Swerve: Belts or Chain?
 

We've got a very young and energetic team (including a freshman programmer), and a new milling machine...understanding that this process may take several years, we are ready to start experimenting with swerve. Our plan is to make four modules and start playing with the programming and implementation.

A couple questions:
(1) The 15 tooth miter gears I'm finding on most robot supplier websites seem way too small to me. It seems to me, as I start to draw it all out, that a larger driven gear and a smaller pinion would make the geometry better - and accomplish a part of the speed reduction in the process. What experiences have others had here?

(2) I'm leaning toward using belts, but notice that most systems use chain. I definitely have a bias toward belt - I'll admit that, but what are the true pros and cons (other than the obvious, that is)?

Re: Swerve: Belts or Chain?
 

Where have you seen chain driven swerve modules recently? I haven't seen them in pictures/videos for at least a few years now, and every one I've seen in my (relatively short) FIRST career has been either gear or belt driven.

Re: Swerve: Belts or Chain?
 

A true swerve design discussion would be a very long ramble. This is what our team has done if you haven't seen it, we published our design.
http://wiki.team1640.com/index.php?title=Swerve_Central

Re: Swerve: Belts or Chain?
 

My apologies for not being more specific...I am referring to the module itself, attaching the driven bevel or miter gear to the wheel. I do intend to use belts for the motor and steering.

Thanks for the link...I have seen...but have not studied it yet.

Re: Swerve: Belts or Chain?
 

Why not avoid bevel gear altogether? One of the oldest names in the swerve business, team 16, uses slip rings to allow continuous rotation without the use of coaxial shafts. This gives them a huge efficiency advantage because they don't have to deal with the power losses of bevel gears. Ultimately it's up to your team to decide what is most important to you. But if I were to design a swerve right now I would build on all the information the the neutrinos, pwnage, and the bomb squad have released about their swerves recently. In particular the cim as an axle style that the neutrinos pioneered is one of the simplest and most compact ways I've ever seen a swerve done.

Re: Swerve: Belts or Chain?
 

Just to clarify before the discussion gets more in depth, is your team specifically looking into a coaxial system?

This isn't coaxial, but I'll link you some of our info and CAD model if you would like to take a look.

http://www.chiefdelphi.com/media/photos/40299?

-Nick

Re: Swerve: Belts or Chain?
 

Are you talking about something like this?

Re: Swerve: Belts or Chain?
 

Great thread.

I appreciate all the resources folks have posted. I have looked at swerve drives off an on over the years and never had the resources to build one. I think we might have enough interest from the kids to do a swerve as an off season project.

I apologize if the answer to this is in one of the documents (I didn't see it during my quick skim). I have wondered why teams would run one drive motor and one steering motor per module (like http://www.chiefdelphi.com/media/photos/40299? or http://www.team221.com/order.php?cat=3). This seems like a waste of weight to me. The concept shown briefly in http://www.youtube.com/watch?v=CAJBC-DDL9w and commercialized as the Revolution Swerve (
http://www.team221.com/viewproduct.php?id=89) seems like a better idea. The reason it seems like a better idea to me is that you can have a single steering motor set and a single drive gearbox.

Why do teams use 4 total gear reductions instead of one central gearbox and a chain to transmit power to all 4 wheels? With a 72T or 80T 20 DP gear attaching to 4 CIMs you could make one big gearbox in the center, perhaps even add shifting for a second overall gear ratio.

Why do teams use 4 steering motors? This would seems to make it hard to steer straight forward (as mentioned in one paper previously linked). With 1 or 2 motors for steering and a worm gear based gearbox and a chain the wheels would be mechanically synced (minus any stretching of the chain). I realize that you would need two steering chains (i.e. 2 sets of 2 wheels chained together) to be able to rotate the bot around its center. By using less motors and therefore less controllers this would seems to save weight

I realize that belts can also be used for power transmission between wheels, but chain has a much higher overall load rating in tension and is easier to make very, very long.

These answers may be obvious to teams that have done swerve before, but are a mystery to us.

