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By: cadandcookies
New: 07-07-2013 23:35
Updated: 07-07-2013 23:35
Views: 2648 times
As an offseason project, my team has been working on creating a new showbot/electronics test platform. We have settled on creating an octocanum drivetrain for this robot for both the experience of designing a drivetrain outside of our norm (we've done a kitbot-style 6wd just about every year since we started), but also for the "wow" factor when we're showing sponsors or for school assemblies.
This is our version two of the actual octocanum module, rendered in with Creo's built in tool and combined in Photoshop. The plates are 3/16" aluminum, and it uses VexPro mecanum and traction wheels. All axles are dead, and it's powered off of a gearbox above the frame to the sprockets on the module. We plan to articulate the drive "Neutrino-style," or, barring that, using the more "traditional" method of attaching the piston to the bar (using a combination of channel and angle stock).
The module is designed to be dropped into a 5" wide channel, sandwiched between two pieces of 3x1 1/8" wall 6061-aluminum tube stock. The top side of the module wall is parallel to the tube stock.
Feedback, critique, and comments are welcome. We're aiming for production around the end of July, with assembly and powdercoating taking place in August.
Discussion
view entire thread
08-07-2013 04:36
CENTURION
Re: pic: Team 2220 Blue Twilight Octocanum Offseason Project
Are you using threaded rod as an axle for the traction wheel?
I ask because we've tried doing that before, and it didn't work very well, the axles bent after only a few minutes of driving. Then again, I think we were using pretty small rod, I think it was 3/8", but it might have been 1/2", I can't remember exactly.
Also, what is keeping your wheels centered on the axles?
The module is looking pretty excellent though! Can't wait to see the finished product! 
08-07-2013 10:08
Boe
Re: pic: Team 2220 Blue Twilight Octocanum Offseason Project
which wheels do you plan on having at the corners of your drive?
Also what are you planning on for a gearbox? It appears that the sprockets on both wheels are the same size is that correct? Overall looks very nice hopefully i can see it sometime at an off season event when its done.
08-07-2013 10:59
lemiant
Re: pic: Team 2220 Blue Twilight Octocanum Offseason Project
Based on the fact the double sprockets are on the meccanums I think it's a safe guess that they have the traction wheels on the outside.
08-07-2013 12:12
Jeffy
Re: pic: Team 2220 Blue Twilight Octocanum Offseason Project
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Originally Posted by lemiant
Based on the fact the double sprockets are on the meccanums I think it's a safe guess that they have the traction wheels on the outside.
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For the OP,
If you have decided, can you tell us why you decided on the placement of the traction wheels on the inside or outside. I think there are a few obvious pros and cons to each, but would like to hear your view on it.
08-07-2013 13:17
Trent B
Re: pic: Team 2220 Blue Twilight Octocanum Offseason Project
One comment I would make is with the sprockets chaining the traction wheel and mecanum together, did you pick the wheel separation distance to ensure an even number of chain links? It is a bit harder to put the modules together under tension, but never worrying about throwing a belt or chain is great.
08-07-2013 14:03
cadandcookies
Re: pic: Team 2220 Blue Twilight Octocanum Offseason Project
| One comment I would make is with the sprockets chaining the traction wheel and mecanum together, did you pick the wheel separation distance to ensure an even number of chain links? It is a bit harder to put the modules together under tension, but never worrying about throwing a belt or chain is great. |
| If you have decided, can you tell us why you decided on the placement of the traction wheels on the inside or outside. I think there are a few obvious pros and cons to each, but would like to hear your view on it. |
| Are you using threaded rod as an axle for the traction wheel? |
08-07-2013 14:33
Trent B
Re: pic: Team 2220 Blue Twilight Octocanum Offseason Project
Fortunately the size of the pistons you need to shift is pretty small, I think ours had a stroke of 1" tops, and a bore of 1 1/16" or so, we also plumbed them all off of 1 or 2 solenoids, I can't remember what we decided on at the end of the season. You don't have to use air to pull the traction wheels back up, robot weight should be more than enough. Also, the bot was downright hard to move if it stopped in traction mode.
08-07-2013 15:01
cadandcookies
Re: pic: Team 2220 Blue Twilight Octocanum Offseason Project
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Originally Posted by Trent B
Fortunately the size of the pistons you need to shift is pretty small, I think ours had a stroke of 1" tops, and a bore of 1 1/16" or so, we also plumbed them all off of 1 or 2 solenoids, I can't remember what we decided on at the end of the season. You don't have to use air to pull the traction wheels back up, robot weight should be more than enough. Also, the bot was downright hard to move if it stopped in traction mode.
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08-07-2013 15:05
Madison
Re: pic: Team 2220 Blue Twilight Octocanum Offseason Project
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Originally Posted by Trent B
Fortunately the size of the pistons you need to shift is pretty small, I think ours had a stroke of 1" tops, and a bore of 1 1/16" or so, we also plumbed them all off of 1 or 2 solenoids, I can't remember what we decided on at the end of the season. You don't have to use air to pull the traction wheels back up, robot weight should be more than enough. Also, the bot was downright hard to move if it stopped in traction mode.
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If you don't use air to retract the traction wheels, won't the end result be having both the mecanum wheels and traction wheels touching the ground simultaneously?
08-07-2013 15:59
Trent B
Re: pic: Team 2220 Blue Twilight Octocanum Offseason Project
Was trying to figure out what the confusion was, then I realized.
We also have a small spring (maybe 1-2lbf tops) that pulls back on a small "finger" sticking up from the plate by the axle lifting the traction wheel the remainder of the way off the ground.
