please, talk to me about robot drive

[Dasistmeinmoped]

sure figuring out gear ratios, max pushing power, current pull and such was pretty easy, but setting up some wheels to do the job.. not so much. I have been trying to research some drive designs but seem to be hitting a brick wall in my progress. to make this easier to read here's just a list of design ideas. PLease comment slash ad!
-6 WD - saw design with all traction wheels all wheel contacts coplaner
-6 WD - Drop center, all traction we prototyped this and found it near impossible to turn while stationary. The center axle was dropped 3 washers (french national measurment, its real look it up..) which was about 3/16" i believe
-8 WD with some funky mechanism that lifts the front, and rear wheels off of the ground as to shorten the wheel base.
-6 Wheel swerve where the front and rear wheels move up to 80 deg. (view mars rover)
-6 WD coplaner- middle and rear traction, front omnis <-(prbly will proto type this withing the weekend)

thank you for any input. We are looking to have a strong stance on the field, so in any design some amount of tractionwheel is desired.

matt

[Laaba 80]

There are also different types of holonomic drives: where a robot can move sideways or other directions without turning. Someone elso on CD posted a great powerpoint with all kinds of info on these drives. I'll look for them and post a link to them for you if I can find them.
Joey

[Laaba 80]

http://www.chiefdelphi.com/media/papers/1836 here is the info I mentioned. It has some great info, along with pics and sketches. It really is a worthwile thing to check out. Great job to the creator of that presentation, it really taught me about them.
Oh yea, nice German in your Username
Joey

[lukevanoort]

I'm curious about your dropped centre wheel 6WD; would you happen to have a picture of it? I ask because that is a very unusual problem to have. In the last two years, my team has done two 6WDs with a dropped centre wheel; instead of having turning be too difficult, we sometimes have issues with turning that is too easy.

Another PowerPoint that might be handy is the one in my team's site's whitepapers section.

EDIT:

Originally Posted by Dasistmeinmoped

1) 6 WD - saw design with all traction wheels all wheel contacts coplaner
2) 6 WD - Drop center, all traction we prototyped this and found it near impossible to turn while stationary. The center axle was dropped 3 washers (french national measurment, its real look it up..) which was about 3/16" i believe
3) 8 WD with some funky mechanism that lifts the front, and rear wheels off of the ground as to shorten the wheel base.
4) 6 Wheel swerve where the front and rear wheels move up to 80 deg. (view mars rover)
5) 6 WD coplaner- middle and rear traction, front omnis <-(prbly will proto type this withing the weekend)

(numbering added by me to make this easier to respond to)


1) What is a saw design? Assuming this is just a coplanar 6WD, here are my thoughts: If #2 is giving you turning trouble this definitely will. Of course, this depends on how fast you choose to drive, but with all traction wheels you are looking at a lot of scrub. 25 does quite well with this design, but they mess with their wheels a lot and their turning still is not the smoothest I have seen.
2) Dropped 6WD is probably my favorite drive design. 61% of world championship finalists since 2005 have run a 6wd. Of those, I believe that only team 25 doesn't lower their centre wheel. Turning is usually very good, traction is very good, weight is okay, and they handle uneven terrain quite well.
3) Can you elaborate more on this design, I'm not sure what you mean.
4) This would be very complicated and probably have minimal improvement over the 6WDs. If you are going to make four swerve modules, I'd just make a swerve drive. With this design you are getting the worst characteristic of swerve designs--complexity--without gaining the ability to drive sideways.
5) This should work well. I'm a strong believer in the capability of the lowered centre wheel 6WD, so I personally probably wouldn't go this route. That isn't to say it is a poor choice, most people who go this route probably find the lowered centre 6WD's rock problematic. I don't have a problem with a little rock, so I choose not to do it. If you do end up prototyping this, would you be willing to try putting the two omni wheels at opposite corners (ie front left and right rear)? I have always been curious how such a drive would perform; it seems like it would be handle very nicely, but, as far as I know, nobody has done it.

[ewankoff]

We used the 6wd system with the center wheel lowered about 1/8" and we had a 0 turning radius. I think this is the most straight forward/most effective method of 6wd that I know of.

Now that I think about it the swamp things had an interesting setup. they had 6wd that had the outer wheels co-linear and the center wheels further apart. I would appreciate it if someone could tell us about how this design worked. Were the center wheels lowered? What is the traction benefit from this design?

[lukevanoort]

Originally Posted by ewankoff

Now that I think about it the swamp things had an interesting setup. they had 6wd that had the outer wheels co-linear and the center wheels further apart. I would appreciate it if someone could tell us about how this design worked. Were the center wheels lowered? What is the traction benefit from this design?

The centre wheels were lowered. There isn't a traction benefit from running wheels like that, they are arranged like that to gain the greatest stability with the octagonal (or hexagonal in 2007) frame design. The frame design is the real magic of that design; when teams try to push or pin SwampThing (or if the robot just runs into something), the opposing robot/object tends to get diverted off to the side instead of heavily impeding their motion.

