Drivetrain First Thoughts?
 

A drive system like the Omni or the Mecanum seems most fit for this year's game. Any first minute thought? The drive train I spend hours making yesterday came out to be useless for this...

Re: Drivetrain First Thoughts?
 

The key to the small robot will be making something that's FAST! I don't know how putting the minibot higher on the pole will play out right now, but it seems interesting.

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My initial thought is KISS. No obstacles or special terrain means my initial reaction is the tried and true 6WD drop.

All subject to change as I go through game and strategy analysis of course.

Re: Drivetrain First Thoughts?
 

First thing I thought was fast, I think speed will be fairly important in this game. The faster you complete the objective the more you get to block the other alliance. My next thought is strength; I feel having a stronger drive train will give you an edge in blocking... then there is the being able to move any direction giving you an advantage in scoring because of maneuverability.
Drive train this year seems really dependent on what you team wants to do, not the field elements.

Re: Drivetrain First Thoughts?
 

Speed seems quite important. I too am inclined to go with 6WD.

Re: Drivetrain First Thoughts?
 

My first thought was

Tank drive will never work, since you have such an awful turning radius, unless your scoring system is seperate from the direction the robot is facing.

Next I thought Mecanum, since they can go sideways, and line up great for an elevator style scorer. But then I realized that mecanums are quite slow, so its quite a conondrum.

Could someone outline why 6WD would be good for this year?

Re: Drivetrain First Thoughts?
 

6WD drop-center is doable with the KOP drive train straight out of the box, that's why it's so good for this year. IT WORKS right outta the box, and at a good, balanced speed too. It's good enough that veteran teams should already have the KOP drive train built by the end of today and can now work on their manipulators.

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Well... go big or go home. 8wd swerve! :rolleyes:

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My team is most likely going to use the same mecanum drive as last year, as it'll allow for much easier self-alignment to the pegs (and therefore less error while placing the tubes). We'll just slide along the endline while everyone else will be trying to turn with their clunky 6WD ;)

EDIT: @Grim Tuesday: D'you mean that mecanums are slow going sideways? I agree, but a crabwalk shouldn't last for more than a few feet to align to pegs, so it would be more advantageous to have the additional maneuverability -- sliding > turning

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I was thinking more like 8 wheel omni... Like a giant octagon:eek:

LOL JK I got something cooler.

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Tank drive has 0 turn radius.
The reason why teams tend to go with 6WD Drop is because it shortens the effective wheelbase, reducing wheel "scrub". Basically it makes it so that the robot is able to turn without jumping around or stalling.

Mecanum drive can be just as fast as any other drive system, its all in the gearing.

Re: Drivetrain First Thoughts?
 

Our team is heavily considering our first (successful) swerve drive this year. We figure that extra maneuverability while placing the tubes on the pegs will be extremely helpful. You can also lock the modules in place and run it as a tank drive so we have that option as well. We're hesitating a little because, although we built it during the offseason, we never have tested the electronics or controls (which people say is the harder part)

Otherwise, we'll go with a standard 6WD dropped center. It's simple, easy to make, and we'll be done with it in the first week.

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I think a 4 wheel drive is sufficient. You don't need as much mobility as last year's game required. As long as you have an accurate racking mechanism, I don't think the drive matters as much.

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Check the 60" rule, make sure you know how accurate you can actually be.

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Before you choose 6 wheel drop think of the slight rocking that occurs due to the middle wheels being lower. Another solution to this is omnis at the corners and having all wheels on the same plane eliminating rocking.

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Actually, in this game EVERYTHING matters. I say that because this is one of those games where for the most part everyone will be able to do everything. What matters is being able to do it better. With this in mind, better means you need to place more tubes than others (aka. you need to be faster.) In the end the teams that excel will be those that took the extra time to shave off seconds from their tube handling process (from the ground/ feeder to peg). The drivetrain is also a huge part of this. If your drivetrain is unreliable you won't be able to excel.

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2415 is considering going with a non tank drive system for the first time this year. In the tight quarters of both the scoring zone and the lane, we want the ability to maneuver around defense, both strafing and turning.

