Crab Drive
 

After our rookie season, we decided to take on building a crab drive. It has taken us a while since none of us knew inventor, it is also fairly primitive. The sprockets used to connect the wheel modules are missing, (they refused to go parallel with each other...) and there is no chain or screws yet. We would like some feedback on our design. Pictures are posted on http://s296.photobucket.com/albums/mm173/rshanor/
Thanks :D

Re: Crab Drive
 

Regarding the sprockets...

Constrain them using the "flush" constraint. That should get the connecting sprockets coplanar/parallel to each other. Then mate them to the wheel assemblies.

So far, it looks good; I'm a little puzzled why there is a ring around the wheel. Is it to provide a rotational platform?

Re: Crab Drive
 

Cool, looks a bit like wildstang's crab design.

How much does it weigh as shown?

Will it be welded, or will it use the angle brackets as shown? If you use the brackets, its in general a good idea to have at least two bolts mounting it to each surface, so that they cannot rotate. It looks like there are several places with only one bolt hole.

Re: Crab Drive
 

Crab Drive is a little iffy to me. Other then high maneuverability there aren't many advantages IMO. I mean if you lose one motor or chain it's game over for the match. Also it's a lot of work to build. I find that keeping a chassis simple is better and often lighter.

Re: Crab Drive
 

That ring will be used to mount the module to the lazy susan. As of welding, we hope to if we can find a location to weld at. We have not yet calculated weight yet, we are going to cut weight anter we build our 1st design.

Re: Crab Drive
 

That is very impressive. Especially from a rookie. You guys are definitely one of the up and coming young FIRST teams.

What sensors are you going to use to get the wheels to re-center? Or are you going to rely on driver skill?

Re: Crab Drive
 

It looks like a really solid start for a crab drive. You have the overall concepts down, but there is a decent amount of detail work that can be improved.

Even with the bottom support for the module, I wouldn't use such a small shaft for the module rotation: It could potentially be taking a lot of load. Also, what is your method for attaching the shaft to the module?

I would definitely get it welded. It will be stronger than those L brackets by far, and won't loosen like the bolts will (You'd have to be rechecking the bolts constantly to be safe).

There doesn't seem to be a tensioner for the steering chain loop, what do you plan on doing?

I would try to make the bottom row of outer extrusion extend all the way into the corners for maximum strength. I see how it may interfere with that ring some, but you'll have to work that out. Since you are taking a lot of the module's load at the bottom with that turntable, the structure really needs to be strong there. I can see how the plate would strengthen it, but I would have the extrusion go the whole way just to be sure.


Don't let my comments get you down, it is definitely a promising start.

Good luck, and keep it up until you get a truly great design.

Re: Crab Drive
 

You can do this fairly easily in inventor, by setting the material through iProperties, and then viewing the physical properties of the completed assembly

I suggest looking at wildstang's 2007 design. It is of the same basic design, but you can probably pick up a few ideas from it.

Re: Crab Drive
 

We didnt think we needed a tensioner. We can tension by hand and there should be about 180 degrees of contact i think...

Re: Crab Drive
 

We were going to mount a POT on the verticle shaft. Still need to work on a mount though...

Re: Crab Drive
 

Depends on exactly how it's set up. It is quite possible to have a motor go out and still be able to drive (e.g. 118 or 111). It is a lot of work to build, yes, but if they build one post-season, then it'll be a little easier.

There are easier ways to get maneuverability, but you can lose lots of power. Any drive system (or any system for that matter) that is designed during the offseason has an element of risk; the rules or game design may force a different setup.

And, in this case, there are definite advantages, if not to applying it for the 2009 game, then simply to doing it. For one thing, they're learning CAD. For another, they're thinking about new tricks to try in their second year. If they do build it, then it'll be a good experience and a pretty good freshman introduction.

Re: Crab Drive
 

Thanks for the support. :D

Re: Crab Drive
 

True, I'm so use to doing things the same way every year when it comes to drive trains. It is good for learning how to build and for using CAD. I was just talking about the drive it self.

Re: Crab Drive
 

I'm not sure what you mean by tension by hand. Do you mean try to stretch the train onto the sprockets? that won't work.

you could do *exact* center to center distances, but that would be difficult with all the different parts, and once the chain stretches it wouldn't work.

Really, you should be a tensioner in there, and position it so you get more wrap on the globe motor. Also, the globe motor output shaft should be supported.

As for the replies to the thread, he's designing crab drive, he already decided that. Help him with that, or don't reply.

