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Unread 16-05-2011, 15:08
Peter Matteson's Avatar
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Re: Mecanum Einstein this year

Quote:
Originally Posted by JesseK View Post
I think the next trick to figuring out how complex drive trains should be is clarifying which of the D/T's on Einstein (or Top25) were:
  • KOP (with perhaps sprocket gearing changes or different wheels, but all else the same)
  • 1 or 2-speed
  • Full-traction versus omni's on the corners
  • Direct-Driven
  • 4/6 motor
  • etc

Speeds in high gear may be interesting to see, but really those are more about what a team's strategy is rather than 'best drive train'. I suspect we'd see a wide variety of skid steer designs in the top ranks.
177 used the latest revision on our 6wd that has gone through a lot of optimization over the last 6 years.

The stats:
4 CIMs
6x 4" AM performance wheels with roughtop tread.
AM shifter "guts" repackaged/repurposed and final ratios modified.
2 wheels are direct drive
~14 fps high gear
~5 fps low gear
Everything is packaged into a 2.5"x2.5" extrusion.

This year we used low gear so rarely that that our driver ocasionally forgot to shift when he got in a pushing match.
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2010 Championship Winners/Newton Division Champions
Thank-you 294 & 67

2009 Newton Division Champions w/ 1507 & 121
2008 Archimedes Division Champions w/ 1124 & 1024
2007 Championship Winners/Newton Division Champions w/190, 987 & 177 The Wall of Maroon
2006 Galileo Division Champions w/ 1126 & 201
www.bobcatrobotics.org
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Unread 16-05-2011, 15:34
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Re: Mecanum Einstein this year

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Originally Posted by Peter Matteson View Post
6x 4" ... wheels
What was your rationale for the small wheels?

I'm thinking 1) less gear reduction necessary and 2) lower center of gravity

are there other reasons?


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Unread 16-05-2011, 16:14
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Re: Mecanum Einstein this year

Quote:
Originally Posted by Ether View Post
What was your rationale for the small wheels?

I'm thinking 1) less gear reduction necessary and 2) lower center of gravity

are there other reasons?

The question I always ask the people at competitions who question are small wheels is, "Well, why do you use big wheels?".

Usually the answer is either they don't know, or they always have.

We use small wheels for a lot of reasons, but they mostly stem from two of ours teams primary design goals; less weight, and less friction.

Small wheels are physically smaller, which is less weight.
Small wheels require less torque to turn to achieve the same force on the ground, less force in the shafts/sprockets means smaller and lighter parts.
Small wheels need less reduction, which is both a direct decrease in weight, but also a decrease in friction losses as we can run less stages of gear reduction total.
Small wheels let you have a slightly longer wheelbase for all other factors the same.
Small wheels are cheaper for us to make, as it's a smaller diameter stock, and has much less wasted material.

Also, experimental data has shown that for rough top tread, smaller diameter wheels have more traction.

To summarize, teams with small wheels usually have done actual engineering and reasoning behind their wheel size choice. Many teams with larger wheels do not (although some do, and decide to use KOP or other COTS wheels of a larger size for some logical reasons (like 111)).
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Unread 16-05-2011, 16:27
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Re: Mecanum Einstein this year

1503 used 6X4" wheels, 1/8" drop. We used blue nitrile tread and didn't change tread once.

Our gearboxes were little innocent CIMple boxes with a secondary 12:18 sprocket reduction.

Fun fact about the CIMple boxes; if you didn't Loctite your motors in, they would work themselves loose after about two minutes of usage.

-Nick
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Unread 16-05-2011, 16:28
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Re: Mecanum Einstein this year

It's also worth noting that it's somewhat easier to use bigger wheels for newer teams.

Personally, I dig small wheels. 4 inch wheels are a biiiig weight cut when you add all the parts up.
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Unread 16-05-2011, 17:25
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Re: Mecanum Einstein this year

Quote:
Originally Posted by Ether View Post
What was your rationale for the small wheels?

I'm thinking 1) less gear reduction necessary and 2) lower center of gravity

are there other reasons?

