Swerve Drive Modules
 

Hi Everyone!
Team 1515 is looking to build a swerve system in the 2011 offseason. Here is a rendering of the CAD so far.

I've been able to source nearly all the parts save for the bevel gears, a crucial aspect of a swerve system. For this design, we need 3/8"bore keyed gears. Does any team (preferably one who has built swerve) know where I might source these?

Also, any comments, or even better, criticisms on the sheet-metal design would be greatly appreciated. I'm considering adding in sides, bent over one side of the swerve, riveted over the next. Is this necessary? Oh, and the vertical shaft is not in this render, but it has all of the snap-ring grooves just as the horizontal shaft does. They'll both (obviously) be turned on a lathe.

Thanks,
Mathew

Re: Swerve Drive Modules
 

You can get bevel gears from McMaster-Carr. the website can be hard to navigate, but once you get use to it, you will find that they really have everything.

As for the design it looks good. I was the head drivetrain designer this year for FRC 79, and we did a coaxial swerve very similar to this. I really like the sheet metal concept. Our modules this year were open on 2 sides like this, but were made from 1/4 inch 6061 Alu plates. we never had any trouble with them.

If you have any further questions feel free to PM me, and I'll be happy to help.

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Yes. I am familiar with the McMaster bevel gears. my opposition is that they cost upwards of $30, and are relatively large. way too large for our needs.

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Boston Gear makes some nice miter gears that might work for you. Check out this PDF.

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Every swerve 1625 has had (alot) has used mcmaster part #6529K14
this has a 3/8ths bore, you'd have to broach a key yourself, we've had them hex broached typically.

Haven't killed one yet.

Also advice on the module, take a long look at the path forces travel and what things need to stay perfectly aligned (bevel gears)

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It's going to be tough to get the bearing holes aligned based on the tolerances on the sheet-metal bends. This will be even tougher because your design doesn't have a flat surface parallel to the bend line that the brake operator can use as a back-stop.

Correct bevel-gear alignment is VERY important in a design like this, especially if you're using a "small" bevel gear. My advice would be to use the one Aren suggested (good size for this application).

You should consider making your bottom wheel-axle into a structural axle (make it into a standoff with both ends tapped, or (148's favorite trick) make it 1/2" OD tube with a 1/4" ID, then run a 1/4" bolt through it. Making the axle a structural member in this way will GREATLY stiffen up your module. Right now I'd be concerned that the forces on the module will "unbend" the sheetmetal -- you should do something to turn the module into a box structure.

Ohh -- I almost forgot: don't do a swerve drive. :) The cost is greater than the benefit in almost ALL situations for almost ALL teams.

-John

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I agree with John here. From what I remember, you guys had some pretty big reliability issues with your drive this year; I think simpler is the way to go, not more complicated.

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What is the thickness of the sheetmetal?

The concept looks good, but that metal is going to flex at the bottom. My team used 6061 T6 with 3/16 wall in 2008 and we had trouble with stress on the axle bending the modules with a similar design. I would try to reinforce it somehow.

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What would you say is a comparable drive train in power and maneuverability like the swerve?

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Power is a silly comparison, since it is based purely on motors. a 4-motor 2WD has the same "power" as a 4-motor swerve.

As far as maneuverability -- I don't think sideways motion makes a swerve drive robot more maneuverable in application than any other drivetrain.

Most teams don't get the software right.
Most drivers can't control it.
Most swerves tend to fall apart at bad times.

Build a reliable skid-steer drive and you'll be just as good as a swerve in most games -- I bet you'll spend less effort to make it work, and I bet it is less likely to bite you in the butt at the wrong time.

-John

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Thanks everyone. Especially John. I'm going to fix that in the design.
And as to not building swerve, I too have seen it work and fail. Of course, 1717 has an exceptional design that we will not be able to match for at least a year or two.
However, We feel that this is a good way to open our team up to the sheet-metal world. If the swerve works well, we may build our 2012 chassis with SM. If we can form a solid relationship with a SM manufacturing shop, its worth a try.

@AdamHeard, yes we did. I'm going to do my best to get our team away from Mecanum drive.