Thanks!

-matto-

Re: Swerve: Belts or Chain?
 

I think what you are referring to is a crab drive.
The biggest difference is that with all 4 wheels being steered together you can no longer have true holonomic motion. With a 4 wheel swerve you can turn all 4 wheels perpendicular to your point of rotation. Causing for 0 scrub when turning with your pivot point being anywhere(large banking turns).
Also using that much chain running around your robot is a nightmare to design around, It also adds up in weight after a while.

The ability to have full holonomic motion is the number one reason our team went with the swerve drive.
We use belts for all of our stagings other then the bevel and the chain in the lower pod. The reason we did not switch the chain out to a belt in the pod was that in the final staging a 9mm belt would not transmit egnuf force with out breaking or wearing.
Here is ours. Our only problem was the encoders slipping and one getting destroyed after we received a catapult shot to the encoder assemble. As always thank you 1640,2471 for cad and good design examples. http://www.chiefdelphi.com/forums/sh...d.php?t=124664 A

Re: Swerve: Belts or Chain?
 

4 gearboxes allows for independent control of each wheel.

To put it bluntly, if your team can't handle putting 4 independent modules in the same direction, your team definitely can't handle the software complexity of a well tuned swerve drive.

Without being able to independently power wheels or turn modules independent of one another, a swerve base has no ability to rotate, only translate.

Assuming a four wheel swerve:
All wheels linked, all steering linked - robot can translate in any direction only, but no rotation.

Pairs of wheels linked, all steering linked - robot can translate in any direction. Robot may be able to rotate the same way a skid steer robot does, depending on which wheels are linked and if turning scrub can be overcome.

Pairs of wheels linked, opposite corner steering linked - This allows for translation in any direction, and rotation without translation, but not rotation with translation.

All wheels independent, all corners steered independent - This configuration allows for any combination of rotation and translation at the same time.

With the wide open motor rules, etc. the 4 wheel independent swerve design has gained a lot of popularity among those whom have mastered the swerve drive. I would not go to a lesser option for the sole reason that you don't think your team can handle independent swerve - if your team can't handle independent swerve, it likely can't handle ANY swerve. Not saying there aren't good reasons to make other choices, though.k

Re: Swerve: Belts or Chain?
 

Depending on the game crab drive can actually be an adequate swerve design. For example 118 used crab drive with a fully independent upper turret for their manipulator in both 2007 and 2008. they had no control of their chassis orientation but it didn't matter because they could turn their manipulator to face whatever way they wanted. Having one giant drive gearbox is a pretty neat advantage. I'm sure a revisit of their "v-6" gearbox with modern motor counts would be beastly. 148 also used crab drive in 2008. Since they had no manipulator (besides a pole to knock the ball down) they didn't need to worry about orientation. Just some thoughts.

Re: Swerve: Belts or Chain?
 

Fun fact, revisiting with current motor allotments would require a bit of caution. Recall that our battery supplies a finite amount of power at any given point in time. I know 125 has both blown our main breaker and browned out our CRIO this season with our 6 motor drive. We've seen current draws dropping as low as 4v out of the battery (before the CRIO shut down since the 24v regulator stops functioning properly at 6).

Re: Swerve: Belts or Chain?
 

We had problems this year when we were geared on the high side for swerve.
When trying to go from a stand still to full power in a different direction we would draw a lot of current and brown out our CAN bus killing us for the match. We traced this to 2-5 year old battery. We got new batterys and implemented a new code feature that would off set the motors from each outher by .005 of a second allowing the magnetic fields to establish in the motors before the next ones would start.
CAN+Swerve...... Be carefully.

Re: Swerve: Belts or Chain?
 

I can see a lot of advantages of this configuration. It would be easier to build and program, it would give a robot the ability to strafe without sacrificing traction, and it would be no worse at pivoting than a standard tank drive (though not as good as a 4-wheel-independant swerve). Do you know of any team that has tried it?