Adam pretty much nailed it below.

Quote:
Originally Posted by AdamHeard View Post
The question I always ask the people at competitions who question are small wheels is, "Well, why do you use big wheels?".

Usually the answer is either they don't know, or they always have.

We use small wheels for a lot of reasons, but they mostly stem from two of ours teams primary design goals; less weight, and less friction.

Small wheels are physically smaller, which is less weight.
Small wheels require less torque to turn to achieve the same force on the ground, less force in the shafts/sprockets means smaller and lighter parts.
Small wheels need less reduction, which is both a direct decrease in weight, but also a decrease in friction losses as we can run less stages of gear reduction total.
Small wheels let you have a slightly longer wheelbase for all other factors the same.
Small wheels are cheaper for us to make, as it's a smaller diameter stock, and has much less wasted material.

Also, experimental data has shown that for rough top tread, smaller diameter wheels have more traction.
Historically we used the 4wd 2001-2005 (disclaimer: I haven't really looked at any of our robots older than this) with 6" pnuematic wheelchair wheels and 6" skyways in 2001 & 2003. In 2004 and 2005 we used 6" pnuematic and 6" omnis with a T-kats based 2 speed.

In 2005 we had a lot of drive issues, marking the carpet with our omnis and breaking KOP skyways when we switched over to them. That was the last year I let our team leader use the excuse "This is easy we've done it before." as a logical arguement in the decision of what drivetrain to use. In 2004 we had pushing power and the robot was designed to operate in the limited perimeter of the field. 2005 we were outgunned and spent the whole season fixing issues.

In 2006 we clean sheeted our drivetrain ideas and arived at going with a 6WD drop center with AM shifters an d 4" wheels. We made bearing blocks that bolted to a 1x1x1/16" box Al space frame. The performance was good for the way we played the game but we saw weaknesses in the design with the reliability, wheel alignment and maitenance. We used the brand new IFI wheels that year, which were good but did not reach the robustness of where they are now at that time.

In 2007 I designed the first parallel plate design that the Bobcats used as a way to keep the performance of the 2006 robot but improve the robustness of the 2006 design. This purpose of this was to improve mainenance, modularity, and reliability. In doing this we used AM shifters with custom output shafts and lost the outer steel plate from AM to shed weight and remove redundancy. We also made our own wheels because we wanted wide wheels in the corners(these can be seen elsewhere on CD). We never lost a chain, replaced a tread or had any issue with the drive. We still use this robot to practice although we probably need to replace bearings and motors at this point.

In 2008 we had to speed up the robot for the game, so we took the opportunity to improve packaging further and tighten everything up. At thispoint the drivetrain was mostly designed by another engineer on the team. We also narrowed our wheels because we did the math and realized the wider wheels didn't really do anything.

In 2009 we did our first live axle and cantilevered the wheels as well. We packaged the transmissions and all in a 2x2 box because we needed to package low and tight for our design that year. After this with the introduction of the AM hex bearings we hex broached everything.

Our 2011 drive was an evolution of the 2009 combined with our parallel plate system from previous years. We put the wheels and all inside the box this year combining the best of the 2008 and 2009 designs. This is what we planned to do for 2010 until we saw that game.

I hope this explains some of the thought behind how we got where we are as an example for other teams. I will openly admit we think there is another jump in highly mobile drive trains that teams are starting to go through right now and we think we have to put some work in to plan ahead for it.
__________________
2011 Championship Finalists/Archimedes Division Championships w/ 2016 & 781
2010 Championship Winners/Newton Division Champions
Thank-you 294 & 67

2009 Newton Division Champions w/ 1507 & 121
2008 Archimedes Division Champions w/ 1124 & 1024
2007 Championship Winners/Newton Division Champions w/190, 987 & 177 The Wall of Maroon
2006 Galileo Division Champions w/ 1126 & 201
www.bobcatrobotics.org
"If you can't do it with brains, it won't be done with hours." - Clarence "Kelly" Johnson
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