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Unread 09-28-2017, 06:43 PM
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pic: 5987 Octocanum CAD

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Unread 09-29-2017, 08:59 AM
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Re: pic: 5987 Octocanum CAD

Some practical considerations:

Are the octocanum modules designed to be assembled outside of your robot and pre-tested so they can be easily and quickly installed in the robot? It would be a total pain in the neck if you had to assemble it in the robot.

You may want to consider using a separate pneumatic circuit with it's own regulator to run the four cylinders. This would allow you to adjust the "stiffness of the suspension" to get the optimal traction in mecanum mode.

The "L" and "T" shaped gussets would be stiffer if you added a triangular part in the corners. It may be difficult with the L's due to the wheels though.
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Unread 09-29-2017, 03:35 PM
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Re: pic: 5987 Octocanum CAD

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Originally Posted by philso View Post
Some practical considerations:

Are the octocanum modules designed to be assembled outside of your robot and pre-tested so they can be easily and quickly installed in the robot? It would be a total pain in the neck if you had to assemble it in the robot.

You may want to consider using a separate pneumatic circuit with it's own regulator to run the four cylinders. This would allow you to adjust the "stiffness of the suspension" to get the optimal traction in mecanum mode.

The "L" and "T" shaped gussets would be stiffer if you added a triangular part in the corners. It may be difficult with the L's due to the wheels though.
Thanks for the suggestions! They all sound like good ideas. To address your suggestions:

1. One of the reasons we chose to put the motors in the modules instead of driving the traction wheel shaft from inside the chassis is so that we could assemble and test the modules outside of the chassis. The idea is that the outside tube can be removed with a few bolts, and then the module simply slides out and we can easily repair or replace it.

2. We designed the modules so that they would work with as low as 30psi, rather than the 60psi max. The reason behind this was that if we designed for 60psi and we use more than half of our stored air, we will no longer have enough force. Whereas with 30psi we can use up to 3/4 of our air and still have enough force. It also gives us the wiggle room to stiffen or soften the suspension.

3. I made those gussets quickly just to hold the tubes together for the render; they likely won't be what we implement (certainly not without bolt holes as they are now). They were modeled after VexPro's gussets, which don't have the triangular part.
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Unread 10-09-2017, 10:22 AM
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Re: pic: 5987 Octocanum CAD

Hey, I'm curious as to whether or not it is safe for the belt to rub against the cim as you have done? I don't personally have any experience in this subject so I am wondering if anyone with more experience has some input or clarification that could help me understand.
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Unread 10-09-2017, 12:05 PM
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Re: pic: 5987 Octocanum CAD

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Originally Posted by Volpidash2770 View Post
Hey, I'm curious as to whether or not it is safe for the belt to rub against the cim as you have done? I don't personally have any experience in this subject so I am wondering if anyone with more experience has some input or clarification that could help me understand.



+1
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Unread 10-09-2017, 12:12 PM
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Re: pic: 5987 Octocanum CAD

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Originally Posted by firecrafty View Post



+1
This is a bad idea for sure.

At best it'll merely be an appreciable source of friction.

At worst the belt will fail quickly and/or the robot will barely drive under load.
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Unread 10-09-2017, 12:15 PM
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Re: pic: 5987 Octocanum CAD

Just want to further emphasize that you definitely, absolutely should not use a CIM as an idler for a timing belt like that. Just don't. It will not work out in your favor, trust me.
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Unread 10-09-2017, 12:34 PM
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Re: pic: 5987 Octocanum CAD

Quote:
Originally Posted by Volpidash2770 View Post
Hey, I'm curious as to whether or not it is safe for the belt to rub against the cim as you have done? I don't personally have any experience in this subject so I am wondering if anyone with more experience has some input or clarification that could help me understand.
I've never done this exactly, but I have run a belt with a small indent in the path from some all-thread with electrical tape over it. It just barely deflected the belt, and the belt wasn't under a lot of tension.

