Drivetrain First Thoughts?

[Hawiian Cadder]

has anyone considered mechanum wheels with rollers not at the standard 45 degrees, if the rollers were at say, 60 degrees, then it would be a much more powerful robot in one direction, however strafing might become difficult.

[MrForbes]

You can play with the friction of the rollers in the wheel assembly to achieve a similar effect.

BUt no matter what you do, it's difficult to get much traction with a mecanum.

[Gdeaver]

Our team could do swerve like last year. Or have a 6 wheel base completed within 2 weeks. The swerve is going to go out to week 4 and the programming team will then go crazy. Does a swerve really give an advantage this year? Or is the pick and place with a mini bot a better focus for resources. Today we will battle these issues.

[midway78224]

My first thought was that its pretty much a wide open field just like aim high with a few odds and ends. So my team thought of a six wheel drive with the center drop. We figure that you need alot of speed but also power if someone defends you. So, we are thinking of a 2-speed transmission to give us that speed and power combination that we need.

[thefro526]

I think we'll be able to go back to 2005 through 2008 style drivetrains.

I'd say the easiest and best drive is probably a 6WD, the Kit-Bot looks extremely promising this year. 8WD also has it's pluses and as do Omni drives.

I'd say as long as it's fast and maneuverable with some finesse it's going to work well in this game.

[Bill_B]

Originally Posted by liam.larkin

Speed and manuverablility is the key here. Additionally, mechanum makes it easy on your programmers gives them that "shuffle/crap" ability to get in line with your target ...

Typos can be such fun sometimes. No, I'm not being crabby about it!

[Cyberphil]

Originally Posted by liam.larkin

Cyberphil I am shocked you would not consider MECHANIUM........Figured 103 would be locked on that for sure.

I appreciate the sarcasm!

I am not a big fan of mecanum, and neither is our team (obviously). I do not dislike other teams for using it though. I actually applaud teams that use them! I think in the right configuration and implementation it could be optimal. I just do not think it is optimal this year (And surprisingly, neither does our team)!

[Kevin Sevcik]

If your argument it that scoring in 2007 was easy with a 6WD, you might want to look closer at how teams were doing it. It looks to me like the 6WD bots were approaching the scoring pegs from the side to line up and drop. Which worked really well with a widely spaced round grid.

This year's grid is flat, and more tightly spaced. I think approaching a peg from the side is going to be significantly more difficult than it was in 2007.

[Walz_706]

The only problem with mecanum is it is slow and not powerful. Im assuming, with four poles, 6 robots, and limited sight (do to game pieces getting in the way) you are gonna hit other robots and you want some power behind it.

http://www.thebluealliance.net/tbatv/team/706

[delsaner]

A road we are considering going down is a chassis similar to ours last year (8WD driven by 4 CIMs). We are considering 6WD, but i am not sure how many motors we are going to put on. Unfortunately, we are only limited to 4 CIMs this year, so we may have to limit to 2 CIMs total, which surely would decrease our speed.

One thought I have had, but not sure how exactly to execute, is to have 3 CIMs power both sides of the drive train.

[meebee]

Originally Posted by theprgramerdude

What gearing ratio would you use this year? The kit of parts uses CIMple Boxes now, instead of the tough boxes, meaning we suddenly have a 4.67 ratio instead of the usual 12.75... While this may not be a big deal to veteran teams, I think the rookies that use it will end up creating bullet-speed projectiles launching all over the field.

Has anyone else noticed this? From my calculations the kitbots will be bolting at a frightening 15.5fps. Anyone remember FPS calculations of previous year kitbots? I'm pretty sure they have been much less than 10fps.

[apalrd]

I did some math on the KOP drivetrain, and with a reasonable speed loss constant (about 85%), it will be around 14 ft/sec (about what you got, meebee). That said, it will have relatively poor acceleration and very poor pushing power (it becomes traction limited at about 100 amps per CIM, with 4 CIMs).

About the 2motor vs 4motor thing - You have one 290 watt Fisher-Price and FOUR 250-watt RS550 motors (assuming the 18v 775 is correct, they are about 260 watts at 12v, but at a lower speed and higher torque than the 550's).

I would say that a 4-motor drive is a MUST. You have enough motors to build a very fast mechanism with the FP and BB motors. Remember:
A CIM does ~340 watts
A FP does ~290 watts
A 550 does ~250 watts
The FP last year did ~180 watts.
You can have almost a CIM with a FP. Two 550's gets you a lot more power than a CIM. You probably don't need to liberate the CIM's from your drivetrain, and if you do, at least put two 550's in to replace them (the andymark FP planetary works for 550's too).

[dtengineering]

For Rack'n'Roll we built a mecanum drive... for many of the reasons that have been advocated here.

It was great... I'm glad we did it... and when we eventually got everything working and our driver learned how to maximize the freedom it gave him, it worked great.

But it came at a cost... mecanum requires much more software development than tank drive, particularly if you want closed-loop feedback for motor speed controls. Or at least it DID... the Jaguars are well set up to offload speed control processing from the main controller, and even then, running PID speed control in labview is next to a "no-brainer" compared to what it was on the PICs. (I actually liked the fact that you had to write efficient code to accomplish complex operations on the PIC...)

Mecanum also required a lot more time for the driver to learn how to maximize it's abilities. A good tank-drive push-bot is easy to learn... but mecanum gives up raw pushing ability for elegant maneuverability. It takes longer to learn to be elegant than it does to be a brute.

In the end, I think from a competitive point of view we would have been better of to go with a 6wd system... but people still love to check out the mecanum wheels and see the robot strafe. If you want a robot that is fun after April, you might consider taking a risk on mecanum drive. Just keep in mind that you're likely building a mecanum because it is cool, not because it is necessarily the best drive train for playing this particular game.

Finally we built a 4 CIM, 8wd for last year's game. We put 14:1 gearing on it, direct drive to 6" VEX Pro traction wheels. That robot could accellerate to full-speed, come to a full stop, turn 90 degrees and get back up to full speed basically as quickly as our less-torquey mecanum drive could. We had basically achieved maneuverability close to that of the mecanum but in a robot with around 200 pounds of pushing force and the ability to "get air" over the bumps.

I'd suggest that high rates of accelleration, rather than top speed will be critical for the game. Consider how much time you will spend getting up to speed, and slowing down relative to the amount of time you will spend at top speed. If you want the best of both worlds, a shifting gear box might be in your design, but that is only useful if you actually shift gears frequently during a match.

Jason

[noa_n_f]

What is everyone's opinions about a swerve drive for this year's challenge? You would have the maneuverability of a mechanum drive but also the traction of a tank drive. Is it a matter of difficulty when it comes to building and programming that prevents teams from using swerve drives or are there other reasons?

[PatrickS]

Originally Posted by noa_n_f

What is everyone's opinions about a swerve drive for this year's challenge? You would have the maneuverability of a mechanum drive but also the traction of a tank drive. Is it a matter of difficulty when it comes to building and programming that prevents teams from using swerve drives or are there other reasons?

The swerve drive is an excellent mobility platform. I think the problem however arises from as you said the seemingly complex nature of the system. This hurdle only becomes more significant if you are attempting it for the first time during the season. You only have six weeks and most teams want to spend as much time as they can practicing and refining - not implementing something new.

For the challenge this year I can see a good old 6 wheeled robot doing fine. Although I will say I really do want to try and implement a swerve drive sometime off season.