Mecanum Drivetrains

[BrendanB]

1058 is definitely one of those teams that has re-defined how to use mecanums effectively in the game. Their 2010 robot with the field oriented drive performed amazingly throughout the 2010 season. It was fast, responsive, very agile, and played some very mean mid-field offense!

I didn't get to see too much of their 2011 robot but I've heard a lot of stories about what they pushed.

Their 2007 and 2008 mecanum drives were also effective players of the game.

While mecanums still aren't my personal choice to use in a drivebase they can make for a great off-season project for a team. Plus they make excellent demo robots.

Use in a competition is a different story but every team needs to make decisions when they design their robots. There have always been times where being able slide sideways is advantageous and mecanums are a very easy way to achieve it. There are downsides that go with them but there are downsides of every mechanism you put on your robot.

In the end, no matter what drivebase you go with the best thing you can do is train your drivers and give them time to practice under realistic circumstances. While our team doesn't use multi directional drives we've trained our drivers to react faster so they can quickly maneuver around obstacles without us giving them another direction. We use old robots, chairs, trash cans, simulations, etc to keep them on their toes so when their path is blocked they don't stop moving while maneuvering.

Practice, practice, practice and the decisions you make at the beginning of the season will pay off no matter what drivebase you use.

[Gabe1511]

Originally Posted by Flak-Bait

If you want to get creative, you can do all kinds of crazy things with mecanums. For example, in 2011, 1058 built this simple 4-cim mecanum drive with a twist. Each wheel had a third mecanum plate with cut-up pieces of truck mud flap on it. This plate could be actuated into the rollers, freezing the rollers in place, but not the wheel. With these activated, the robot would be a high-traction 4WD that could push other robots around. At the press of a button this would switch to the normal mecanum drive, giving it unmatched speed and maneuverability.

How effective was that strategy? It definitely sounds like it would appeal to my team, giving the best of both worlds.

[Jay O'Donnell]

Originally Posted by Gabe1511

How effective was that strategy? It definitely sounds like it would appeal to my team, giving the best of both worlds.

The locking mecanums were both effective and cool to show off and demonstrate. It's also fun being able to tell people we pushed a traction drive with mecanums.

[Aren_Hill]

Originally Posted by Jay Money 1058

The locking mecanums were both effective and cool to show off and demonstrate. It's also fun being able to tell people we pushed a traction drive with mecanums.

Well considering you pushed a traction drive with a traction drive....

I'm also curious for any pictures/videos you have of this system in action, I've played with some locking wheel concepts, but never found anything truly elegant.

-Aren

[Jared]

When building a mecanum drive, it can be helpful to create some type of suspension so that each wheel has the same weight above it. It can be as simple as having the pair of back wheels on a pivot.

Remember, driver practice is really important.

Also, be careful of any bumps on the field. When aligning with the pyramid this year, the little 1/2" bump in the floor caught our mecanum wheels, and made it really hard to line up.

Finally, realize that there are many teams that will immediately disregard your robot in alliance selections. While I don't agree that all mecanum robots are bad, many teams have this opinion, and just won't choose a mecanum robot for eliminations.

[Andrew Schreiber]

Originally Posted by Jared

Finally, realize that there are many teams that will immediately disregard your robot in alliance selections. While I don't agree that all mecanum robots are bad, many teams have this opinion, and just won't choose a mecanum robot for eliminations.

This opinions is based on the fact that given a robot with 6wd or an mecanum drive the 6wd offers more tactical flexibility in that they can hold their ground better should I need to stick them in the way of someone else for a couple seconds. The exception to this was in 2012 - Mecanums were practically DNP for me due to their difficulty with the bridges.

Please don't assume it's a bias AGAINST mecanum, perhaps it is merely a difference in priorities and needs.

[MechEng83]

Originally Posted by Andrew Schreiber

The exception to this was in 2012 - Mecanums were practically DNP for me due to their difficulty with the bridges.

I would put our 2012 driver using mecanums against any other robot in terms of bridge balancing. We nick named him "bridge-dancer" that year because he could do fancy manuevers on the bridge, including driving across while turning 90 degrees to accomplish a triple balance.

[pfreivald]

Originally Posted by MechEng83

I would put our 2012 driver using mecanums against any other robot in terms of bridge balancing. We nick named him "bridge-dancer" that year because he could do fancy manuevers on the bridge, including driving across while turning 90 degrees to accomplish a triple balance.

Yup. Our mecanum wheels had no issues whatsoever with the bridge.

[Jared]

Originally Posted by Andrew Schreiber

This opinions is based on the fact that given a robot with 6wd or an mecanum drive the 6wd offers more tactical flexibility in that they can hold their ground better should I need to stick them in the way of someone else for a couple seconds. The exception to this was in 2012 - Mecanums were practically DNP for me due to their difficulty with the bridges.

Please don't assume it's a bias AGAINST mecanum, perhaps it is merely a difference in priorities and needs.

For 2012/2013 I agree that mecanum wheels were a poor choice. Our team used them, and now every member on the team hates them with a passion.

