27-11-2012 20:18

Re: pic: Another concept study

Beautiful design and render, as always. One question, though - Any reason to have the traction wheels on the inside instead of the outside?

27-11-2012 20:22

CalTran

Re: pic: Another concept study

Shorter turn base?

27-11-2012 20:23

Re: pic: Another concept study

I thought that, but it seems like a really small wheelbase then for only two 4" wheels.

27-11-2012 20:37

CalTran

Re: pic: Another concept study

It's not too small. If that's modeled as the long side of the robot, that'd be somewhere around 12"-15" between wheels.

27-11-2012 20:38

Re: pic: Another concept study

Oh, looked short sided to me.

27-11-2012 20:44

CalTran

Re: pic: Another concept study

I might be wrong that it's long side. I just can't imagine doing octocanum for a wide robot.

27-11-2012 21:00

ehfeinberg

Re: pic: Another concept study

I guess that when driving with Octunum, you would drive it like mecanum with traction wheels. Thus you would be mostly driving with mecanum, using your traction wheels only if you need more traction.

Because of this, and with a much smaller wheel base, You might encounter some rock when on the inner wheels. Since you would want your main drive orientation to be as stable as possible, you could want your mecanum wheels on the outside. Also, for mecanum to work well, you really want all 4 wheels to be in contact with the floor at all times. If the mecanum wheels are the inner sets of wheels, during rocking, one or two of the wheels might lose traction with the floor impeding on your motion. (This rock might be negligible but hey, there is no disadvantage on having the mecanum wheels on the outside (And imo it looks cooler))

27-11-2012 21:10

ENeyman

Re: pic: Another concept study

I'm not sure if this is the long side or the wide side, but in this render by the same person, he says that his team uses a mini-octocanum because they have a wide robot.

27-11-2012 21:52

Tom Ore

Re: pic: Another concept study

I did the concept study this way because it made sense for last year's game. We wanted maximum stability on the Mecanum wheels. You would have to decide what made sense based on the game. It could be wide or long - again depending on the game.

27-11-2012 22:02

Re: pic: Another concept study

That makes a lot more sense. What are the speeds for each wheel? How much does it weigh?

27-11-2012 22:16

Tom Ore

Re: pic: Another concept study

I believe I have the Mecanums at around 13 fps and the (3.25") traction wheels at around 7 fps.

The module we competed with last season weighed 23 lbs for each side so 46 lbs total - very heavy. This one is 18 lbs for each side for 36 lbs total. We sure could have used that 10 lbs last year. For example, we only had 1 motor driving our shooting wheel because we just didn't have the weight allowance to add a second motor.

Also, last year's version would have been nearly impossible to maintain if anything had failed. This one is very easy to maintain. The Mecanum wheel can be removed by just removing the one screw. The gear boxes have just 4 screws and the clevis pin. The CIMS stay with the frame rail when you drop the gearbox.

28-11-2012 10:18

JesseK

Re: pic: Another concept study

Is there any concern that the shaft on the reduction just prior to the 3.25" traction wheel won't hold up to the stresses of having 1/4 the robot's weight on it while transferring higher torque through it?

The shaft looks like it's 0.375" even though the wheels themselves are on a 0.5" shaft (based upon bearing holes, so it's just a guess), which is the only reason I bring it up. Seems to me that if the wheels need to be on a 0.5" shaft, then that particular reduction shaft needs to also be 0.5" since it will hold the weight of the robot when the traction wheel is down. Or maybe the wheel shafts are 0.5" since the 1/8" keyway is more preferred for the wheel?

Love the render.

28-11-2012 12:29

Akash Rastogi

Re: pic: Another concept study

I really like the compact packaging of your design. Do you think, just for kicks, you'd try out a vertically oriented CIM + bevel gears to have all the space in the center free? Might be fun, although not necessary.

What are the distances between mec to mec and traction to traction?

28-11-2012 12:49

akoscielski3

Re: pic: Another concept study

You may want to consider making the macanum's slower. 13 FPS will not give you a lot of torque on the wheels that is needed to make your roboto strafe. In 2011 we used CIMple boxes on our 6inch mecanum's and we were unable to strafe. I may be wrong because I am not a Mecanum master or anything, but this is just from my experience.

28-11-2012 13:47

Re: pic: Another concept study

Yeah, but you forget to mention only 3 of your wheels were on the ground at once. Aaron =/= trusted source on mecanums.

28-11-2012 14:07

Madison

Re: pic: Another concept study

My software-addled brain is having trouble visualizing how face-mounted cylinders can follow the arc the traction wheels make when extended downward.

28-11-2012 14:16

AdamHeard

Re: pic: Another concept study

It looks like they aren't facemounted, but rather front pivoting cylinders. The crossholes in what appear to be the mounts must be what they rotate on.