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If you do decide you really love swerve and want to make one, spend an entire offseason just mechanically making it, then set it aside and ignore it for a season. Then spend the entire next offseason making it drivable.

The biggest reason I'll tell people to not go after swerve, is the control is never quite good enough, it lacks the direct mental connection most tank drives achieve and from that loses effectiveness.

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I dont know the efficiency on the bevel gears, but i bet a 4 wheel omni drive with an rs775 and a cim on each wheel would be more maneuverable, and probably push better too. I thought disco-bots did something like this year.

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I updated the design a bit to fit everyone's suggestions. (I did not fix the drive shaft yet, though. I am a little worried that it'll be difficult to assemble like this, so I may make the dual-triangles on the side into large rectangles with filleted corners.
You may notice that I removed the riveted sprocket from the top due to the fact that it is very obviously ineffective. I am going to eventually replace it with timing-belt pulleys from McMaster. Does anyone have an idea as to how I might attach these to the frame?

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Also, I know 1717 uses the main bent SM piece with open sides, in additional to two side plates which rivet on. This makes assembly way easier, but makes repair quite difficult. Of the two designs (one piece, three pieces), which is preferable? Also, would it be a terrible idea to weld the timing-belt pulley to the SM? I know it would warp considerably.

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There is no way that you would be able to bend that module in one piece.

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What method are you using to hold The output shaft to keep the module standing straight? This tends to be one of the more important things to get right in swerve drive. As it is i don't see any method in place to hold the module against side force.

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I'm not sure what you mean. The output shaft infers the bolt holding the wheel/sprocket, correct? What do you suggest I add to prevent unwanted sheet-metal bending?

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I was actually referring to the shaft that the whole module pivots on, as it it i see it being held by one bearing at the top of the module, normally teams will have a second shaft around the drive shaft that the module turns on. Then this shaft can be held by some sort of bearing system that attaches to the robot.

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Okay. Well, yes, in this design it is effectively held by one bearing. Where exactly do they place that second shaft? I can also, of course, just change it to a roller bearing, which are longer.

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The drive shaft is placed in the center of the shaft the module rotates on. In most systems there are two bearings hold the drive shaft. There should also be a system to hold the shaft the module pivots on. if your confused about this you should look at 221's Swerve system Revolution swerve http://www.andymark.com/ProductDetai...ctCode=am-0760
They have one of the more reliable swerve systems around

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Has anyone ever build a two wheel swerve drive with omni's on the corners? Make the swerve wheels 2 or 3 inch wide on the center.

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I seem to remember a team doing something like that in 2008, can't name the team though. I would be careful about making the wheels 2/3 inches wide because the wider grippier wheels make rotating the modules more difficult.

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The current design shows several problems. It is not easy to make a module that can hold up to some serious stress. We have done swerve for 2 years now and this link will show a design that has proven very durable.
http://wiki.team1640.com/index.php?t...II_Drive_Train
It's only May. You have several months to refine the design.

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What trouble did you have with Mecanum drive? We did direct-drive mecs from AM nanotubes and had our most reliable, successful drivetrain in 7 years.

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The reason we chose 6wd this year was purely based off our strategy for this year's game. We deemed early in the build season that for this year, speed was far more important than maneuverability.

Every drive system has it's advantages and disadvantages. You should base your drive train decision off what will complete the task at hand the best, not on what is cool or different or easy.

During our initial brainstorming seasons we had at one time 8 different drive systems up that we were considering. Hooray for double sided white boards!

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We were selected to be on an alliance by Teams 1717 and 330 (seeds 1, 3. obviously both powerhouses). We competed well in the first 2 matches of QF. After that, our bearings on the AndyMark Mecs shot out one at a time. 3 or 4 matches later, after the second SF match, we had no working wheels and were dropped from the Alliance. Sure we could have put in better quality McMaster bearings (that's what we did before the next regional, Denver), but it was still more than frustrating.

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Did that fix the problem? Were there some other issues beside that?

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Yes. It fixed the problem, at least for the most part. When it gets down to the root of the problem, though, we had welding displacement and inaccuracies which inevitably caused a misalignment of chain, and thus an unnecessary torsion force on the bearings.

