Different Drivetrains

[GeeTwo]

It's not nearly as cool as a swerve, but we did not use pneumatic wheels, nor treads. We beefed up the usual 6WD to a 10WD, featuring three different wheels (2 2015 KoP wheels, 4 2010 KoP wheels, and 4 8" plaction wheels with pebbletop tread). They were staggered into four lines because they were too big to put ahead of each other. Here's a pic of the drive train before we installed any manipulators. The pebbletop does most of the work on the carpet and the slicks do a fair share of the work getting over the defenses.

[Steph342]

Not the best picture but here is what we did on Team 5508. It is 2 drive trains. One with treads and one with 4 wheel (2omnis and 2 traction). It worked good and only broke a tread once.

[ahartnet]

A pretty good picture of all our wheels + chain. It did pretty well for us, and I was amazed at how easy it was to turn the wheels by hand on an unpowered robot. Base team did a great job of chaining it. essentially 6WD, center drop, but each side had an additional 4 raised wheels to help with rockwall/moat/ramparts. We used the vex wheels in a 1.5" configuration so they couldn't get stuck in the rough terrain.

[emeraldstorm]

Originally Posted by ahartnet

A pretty good picture of all our wheels + chain. It did pretty well for us, and I was amazed at how easy it was to turn the wheels by hand on an unpowered robot. Base team did a great job of chaining it. essentially 6WD, center drop, but each side had an additional 4 raised wheels to help with rockwall/moat/ramparts. We used the vex wheels in a 1.5" configuration so they couldn't get stuck in the rough terrain.

That paint job is sexy. 1x1 steel tubing I assume?

[ahartnet]

Originally Posted by emeraldstorm

1x1 steel tubing I assume?

1x1x.125 aluminum tubing, welded, powder coated. We have a sponsor that can do welding for us so we were able to get by with 1/8" aluminum.

Also, this powder coated frame became our practice bot because we didn't come up with a rig for keeping axle holes aligned during the welding process =(. It was late in the season so we tried a fix by match drilling some plates and drilling out the axle holes on the inside runner for our practice bot (I'm surprised, but I can't find a good picture of it). Since it worked, we spray painted our frame originally intended as a practice bot frame and implemented the fix on the powdercoated frame during/after bag and tag. I liked to refer to our comp bot frame as the Ugly Duckling.

[emeraldstorm]

Originally Posted by ahartnet

1x1x.125 aluminum tubing, welded, powder coated. We have a sponsor that can do welding for us so we were able to get by with 1/8" aluminum.

Also, this powder coated frame became our practice bot because we didn't come up with a rig for keeping axle holes aligned during the welding process =(. It was late in the season so we tried a fix by match drilling some plates and drilling out the axle holes on the inside runner for our practice bot (I'm surprised, but I can't find a good picture of it). Since it worked, we spray painted our frame originally intended as a practice bot frame and implemented the fix on the powdercoated frame during/after bag and tag. I liked to refer to our comp bot frame as the Ugly Duckling.

That stinks. Very few teams powdercoat their chassis, and it's a shame that yours couldn't be displayed in-comp. Performance over looks anyway, right?

It's a shame you guys couldn't make it to St. Louis, you've got a fine looking bot.

[Paul Richardson]

Originally Posted by emeraldstorm

Did your bot turn easily with the grippy wheels in the center? Did it move quickly? Did you have to worry about the omnis taking the force of hitting the obstacles?

I feel like I'm missing an obvious design cue with this. My first thought was "they should've put the pneumatic wheels on the ends and the omni in the center so it could absorb the hits while still being able to turn (with the raising and lowering of the front and rear)", then I wondered if rotation would still be feasible with the friction.

When you think about turning scrub (the resistance to turning from friction), it helps to look at the path each wheel will take when the robot pivots, because this determines the friction force you'll see.

