31-12-2007 14:43

wilsonmw04

Re: pic: 6WD Design Iteration

Being from a team that does not have an in-house machine shop, this design is very interesting. What are the outside rails made of? Is there some sort of track that the wheels are sliding on to provide tension?

31-12-2007 15:00

lukevanoort

Re: pic: 6WD Design Iteration

The outer rails are made out of T-slot aluminum extrusion, probably made by either 80/20, Bosch Rexroth, or Item (In this case, I think it is 80/20). Specifically, it looks like either 1.5" (40mm if metric) or 1" (25mm if metric). The pillow blocks are probably held in by t-nuts and you'd just loosen the t-nut and slide the wheels to tension. One of the wheel pairs (looks like the front ones) is usually powered directly off the output of a transmission (like an AndyMark SuperShifter). This one could also power the rear wheels by chain and transmit power from the rear wheels to the front.

Although this is much simpler than the West Coast drives that you see made by teams like 254, 4, 968, 60, etc., it probably would be a challenge to make without a machine shop (due to the custom machined pillow blocks). You could probably make it without a machine shop if the outer wheels were running on dead axles (eliminating the need for accurate pillow blocks) and if the center one was directly mounted onto the output shaft an AndyMark SuperShifter (probably using the extra-long output shaft upgrade). You could then use bumpers to protect the chain runs from the center-front and center-rear wheels from being damaged by other robots.

BTW, this design is great! It is definitely going into the collection of possible drive designs for next year. If we are collecting balls again (or something else that requires a ton of vacant space in the center of the robot) I'm definitely going to propose this. Great work, Madison!

EDIT: Okay, now I'm almost 100% certain that it is 80/20 extrusion, because I see some 80/20 Quick Frame in the middle holding in the battery. Between the the smallish extrusion on the sides, C-channel front and rear, and the 1/16" wall tube in the middle, this thing must be pretty dang light.

31-12-2007 15:19

maclaren

Re: pic: 6WD Design Iteration

I like the design!

A couple of questions:

What wheels are you planning on using?

Do you have a weight estimate?

What transmissions are you planning on using?

01-01-2008 11:06

Gdeaver

Re: pic: 6WD Design Iteration

The inner rails appear to be 8020 quick frame material. I would be careful with using the 8020 quick frame connectors when they are going to be subjected to allot of stress. This year our team may use the quick frame material however for stressed and load bearing connections The E-Z tube connectors are much better. They do not match the Quick frame tube dimensions but, a little filing takes care of that. They are available with and without internal steel welded reinforcement.
http://www.eztube.com/connectors/connectors.html
If the Quick Frame flange tube was used you would have a support for a board to lay in the center to add stiffness and mount stuff. How are you going to attach the quik frame to the c channel?

01-01-2008 11:59

Tim Delles

Re: pic: 6WD Design Iteration

Few things:

Looks like a lot of stress to put on 2 "rails"... Have you checked to see how this set up will take the stress (seems like thin material to be using)

Also as for cantilevered wheels: i wouldn't do this. First off if they aren't very well protected it is easy to play defense on you (i.e. stopping you from driving around)

Also why not use all 6 traction wheels with the centered one lowered (similar to the kit chassis) as this will make sure you have full surface contact and will make it harder for a team to "spin" you around.

01-01-2008 12:10

Al Skierkiewicz

Re: pic: 6WD Design Iteration

Maddie,
As you probably know, I am nuts about high current drive systems. Six wheels on the floor is one of those, but I think having the omni wheels in front will go a long way to helping with the current problem. I bet you are planning on setting the middle wheels a little lower as well. But, I bet if you have the ability to move the center trucks back about six inches, you can keep all wheels on the ground, lower the current in turns, still have a stable platform for pushing from the side and have plenty of traction in the forward and reverse directions.
I very much like the approach of minimum machining. I am sure there will be some interest in this design for many teams. If we are at an event together, please drag me over to look at this design, should you use it. I would love to see it in action.