Re: Crab Drive
 

its a good idea on any chain run of this distance, regardless of wrap. Chains stretch considerably, and do crazy things under power. If you are planning on having seperate chains running to each module, and mounting space is a concern, then you can use this design, with a sprocket floating within the chain

http://www.youtube.com/watch?v=6fLf71xlVhE

If it is just one piece, then you will probably need a idler to get enough wrap around the globe anyways.

Re: Crab Drive
 

Awesome job! Even if you never build this, you'll have learned a lot!

My Opinion on Crab:

Crab will only offer an advantage when you must maintain a specific angle of orientation with the game piece or goal. Crab will NOT offer any major advantage when the game pieces can be picked up from any angle, and/or the goal can be accessed from any angle. A good example of this is 2008. The balls were round... It didn't matter what angle you approached them from (same for the overpass). There was no need to go sideways, you could just turn and approach them from a different angle. As a result robots that used crab were almost exactly as successful as robots that didn't. Of course this is all just my own opinion, take it or leave it.

On another note...

I was recently researching a battlebot when I found something very cool which yall might be interested in seeing. If I didn't know better I'd say they built this for FIRST. (click 'robots,' and then 'Mechanical Maniac'). Its certainly a very innovative Crab design - uses no gears and looks like it might be easy to build.

Check it out! They are using some neat bearings you might want to incorporate into your swerve design.

Re: Crab Drive
 

We were able to tension by hand two years ago--we had #25 chain running on two sides of our robot in two different loops to turn the wheels, and we used turnbuckles as our "master-links" in order to tension it by hand. Easy as pie.

It seemed to work okay, although after a competition or two the turnbuckle would break because it was some cheap thing from a local hardware store. If you got a bigger/nicer turnbuckle, that could work. But you should be sure that you have enough chain that the turnbuckles don't go anywhere near the sprockets!

Just an option.

Nikhil

Re: Crab Drive
 

Maybe next design, but that is an awsome drive system. I think you have to use gears just to gear down the CIM enough. We were shooting for 12 fps, a big reduction for just 1 sprocket.

Re: Crab Drive
 

Yeah, that guy makes amazing battlebots. Greg actually got his big one destroyed by one of his robots. :P

Re: Crab Drive
 

The main feedback I wanted to give you is NOT to do a crab drive until you've successfully prototyped one during the off-season. There is a lot of development involved and you don't want to be learning how to make a crab drive behave when you're in the middle of robot design for a new game. This is a lot for a team to handle, especially a team as young as yours.

Don't bite off more than you can chew, especially for minimal gain. The best way to be successful is to simplify!

Good Luck,
John

PS - I lied a little bit; my real advice is to never to a swerve drive at all, they don't provide any huge benefit to your robot design beyond a basic skid steer.

Re: Crab Drive
 

just to give you guys a idea of how far these kids have come. at the close of 2008 Championships.....these kids knew NOTHING about inventor besides...draw it and that i would put it into inventor for em.

Now they have ON THEIR OWN designed a crab drive. :yikes: :D

I originally started this project so the kids could get some GREAT engineering experience. They have self taught Inventor and i am now greatly encouraged for the 2009 season. This comforts me as the mentors have decided to take a step back, and push the kids forward for the 2009 season

thanks for the comments!! keep em rollin!

Re: Crab Drive
 

Speaking of feedback on your design, it doesn't look like you have any! (At least not in the model!) I just wanted to remind you guys that although crab drives are usually tougher to design mechanically, they are also more complicated to control. If you want to control your wheel speeds or count distance during autonomous mode, think about where encoders or other such sensors need to go.

As an example, you will want some sort of potentiometer/encoder that keeps track of current wheel angle. How many chain loops will you have turning the pods, so how many potentiometers or encoders will you need in order to make sure all the wheels are pointed the same direction? Will you use a gear ratio to increase or decrease sensor resolution?

So, as you're doing the mechanical design, think about how you will code/control the crab drive, and what information you will need in order to do that and then put those sensors in the mechanical CAD design to make sure they can get mounted and the wires routed and all that sort of good stuff. Adding sensors to a design that wasn't designed with them in mind can be a terrible headache. You may even want to be picking sensors as part of your design exercise just like picking gears or sprockets.