28-11-2012 15:16

Garret

Re: pic: Another concept study

What ratio were you using? As Andrew suggested, That sounds more like a problem with the implementation than with the gearing. Our 2011 mecanum drive was around 8:1 and we had plenty of torque and speed to strafe. And besides gearing for torque on mecanum is like gearing for torque in Lunacy, you have just above no pushing power (in most cases) so gearing for high torque doesn't really do anything except slow you down and make you easy to defend against.

28-11-2012 15:16

Tom Ore

Re: pic: Another concept study

You are correct - the cylinders pivot.

Last year we did use fixed cylinders with the traction wheels. The arc error was only about 0.010" so we made the holes oversized to accommodate it. With this concept the tractor wheel rotates too much for a fixed cylinder.

28-11-2012 15:26

Tom Ore

Re: pic: Another concept study

When we run a simple Mecanum drivetrain we run at around 9 fps. This seems to be about as fast as you can go and still get maximum pushing force from the Mecanums.

Last year we ran at about 10.7 fps and had no trouble. With the traction wheels we don't need to worry about the Mecanum's lower pushing force so we can run faster.

This concept is at 13 fps but that doesn't mean we would ever build it that way. Our driver said he didn't go to full speed at 10.7 fps very often so faster than that would have be a need driven by the game.

28-11-2012 19:01

Garret

Re: pic: Another concept study

I love this design (and the detail in the model), but I cannot see how are you going to make the hollow round housing like that for the traction wheels? I am sure I am missing something but it looks like it is just machined from one solid block of aluminum.

28-11-2012 19:22

Tom Ore

Re: pic: Another concept study

Yeah - that one is a bit messy. I have it designed as a tube with end plates welded on and then machined.

28-11-2012 22:41

Greg Needel

Re: pic: Another concept study

I would recommend changing the setup so that the pivot is on the traction wheel and not the mechanum. The reason is that you will see a large bending force on your pivot if you are in traction mode and you get pushed from the side. The distance from your traction wheel to the pivot will act as a moment arm and can reek havoc. Now you are using box tubing which has a great bending strength, but you are also using that same box tubing to house a gearbox, so even a slight bend can really foul up the works.

This is a lesson learned from experience on 148. Take a look at the differences in the drive train between 2010 and 2011.

29-11-2012 17:47

cgranata

Re: pic: Another concept study

With what program did you make this with? About how much money is it to have the program and how much skill is needed to create this?

Our team is looking into this kind of designing.

29-11-2012 17:51

Tom Ore

Re: pic: Another concept study

I used Creo Parametric 2.0. It's free to First teams. It does take skill but no time like the present to start learning.

29-11-2012 17:54

cgranata

Re: pic: Another concept study

Does this program come included in the Kit Of Parts? Or must we request it separately? Also, how do you get FIRST pieces into the program (ex: the CIM motors, omni wheel)?

29-11-2012 18:08

Tom Ore

Re: pic: Another concept study

Start here:

http://www.ptc.com/company/community/first/

30-11-2012 11:56

Siri

Re: pic: Another concept study

I don't see the FIRST KoP on there (Tom's PTC link), so I'll just mention that you can get the files from Autodesk FIRSTbase (you want the STEP format if you run Creo). AndyMark and McMaster-Carr also have a lot of CAD files available for their products.

FIRST teams have access to a lot of free CAD software: Autodesk Inventor, Creo, and SolidWorks, at least. You can do things like this in any of them, if you practice! People here would be happy to help you (but you'll want to find/make the relevant threads).

...

And now back to our regularly scheduled ogle-at-octocanum thread. This is gorgeous! I never even thought about doing a concept study like this; brilliant insight. What's your cylinder bore?

30-11-2012 22:05

Nemo

Re: pic: Another concept study

I've been studying the details of this render in several sittings, and I'm still in the process of absorbing some of them. This is really neat stuff. Thanks for posting!

It looks to me like the speed on the traction wheels would be something like 3.5 times slower than the mecanums, based on wheel size difference and a guess that the traction wheel's gearing is about 2:1.

30-11-2012 23:00

Tom Ore

Re: pic: Another concept study

The speed difference is mostly due to the size difference of the wheels. The CIM drives the center gear. The Mecanum wheel is driven off a second reduction going one way and the traction wheel is driven off the #25 chain and a second reduction going the other way. (I don't have the exact gear ratios handy - must have overwritten the spreadsheet.)

30-11-2012 23:03

Nemo

Re: pic: Another concept study

Ah, yes. I missed the fact that the traction wheel doesn't get the second stage reduction that the mecanum wheel gets.