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Mecanum vs. Tank Drive?
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?
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Re: Mecanum vs. Tank Drive?
Mecanum
Pros: precise maneuvering Cons: requires precise robot mass distribution in order to work. Also harder to program Tank Pros: easy to build Cons: go up to feeder station. You're not aligned. Drive back and turn. Go up again. Still not aligned. Drive back and turn again... |
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Tank.
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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! |
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A tank drive will give you the ability to literally push your way across a field and hold your ground if you need to. Mecanum is a really good drive system if there's a reason. The field this year is flat, making mecanum a viable drive system to pursue. With limited visibility though, there is a possibility of you getting into situations such as being pushed around (perhaps while on the way to get fuel or gears), and not even recognizing what's going on. 2014 was a strong year for mecanums because the field was flat and clear. 2015 was a good year because there was no defense. This year, on the other hand, I'm not so sure. My vote is for tank.
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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. |
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The biggest downside by far to mecanum drives are that they can and will get pushed by everyone. In a game like Steamworks where there's an open field which most robots will regularly have to traverse, and where teams without functioning devices have nothing to do other than play defense, that one downside overshadows any and all minor advantages that mecanum drives have.
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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. |
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tank
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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. |
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Actually, woody was the only robot we have ever built that I have over an hour of stick time on, due to a demo I did at work. All I recall was having to account for a slight pull to one side. |
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I would strongly recommend not using mecanum in any game where defense will have the potential to have an impact. The ability for a robot to be completely shut down by defense with little effort is a huge drawback. There is a reason why there has never been a mecanum drive on Einstein (Except 2015).
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Tank.
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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. |
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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.
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I think this year's game may require defense. So I'd steer clear (get it?) of mecanum. |
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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" |
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The biggest thing that kills me about Mecanum is when I am at a competition and I see a Mecanum robot being driven around like a tank. Unless your design, driver, and auto really utilizes strafing then you should not be using mecanum at all. Look at your strategy and ask yourself "do we need mecanum or will drive practice also get us where we need to be"
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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. |
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I suppose I have a few questions...
If teams are in a position to pick and simply refuse to select an alliance partner that uses Mecanum simply because they use Mecanum then isn't that a problem? I would hope that teams are utilizing scouting and performance in order to determine alliance partners...though that may not be the case. One thing that we might consider is, if a team on the opposing alliance is tied up defending however successfully against one alliance partner, then that is one less team member on the opposing alliance that is off scoring points... Finally I think it comes down to strategy. Saying that mecanum means that a team will be easy to defend against isn't exactly accurate. A mecanum drive robot can work to avoid conflicts instead of engaging in them. I hope I haven't tipped my hat too much...Oh...and best of luck to all. Edoga |
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-Mike |
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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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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?
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Do you have experience building a mecanum drive? If not, I would definitely build the KOP chassis.
The clear field is great for a mecanum, however you want it done quickly so your drivers can practice driving with defense. |
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I think it is easier to do well with Tank, but there is more potential in Mecanum. I also feel that most of what Mecanum can do, Swerve can do better (except module complexity).
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We did mecanum wheels twice in our team's existence and both times regretted it. I agree with others that if you need precise alignment in the lateral direction(or any direction really) something is wrong with the mechanism design and not the drivetrain. Even if you do use mechanum to fix that problem, the weight of your robot is not likely to be balanced efficiently and you probably won't have a good enough driver to make mecanum worth it in the first place. Literally the only team that comes to mind that used mecanum drive effectively was 4522 in 2014. There may be a few others that actually pulled it off but they are not the norm. In fact most of the time, teams with mecanum just end up driving it like a tank anyways.
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We won a regional in 2015 teamed up with a mecanum bot as the #1 seed.
So while we are not opposed to teaming up with a robot that uses it, we are definitely against using mecanum wheels for our own robot. Tank drives have way more advantages than the pluses for using mecanum. Mecanums should only be used for intakes! |
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I'm confused:confused: -Aren |
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The advantages of mecanum over a tank drive become largely irrelevant with good manipulator design.
Your mechanisms should not require so much precision that it becomes a basis on which to justify mecanum. |
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I'm not going to try to convince you with logic, because it's easy for logic to make the advantages of mecanum look bigger than they are. So here is my testimony having driven in competition gyro stabilized mecanum, swerve drive, and tank drive:
A good drive train should get your robot where it needs to be, reliably, quickly, and easily. The bottom line is that a tank drive gets the bot to point B more reliably, more quickly, and more easily, so there is no real reason to build a mecanum drive. Just build a tank drive. It will make your season better. |
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I advocated against Mecanum drive this year, however the students preferred "maneuverability" to the "reliability" of west coast drive.
Here's my belief: whatever you do, you do it well. As many people have said before— a good driver can be the decisive factor.. having a mecanum bot that goes 20 ft/s and a driver who knows how to control it... can be amazing for traversing the field and avoiding attacks. Best of luck this year— let's go team mecanum! |
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We like mecanum wheels - if it's feels right for the game we'll use them. We have a total of 12 regional wins and 9 of the wins were with mecanum wheels.
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Mecanums are great for an intake, past that I definitely would not recommend.
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As much of a fan as I am of using mecanum and not picking tank without proper game analysis, if you use pure mecanum this year you will have a bad time. From what I understand, you are not protected while placing gears, so all the fine adjustment in the world isn't going to help when you are being clobbered whenever you get close. If you can't do swerve, octocanum, nonadrive or butterfly drive, then you should stick with skid steer and focus on compensating for the less maneuverability.
