Mecanum vs. Tank Drive?

[Max Boord]

Originally Posted by Kevin Sevcik

I'm curious if butterflies and octocanums collect alliance selection dings like straight mecanum. Or is it not relevant because teams willing/capable of these drives are usually pickers?

H drive and mecanum are treated worse than any other drivetrain (automatic DNP list most of the time). Buttterfly, octocanum and swerve are a little more complicated. Basically if I wasn't confident that they could fix it if it somehow broke, they would get lumped into the mecanum drive pile. If I was confident that the team could fix it if it broke, I would still put them below tank due to the inherent reliability issues but it would not completely prevent a team from being picked.

[bEdhEd]

Originally Posted by niklas674

While our best year involved us using a mecanum drive (mistake), we practically never used it in matches, the positioning was close enough that we didn't have to make large changes. Bottom line is that with plenty of drivers practice, you won't need to be able to strafe because your drivers will get to the exact spot almost every time.

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TL;DR: Even in a game free of opponent pushing where mecanum was a viable option for strafing motion, it still was not our first choice in the design of the drive system. In the end it failed to work as well as we wanted.
__________________________________________________ ___________
I recall a few factors in 2015 in deciding to use mecanum. One was the fact that we were trying to be a landfill clearing robot, which required fine maneuverability due to the perpendicular orientation of the totes. We also constructed our robot to ride along the alliance wall behind the yellow totes as part of our autonomous, which required the strafing motion. But even with that in mind, we didn't even opt to use mecanum first.

Our first drive system iteration was an H drive with 4 omnis on the outside and one perpendicular omni in the middle. We wanted H drive over mecanum first, because we didn't want to bother with programming mecanum when H drive was a simpler option, and furthermore, H drive strafing is not as dependent on mass distribution as mecanum, if at all (as far as I know, correct me if I am wrong please). We knew that grabbing totes and making stacks at the forward area of the robot would drastically change the distribution, so H drive seemed to be the smarter option.

Unfortunately, the center wheel of our H drive would cause the robot to lose contact with some of the floor when driving up the scoring platform. At this point of failure in our prototyping, we settled for mecanum, and I say "settled" for a reason. Through all our days of practice and competition, the strafing abilities were hampered due to poor distribution of mass, especially with the addition of totes in a held stack. Because of this, our intial idea of a strafing wall-riding autonomous was given up for simpler options.

As Niklas mentioned, we barely strafed at all and drove the robot more like a tank, and I feel like we should have just designed for tank. After all, the only teams who were major scorers that beat us in all three of our regionals in finals did not have mecanum (118: 2 tread 2 omni tank, 254: 6 tread tank, 624: 5 omni H, 1678: 6 tread 2 omni tank) and two of them were the world champs, 1678 and 118, both of which we competed against at two separate competitions in the finals. I think I can chalk up our losses to these teams partly to a lack of proper control due to an improperly implemented mecanum drive.

The lesson I got from this was that even in a game where mecanum may seem like a good option due to the game design (no opponent pushing and lots of opportunity for fine control), it still might not be that great of a choice, so be careful with that decision. Some of the best teams use some form of tank drivein every game without fail, so I like to go their directions.

[aguzauckas]

Originally Posted by svince

Our team is currently trying to decide between using a mecanum drive and a basic tank drive from the KOP in order to effectively make a cycle for placing gears. Would mecanum be a viable alternative because of the precision boost, or would tank style overpower us in our runs across the field to the point where it isn't worth the change in style?

I would strongly suggest to go with what you are stronger with. If your team can code and drive mecanum, then that would be a great idea. There are downsides to a tank drive as you would have to reverse and go back forward if you aren't aligned correctly.

[Taylor]

Great question!
There are two schools of thought regarding mecanum wheels. One is that mecanums are an easy way to gain an extra degree of freedom when driving (easy as compared to swerve). It does have a bit more complexity than a standard 4 or 6wd setup - more refined versions of it require use of a gyroscope. However, this does allow a team to get many of the translational properties that swerve allows, at a reduced cost and a fraction of the resources (labor, motors, programming, electrical).
Another view is that mecanums are a lesser drive system - by virtue of the rollers on the wheels, they are subject to heavy defense, and cannot play defense themselves. This view is usually the more loudly proclaimed, usually by people who heard this view from somebody else, and by people who have no direct experience with mecanums.
I will say that a number of high-end teams have experimented with mecanum drives and felt they have found a better solution elsewhere.
I will also say there is a stigma with mecanums, with questionable merit, and some teams say they would not pick a mecanum robot to ally with in eliminations. However, I have noticed that the people who proclaim they'd never ally with a mecanum robot, aren't usually in a picking position.
The general rule of thumb is if somebody doesn't know how to spel "mecanum" then their opinion is mostly based on hearsay.
Welcome to Chief Delphi!

[FrankJ]

The stigma against mechanum is very real and a bit undeserved. You generally going to have to seed well or be an alliance captain to play in the afternoon.

Mechanum is very good for precise aiming and locating for tasks like placing gears.

Mechanum will do poorly against a properly geared high traction robot. Varying results against others. On the other side of the coin, you can't be pinned by being pushed sideways like a high traction bot can. You can use defensive strategies that involve deflection rather than pushing matches.

Effective use of mechanum takes a different mind set than tank drive. Key is lots of driver practice in a full size space. But then effective use of tank drive requires lots of practice in a full size space.

[svince]

Originally Posted by FrankJ

Mechanum will do poorly against a properly geared high traction robot. Varying results against others. On the other side of the coin, you can't be pinned by being pushed sideways like a high traction bot can. You can use defensive strategies that involve deflection rather than pushing matches.

