/R steered and L/R driven. http://i.imgur.com/95CrP.jpg
AdamHeard
09-08-2011, 10:06
If the wheels are not steered independently, (which I don't remember them being on 1625's 2010 robot) then it allows the robot to rotate about its center much easier, with far less turning scrub than a 4WD crab setup. It also gets you to Einstein and turns lots of heads and costs a small fortune to make.
-Nick
Ah, I didn't realize all the wheels were steered together, that makes more sense now.
If the wheels are not steered independently, (which I don't remember them being on 1625's 2010 robot) then it allows the robot to rotate about its center much easier, with far less turning scrub than a 4WD crab setup.
That part makes sense.
But if the wheels are not steered independently AND you have drop-center wheels, of what use is the steering? (Unless the center wheels are omni or low traction?? In which case, what's the point?)
That part makes sense.
But if the wheels are not steered independently AND you have drop-center wheels, of what use is the steering? (Unless the center wheels are omni or low traction?? In which case, what's the point?)
All six wheels are steered. (that might not be the answer to your question, it confused me)
Dillon Carey
09-08-2011, 16:56
But if the wheels are not steered independently AND you have drop-center wheels, of what use is the steering? (Unless the center wheels are omni or low traction?? In which case, what's the point?)
All of the wheels are steered together. The center wheel is dropped. All of the wheels are traction wheels.
I believe the part your missing is that the left and right sides are powered seperatley. This allows for operation similar to a normal 6WD drop center tank drive when all of the modules are pointed straight. However this still allows us to operate like a swerve with all modules steered and powered together.
Hope that helps.
All of the wheels are steered together. The center wheel is dropped. All of the wheels are traction wheels.
OK, got it. Lemiant (http://www.chiefdelphi.com/forums/showpost.php?p=1072574&postcount=10) beat you to it by about an hour :-)
I believe the part you're missing is that the left and right sides are powered separately.
No, that part was pretty obvious :-)
AdamHeard
09-08-2011, 17:15
All of the wheels are steered together. The center wheel is dropped. All of the wheels are traction wheels.
I believe the part your missing is that the left and right sides are powered seperatley. This allows for operation similar to a normal 6WD drop center tank drive when all of the modules are pointed straight. However this still allows us to operate like a swerve with all modules steered and powered together.
Hope that helps.
A traditional 4wd swerve chained in the same fashion turns in the wide orientation, you just did the 6wd to shift the turning orientation to long? Interesting solution to the problem. Did you evaluate the cost/resources of doing that versus splitting up the steering/driving more?
EDIT: No matter what power arrangement, the robot won't easily turn in place in the long orientation. Was the time it takes for the wheels to rotate too long for your liking?
PAR_WIG1350
09-08-2011, 17:24
No matter what power arrangement, the robot won't easily turn in place in the long orientation. Was the time it takes for the wheels to rotate too long for your liking?
if the drive power is split left/right (relative to the long orientation) then turning in the wide orientation is impossible. The reasoning behind such a configuration would be heavily dependent on the other mechanisms on the robot.
Aren_Hill
09-08-2011, 17:31
It was kinda a mechanical solution to a problem that could've been solved by software and different power arrangement for modules. We were better suited at the time to take on the additional mechanical difficulty vs the extra programming development
Dillon Carey
09-08-2011, 18:41
No matter what power arrangement, the robot won't easily turn in place in the long orientation. Was the time it takes for the wheels to rotate too long for your liking?
I would have like the modules to have turned a little bit faster, but they we had to gear them down a little bit from the original design due to drag. If we had chosen to use colsons instead of roughtop, I believe we could have reached a rate with which I (the driver) would have been content with. Although I can't complain to much, as that was still the fastest we had ever geared the turning on of the modules.
As far as the not being able to turn along the long orientation. Do you mean turning like a wide ways bot, or a normal "skinny" bot?
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