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Everything does matter and if the max is scored of 158, the minibots are worth 32% and 68% for scoring tubes. The best option for drive trains is mecanum because of the maneuverability and being able to move to each of the different pegs left to right with out having to turn. I think it is going to be really hard to be fast just because the robots will constantly get in the way of each other making high speeds fairly dangerous if you hit another robot that is also going full speed. Also unlike last year robots aren't competing against the same objects.

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6wd would never work for hanging tubes

http://www.youtube.com/watch?v=p05UsZMKd8I

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Personally, I think speed is the main factor here. Although mecanum is maneuverable, I have a feeling that it is not ideal.

The faster you get across the field, the more pieces you can hang. Period. I think everyone can agree with that.

I have a prediction that all but maybe one of the robots on Einstein will have a 6wd, or 8wd, dropped center. Look at 2007....

http://www.thebluealliance.net/tbatv/event/2007cmp

Re: Drivetrain First Thoughts?
 

Six wheel drive is my first thought.

Re: Drivetrain First Thoughts?
 

In this game, the benefits of mechanum/omni/swerve are most pronounced when considering a manipulator that can only hang tubes on pegs directly in front of the robot.

If your robot's manipulator can hang tubes on pegs not directly in front of the robot, then tank style drive should be sufficient.

Personally, I would keep the drive train simple.

Re: Drivetrain First Thoughts?
 

Why wouldn't you just turn towards the peg with a 6wd system? That would probably be faster than adjusting your manipulator and would make your robot significantly more simple. Like I said in a previous thread, look at 2007. All of the robots on Einstein had very simple, fast claws. They were not necessarily the most complicated. In fact, none of the robots had articulating arms (rotating arm on base).

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Didn't most if not all of the finals robots from 2007 have 6w drive?

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Yes, most bots were long 6 wheel in '07. However, there was an added issue: to be a good robot you had to be able to ramp easily, and most ramps couldn't handle bots in a wide configuration.

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Have you guys realized that there were no height limitations during the game? that means technically if we can physically do it, we can make a 15 ft robot? Now what is the physical limit of the height before it become unstable? Now its some calculation with torque or something, but unfortunately I got an F on that test...

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I believe Squirrel was making a point regarding the 6wd being sufficient for the game as the video shows his team doing fairly well in placing tubes.

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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.

Re: Drivetrain First Thoughts?
 

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

As Mickey from Rocky said "WE NEED FAST GREASE LIGHTING ITALIAN SPEED"

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 and if built correctly provides for tremendous speed. For those who want to pin; cant pin a mechanum they always have a way out. SPEED SPEED SPEED and did I mention SPEED.

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But you sure can block a mecanum.....the ZONE is not very wide....

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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.

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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.

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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.

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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.

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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.

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Typos can be such fun sometimes. :D :) No, I'm not being crabby about it!:rolleyes: ;)

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I appreciate the sarcasm! :D

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)! :p

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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.

Re: Drivetrain First Thoughts?
 

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

Re: Drivetrain First Thoughts?
 

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.

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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.

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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).

Re: Drivetrain First Thoughts?
 

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

Re: Drivetrain First Thoughts?
 

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?

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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.

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That and it's a lot of weight, time, and resources that could be better spent on other parts of the game (for a low resource team like mine).

Re: Drivetrain First Thoughts?
 

Good point. However, from a driver perspective (I drove 3 competitions in 07) we used the side approach often because it was simply a straight line from where we were to where we wanted to score. Also, space was limited between the rack and the side walls, so scoring sideways was almost required since turning and going straight on would require careful positioning. Remember how two bots couldn't even fit through the gap between the rack and wall sometimes?

Side note, holding the tube horizontally allowed us to come at the spider from almost any angle (straight on or otherwise) and score with equal effectiveness and reliability. This year, scoring straight on, or close to it, will definitely be more common, as there is essentially a lane that forces a right angle approach.

Back to the thread. My first thought, 6wd. Second thought? 6wd. You get the picture :)

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I am 100% certain on my design. Its just a simple 4 wheel drive. I will post a picture of the robot later on when it starts coming into shape. It will blow your mind what can be done with something so simple as a 4 wheel drive.

Re: Drivetrain First Thoughts?
 

Here's what 2181 has come up with:

We decided on an omnidirectional drive, so that we can let the robot, in essence, drive itself. When we get to the guidelines, rather than us lining it up, have the robot set itself to angle 0 (via gyroscope) and follow the lines from there. Little room for human error and we'll always be facing the wall with omni drive.