One catch - as the wheels turn 90 degrees to strafe, the wheel pairing pattern would need to change to allow differential steering. You might still need 4 independent drive motors (need to think about this a bit). Even so, it would be simpler to implement the steering.

Re: Swerve: Belts or Chain?
 

This is what I was going to ask about. It seems lighter to make a "crab" (4 mechanically linked wheels with linked steering) with a rotating (turret) manipulator. To me this makes a lot of sense for control as well since the driver and co-driver can move each piece (drive base, manipulator) independently.

For those who did a swerve this year - do you think a "crab" with turret manipulator would have worked well? Seems like it would be great for passing and strafing for a shot (co driver could align to goal while driver is moving away from defense)

Has anyone measured the efficiency of a swerve or crab? There are a lot of interfaces for power transfer!!!

Thanks!

-matto-

Re: Swerve: Belts or Chain?
 

One more thing about crab is that 116 inches of chain adds up to a lot of weight.
If you loss that chain your robot drops dead.
I would never chose to run a crab over a swerve the advantages of the swerve when in my opinion it would be easier to build a 4 wheel independent swerve over a crab with a turret on top. With the 4 wheel swerve you can do so much more.

Programming is harder. but mechanically building 4 identical gear boxes is easier then that plus a rotary turret that can hold weight. Add on that the system you build to turn the wheels/drive if that fails then you loss the drive.. (most of the time a loss drive means a dead robot).

Re: Swerve: Belts or Chain?
 

The 1625 robot in 2010 had 324" (27') of #25 chain on the 6wd swerve...It's not that bad....

Re: Swerve: Belts or Chain?
 

In 2012, my team did crab drive based on the wild swerve module. I was easier to implement, but the lack of steering ability made the extra weight marginally worth it. This year we did a custom independent swerve drive. I took some more development, but we are much happier with the results.:D

Re: Swerve: Belts or Chain?
 

I agree, but how is this different than a tank drive? If you lose one side of the chain you're pretty much stuck (although you can do a funky circle). In my experience the chain typically fails at the connection point (master link) and with a long chain there will be a few more chances for failure because there are a few more master links. If inspected periodically this should not be a major issue. You can also get 50' or 100' lengths of chain at McMaster

Re: Swerve: Belts or Chain?
 

I recall some teams linking opposite corner modules in order to turn. I can't recall which team it was but it allowed you to strafe or rotate but not do both at the same time.

Re: Swerve: Belts or Chain?
 

We have gone to the module form. If any of our pods fail or are suspect of not performing properly, they are immediately swapped out. Our pit crew can swap a module in < 5 minutes. Repair later. 4 bolts and 3 wire connectors and it's out. How fast can a mech, West coast drive or any other drive be repaired? I have watched teams struggle with repairs to other drives and like the repair ability of the swerve Modules. For us the goal of our team is not to win but, to mentor and teach our students to work together and accomplish hard technology tasks. 4 driven wheel, independently steered swerve is a good one to teach this. Yes the kids are pushed hard to do this each year. The pay back in their growth is great. Have to say we started to win more after the switch to swerve. To do swerve forced us to develop processes and a mindset to do it. The same processes have help with other aspects of the team. Swerve is hard but for us the pay back has been great.

Re: Swerve: Belts or Chain?
 

The swapping is a great point. Have you ever had a module fail in a match? How does yours drive with 1 dead module?

I agree that a swerve is a good way to teach many important skills. I have never been willing to take the risk of doing swerve during the season (if it fails then you're really in trouble if you can't drive forward!). What was it like the first year you tried swerve? Did you do a trial run during the off season?

Re: Swerve: Belts or Chain?
 

Love where this conversation has gone. Here's my design thus far.



Before everyone piles on...the prototype will use stuff we have laying around, like the Andymark wheels with the plastic pulleys, and mini cims for steering. It's just the start.

Re: Swerve: Belts or Chain?
 

You could do gears instead of belt or chain.

Check out 973's iteration of the West Coast Swerve, "Emperor Swerve" & "Encore". 1717 uses a very similar and much more refined West Coast Swerve.

