Swerve Drive

[Al Skierkiewicz]

Originally Posted by Ether

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.

As a vet here on CD, I respectfully disagree.

[Ninja_Bait]

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.

[Aren_Hill]

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.

[ajlapp]

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.

[Joe Johnson]

Originally Posted by ajlapp

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.

Pictures or it didn't happen ;-)

Originally Posted by ajlapp

We have much more trouble with the roller chain final drive than we do with bevel gear alignment and spacing.

Now I KNOW you are pulling my leg!!!

Originally Posted by ajlapp

BTW, thanks to Dr. Joe for bringing us swerve/crab technology in 1998. I was truly blown away and inspired by the design.

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.

[Aren_Hill]

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)

[Peyton Yeung]

Originally Posted by Joe Johnson

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.

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!!! ), 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?

[Aren_Hill]

Originally Posted by SuperNerd256

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!!! ), 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.

[Andrew Schreiber]

Originally Posted by SuperNerd256

Well, in all of my years in FRC (Almost 2!!! ), 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?

Look at FRC118 in 2008 or 2007.

Originally Posted by Andrew Schreiber

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!)

[Joe Johnson]

Originally Posted by Aren_Hill

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.

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.

[Aren_Hill]

Originally Posted by Joe Johnson

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.

[AdamHeard]

Originally Posted by Aren_Hill

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.

1717 does as well. Good stuff.

[JDNovak]

Dr Joe,
For 2006, 07, and 08, we used your Nothing But Dewalt CIM setup with a bevel gear on the drill output shaft. We just trued up the casting designed for the handle and shoved it into a collar that aligned it with the horizontal gear mounted to the wheel like a sprocket. I think it was a 2:1 bevel gear set from Martin and it wasn't cheap. It did work pretty well and never failed. we just set the lash by feel and clamped the drill nose with set screws. It worked well enough to make it to the Einstein finals in 2008.

I just couldn't stand the inefficiency and we moved away from the transmissions and bevel gears the last few years. The motors sure stay a lot cooler these days.

As for crab vs swerve. The code to steer from straight through an arc to rotating around the center of the frame has taken years to perfect. Actually, we had the trig working on the Basic Stamp in 1999 but changed to look up tables through the PIC years. We finally got a better version of real time trig working last year. Crab, on the other hand, takes about 10 minutes to point all the wheels in the same direction and turn all the wheels the same speed.