[EricH]

A 6WD drop style should turn on a dime. If it doesn't, there may be other problems. Ways to fix a non-turning one include: increase drop (not applicable here, you may even have too much, generating a huge rock), add power, and decrease traction on front and/or back. 330 (one of the champions in 2005) once tried six high-traction wheels. They didn't like the resulting maneuverability, so they put kit wheels on the front. Problem solved. They had both front and back less traction in two different years and never had a problem.

[s_forbes]

Originally Posted by EricH

A 6WD drop style should turn on a dime. If it doesn't, there may be other problems. Ways to fix a non-turning one include: increase drop (not applicable here, you may even have too much, generating a huge rock), add power, and decrease traction on front and/or back.

In addition, it helps if the robots center of gravity is directly over the middle wheels (or as close as possible).

[EricH]

Originally Posted by s_forbes

In addition, it helps if the robots center of gravity is directly over the middle wheels (or as close as possible).

Yes and no. Granted, it'll turn better. It'll also rock better. Having it on one end will simply shorten the wheelbase and you'll turn on four wheels. Take a look at 330 in 2005 and 2007. The CG was aft, and they still turned well.

[Dasistmeinmoped]

Originally Posted by ewankoff

We used the 6wd system with the center wheel lowered about 1/8" and we had a 0 turning radius. I think this is the most straight forward/most effective method of 6wd that I know of.

Now that I think about it the swamp things had an interesting setup. they had 6wd that had the outer wheels co-linear and the center wheels further apart. I would appreciate it if someone could tell us about how this design worked. Were the center wheels lowered? What is the traction benefit from this design?

could i ask what size/kind/tread wheel you used, and what your gearing was? one motor or 2 per side?

matt

[Dasistmeinmoped]

http://s2.photobucket.com/albums/y20...t=DSC00825.jpg

above is a pic of the drive we prototyped, thos are 8"X2" wide rough top ifi wheels, 1 cim per side, with a gearing of ballpark 14:1. We use dthe existing gearbox from our 2006 robot, yes that our shooter sneaking into the pic, hah

[Swampdude]

To comment on the swampthing design: our 2005 design had inset front and rear wheels, which was pointed out earlier, only because of the hexagonal frame shape. I do not believe it improved manuverability, just helps you get out of pin holds (and it does). However the center wheel was lowered about 3/8-1/2", which is quite a bit more than most teams do it. This was an outcome of excess frame warpage during welding, but turned out to be the perfect amount. Reason is those center wheels sink into the pile of the carpet and let the outer wheels drag if you don't lower them enough. But that 2005 chassis had a very well balanced cg in the center of the bot.

I also wanted to bring up our 2007 chassis, which is very interesting and I haven't seen anyone else try it. It actually has the same performance as our 05 chassis, but much easier to design, build and maintain. basically only the rear 4 wheels are driven, and an extra set is added in the front to give the 6w stability. The thing is keeping the cg over the center wheels. This performs exactly the same as a 6wd but is lighter, simpler and more durable. The misconception I think is more traction is had by driving all 6. I think we showed the performance of this drive system in the final matches of the championship to everyone that it was more than capable of pushing bots and getting around. Where the majority of your CG is, is where your traction is, so keep it to the center on a 6wd. Also I'll admit, we never once tightened or messed with the chain the whole season. Not 1 single drive train maintenance issue, unlike all of our previous years. Special thanks also goes to the amazing Andymark transmissions.
With 6wd bots you need to have a rigid chassis to keep the teeter action. Also I don't know about other regionals but at nationals, the floor planks are all warped so at times all 6 wheels may touch down. So the more your vertical wheels are offset, the less impact this has. Of course too much wheel offset is a bad thing also, I wouldn't go over a 1/2".

[Ben Piecuch]

Based on the numbers you presented, your prototype drivetrain is geared for about 10-11 feet per second. With only one CIM motor to drive each side, it's a little too fast and a little too weak to effectively turn that robot. I would suggest:

- adding a 2nd CIM motor
- reducing the speed (going to a larger gear reduction)
- going to a smaller wheel (effectively changing the ratio again)
- going to a less "grippy" wheel

Many teams try to design a single speed drivetrain for around 6-8fps. Multispeed bots will see ranges from 4-10fps. Some teams push for that 10-12fps, but they're typically running no less than 2 motors per side. Best of luck.

Bengineer

[GMKlenklen]

And if you feel like moving on to 4wd... I'd suggest four cims, two gripies, two omni-wheels. arranged like so:
front left = gripie
front right = omni
rear left = omni
rear right = gripie

or a mirror image...

this reduces scrub to a negligible amount, while keeping it hard to apply torque to the drive train in order to turn you around (a problem if you load both omni wheels in the front or back, or less noticeably on one side). the only draw back is that your robot may have a bit of a "pimpwalk" as it bounces around on the omni-wheels... or your heavy enough that it doesn't happen.