I wouldn't do it for a real robot though. With the model my students are designing (the one they are actually building), they are putting the CIM higher so it won't interfere with the belt. I had already designed the plates and was too lazy to change them, so I just did that for my model to give them a 3D example of what we were designing on paper.


edit: nice catch by the way, none of my students caught that
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Last edited by AriMB : 10-09-2017 at 12:47 PM.
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Unread 10-09-2017, 08:40 PM
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Re: pic: 5987 Octocanum CAD

Those pistons look enormous and expensive. Also, expensive.
Are you sure you actually need to size to 30psi? Putting on the 4lbs of air compressor to maintain your reserve pressure will probably save you 4lbs of piston & rod-end steel, plus your peace of mind. All you need is to save 1lb at each corner...
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Unread 10-09-2017, 10:17 PM
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Re: pic: 5987 Octocanum CAD

16.7 fps mecanum with only one CIM per wheel? You'll likely have some real trouble strafing in that configuration.
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Unread 10-19-2017, 10:55 AM
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Re: pic: 5987 Octocanum CAD

A quick note on the pivoting mechanism:

You might be able to drop a considerable amount of weight by losing the (usually steel) eye bolts for some aluminum clevises, like these:

They're easy to make as well if you can't find one of the correct size. This will also allow you to drop the size of the 'axle' they pivot around to a ~1/4" bolt.
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Unread 10-19-2017, 10:57 AM
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Re: pic: 5987 Octocanum CAD

Quote:
Originally Posted by Brandon Zalinsky View Post
A quick note on the pivoting mechanism:

You might be able to drop a considerable amount of weight by losing the (usually steel) eye bolts for some aluminum clevises, like these:

They're easy to make as well if you can't find one of the correct size. This will also allow you to drop the size of the 'axle' they pivot around to a ~1/4" bolt.
Where are you buying these?
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Unread 10-19-2017, 11:42 AM
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Re: pic: 5987 Octocanum CAD

Consider a spring pulling up on the traction wheel so if you lose air altogether you still have an unobstructed mecanum drive.
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Unread 10-19-2017, 04:49 PM
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Re: pic: 5987 Octocanum CAD

Quote:
Originally Posted by Brandon Zalinsky View Post
A quick note on the pivoting mechanism:

You might be able to drop a considerable amount of weight by losing the (usually steel) eye bolts for some aluminum clevises, like these:
...
They're easy to make as well if you can't find one of the correct size. This will also allow you to drop the size of the 'axle' they pivot around to a ~1/4" bolt.
I had considered using clevises, but for some reason decided not to. I don't remember what the reason was anymore. We might need to revisit that when the students make their version.


Quote:
Originally Posted by pfreivald View Post
Consider a spring pulling up on the traction wheel so if you lose air altogether you still have an unobstructed mecanum drive.
The traction wheel stays in the same place; the mecanum wheel pivots around the traction wheel axle. We debated adding a spring, but decided the extra parts and complexity isn't worth it. We figure in the rare case that we lose air pressure, having the mecanum wheel floating isn't the end of the world. Unless someone has some experience that would argue with that line of thinking.
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Unread 10-19-2017, 06:53 PM
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Re: pic: 5987 Octocanum CAD

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Originally Posted by AdamHeard View Post
Where are you buying these?
We usually buy these with the cylinders. Bimba offers these as a standard option on many (if not all) of their lines. We typically buy these even if we don't anticipate using them right now, because those cylinders last a long time, and we just keep the clevises in the pneumatics parts box.

Edit:
Quote:
Originally Posted by pfreivald View Post
Consider a spring pulling up on the traction wheel so if you lose air altogether you still have an unobstructed mecanum drive.
I was actually thinking the other way - put the spring so that if you lose air altogether you have a traction drive. Either way, it is definitely better to make sure that your system fails in a direction you DECIDE, not just however, and not to put all the wheels on the floor. If you did want mecanum as your "failsafe", you would probably do better having that as the fixed axis, and rotating the solid wheel into the carpet. Actually, this is really a better option anyway, because mecanum wheels inherently produce strafing forces, so mecanums are (IMO) better placed on the fixed axle. In any case, if you have the "fixed axle" wheel as the failsafe, this spring does not have to be (and should not be) really stiff, just enough to pull the other wheel off the carpet in the absence of pneumatic force. If you do it this way, you do not even need to pressurize the opposite side of the cylinder, saving a bunch of air.
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