Re: Crab Drive
 

What would be a good speed for the modules to turn at. We were thinking about 60 RPM's :D

Re: Crab Drive
 

I would suggest doing an expected torque calculation (just ask if you need help) for turning the wheel, and then seeing what kind of speeds that you can get using candidate motors using that. I know it is kind of backwards, but it may be an easier and better approach, because you may say "I want 150 RPM" and the only way you can get that is to use FP motors--which you might not want to dedicate to the drive train. So get an idea of what motors you'd want to dedicate, then run the numbers with them to see how fast you can turn the wheels at appropriate current levels, and then choose the speed. Obviously you will want it at a certain speed or above. From my experience, Globe motors and Window motors are fairly appropriate for this task, but you should calculate it with other motors just to be sure you don't want to use them. I would personally be wary of investing my FP's or Banebots RS-550 into the drivetrain--the rest of the robot usually needs some oomph, too.

Re: Crab Drive
 

I'm not sure I understand this. Do you mean:
a) What speed should the wheels turn to move the robot at;
b) What speed should the modules for the wheels rotate at (to translate);
or c) what should the output speed for the motors be?

If it's b), which is the impression I get, that's way too fast. 60 RPM = 1 rev/sec. That's going to be uncontrollable, or close to it. You tell it to turn, and by the time you tell it to stop turning, you've done a 180-degree turn.

If it's a) or c), you're too slow by a long shot. With a), 60 RPM would get you (assuming 6" wheels):
6" * Pi = circumference "
Circumference " * 60 RPM = x" / minute
x" / minute / 60 sec/min = y"/second
y"/second / 12 = z fps

If I did my math right, that's about 1.5 fps. That's not fast enough.

Now, if it's motor output, then you need a faster motor, or one that can take being geared up. Otherwise, you'll be even slower than the above calculations indicate.

Re: Crab Drive
 

Yes, I was talking about B. I was thinking that we would have preset controls to go to 0, 90, 180, and 270 degrees. We would turn at these speeds for the preset commands. We would do some testing and would have it programmed to turn at slower speeds when we turn the modules manually. Would this work?
I will try to calculate the torque needed, if i can find the coefficient of friction for the wheels we are going to use.
As for the pots, we have discussed sensors but have not planned mounts yet. I was thinking for mounting purposes that we should attach it to the shaft of the globe motor. I am kinda woried though about the wheels shifting into slightly different angles. Should this be a concern. As for other sensors, for the test system we are going to throw on as many sensors as we see a possible use for, for programming practice and to see what works well and doesnt. :D

Re: Crab Drive
 

I would say that it would work, or a function that "steps" the modules x degrees left when a given command is sent and x degrees right on a different command could also help. Definitely slow it if you're rotating them manually.

For a pot or encoder, you would want it as close to the rotating part as you can get it reasonably for best accuracy. One per wheel would be good, though you could probably get away with one for the system.

Re: Crab Drive
 

umm about that... Last year our module turning rate was set at 60rpm, worked out great, quite responsive, this year it was 56rpm.
So its not "way too fast" it actually works quite nicely

plus ma3 absolute analog encoders from USdigital work very well for the steering control.

Re: Crab Drive
 

So you're telling me that you can control it when it'll make one complete turn in one second. OK. Automatic or manual? Was it always at 60 RPM or was it often slower? More info would probably be a good thing in this thread.

Re: Crab Drive
 

all 4 modules were turned by 1fp on both of our swerves. This year was geared at 60rpm free speed. We have a control loop running in which i point the joystick whichever direction i want the wheels to point in relation to the robot. so forward=forward and 360deg all around. It involved some nifty programming.

if you steer 148's style then i wouldn't go at 60rpm

Re: Crab Drive
 

Oh, 60 RPM free speed! That makes a difference-- I'd say you turned at less than that under load, right? And with two sides/ends, it's a bit different than with, say, the V6 system. (For example, it's possible to drive it like a skid-steer...)

Re: Crab Drive
 

Here's my two bits:

2008 was 68's first year experimenting with crab drive. I'd say we were very successful. In fact, I'm almost entirely sure we had zero downtime. That's really good for an experimental system!

The mechanical side: (keep in mind I'm the programmer, so I don't know a huge amount about the mechanical side, but I'll try) Don't think you necessarily need to use chain. We used cable. It went around a spool in the middle of the two wheels, and then out to each of the two front or back wheels. (the two front were tied together, and the two back were tied together) We mechanically limited our drive to +/-90 degrees so that if there was anything wrong with software, nothing got destroyed.

The programming side: We used straight-up potentiometers, gearing them so we got almost the full travel of the pot per full travel of the mechanism. I broke the control software into two pieces. The first piece read the pot values, scaled them to degrees, and drove the PWM outputs to keep the mechanisms at the stored degree values. The second piece read the joystick and set the degree values for the first piece to use. This setup was really good because it was easy to write auton--stop executing the piece that reads the joysticks, and set the degree and velocity values for the first piece to use.