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I'm a bit puzzled then by your earlier statement: "I'm going to do my best to get our team away from Mecanum drive". Since it wasn't the mecanum's fault, why do you say this?

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Final-Draft for the CAD! I added an extra bearing to hold the drive shaft. The sides are now riveted on, as opposed to bent from one piece. After creating a BOM, I found that 4 modules will end up costing about $750.

The entire module, from the top of the Timing-Belt pulley to the ground is exactly 7" tall. it is 2.65" wide, and 3.5" long.

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You are still likely to see misalignment between the bearing holes on either side of the module that will cause grief for you in the bevel gearset.

If your team isn't yet capable of building a 4WD frame that doesn't wreck ball bearings, you should not expect to have success with a swerve drive yet. Focus on improving your deficiencies instead of replacing them with all new ones and you'll find it to be a valuable experience.

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Again, this is an offseason project. It is to test whether or not we will use Swerve on the 2012 robot. It is very possible that we will put it off for a year and use it on our 2013 bot. We are simply building this to test our abilities and form a relationship with a SM shop.

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It looks like a nice module. Do you have a weight?

Also, I see the gear for a belt to rotate the module, but how are you actually going to attach it to the robot?
From personal experience, that was the toughest part to get working right.

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We've used Martin Gears' HM1616 (16 teeth, 16 Diametrical Pitch) miter gears for the last 2 years, but will probably switch to M1616 (not hardened) if we do this again. Gears have 3/8" unfinished bores. We cut our own keyways (finished bore gears are too expensive). We've had good experience with these.

Here's a view of our 2011 Pivot Module. Our Module includes the motors and angle sensor:

http://wiki.team1640.com/index.php?t..._annotated.jpg

Mass is 9.1 lb including motors, gearbox & sensor.

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I would have doubts on the ability of the single output shaft to be able to take the side force from impacts regularly incurred in FRC competition, also how would you be taking thrust load from the weight of the robot? In general these are both really important factors for swerve drive. I would recommend looking into Delrin and other bearing systems to attach it to the robot.

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Just a good thing to keep in mind when building drivetrains:

The best drivetrain is the one that gets you where you need to go, when you need to be there.

Ask yourself:
-Does a 6wd get you there? In a timely mannor? Consistantly?
-Does a swerve get you there faster than a 6wd? Every time, Consistantly?
-Is the difference in performance between a 6wd and a swerve enough to outweigh the potential driver practice that comes with finishing more quickly?
-Could the effort put into the drivetrain be better placed in another part of the robot?

Just trying to keep you honest :] , Bryan

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Love it! Yeah... what Bryan said.

-John

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The best drivetrain never has issues. If there's one part to make reliable, make it your drive.

If you have issues with non-swerve drivetrains, don't waste your time with a swerve prototype. You won't be able to pull it off. You have to learn to walk before you learn to run.

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for 750 per set, why not just buy wild swerve modules from andymark. if you want to go further, get the revolution modules.

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I resourced a bunch of the parts, and we're going to be paying $640-$660 now. The revolution costs $990. And again, we're not building this necessarily for the season. This is an offseason project designed to test our abilities, teach freshmen/newcomers, and form a relationship with a SM shop.

All in all, I think that the value of the project outweighs the cost. However, that is subjective, and is my personal opinion. To those who promote 6WD or 4WD, that's more than likely what we will use in the 2012 season. It depends both on the game and whether or not the swerve is extremely successful and reliable.

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Test the living daylights out of it. Weigh it down, run it hard, long (lower voltage), and against defense. Complex components have a nasty habit of seeming really great until you actually need them to be.

And, just to clarify what I think a lot of people have been saying (I think you realize it): Don't build a swerve to test if you can do it. Build it to learn the skills you need to build great drivetrains: testing, troubleshooting, consistency, accuracy, robustness. (not exhaustive) Emphasize those lessons over the result of the actual device. They're more important, for FIRST and beyond.

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Ding ding! I have to spread some rep around before giving Ms. Madison any more, but she deserves it.

