I’m not sure, but I think in a full swerve, all modules can rotate individually (you get the magic “rotate and and translate at the same time” motion). In a crab, modules are linked rotationally (less motors?).
Nope, that’s not the difference. 118’s coaxial swerve drives have had every module linked rotationally in the past.
I think it has more to do with the way the rotation happens and the various linkage schemes. There are quite a few of those, including all, front/back, and left/right.
Technically 118’s '07 and '08 bases were crab drive, because they had no ability to rotate. The steering of all the wheels was linked together with one chain, driven together. They also powered all the wheels together, and were able to do this because their manipulated was turreted. 148’s '08 drive was also crab for the same reasons.
from my perspective (3 competition swerves (one with too many wheels), and a bunch of prototypes)
advantages:
Elusive to defenders (the biggest pro IMO)
Many defensive robots aim to lock up opponents with a side tbone, or simply playing moving wall, its much harder for them to accomplish both of these if you have a well driven swerve and are unpredictable.
- great in tight quarters (in theory)
- grace and fluidity are possible (though rarely accomplished IMO, watch any 16 matches for an example)
- great creative outlet for a bored designer
Disadvantages:
- control: Getting that direct mental connection from operators mind to robot movements is quite difficult, and takes alot of development and practice time
- labor intensive in most cases (COTS ones available)
- expensive
- typically take up much of the base (I’ve found ways around this)
- require many motors (I don’t think i’ll ever do a non fully independant one again meaning at least 8 motors on drive)
- Number of moving parts/failure points is fairly high
You mean the ones that lost a few teeth each regional?
Ether,
Crab mode robots have the potential for changing robot orientation with respect to the field with a simple descending foot that lifts two wheels off the floor and allows the drivers to move one side of the robot (Wildstang 2003). Another method is to jump to a software routine that allows the robot to drive in “tank” mode to change orientation and jump back to crab mode when the driver is satisfied with the orientation. In rare cases designers have allowed two steering motors (connected to opposite sides of the robot drives) to change the turning radius of the robot (Wildstang 2007). This allows changing the orientation over a greater distance but still accomplishes a design with the advantages of crab for other parts of the game.
Personally, I find the terms “crab” and “swerve” interchangeable. If I was asked to make a distinction I would be inclined to call drives where the wheels rotate 180 degrees or less “swerve”. Drives that turn up to 360 degrees “crab” and drives that turn greater than 360 degrees “continuous crab”. Drives that are any of these where one set of wheels turns independently of the others can use the modifier “steerable” e.g. “steerable crab”. As Joe has pointed out above (possibly the first to use a crab drive in First) each system has distinct advantages and disadvantages.
How you use the drive or implement the design can have a huge impact on the drivers and how the game is played. Crab will not win the game all by itself. If it takes your drivers a lot of time thinking where to go and how to get there, you will lose precious seconds you could be scoring.
That’s unfortunate. It makes the words redundant and far less useful.
To many vets here on CD, crab is a subset of swerve, wherein all four wheels are steered in the same direction, whether by mechanical linkage or by software design. That is a very useful distinction and one worth supporting.
http://www.chiefdelphi.com/forums/attachment.php?attachmentid=11113&stc=1&d=1321972474
http://www.chiefdelphi.com/forums/attachment.php?attachmentid=11111&d=1321936036
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As a vet here on CD, I respectfully disagree.
Let’s not get too crabby, guys.
I’ve always felt that “crab” holds the connotation that the robot is primarily meant to translate without rotating, much like a crab’s motion. While there may be ways to make a crab drive rotate, it doesn’t change the fact that the wheels almost always all point in the same direction and are meant to be used that way. (In this case, crab drive can also describe a control system.)
Swerve (and I’ve also heard vector drive) is the broader definition where the wheels actually can have different orientations relative to each other. That allows a wider variety of motions that a crab drive might struggle to achieve.
Honestly, though, it’s just finicky semantics. We could also call them all “powered office chair caster wheel drives” but that would be lame. They’re all basically built from the same idea, and the name doesn’t matter as long as you’re specific enough to differentiate the mechanism.
I ignore any difference between the two, and use them interchangeably, as enough variations exist for steering and power distribution.
Swerve/crab to me is just a drive that can pivot all its wheels, and just follow it up with what configuration you’re running for steering and power.
On the one hand, I hate bevel gears (in FIRST applications especially). They can be done well
You should try using a set this upcoming season. We build our Revolution Co-Axial style crab/swerve modules with a bevel set using semi-precision components and they work well in my opinion.
We have much more trouble with the roller chain final drive than we do with bevel gear alignment and spacing. 
Also, crab and swerve are the same thing to me. King Crab means something, but otherwise I don’t adhere to any difference between them.
BTW, thanks to Dr. Joe for bringing us swerve/crab technology in 1998. I was truly blown away and inspired by the design.
Pictures or it didn’t happen 
Now I KNOW you are pulling my leg!!!
Now you’ve got me blushing. To my knowledge the only thing you may be able to argue was earlier was the robot that Tony and Bob’s old Raytheon Team built in 97 which docked with the goal (the tube tree) and spun it. Man, nobody wanted to play against them on Saturday (this was the days of 1 on 1 on 1 and Saturday morning was a double elim tourney). As recall, they defeated Wildstang that year when a branch of the spinning tree knocked them over as it tried to dock with the goal after Raytheon had docked and the dervish had begun. Back to being first, it is a fleeting thing. The technology was advancing such that it was going to happen sooner or later.
While I am posting, I have an idea for a crab chassis morphology that I have never seen in FIRST before. I think it could be 4 ways of awesomeness.
But it is only a twinkling in my eye at this point. If I can get my rookies organized enough that we can prototype it before the season starts, AND if it works like I think it will, AND if the game is sideways motion friendly… …all ya’ll may get to see it at the Boston Regional
Joe J.
Dr. Joe,
I’ve used bevel gears 3 times in competition and have never had a problem with them, in the above pictured setup we only had issues with the utterly ridiculous chain runs.
(^that one hit einstein in 2010)
I’m so excited. It’ll be interesting to see what you have in mind.
So it looks like this:
Crab drive keeps the omni-directional part, but doesn’t change the orientation of the robot.
Swerve drive has omni-directional motion, as well as the ability to change the orientation of the robot.
Well, in all of my years in FRC (Almost 2!!! :D), I’ve never seen a match where robots didn’t need to change their orientation, so therefore, why would one use a Crab drive if it can’t change robot orientation?
Are there any advantages/disadvantages to the loss of the ability to change orientation?
Usually people without the ability to reorient the base turret the entire auxiliary.
Look at FRC118 in 2008 or 2007.
Point taken. I did not think of that. Thanks Andrew!
And thanks to Aren, since i don’t want to leave him out (he answered too!)
I concede. Those chains DO worry me more than those bevel gears! But that doesn’t mean I am not worried about the bevel gears
FYI what pitch are those bad dads and where do you get them/make them?
Joe J.
The ones we’ve used since 2007 for every swerve has been Mcmaster part # 6529K14
(to be honest they were a shot in the dark guess from a naive designer that happened to be a bulls eye)
12DP, Never once have we lost any teeth or damaged one of them past functionality (we also completely remove the hub off the back leaving JUST the teeth, steel is heavy)
I believe they are the same ones that FRC118 uses, and 973 has used them several times.