At our regional, we only picked other robots that could function as a defensive robot, so mecanum wheeled bots were off the list, even through we had mecanum wheels on our robot, which we quickly decided to remove/replace with hi-grip wheels.

However, for 2011, there were some good first picks that had mecanum.

[Gabe1511]

Thanks for offering to post the video! My team has been wanting to rig up a mecanum drive train for the past few years, but we've never done it, and I've decided to spearhead the effort this year. Through scouting I've learned the advantages and disadvantages that accompany mecanum drive trains, but I've just wanted to be able to play with one.

[BBray_T1296]

Several Very VERY critical points for a successful mecanum drive:

-SQUARE FRAME. Not as in length (though a square shape IS the most efficient for mecanum), but as in straightness and lack of vertical warping. That is, all 4 wheels need to be perfectly level to get ideal force distribution. If you are worried about having a lifted wheel, put some sort of rubber or spring mount to allow the robot to settle in a planar configuration. When you design the robot, try to create close to a square shape too. We did rectangles both years, but the only way that works is with a "narrow 'bot" configuration. ("wide 'bots" will not work well!)

-WEIGHT DISTRIBUTION. When you construct your 'bot, try to keep your CoG in the center. Not forward, not backward, not left/right, but dead center (ideally). Having a shifted CoG will cause one wheel to have more traction than others (f=uN) and will cause the robot to skew, particularly when strafing. In all drivetrains, low CoG is ideal, so that too, but less importantly.

-DRIVER EXPERIENCE. your drivers need TONS of practice, even if that means playing around with just a base for a while. Know the ins-and-outs of mecanum, and it is advantages. As previously mentioned, mecanum has poor pushing power, but with a skillful driver and well made mecanum (coding is a major factor), you can use the agility to dodge even the toughest defensive 'bot. In 2011, we burned a jag mid competition, and had to compete a match with just 3 operating wheels. From a spectator's perspective, you would not have even known the difference. Of course, holding "up" on the joystick did some funky things, but it was easily correctable for a guy who knew what was going on.

-CODING. Make sure you use encoders, as wheel speed is the single most critical factor. I'm not a coding expert (hardly a grasshopper), but perfectly calculating wheel speeds in all situations (try driving forward, while strafing left, while turning clockwise. Imagine what the wheels have to do!) will really give you an advantage. This is probably the most daunting problem teams have when building mecanums, and where most teams who have tried it and hated it went wrong.

If you don't already, depending on your driver's preference (Xbox or Playstation) get either a wired Xbox controller, or a Logitech controller. (wireless Xbox 360 and PS3 controllers are illegal, and I dont think there is a wired PS3 controller, hence the basically identical Logitech). Driving a mecanum robot is a lot like playing a first-person-shooter. Configure the joysticks to replicate how you would play Halo or Call of Duty, except without the "look up/look down" axis. This will make driving feel far more natural to drivers, and will let almost anyone have the basic ability to drive the robot instantly

Both years we did mecanum (2011, 2012) we used toughbox nanos (I dont know the ratio) with mecanum wheels directly on the output shafts. We built custom assemblies to house each wheel including our deflector plates for hopping the bump in 2012.

[Taylor]

Originally Posted by BBray_T1296

Several Very VERY critical points for a successful mecanum drive:

-SQUARE FRAME. Not as in length (though a square shape IS the most efficient for mecanum), but as in straightness and lack of vertical warping. That is, all 4 wheels need to be perfectly level to get ideal force distribution. If you are worried about having a lifted wheel, put some sort of rubber or spring mount to allow the robot to settle in a planar configuration. When you design the robot, try to create close to a square shape too. We did rectangles both years, but the only way that works is with a "narrow 'bot" configuration. ("wide 'bots" will not work well!)

I disagree with this. Many teams encountered difficulties when driving over uneven field. When field components are placed under the carpet, as they often can be, this takes your carefully tuned 'square' base and lifts up a corner or two. I'd recommend taking Nathan's advice and leaving the chassis 'loose' or having some sort of suspension to negate 'imperfections' in the field.

-DRIVER EXPERIENCE. your drivers need TONS of practice, even if that means playing around with just a base for a while. Know the ins-and-outs of mecanum, and it is advantages. As previously mentioned, mecanum has poor pushing power, but with a skillful driver and well made mecanum (coding is a major factor), you can use the agility to dodge even the toughest defensive 'bot. In 2011, we burned a jag mid competition, and had to compete a match with just 3 operating wheels. From a spectator's perspective, you would not have even known the difference. Of course, holding "up" on the joystick did some funky things, but it was easily correctable for a guy who knew what was going on.

We found quite the opposite during our offseason event - rookie drivers with very little practice driving a robot had no preconceived notions about how a tank-style drive handles, so they bobbed and weaved and spun around traffic with ease using our sensor-less and simply-programmed mecanum drive.

[Andrew Schreiber]

Originally Posted by Taylor

I disagree with this. Many teams encountered difficulties when driving over uneven field. When field components are placed under the carpet, as they often can be, this takes your carefully tuned 'square' base and lifts up a corner or two. I'd recommend taking Nathan's advice and leaving the chassis 'loose' or having some sort of suspension to negate 'imperfections' in the field.

This. 1000 times this.