To start simple, imagine a Segway rotating in place about its center. If you traced the path of the wheels, they would draw a circle, right? If you looked at the wheels at any point along that circle, they would be pointing tangent to the curve. In other words, in order to follow the curve, the wheel just needs to roll forwards as the Segway rotates, and there is no sideways movement (which would cause friction). So you can see that the center wheels of a robot have almost no contribution to turning friction.

However, if you switch to a 4/6/8 wheel robot and imagine the circle traced by the corner wheels, you see that the wheels are not tangent to that curve. The corner wheels can't just roll along, they have to slide sideways, too. This is where nearly all turning resistance comes from.

[emeraldstorm]

Originally Posted by Paul Richardson

When you think about turning scrub (the resistance to turning from friction), it helps to look at the path each wheel will take when the robot pivots, because this determines the friction force you'll see.

To start simple, imagine a Segway rotating in place about its center. If you traced the path of the wheels, they would draw a circle, right? If you looked at the wheels at any point along that circle, they would be pointing tangent to the curve. In other words, in order to follow the curve, the wheel just needs to roll forwards as the Segway rotates, and there is no sideways movement (which would cause friction). So you can see that the center wheels of a robot have almost no contribution to turning friction.

However, if you switch to a 4/6/8 wheel robot and imagine the circle traced by the corner wheels, you see that the wheels are not tangent to that curve. The corner wheels can't just roll along, they have to slide sideways, too. This is where nearly all turning resistance comes from.

That's some interesting information. On my way to redesigning my concept drivetrain due to this; Thank you! (is it ok to mention that I repped?)

[Thayer McCollum]

We built a pretty interesting drive-train this year, it was a 8 wheel, drop center, articulated tank drive. The articulation was what made it really interesting. One side of the drive-train (4 wheels-2 CIMs) was solidly fixed to the frame, but the other side was on a pivot underneath the frame. This meant that the sides of our drive-train could be in different planes at the same time. This was really effective at getting over the ramparts specifically because we could drive right up the middle and have both sides of our drive-train flat against both slopes of the ramparts.

https://www.thebluealliance.com/match/2016code_qm3

You can see in this video our robot really bounced back and forth, this was because it would rock on the pivot point due to it not being perfectly balanced.

[happyWobot]

We went with a 35" long x 24" wide perimeter with 4 wheels, 12.5 inch pneumatic configured with #35 chain and two AM Tougbox Minis at 12.75:1 with all wheels dead axle and using Tank Drive as the logic. Wheelbase was about 20 inches axle to axle with a 7 inch separation between tires. This might seem fairly straightforward but here is what we did different.

Chassis floor was 7 inches off the ground to mount the wheel axle below the chassis and bumper was 6 inches off the ground to clear the rock wall. However, while crossing most defenses was silly easy, turning was a problem with significant shudder. The wheels were plastic frame and would bend on turning while the tire shell had knobbies with a lot of grip. The wheel frames would bend so much in a turn that significant energy would get released when the tire knobbies released from the carpet causing the bot to literally bounce several inches off the floor on carpet.

So we tried something different, we dropped the front left and back right wheels 1/8 inch simulating a drop wheel config but on the diagonal. This was highly effective in taking a lot of the shudder out. The diagonal list was barely noticeable and proved not an issue in driving straight. But since the wheels were pneumatic and flexed enough to still maintain some contact with all 4 wheels, the bot still shuddered some, just not violently anymore. We found a slick black duct tape and covered the rear wheels to permit them to drift. Turning was no longer a problem and the config was ridiculously effective at crossing defenses permitting us to avoid the Evel Knievel breaching method.

The diagonal wheel drop and tape cover worked remarkably well even on wheels that have a lot of surface contact due to their diameter and any rocking action was practically unnoticeable. I spent significant time trying to find some reference to someone else having done it before to see if there was any insight on the best wheelbase separation and drop height. Could find no evidence that it had been tried before. I joked a couple times that we should call it the East Coast drive.