Let me know if you'd like to see some code--I am willing to release fragments of the crab modules I used this year.

As a side note, as a programmer having an interface to the robot through serial was HUGE. I could poll the values of all the pots to figure out why software was pegging the mechanism at +90. I could re-calibrate easily without re-compiling code. I am also willing to release pieces of that code.

I can also get pictures of our setup.

Ahh...crab drive. This was a good year--that was really fun to program! :) I'm not a nerd or anything...when pigs fly...

JBot

Re: Crab Drive
 

the modules could rotate forever, so that should not be an issue. As for programming, our team needs a bit of help, we have a mentor who did it all last year, so we might take you up on that. Thanks a lot. :D

Re: Crab Drive
 

*hits self over forehead* Belts! Same deal as cable, with the sure grip of chain, or close to it. Either way, keep them tight or you'll get some slipping modules.

Re: Crab Drive
 

Unless you are going to be using slip rings, your modules will be limited by the wires to the CIMs. They can only move and twist so far. Therefore, most crab designs only allow for 180-360 degrees of motion.

here's a good example of what I think he was describing with a cable system.

Re: Crab Drive
 

Which would you recommend?:D

Re: Crab Drive
 

Actually, our earlier prototype used belts. They were screwed to the modules in one place per pod, so they did not slip. We only ditched them because the design we used took up more space than cable did. Honestly, I think belts would've been a workable solution this year.

I'm not so sure you want to do that. I've seen cases in the shop where a pot has come unplugged or some other disaster has happened in which the mechanism got pinned to one of the limits. Are you absolutely certain your device can withstand two minutes of spinning full circles? That's worst-case, of course, but it is something that needs to be thought of. We had limit switches planned, but alas, time constraints kept me from implementing them...luckily we didn't need them.

That looks to be the same basic design I was describing.

I will try to scrape together a good list of things to check out and a basic programming guide for crab. It may not happen today or tomorrow, though, as I have my open house to prepare for.

JBot

Re: Crab Drive
 

Anytime is good, I will be out of town for 2 weeks. You could send it to davidallen@westminster.net whenever you get around to it. I really appreciate it. :D

Re: Crab Drive
 

I'd say use a timing belt, properly tensioned; some teams have used polypropylene belts (I think) in their drivetrains. A quick CD-Media search could help.

For a cable, you would probably need a crimper. JBotAlan, am I right about that? If so, that's more money to spend...

Re: Crab Drive
 

I don't think we used any kind of crimper, though I would have to ask the mechanical guys. I'm pretty sure it was just screwed down, possibly under a washer? I'll see if I can get you more info...sorry for the lack thereof.

JBot

Re: Crab Drive
 

What about the plastic belting like that used for car windows? I've seen it used on 71's bot for swerve steering and 27 used it to elevate thier shooter this year. I've been told several times that it is available from McMaster-Carr, but my memory doesn't seem to be long enough to remember what it is called.

Re: Crab Drive
 

Window tape is what that is called, I think.

Re: Crab Drive
 

well rick all i can say is wow we (your mentors) are really proud of you for taking a bit of independence in this thin.
I would look into belt also but as you know we have had issues in the past with belt.
something i discussed earlier with the thread makes was weight. does anyone think this looks at all bulky? I have never done crab before so that is why i am asking.

Also yes, we are building this crab over the summer so that we can get a feel for it in the event that we wish to pursue it in the next build season. And this crab may also serve the purpose of helping the newer members get involved with building as well as learning about the different systems involved in FIRST, and yes of course, the main thing, outreach to get others in the community excited about FIRST.

Re: Crab Drive
 

Here is a picture of our first module, which we have (finally) gotten to build. Everything works well so far. We have ordered parts for the other 4 modules, and we already have those plates cut. More pictures are at http://s296.photobucket.com/albums/m..._tab_my_albums
It is now about 7 lbs, we are hoping we can take off at least 1.5 pounds for the other modules. :D

Re: Crab Drive
 

I am by no means a gearbox expert, but those gears look quite thin compared to many other gearboxes and Crab drives I have seen.

Any particular reason for that, other than weight? Sure they will work?

Re: Crab Drive
 

They are skinny for weight reasons. We have done some stress tests and they have worked so far. If we need to we will change them out when we make this for the season. As long as they are alligned well, we shouldnt need them.

Re: Crab Drive
 

Take my advice, the last stage loks like it is about >125" which is much too thin. You may find that it works for a while but it will wear out prematurely due to very high gear tooth stress on that last stage.

Also, the smaller pinion gear should be wider than the larger mating gear to ensure the thinner teeth are always fully in contact.