Rigid frames are the death of holonomic systems like this (omni or mecanum drive, swerve is a whole different ball game I don't have enough experience with to talk about).

You want flex because you want each wheel on the ground at all times. Not doing this will result in unpredictable behavior... well, ok, it's completely predictable given that you know that you're driving over uneven terrain and what the normal force on the wheels is and... blah blah blah.

[pfreivald]

Originally Posted by Taylor

We found quite the opposite during our offseason event - rookie drivers with very little practice driving a robot had no preconceived notions about how a tank-style drive handles, so they bobbed and weaved and spun around traffic with ease using our sensor-less and simply-programmed mecanum drive.

Our first mecanum drive (2010) used 9:1 Banebots p60 gear boxes direct driving four mecanum wheels (and supported on the far side with pillow blocks), and default mecanum code without sensors. The drivers had instant, intuitive control of the machine, because it handles exactly like a 3rd-person shooter with move/strafe--if you play COD or Fallout or whatnot, you've got the basics down already.

The thing to realize about uneven terrain is that mecanum drives work just like tank drive (or close enough that it makes little matter to a human driver) when going forward, backward, or turning normally...the concerns about uneven terrain are, by and large, overblown.

We've since gotten much more sophisticated, with gyros and encoders and octocanum, but the initial mecanum drive we played with was the easiest drive train we've ever built, programmed, or driven--it barely qualified as an afternoon project to get it up and running.

[BBray_T1296]

Originally Posted by Taylor

I disagree with this. Many teams encountered difficulties when driving over uneven field. When field components are placed under the carpet, as they often can be, this takes your carefully tuned 'square' base and lifts up a corner or two. I'd recommend taking Nathan's advice and leaving the chassis 'loose' or having some sort of suspension to negate 'imperfections' in the field.

We found quite the opposite during our offseason event - rookie drivers with very little practice driving a robot had no preconceived notions about how a tank-style drive handles, so they bobbed and weaved and spun around traffic with ease using our sensor-less and simply-programmed mecanum drive.

Brace yourselves, a text wall is coming!

After some distraction, that post really was badly typed. Red: You will notice my post did mention the recommendation of using of some sort of springy material to absorb deformities. That being said, on both of our robots, in 2011, and in 2012, we had our typical solid-as-a-rock welded aluminum box tubing frames. These were set and welded by one particular employee at our sponsor's shop, with precision as a top priority. For this reason our frames are extremely rigid and (realistically) perfectly 'square'. Like the instance I mentioned in the first post, we were subject to, and did experience raised/disabled wheels in the competition. But! because (Blue:) we controlled our drive not with motor power, but with motor RPM (using encoders), we had almost no trouble at all. When a wheel is lost (for whatever reason) and the driver attempts to do anything, the left side with only 1 wheel suddenly finds itself with half the torque of the right side with both wheels at matching PWM input. The code detects that the only left wheel is not spinning as fast as the right wheels while the robot tries to go, so it amps up the power to the lacking wheel to match RPMs, and thus balances the torques*. This operation was also the case when the robots were strafing. This whole scenario was a very rare occasion though, as most obstacles in 2011-2012 were faced head on anyways (ramps/key in 2012, minibot poles in 2011), so 2 wheels climbed the bank at the same time, making the leveling problem a non-issue. I concede the obstacle of 2013's pyramid, and who knows what 2014 will have in store, so I concur, I am making the recommendation to put springs or some other form of 'loose' configuration on future mecanum 'bots.

*while the 3 wheels are not slipping (or in mecanum's case: equally slipping), the only way they can all turn at the same speed is with the lone wheel compensating for it's disadvantage with double the torque

As for (Green:) driver experience. Like I said,

Originally Posted by BBray_T1296

This will make driving feel far more natural to drivers, and will let almost anyone have the basic ability to drive the robot instantly.

That being said, a driver with hours of practice will be able to more efficiently and more effectively traverse the field, avoiding defense 'bots and evading traps. Magenta: These guys are trying mecanum for the first time, likely from some variant of tank drive last year. Any drivers-to-be on the team (who will more than likely NOT be rookies) will have probably driven this year's robot, an thus will, likely, have "preconceived notions about how a tank-style drive handles." Sometimes it can be easy to forget the strafing ability, especially in the heat of a match when you are used to driving tank from an event in the past (or simply spending hours driving a tank 'bot while waiting for the mecanum base to be finished).

One last rant: Green again: In every single last instance possibly imaginable, tons of practice is superior to a lack thereof. Commercial pilots spend thousands of hours training in flight simulators before they operate a real jet. Imagine handing me a copy of Flight Simulator X and saying "you have one hour". After that one hour, you stick me in the captains seat of a 747 and tell me to fly to Seattle. Not the best idea, I have to say. Same goes for a FRC robot. If practice is so worthless, and anyone can drive the robot as well as a trained 'pro', why not hold a school wide raffle on Thursday before your regional. One lucky student gets to drive the robot in competition, be it a band kid or a cheerleader. Maybe you could sell the tickets-what a great fundraising idea!

Originally Posted by Flak-Bait

have fun, drive fast, and forget the haters.