I don't think we'll lose on speed but has anyone ever thought of acceleration being an issue with the omni drive? Just a thought since there isn't as much power in those wheels.

Re: Drivetrain First Thoughts?
 

We had 4wd our first year. We also had a reliable autonomous (so we could get the bonus and usually win matches and qualified highly), and great scouting (so we could get two great 6wd robots to help us) and we won a regional.

If you haven't driven a few different robots, including mecanum, 4wd, 6wd, etc then you might want to do a bit more research before deciding on a drive configuration.

Re: Drivetrain First Thoughts?
 

Hi,
What about a 4WD will blow my mind? I've got a pretty good idea of the physics involved, so I tend to understand the tradeoffs pretty well...

In a narrow configuration:
You'll have either higher turning scrub than a 6WD so you won't be able to turn as well, OR you'll need to reduce lateral traction on some of your wheels (and likely your primary traction as well) OR you'll need to reduce your wheel base which will make you less front/back stable OR you'll need to shift your CG drastically forward/back which will make you less front/back stable.

In a wide configuration:
You'll be MUCH less front/back stable and wider (which makes it harder to get through traffic).

So yes, you can make a 4WD work great for the game -- but there are tradeoffs involved. Typically those tradeoffs involve reducing traction (at least partially) or reducing wheel base or reducing stability.

A "rocking" 6WD allows you to keep full traction while keeping full wheelbase (because it essentially has two short wheelbases that it alternates between. The tradeoff there, is the added complexity of additional wheels.

Note, this post only applies to "skid steer" robots commonly known as "tank drives" -- when the wheels can turn it is a whole different animal.

Please try to remember that you're preaching (in part) to a group of veteran robot builders and experienced engineers on this forum. Conduct yourself accordingly, and you'll get additional opportunities you can't possibly imagine.

-John

Re: Drivetrain First Thoughts?
 

A note for rookie teams:

It isn't uncommon for teams to get to competition for the first time ever, and then learn that their skid-steer robot cannot turn very well (or in some cases at all!)

It is CRITICAL that you test the turning ability of your robot ON CARPET comparable to the carpet used in FIRST games AS SOON AS IT IS BUILT, so that you can make modifications if necessary. (4WD with omni wheels on the rear are simple, lightweight, have good forward/reverse pushing power, zero turn radius... and unfortunately can be spun like a top when pushed laterally in the rear.)

Re: Drivetrain First Thoughts?
 

For what it's worth, 4wd with omnis does have a small front-biased turning radius. They are vulnerable to being pushed, but they are probably the simplest drivetrain a team can build. Low resource teams should look into this drive if they feel they don't want to take any chances with their drivetrain this year. And really, when do you want to take a chance on the most important part of your bot?

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Make sure you test your robot turning on carpet at full weight, as well (including bumpers, battery, and mini-bot).

-John

Re: Drivetrain First Thoughts?
 

Ditto, especially with a potential of 160+lbs of weight on the robot this year.

-Brando

Re: Drivetrain First Thoughts?
 

FWIW, if you're testing early enough, you can play around with where that weight is to see how it impacts your drive as well.

-Sean (joining the cool kids club of signing my post outside of my signature).

Re: Drivetrain First Thoughts?
 

Your post really made me say 'hmm'. It's a very valid concern.

Yet after a bit of thinking, I think robots will not NEED to approach from the side this year due to the very large differences in the plates on the front of the pegs. The room for error this year is quite a bit bigger for the Circle, and still rather large for the Square.

Re: Drivetrain First Thoughts?
 

I am surprised to see so many people referring to Mechanum drives as being slow. In reality, yes, they are inefficient but that doesn't make them slow, gearing is what ultimately determines speed. Like others, I think there is merit in exploring them as an option. Elegant driving can quickly get you away from even some of the best defensive bots.

Not saying we're going with with this system, just saying I don't think it's something i would write off.

Re: Drivetrain First Thoughts?
 

What do you mean by the word "inefficient" in this context?

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I mean that you lose energy through a vector other than forwards. Those mechanum rollers are rolling even when you're going straight, so you're losing energy and it could be taking you more wheel rotations to cover the same distance as a non mechanum wheel of the same diameter.