In 2005, 2006, 2007, and a little bit in 2008, we struggled with throwing chains on our drives. At the single most inopportune moment, the robot would decide to drop a chain and do the robopeepee dance. in 2009, we put a priority on chain routing - the routes were clean, properly lined, and shortened - we had a great year with no drive problems. In 2010, our chassis flexed in ways we didn't realize until after the FRC season had ended, causing us to throw what we thought were properly aligned and tensioned chains; we put on structural supports on before the offseason events and ran them smoothly.
However, the notion around the team became "chains are nothing but trouble" - the truth is chains are fine. We just weren't implementing them correctly.
As I mentioned, this year we went to direct-drive mecanums right out of the gearboxes, with absolutely no complaints. This year taught us that simplicity brings success. I applaud you for pursuing the swerve drive as an offseason project - I think your team will learn a lot about iterative design, programming, and machining. However, I'd be careful about creating these with the 2012 or 2013 game in mind - as others have said, let the robot fit the game, don't force the game around your idea of what a robot should be.

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Others have said this and I will echo their sentiments: if you do swerve drive, it needs to be reliable, and built like a tank! We spend many hours at our (only) competition fixing little things that kept creeping up on our swerve drive system. Also, I have trouble believing that the module you disigned will hold up against any amount of side load (imagine a 200 lb robot traveling 20 fps raming you at a dead stop. If you are moving, that net speed goes up, and bumpers will not do everything.) without torquing or just sheering the ouput shaft. Personally, it was really a nice feeling to have the bottom assembly of the WildSwerve modules there, where we could not sheer the output shaft.

In addition, decide on a control scheme and programming strategy, and if at all possible, have the driver be the programmer. I both programmed and drove my team's swerve drive this past season, and I was able to quickly and easily diagnose software problems just because I knew the program so well.

If you are interested, I can point you to the team code from this year, as well as answer any questions you have via PM.

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I love it. :] That's an excellent way of putting it. If you don't mind, I'm going to use that when teaching younger and newer team members. And that is exactly why I wanted to build it in the first place.

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I really like that idea. Our current driver is graduating this year and our programmer (at least the one programming the swerve) is going into 11th grade with me. (next year will be my 5th year, so I have quite some experience.) I will talk to him about driving. Also, does your team use a 3 axis joystick or 2 axis and a potentiometer/dial? And we have 1717's code (on their website) so I think we're okay with regards to reference.

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Have you looked at that code yet?

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No problem. Not to be a buzzkill, but just do realize you're trading making something potentially cool (which unfortunately likely won't work well enough) for concentrating specifically on what will help you next season.

The former seems to be most inspiring though, so who am I to complain. Just don't get discouraged if/when you don't accomplish the device's goal, because hopefully you do accomplish the learning & inspiration ones. If not though, I would consider a different course for the next off-season. Good luck, and you know where to find us if you need anything. :)

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I would strongly urge you to consider designing and manufacturing a bulletproof, reliable 6wd before even considering trying a swerve drivetrain. It sounds like you guys have yet to master that so why make the jump to the most difficult drivetrain. While it's fun to design and think through, there is rarely a game when having the abilities of swerve outweighs the tradeoff of added cost, time to manufacture, time to code, difficulty of control, and reliability.

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Has anyone suggested using Team 221's Revolution Modules?

If you've got your heart set on doing a swerve, you could always use 221's modules to take a large chunk of the hard (mechanically) stuff out of the equation then spend more design time on the frame, power transmission to the modules, steering and code.

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Right now, we use a 2 axis joystick for the movement/strafing and another joystick for rotating around the central axis of the robot. As for a pot, we used the MA3 from usDigital. I caution you about being presumptious over programming/driving though. Make sure it will not tear your team apart if one person fills these two critical roles. Again, I must reccomend spending one year mastering a 6wd. You will make some mistakes, and the knowledge gained will help even more when you go to make a swerve. For my team, mechanical issues were the problem in the end, not the software. Make sure you don't forget either or else you will have a very expensive paperwieght come competition.

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And if the goal is to see how your relationship with the sheet metal shop will work, maybe design a six wheel drive sheet metal chasis similar to 148, 217, etc. Still no small accomplishment but more likely to be something you can get working well enough to use in the competition season.