I've heard that we can use gearboxes with a different ratio to increase our force, but is there any truth to that rumor?

[TheNerdJedi]

Do a decision matrix with members on your team. List various traits you want your drivetrain to have, like "mobility", "speed" , "complexity" , etc. and compare the various drivetrains in question. Assign a number for each trait to each drivetrain and see which drivetrain has the highest at the end.

Think about your team capabilities and what would be best for the team.

[Ben Martin]

tank

[Kevin Sevcik]

Since our team has used mecanums a couple of years, I'll chime in.

First, exactly centered weight distribution isn't 100% necessary. Things will go wrong if your CG is 4 times closer to one pair of wheels than another, but a CG that close to the edge is terrible on any drivetrain. Follow normal CG location principles and you'll be fine.

Flat floors matter somewhat more. We used mecanums* in 2011, and the undercarpet plywood support for the towers would often often skew us when we hit it crooked. Closed loop yaw rate control will help a lot here. (*Technically half octocanum. Only 2 sticky wheel modules because weight.)

You're not going to get pushed entirely across the field by every robot, but you're definitely going to lose to heavyweight well designed tank drives. You do need to account for this in your drivetrain evaluation. Expect it to add extra point-to-point seconds to your cycle relative to a tank drive. On our 2011 bot, even just the two extra sticky wheels helped. The driver would drop them to stop a sideways shove and power out of it. They didn't get fast and good at this till Champs.

Since you're probably pitting a 4-CIM mecanum vs a 6-CIM tank drive, the mecanum is going to have less push and acceleration even before tracion comes into play.

Mecanums are not actually complicated to program. There are labview, C++ and Java blocks/classes for open-loop mecanum control. I know there's also C++ and Java code for closed loop yaw control and field centric control as well.


All that said, I'd still go with a 6-CIM or 4-CIM + 2 miniCIM tank drive. It's pushier and has less moving parts and breakables than mecanums. It will inevitably get you where you want to go faster. If you need a system to quickly line up a gear without backing up, add something to slide the gear side-to-side on your robot. Moving the gear side-to-side for alignment is the goal. You don't have to move the entire robot to do so.

[Lord Basket]

Originally Posted by Taylor

However, I have noticed that the people who proclaim they'd never ally with a mecanum robot, aren't usually in a picking position.

While that may be the case sometimes, look at what members of consistently great teams are posting in this thread. Members of Citrus Circuits, Techfire and Robovikes literally posted one-word answers that just say "tank" and a member of the Cheesy Poofs posted "Just say no to mecanum drive." A member of TORC posted a more detailed response, but the conclusion was the same. Those teams are some of the best in the world and are in a picking position every year. While they haven't explicitly said they'd never pick a mecanum robot, they don't seem to think too highly of mecanums, to say the least.

[Koko Ed]

Mecanums are effective if there's no defense present. There will be lots of defense this year and it will be brutal. Especially since there are no safe zones when you are looking to score gears.
Go with tank drive.

[Ben Martin]

Originally Posted by Lord Basket

While they haven't explicitly said they'd never pick a mecanum robot, they don't seem to think too highly of mecanums, to say the least.

Only in contact games is it a factor. Mecanums make you both easier to defend against and a worse defender.

[scaryone]

Respectfully speaking, mecanums get a bad rap. While it does have a slight disadvantage in a pushing contest, in my opinion, it is way overstated on CD. Weight distribution is important but not mission critical. Hard programming? How is it harder? Practice is critical, but isn't it for every team/drivetrain? Humbly speaking, the students have 2 Blue Banners using mecanums. Not sure what we'll do this year but mecanums are in the running.

[Michael Corsetto]

Originally Posted by Lord Basket

While that may be the case sometimes, look at what members of consistently great teams are posting in this thread. Members of Citrus Circuits, Techfire and Robovikes literally posted one-word answers that just say "tank" and a member of the Cheesy Poofs posted "Just say no to mecanum drive." A member of TORC posted a more detailed response, but the conclusion was the same. Those teams are some of the best in the world and are in a picking position every year. While they haven't explicitly said they'd never pick a mecanum robot, they don't seem to think too highly of mecanums, to say the least.

+1 to this. We do a first pass filter on our second pick list based upon drivetrain style. If it's not tank style, we often drop the team from our list right there.

Originally Posted by Taylor

I will also say there is a stigma with mecanums, with questionable merit, and some teams say they would not pick a mecanum robot to ally with in eliminations. However, I have noticed that the people who proclaim they'd never ally with a mecanum robot, aren't usually in a picking position.

My experience has been the opposite, but YMMV. I know to us, and a few other teams usually in picking position, mecanum is almost always an instant "do not pick".

There's lots of jokes/meme's around mecanum, because for contact games, it's just not what teams in picking positions want out of a second pick, so those mecanum robots get instantly dropped from pick lists. Carefully consider if this is a risk your team is willing to take.

-Mike

"Friends don't let friends build mecanum"

[Rosiebotboss]

Interesting thread.

Mecanums get a bad rap. With a skilled driver a mecanum drive train can be very effective, ease of alignment to the high shot and placement of a gear negates the use of a turret for a quick "lock on" of the high boiler and a passive gear holding system can be used for the "lift."

As far as defense, I look at it this way, a running back on a football team charges the defensive line, usually made up of bigger, stronger guys. He'll hit the line, bounce off, rotate out and start upfield again, being more agile sometimes is better than stronger. As far as playing defense, All you have to do is hit the opponent on a corner of the frame to knock the robot out of their 'spot' of slow them down and prevent them from completing all their cycles. It's not always about brute force.