[FTC]: Questions about 4 motor drivetrain

[Techhexium]

Hello, I am currently prototyping a basic direct drive 4 motor drivetrain with 4 traction wheels (4 inches). Here are my questions:

1) Is there a difference of speed for using 4 motors instead of 2? Like if the robot is heavy, would the 2 motor drivetrain be slower than normal but the 4 motor drivetrain not be as affected?

2) Is there a good way to put in two wheels between the 4 wheels for a six wheel configuration? They won't have any motors installed on them. I have read from the FTC Robotics book that these two center wheels should be slightly lower than the outer wheels (these wheels have motors). I have done that but then b/c the outer wheels aren't really touching the ground they just move in place.

Pictures:
http://imageshack.us/photo/my-images/815/img5649.jpg/
http://imageshack.us/photo/my-images/51/img5650x.jpg/


Thanks for the help!

[TRWSHSHLX]

1) Yes that's correct. The 2 motor drivetrain will be slower than the 4 motor if the robot is heavy.

2) The "drop-center" configuration is good for when the robot have trouble turning because of the 6 wheel drive. When you drop the center wheel, only 4 wheels will be touching the ground at all times so that it's easier to turn. However, this configuration is usually implemented with all wheel powered so that you can utilize all the motors on the drivetrain and won't have trouble turning because of the # of wheels.

For HotShot! our team's robot had a 4 motor 6 wheel drivetrain and it had no trouble turning at all. So maybe you can try just having the center wheel at the same level with the outer wheels first and change it later if needed.

Also, it's nice to have all wheels powered.

[Andrew Remmers]

Thats what I have found out, The motors in FTC are pretty powerful and can handel the stresses of even 8 wheels on the ground, I'll upload pictures of our bot to my google account later and post a link.

But You should be fine even with all wheels on the ground. 1902 has built every drive train so far with the same gear configuration actually just different output ratios or wheels configs.

- Andrew

P.S. Just uploaded the pictures check out the album there are lots of pictures

https://picasaweb.google.com/1140571...73992776004930

[Coach Doug]

We went with a six wheel drive 4 motor fully geared drivetrain this year and was quite pleased.

The drop center is a FRC standard and has logic for their wheelbase but i don't think our 18" square robots really need it.

4 motors vs. 2 will help with speed but that is a function of where each motor is on the tq curve ( less work per motor ).

The real benefit of 4 motors and fully geared is pushing force. We pulled down a white paper in summer of 2010 to concept our drivetrain. Unfortunately I can't find it now when I search but had it on my local so it is attached. It walks thru the types and forces ( pushing and turning ) of each type.

We ended up with 6WD - 2 motors, each side fully geared for added robustness against any type of failure mode.

Didn't turn well on a regular floor - drove and turned like a dream on the FTC mats.

[JesseK]

Originally Posted by Coach Doug

{Attachment}

Great paper!

The last time I coached FTC was for HotShot. The drive train the kids built had 4 motors, 1:1 to 3" wheels. For what it's worth, if maneuverability is all you want, 2 motors 1:1 to 3" wheels is just as good.

The kids wanted to push through defense, so the obvious choice was 4 motors that year. Yet by accident, they discovered (and I learned by watching/coaching) what it takes to make a great defensive robot.

The drive train wheel base was, from what I could tell, somewhat constrained by the rest of the mechanisms. There were 4 drive traction wheels and 2 caster omni wheels. The 4 traction wheels were close to each other and were located at the front of the robot. The students understood that this helped put their center of rotation closer to the front of the robot -- thus the rear wheels were the casters.

This configuration meant that we couldn't push while going forward. So any time we were required to push, we pushed while moving in reverse. That's really when we realized that pushing in FTC is not only about having enough torque and traction, but also about point of contact. You want your point of contact with the other robot done in such a way that it pushes down on your traction wheels, gaining you more traction. This meant that we reoriented one of our U-Channels so that it'd do exactly that -- it's not an angled wedge, but it was able to get under a robot or two just enough to provide us a traction advantage. It's how we pushed two great robots at once in a match during Championship Quals that year.

The third key to our success with pushing that year was the unorthodox wheels we had. There are a great variety of LEGO wheel/tire combinations on ebay and LEGO websites. Finding a tire that fits around a TETRIX wheel is a great challenge, but it's one that's well worth the search. It basically gave us a 1" wide flat tire that meshed perfectly with the rubber foam tiles to give us a great amount of traction. We discovered the tire on accident since a student brought it in from his old home LEGO kit for use as a shooter wheel. Yet I remember reading about another team this year that also used them.

NOTE: no matter how great your defense is, in FTC if you do not have some sort of offense you will fall in the ranks very quickly. If you're not the alliance captain for an alliance, you won't make it to the championships -- so don't put defense as your only strategy . The offensive mechanisms on our HotShot bot also added a good deal of weight right above the traction wheels.

[Techhexium]

Thanks for the advice everyone! From the photos I seen from Joyride's link (I like how your team's robot used Lego structural pieces for the baton dispensing mechanism) and photo albums of other FTC robots, many teams use long channels to put in axles and gear configurations. I think that makes adding wheels easier but requires a lot of gear hub spacers which my team only have 4 of. (My team has two Tetrix base kits worth of materials but that's it)

I also find from previous experiences that gear configurations without anything heavy installed on the chassis tend to un-mesh easily in a short amount of time driving.

I find my 4WD drivetrain (without the two center wheels) can turn well on both the floor and FTC mats. I'll try putting the two center wheels without drop centering them and see how that goes.