Here is a side profile picture with the "finger" which sticks up through a milled slot in a chassis tube.
http://www.teamneutrino.org/assets/r...drive/side.png
10-07-2013 01:10
cadandcookies
Re: pic: Team 2220 Blue Twilight Octocanum Offseason Project
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Originally Posted by Boe
Also what are you planning on for a gearbox? It appears that the sprockets on both wheels are the same size is that correct? Overall looks very nice hopefully i can see it sometime at an off season event when its done.
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We're planning on either running them through something like the VexPro double reduction gearbox or a custom solution (we can always use more practice!).
The sprockets are indeed the same size-- and they'll probably stay that way, unless we find a good design reason to change it. We have a bunch of 32t #25 sprockets lying around, and might as well use them for this!
I hope we get a chance to show it off at an offseason event!
10-07-2013 01:39
Boe
Re: pic: Team 2220 Blue Twilight Octocanum Offseason Project
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Originally Posted by sentientfungus
Sorry I missed a couple of your questions.
We're planning on either running them through something like the VexPro double reduction gearbox or a custom solution (we can always use more practice!). The sprockets are indeed the same size-- and they'll probably stay that way, unless we find a good design reason to change it. We have a bunch of 32t #25 sprockets lying around, and might as well use them for this! I hope we get a chance to show it off at an offseason event! |
10-07-2013 02:20
Gregor
Re: pic: Team 2220 Blue Twilight Octocanum Offseason Project
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Originally Posted by Boe
You could gear the traction wheel slower because i would assume in a competition the mecs would be the primary drive mode except during auto and when you encounter defense. changing the speed of one of the wheels would essentially give you a two speed drive train with your high gear being omni directional.
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10-07-2013 10:39
Trent B
Re: pic: Team 2220 Blue Twilight Octocanum Offseason Project
I don't have the exact dimensions off the top of my head but I think neutrino went from a 4" omni to a 2.75-3" traction wheel which provided a substantial speed difference. With the current size difference between their mecanums and traction wheels they should see a similar if not larger effect.
10-07-2013 10:50
Boe
Re: pic: Team 2220 Blue Twilight Octocanum Offseason Project
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Originally Posted by Gregor
The traction wheel is already smaller, which will make the robot slower already in traction mode. Whether that is slow enough the team needs to test and play with.
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10-07-2013 13:17
cadandcookies
Re: pic: Team 2220 Blue Twilight Octocanum Offseason Project
The way we have it geared currently, by my calculations (using a 1:9.52 gearbox like the the VexPro double reduction), we would be running at ~12 fps adjusted on the mecanums and ~8 fps adjusted on the tractions (using the JVN 2012 design calculator).
To me, those both sound like rather reasonable speeds. There's also that we have a maximum sprocket size on the 4" wheel (which I believe is something like 44t sprocket). Running that reduction would bring us down to ~6 fps adjusted speed. That's something we might consider if we were to run this in a competition, but probably won't deal with on the prototype, at least initially (because of the aforementioned large amount of 32t sprockets lying around). We might also get a variety of sprockets and test out how it feels in different configurations.
11-07-2013 09:27
Jeffy
Re: pic: Team 2220 Blue Twilight Octocanum Offseason Project
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Originally Posted by sentientfungus
The way we have it geared currently, by my calculations (using a 1:9.52 gearbox like the the VexPro double reduction), we would be running at ~12 fps adjusted on the mecanums and ~8 fps adjusted on the tractions (using the JVN 2012 design calculator).
To me, those both sound like rather reasonable speeds. There's also that we have a maximum sprocket size on the 4" wheel (which I believe is something like 44t sprocket). Running that reduction would bring us down to ~6 fps adjusted speed. That's something we might consider if we were to run this in a competition, but probably won't deal with on the prototype, at least initially (because of the aforementioned large amount of 32t sprockets lying around). We might also get a variety of sprockets and test out how it feels in different configurations. |
If you really want to save weight, you could integrate your gearbox into the wheel module.
11-07-2013 10:32
PAR_WIG1350
Re: pic: Team 2220 Blue Twilight Octocanum Offseason Project
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Originally Posted by Jeffy
It sounds like you are not running a reduction between the gearbox and the wheels. I urge you to do this. If you do, you can switch to a single reduction box which will save some weight and efficiency (especially important on a drive like this).
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Direct drive eliminates one axle, 2 sprockets, and one length of chain per module, while adding one drive hub (and possibly an extended output shaft) per module.
Changing to a single reduction and changing the sprocket ratio removes 2 gears, a stub-shaft, and 2 bearings. One sprocket and the gear box housing are also reduced saving additional weight.
| If you really want to save weight, you could integrate your gearbox into the wheel module |
All possible solutions should be evaluated to determine which is best for the team and its goals.
11-07-2013 14:06
Madison
Re: pic: Team 2220 Blue Twilight Octocanum Offseason Project
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Originally Posted by PAR_WIG1350
This is yet another solution with its own pros and cons to consider. While lighter, it could also be more complex and more difficult to actuate due to the additional weight (not an issue with some methods, but if it is retracted with springs, it might not be worth it).
All possible solutions should be evaluated to determine which is best for the team and its goals. |
o -- X -- x
You can make this sort of thing work by either integrating the gearbox entirely or by continuing to use the pivot point as the link between gearing in the fixed gearbox and gearing to make the wheels spin.
I have a CAD model at home of the module I've been refining. It's not quite done yet, but it is based upon this idea and uses an integrated gearbox. If the above description isn't clear, I'll post a picture when I get home later tonight.
11-07-2013 17:05
Trent B
How much piston stroke would that comparatively require to push the traction wheel down far enough to pass the mecanum wheel?
11-07-2013 17:12
Madison
Re: pic: Team 2220 Blue Twilight Octocanum Offseason Project
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Originally Posted by Trent B
How much piston stroke would that comparatively require to push the traction wheel down far enough to pass the mecanum wheel?
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