Re: Swerve: Belts or Chain?
 

Thinking about it...but afraid. Trying to keep as much of this in the "comfort zone" as possible. The real goal is to get something that works so we can start figuring out how to program it all - that is frightening!

Re: Swerve: Belts or Chain?
 

Teams are always focusing on the mechanical and programming aspect of swerve. You can design and create a perfect swerve and nail the programming.
Guess what, your only half way there. Teams forget the human part of this system. Control presentation and driver training are just as critical. You also have to be prepared to devote as many resources to your drivers.

Re: Swerve: Belts or Chain?
 

In our case, it is not that we are neglecting the importance of driver training...but this project is a sequential endeavor...we can't program until we have something built mechanically, and we can't train until it is built and programmed. Right now we're trying to get something built - everything else comes afterward. Knowing my team's strengths and weaknesses, though, the programming is going to be our most difficult part by far.

Re: Swerve: Belts or Chain?
 

For programming start with Ether's paper on the kinematics of swerve. Several teams have published their code. You have to understand the mechanical and sensors to follow the code. Big choice is incremental with indexing or absolute encoders. Big difference in the steering algorithm. Also, your programmers must understand and embrace the principles of real time-- data flow methods. You will have a good bit of code running just for the swerve. For other mechanisms on the robot the the efficient execution of those blocks of code is critical. Your team should master a fundamental knowledge of state machines. We had problems our first year. Our swerve worked OK at home. Went to the finger lakes regional and had bad FMS lag. Our robot several times did what our team calls the happy dance. Steering goes wild. To me it looked like a grand mal seizure. We did not give up and have been driving forward ever since. Make sure your team is ready for problems and are ready to not hit a home run the first time. Are you willing and ready to make a long term commitment to perfect it? If not then basic tank drive what ever the wheel configuration can take you far.

Re: Swerve: Belts or Chain?
 

I would be extremely cautious about that drive belt ratcheting given the small amount of wrap on the tiny top pulley.

Re: Swerve: Belts or Chain?
 

Also 9 mm pullys may not handle the thrust at that end of the gear redction. Belts like high speed over high tourqe.

Re: Swerve: Belts or Chain?
 

Our swerve design is somewhat similar to this. We use gears, though, and it's been working nicely.

For my team, even with one programmer (myself), the code was quite manageable with the assistance of some papers/presentations.
But I must admit, debugging errors in the code got pretty annoying.

Re: Swerve: Belts or Chain?
 

Thanks everyone for the great responses:

I am a bit worried about this.

That's the kind of information I was looking for. I may make the module a bit wider and use a 15 mm belt here - at least for the prototype.

Gears between the bevel and the wheel is something I had not even considered...I like that idea.

I'm heartened that the programming was manageable. It is my biggest fear.

Re: Swerve: Belts or Chain?
 

Definitely use 15mm rather than 9mm belt, and if possible make the top pulley larger and change the ratio from the CIM to the coaxial drive shaft to account for it. Or, as mentioned, use gears; they're heavier, but have fewer failure modes. Switching to VexPro VersaWheels would make gear-mounting a breeze (in fact, they now even have gears with 9/8'' bearing holes so you can keep it dead-axle).

Re: Swerve: Belts or Chain?
 

We did a project in 2008-2009 and created a paper and did a presentation at the CHP. Some of the technology has changed and improved, but the basics are the same.

We use purchased modules. For our competition robot, we paired the front wheels together and the rear wheels together for rotation. Even with this "simpler" design we worked the entire fall season before deciding to use the design on a competition robot.

These documents might help.

http://www.chiefdelphi.com/media/papers/2381

http://www.chiefdelphi.com/media/papers/2378

Re: Swerve: Belts or Chain?
 

How are you attaching the vertical tube with the hex bearing to the U structure that holds the wheel? A cross section view might help. Also, if you are using a MiniCIM for turning you are going to need something like a 50:1 reduction, thats pretty hard to get with 1 stage of belt or chain.