Supershifters and Mechanum wheels

[sdcantrell56]

Just to add to this conversation...unless it is a particularly special case (2011 our first pick at north Carolina had them) , any team with mecanum wheels is automatically off our pick list. In fact the only thing our pit scouts do is create a list of all mecanum robots.

[Anupam Goli]

Originally Posted by bs7280

Please keep into consideration that our season is over (we had a good run, first time getting picked) and we are currently working on a new frame from scratch (preserving this years bot) in order to test drive trains. We also have a good amount of money left in this years budget that we can most likely buy the gear boxes (we already had the wheels) especially considering that they are reusable.

This years drivetrain was a 6 wheel tank drive on the andy park standard KOP frame using the long formation with omni wheels in the front and back of the bot, and kit standard wheels from andymark (highgrip) in the middle, all 6 inch. we had 4 cim motors on two cimple boxes. We added the omni wheels because it could not turn for its life on carpet. It worked well in competition, as it could run very well when it was just going back and forth collecting and shooting frisbees. I was pleased with it because last year we were basically a sitting duck with our 6 highgrip wheels powered by 2 cims.

The problems came up when we had two line up our bot to shoot/load frisbees. Unfortunately our bot could not change the firing angle at all, and had no sensor, so the shooting had to be lined up by shooting very specifically at the bottom of the pyramid. It worked great in auto mode, but telop was tricky because with tank drive it was impossible to make slight adjustments to alignment. Especially with lateral movement.

The feeding was the same thing, but easier because we could communicate, still took way too long. I will admit our feeder could have been much better designed. On the optimistic side, lining up on the pyramid was easy and took no moving parts at all for climbing.

I don't have any great videos, but we have some O.K. ones on our website with more elsewhere that need to be uploaded. there really are not any obvious problems at first glance, but it is defiantly not fast, and it does not block/handle blocking well.

Rather than looking at a new drivetrain, let's analyze what was going on with your setup. You said the standard kitbot drivetrain was not turning on carpet. Where was your center of gravity in this setup, and are you sure you dropped the center wheels in the 6 wheel drive by 1/8"? Make sure your center of mass is low and over the center two wheels. If this still does not remedy your issues, look into going to Toughbox gearboxes, or something with a higher ratio. I think the CIMple boxes house a 4.3:1. This ratio is meant to give you a lot of speed, but not much torque. You might need to drop down to 8 or 9:1 in your gearboxes. The toughboxes should be enough, even with high traction wheels, to allow your robot to turn smoothly, given that the center wheels are dropped 1/8" and the center of mass is low and over the center wheels. This is in fact what the kitbot on steroids project by Team 1114 is based on.

Alignment issues are a pure driver practice thing. It's much easier to get your driver to practice lining up in one sweet spot (and with the goal this year being so wide, you don't need to exactly have only one sweet spot) than it is to invest time and resources into an omnidirectional drive, only to use it just to strafe a couple of inches every match. Try expanding your feeding are on the robot. My team's loading mechanism on the robot is the length of 2 frisbees. We had very rare issues with lining up to the feeder slot, and we ran a standard 4 wheel wide drivebase.

[Ether]

http://youtu.be/LlSoEfAW47I?t=3m33s

[Jeffy]

Building a competitive drive train is about building an efficient, stiff and robust base with the correct math done regarding gearing. There are resources a-plenty for learning to do this.

At 159, we had such incredible success with our traction drive last year, that it's unlikely we will ever use a drive without traction wheels.

As per lining up...
I build bots with a philosophy that if you can't line it up with a tank drive and a few minutes of practice then your mechanism is broken and not your drive train.

[z_beeblebrox]

Don't underestimate the power of the kitbot. 2013's new and improved kitbot performs better than many, if not most, custom drivetrains and, possibly most importantly, is quick and cheap, so you have more time to spend on developing other systems for the robot.

Additionally, with simple modifications, such as building 1114's kitbot on steroids, adding higher traction wheels or perhaps shifting transmissions.

Personally, I haven't played enough with Mecanums enough to give a verdict on their performance, but from what I've seen, a decently constructed, well driven tank drive can often outperform all but the best Mecanum drives in everything but going sideways. When considering Mecanum, you have to decide if going sideways is worth being slower and having less traction.

Post #254! Go Poofs!

[Woolly]

Originally Posted by z_beeblebrox

Don't underestimate the power of the kitbot. 2013's new and improved kitbot performs better than many, if not most, custom drivetrains and, possibly most importantly, is quick and cheap, so you have more time to spend on developing other systems for the robot.

Additionally, with simple modifications, such as building 1114's kitbot on steroids, adding higher traction wheels or perhaps shifting transmissions.

Personally, I haven't played enough with Mecanums enough to give a verdict on their performance, but from what I've seen, a decently constructed, well driven tank drive can often outperform all but the best Mecanum drives in everything but going sideways. When considering Mecanum, you have to decide if going sideways is worth being slower and having less traction.

Post #254! Go Poofs!

Mecanum is good for when you need an omnidirectional drive system but don't have the time/resources/money/programming team to devote to swerve or butterfly drive. For example, we didn't make the decision to go mecanum until just before the start of week 2 during the build season, after realizing that an omnidirectional drive could be very advantageous.
I'd take a mecanum kitbot with basic WPI code over a hacked together and poorly programmed swerve drive any day.

If you really, and truly need an omnidirectional drive train, I'd start with mecanum and plan to go to swerve or butterfly later.

If you don't, look at some of the tank drives with traction (and sometimes omni) wheels around FRC, such as Team Titanium's Tremendous Turning Twinspeed Tank Transportation in use on this year's robot.