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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. |
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I do have a question, we're using a single reduction clamp gearbox overall 3.8:1 ratio, and the torque load seems to be about 2.8... while each CIM has a stall torque of 2.22--- I believe this is an issue... but also I'm not super familiar with stall torque... is it correlated to the torque load? Quote:
One of the discussions we had while choosing our d-train was in regards to the precision required to place the gear. I believe that butterfly or octanum would be ideal for this game, and it's relatively simple w/ COTS from VEX or very simple machining with a mill. One of the counterpoints our students presented for defense was that, even if you have a tank drive and you're trying to place a gear, an opposing bot can knock you off of alignment as well. I think their reasoning to use mecanum wheels was to more easily avoid robots, and quickly align. |
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I would say tank because the cons of mecanum out weigh the benefits
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It all depends on what strategy you are going to go for of course, but if you are going to play any defense or fight for balls in the neutral zone, I highly recommend tank over mecanum. 99% of the time, you will get tossed around like a rag doll when facing non-mecanum robots. No traction. But as one user suggested earlier, if you are very familiar with mecanum and not familiar at all with tank, or your driver is skilled enough to handle the challenges of mecanums in this game, then go with mecanum. Otherwise, I say tank.
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Our best regional year was 2014 with a mecanum |
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heh...our best regional year was 2009, with those hard plastic wheels.
Anecdotes aren't real helpful, some times. |
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It likely is not the right drive train if you are trying to win Einstein, but it can perform very well at a regional level with good driver practice. |
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I'm going to be doing some 6" 6WD 4 motor tank vs 6" 4 motor mechanum push tests this weekend. I hope I can bring back some useful information to settle this debate in some way. |
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A real A-B test could be run at a scrimmage or off-season event. Make up an alliance with 2-3 butterfly drives vs. a tank drive alliance and do 2-3 matches with the butterflies all mecanum vs all sticky. Although yes, there may need to be some adjustments for the typically lower gearing of the sticky wheels. |
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We used octocanum in 2014 and had a good amount of success. However, we tried to be strategic about switching modes to conserve air so we only deployed it when we absolutely needed it. If I were able to count the teams that we were able to push on mecanums, even at IRI against some of the best of the best tank drives in the world, there would be a lot of people who would be covering their faces in shame.
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4 out of the 6 years since mecanum wheels were commercially available (2007-2012), there was an element of the game discouraging or prohibiting use. I think 2011 was the first year there could've been wide use. 2013, for some robots, drive train didn't matter a whole lot (full court shooters). 2014, single game piece and ferocious defense, wasn't a good idea. 2015 never happened, 2016 favored 8" pneumatic wheels. This year I'm thinking will be more favorable to mecanum wheels. |
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As it looks like most people, I would recommend tank. Using the KoP, it is very simple to do and does't really take much thinking or programming to do. Mecanum from my understanding isn't overly complex but it takes more work. A downside to Mecanum is that it can't really hold it's ground that well, and you could be pushed a lot. A tank drive may not maneuver as sharp or nice but with the defense ability this year, I would highly not recommend mecanum, but that's for the strategy my team wants. My team did mecanum in 2015, but we also didn't have robot-to-robot contact.
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I think a small 6 wheel tank drive robot, in the hands of a well practiced driver, will be able to do amazing things.....
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TL;DR, given the same coefficients of friction for a standard and ideal mecanum wheel, the mecanum wheel can exert 30% less force on the floor before breaking static friction in the forward direction. (Non-ideal mecanum wheels - with friction in the rollers - suffer less in the forward/backward direction, but more when strafing). Moreover, in practice you can find standard wheels that are far more tractive than you can get in a mecanum roller due to the size and geometry, which amplifies the difference, potentially to 50% or more. Static friction provides an upper bound on your ability to accelerate, change direction, push, and resist pushing. Even on an open floor without obstacles and defenders, the traction-limited tank drive matches or exceeds the maneuverability of a traction-limited mecanum drive in many cases. For example, if you want to move 10 feet to the left, a mecanum drive can strafe for 10 feet, but a tank drive can reorient, drive, and reorient again (or drive any number of smoother maneuvers that accomplishes the same thing) and be able to execute each segment of the motion more quickly because of its superior ability to exert force against the floor. Which one gets you to a given arbitrary goal location first? There are going to be cases where a traction-limited mecanum clearly wins (e.g. move 1 inch to the left) and cases where a traction-limited tank drive clearly wins (e.g. turning in place or moving purely forward). For hybrid motions, it depends on the details of the respective robot dynamics. The further away the goal, the more likely it is that the tank drive can drive a path that gets it there more quickly. (The actual study of the optimal path/time to get from one spot to another for different types of drivetrains is fascinating and complex...see http://planning.cs.uiuc.edu/ch15.pdf for a good [graduate level] introduction). So it may make sense to use a mecanum drivetrain if you are optimizing around short, controlled sideways movements in tight spaces. But most FRC games (2015 being an exception) are not dominated by these types of maneuvers. Once you add in the presence of "adversarial dynamic obstacles" (defense), things skew in the tank drive robot's favor in most cases. |
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Also I think Ether had some math that described turning with mecanums, the force vectors from the wheels naturally align (there's little to no wheel scrubbing while turning, this is also a feature of dropped center 6wd). I'll edit this post if I can find it. EDIT: https://www.chiefdelphi.com/forums/s...3&postcount=18 |
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I love how the beginning of my year is always punctuated by this discussion.
Anyway mecanun and tank are tools to execute a strategy, and you always use the right tool for the job. Also omnidirectional capabilities on a robot should be only put on when it is absolutely needed cause if it isn't crucial you aren't going to use it. If your strategy might need it go back and refine the strategy and come back with a yes or no. |
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