Re: Drivetrain First Thoughts?
 

You can model a mecanum wheel as a regular wheel of the same diameter and tread material but with 71% the coefficient of friction (assuming 45 degree, friction-free rollers. In actuality, it is >71% for F/B and <71% for L/R due to friction.).

When you aren't slipping the wheel, you get the same speed out of a mecanum wheel as you would with a regular wheel of the same diameter. It is a common misconception (and one I had believed until recently) that a mecanum wheel is necessarily slower than any other wheel.

See here for the math: http://www.chiefdelphi.com/media/papers/2390

Re: Drivetrain First Thoughts?
 

I believe we competed with you guys last year using a mecanum drive and anyone who saw it wouldn't say that it lacked for either acceleration or speed. If I remember correctly you guys do some pretty extensive scouting that involves taking pictures of every robot at competition I encourage you to look us up, also team 2052 from our area produced another very effective mecanum last year.

We actually will probably be going with a similar set up this year. If you want some help with anything feel free to PM me and we can probably provide you with some of the info you need.

Re: Drivetrain First Thoughts?
 

Does anyone know, then, if the friction coefficients listed for AndyMark mecanum wheels are scaled in this manner? You wouldn't want to aim for 12 ft/sec and get 17 ft/sec because the values were already scaled. :)

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Light cycle, anyone? Attach the arm mechanism to the center of the robot, in between wheels?

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In theory, there is no difference between theory and practice. In practice, however...

Mecanum wheels' rollers do spin when driving in a straight line on carpet. This robs them of forward speed. Perhaps on a rigid surface like tile or concrete the "just like a traction wheel" model is adequate, but it falls short on a typical FRC field.

Re: Drivetrain First Thoughts?
 

My team (3154) is seriously considering a mecanum drive, but I would like to know if the drive turns as well as moves horizontally.

Re: Drivetrain First Thoughts?
 

If you build it and program it to, a mecanum wheeled robot can move forward, backward, sideways, at an angle, and also rotate about it's center (or any combination of these at once)

What it can't do quite as well as a robot with "normal" wheels, is get a lot of traction.

Re: Drivetrain First Thoughts?
 

By the very nature of the wheels, you're transmitting some of the force supplied by your motors into vectors other than your desired direction of travel. Because of the angeled rollers, you don't get 100% of the power of your motors at any given time (aside of rotating in place).

Re: Drivetrain First Thoughts?
 

This is where someone will claim that those elseways-directed force vectors don't actually result in a loss of power because there's no motion in the undesired direction. That someone will be wrong, because the field surface is not perfectly rigid and does move sideways when the mecanum wheel tries to drive forward.

Re: Drivetrain First Thoughts?
 

Heh Heh Yeah...


If you keep your arm light 15 feet is pretty simple to do. You can also deploy various outriggers to help keep you stable. But I have to ask, why would you consider this? You never need to go higher than 10'.

Re: Drivetrain First Thoughts?
 

So you mean "normal" wheels aren't 100% efficient either because the field surface shifts backwards when the wheel drives forwards, right? I'm not sure that makes much of a difference between the two wheels.

Do we even have a definition for "efficiency" in this case aside from the arbitrary demand that all force vectors should be pointed in the direction of travel because we think that's efficient? In that case, I'll point out that mecanums are vastly more efficient at turning than a 6WD, and thus are obviously superior by my idiosyncratic definition of efficiency.

Re: Drivetrain First Thoughts?
 

i was considering omni drive but thought of the loss of power our team has done 4 wheel with rear omni's the last 3 years other than lunacy and have had a decent amount of success. but i was wondering how hard it would be to line up to the pegs with a non articulated arm.

Re: Drivetrain First Thoughts?
 

Yes, indeed it does. But it can probably be safely ignored for the purposes of FRC robotics.

In the case of mecanum wheels whose rollers have axial free play, the problem is exacerbated by the axial motion of the rollers. As each roller comes into contact with the carpet, it is forcibly pushed along its axle until the free play is taken up. As it does so, there will be some rotation. When the roller leaves contact with the floor, gravity and vibration re-open the gap. High quality (expensive) mecanum wheels with well-designed bearings in the rollers suffer less from this phenomenon.