[bs7280]

Ok, say a team decides to go with a drive base (nothing specific for the sake of helping other teams who view this) for each base, how does one select the "best" gear ratio to be chosen on a bot? Looking at different gearboxes for different drive base types, there are alot of choices and it is quite intimidating to choose considering how expensive they are.

[Anupam Goli]

There is no single blanket "best ratio" to use. It all depends on your strategy and what you want to do. Do you want to be quick? Do you want to have pushing power, but don't want to be necessarily quick? If you are using a single speed gearbox, you can look between 4.67:1 to 13:1. We ran an 8.46:1 ratio in a Toughbox mini. We got about 9 fps and we had decent pushing power. The toughbox might be a good starting point. The good thing about the toughbox is that you can just buy the gears and change the ratio based on your results. You can also adjust the ratio in the sprockets. Remember, the higher the ratio, the less speed you have, but the more torque you will have behind those wheels. Striking the balance is something you and your team have to figure out. If you search in the CD papers, you can find JVN's drivetrain calculator, which allows you to learn everything you want to about a drivetrain before you build it.

[Jeffy]

Originally Posted by bs7280

Ok, say a team decides to go with a drive base (nothing specific for the sake of helping other teams who view this) for each base, how does one select the "best" gear ratio to be chosen on a bot? Looking at different gearboxes for different drive base types, there are alot of choices and it is quite intimidating to choose considering how expensive they are.

We use a two speed and mostly because:
We can set a gear used for acceleration and pushing matches to be traction limited (Torque created at ~40 amps times wheel radius > force of static friction of wheels on carpet)
We can set a gear to get us where we want to go as fast as possible. (balance of acceleration from top speed of gear 1 with top speed)

If you choose a single speed, a balance of top speed and pushing force is ideal. Usually teams set there bot to be right on the edge of traction limited. This is not inherently ideal, but is simple and can give a successful result.

JVN design calc as suggested above is the way to go.

[jbsmithtx]

As above said, there is no "best ratio". It should be fit into your strategy. So, with a wide open field like this year, you might want a higher ratio if you were going to be running cycles to and from the pyramid, so you can cross the court swiftly. Whereas if you were a team that wished to block a cycler, you might have a much lower ratio, so that you could at least push them around and slow them down. However, with a divided field like last year, you would be looking at a totally different setup, with a lower "speed" ratio (because of acceleration up to your top speed). It all depends on strategy and how you think matches will end up being played out.

[craigboez]

Originally Posted by Jeffy

I build bots with a philosophy that if you can't line it up with a tank drive and a few minutes of practice then your mechanism is broken and not your drive train.

Solid philosophy.

In the past few years we have used both AWD robots with traction wheels and robots with mecanum drives. The team has tended to favor mecanum for its alleged maneuverability strengths. I think this is a bit of a fallacy, as we haven't been able to drive or align the mecanum robot any better than the AWD versions.

Building a mecanum robot is a great learning experience from a build and a programming standpoint but don't expect a huge boost in driver performance. It isn't as maneuverable as it looks on paper.

[bs7280]

Sorry about the "H" I though that I had fixed all of them.

What was wrong with the mecanum super shifter combo? or what was wrong with just the mecanum wheels?

if one were to make a swerve drive, what ratio would be best for the speed/power combo (as I saw a lot of options on andymark)? I feel like part of our shoving problems came from our omni wheels (so that takes care of its self). I am tempted to try and make one as a summer project.

could you please go into team 254's drive-train, or at least just something to use as a search query.

thanks so much! -3734

[Anupam Goli]

Originally Posted by bs7280

Sorry about the "H" I though that I had fixed all of them.

What was wrong with the mecanum super shifter combo? or what was wrong with just the mecanum wheels?

if one were to make a swerve drive, what ratio would be best for the speed/power combo (as I saw a lot of options on andymark)? I feel like part of our shoving problems came from our omni wheels (so that takes care of its self). I am tempted to try and make one as a summer project.

could you please go into team 254's drive-train, or at least just something to use as a search query.

thanks so much! -3734

I personally have something against mecanum wheels (insert dead horse to be beaten), but what I'm really trying to get at is to start simple and learn from your mistakes. It's easy to jump around ideas and only implement them halfway, but you learn more about engineering and gain more experience when you iterate your design to the best possible iteration. Do the changes I mentioned with your kit drive before exploring swerve and mecanum. If a team can't master the kit base and the standard 6 wheel drive base, they certainly don't have the resources or time to master an omnidirectional mecanum or swerve drive. If you want to learn more, A simple search in the whitepapers for a drivetrain design can help.

[Ether]

Well we got rid of the "h". Now if only we could get rid of that extra "c" :-)

[bs7280]

I will admit that our loaded could have been much better for sure. it was about as wide as a frisbee, not very helpful. We made some changes at competition that allowed us to have a "flapper" that would allow us more surface area to collect frisbees, but anyways, lesson learned there (practice more).

We could try making those changes (as well as checking which need to be fixed) once we meet again. I definatly will push for the gearbox change. If it helps, it could turn its self way better than any human could (on carpet) without us lifting it up.

I like that philosophy. Next year we are going to drive and make some competition pieces so we can get a better idea of what problems we are having.

EDIT:
Almost forgot to mention. Our center of gravity is centered very well (climber is in the middle) and the main problem from driving is that the front and back wheels of the tank drive where having a lot of lateral resistance on the carpet. enough so that no person could push it. So when it tried to turn, that was what it was fighting.