01-01-2008 14:00

Madison

Re: pic: 6WD Design Iteration

Thanks for the interest, everyone. I'm going to answer as many of the questions you've asked as I can, but I wanted to reiterate that 488 won't be using this design for the upcoming season. We've developed a different iteration of the 6WD design that requires a bit more work to build, but it lighter. The prototype we built in September and October involved a lot of machining, but still was finished in less than 20 hours of work and weighed, with electronics, 30 lbs. The design shown here is about five pounds heavier and the iteration we're hoping to use is somewhere in the middle of those two extremes. Our goal is to have a <33 lbs., functioning drive train that is completed in 12 hours. I'm not saying anything about our wheels or gearbox design because the team put me under a gag order. It came to our attention that a team copied our lifting mechanisms based upon detailed views that I shared here and, while we were all flattered, we're not sure we want it to happen again.

This does require some ability to make the bearing plates or bearing blocks. It may be possible to use pillow block bearings sitting atop the extruded rails, but I'm not sure supporting the axles across one bearing -- unless it's a full 1" thick -- is wise.

I'm not aware of anyone that's run dead axles on a cantilevered wheel system, so I can't speak to its effectiveness. It's an interesting idea, certainly, and something I'd want to test on a prototype before putting into a competition robot. I agree that there are all sorts of opportunities for adapting this for different transmission designs and power transmission schemes, though.

Maybe sometime after the intensity of the early build season goes away I can get the CAD files hosted somewhere, though there's nothing particularly interesting about how this goes together. It's reasonably light by almost anyone's standards. The frame is about 4 lbs. lighter than the kitbot frame at 9.1 lbs., including the bearing plates, bearings and appropriate hardware.

The wheels shown are stand-in models based on what I had on hand. For appropriate ground clearance, I'd suggest wheels that are 5" or larger in diameter.

Weight without transmissions or motors is 15.75 lbs. You could use whatever transmissions you wanted with this, really.

I've never used the Quick Frame system, so this is sage advice for anyone interested in using it in the future. I agree that a weak point of this design is the unsupported cross member that creates parts of the battery cage. If it's not somehow supported by the rest of the robot, the weight of the battery may, eventually, deform some of that inner frame, though since its square tubing it should remain pretty rigid.

Which rails are you speaking of? The chassis design is based upon experiences with our prototype mecanum drive. It shares a very similar frame construction and has held up well to abuse, but it hasn't been used in competition.

We had good success with cantilevered wheels last season and there's a long history of success with the arrangement by teams out here. The advantage, really, is lowered weight because there are fewer chassis components. Bumpers go a long way toward protecting the wheels from impact, but our '07 machine didn't have bumpers and managed just fine.

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I hate the "rock" with a passion, really, and that's the only reason we don't do it that way. We built our prototype chassis using no omniwheels and raised the end wheel 1/8" and I'm not happy with its performance. Inconsistency in the tread material and in the carpet means that its performance varies too much for my taste. We've never had trouble with teams spinning us in place, even with omniwheels on all four corners.

We haven't run a drive train like this before -- in that is has omniwheels on one end. We have, of course, run for two years with omniwheels on both ends and some sort of "traction" wheel in the middle. Those setups have all the wheels on the ground and draw very, very little current even compared to our 45 lbs. prototype with a raised end wheel.

We pretty well can build functional, reliable drive trains by now, and lots of other folks have spent lots of time and money on the traction debate, so we've decided that manufacturability and maintenance are the areas we want to improve. We've ended up with something beyond this that is lighter, stronger, though a bit harder to make. I'm pretty satisfied with it and we're looking forward to seeing the new game and adapting and redesigning things to fit. Designing for the challenge is fun, but streamlining your process and getting so good at things that it's second nature is kinda fun, too.

P.S. -- yes, we raised an end wheel on our prototype instead of lowering the center. The distinction probably isn't too important, but I think it's a cleaner design.