Re: Swerve Drive vs Mecanum Wheel drive?
 

Then what difference(s) were you assuming? What did you mean by "a good and a bad mecanum drive" in your post #81 when you claimed that "There is no difference in traction between a good and a bad mecanum drive" ?

Re: Swerve Drive vs Mecanum Wheel drive?
 

From what I've seen in competition the difference between a bad an good mecanum drive train largely has to do with gearing. Placement locations of the wheels, wiring work, and programming. I've never seen a team in person that didn't at least mount the wheels correctly to their chassis (not to say there aren't teams that have made this error, but it's not something I commonly see).

Re: Swerve Drive vs Mecanum Wheel drive?
 

Here are some of the factors that could cause a "good" and a "bad" mecanum drive to have different traction:

- type of wheel (model number)

- manufacturing tolerances of wheel

- manufacturing defects of wheel

- improper adjustment of wheel rollers

- improper alignment of wheels on vehicle

- frame warped or too rigid

- poor weight distribution

When it comes to traction, all mecanums are not created equal

Re: Swerve Drive vs Mecanum Wheel drive?
 

Of course these are going to make a difference. If you're not using the same wheels, I'd assume that they're not the same.

Quote:

- improper adjustment of wheel rollers - improper alignment of wheels on vehicle

This would affect your performance, however I doubt it would be to the degree that would make the difference between getting pushed and not getting pushed by an opposing robot with a strong drivetrain.

Quote:

- frame warped or too rigid - poor weight distribution

These are the most likely differences I see appearing and having the largest affect. However for the purpose of discussing pushing in mecanum vs skid steer, I don't think they'd turn the tables.
Exactly. As I said before, if they're not equal, they are not equal. Of course there's a difference. If I built a tank drive on lunacy wheels, and another on 2" blue nitrile traction wheels I'd expect them to be different.

The point I was initially trying to make in my original post was that a well built skid steer drivetrain can push through any mecanum drive train. I would love to see a drive train on four mecanum wheels that could outpush the tank drive on any first pick robot at CMP. If I saw one of those, I would convert to mecanum immediately, no doubt.

Re: Swerve Drive vs Mecanum Wheel drive?
 

One problem I see in this conversation is the assumption that most tank drives any given mecanum drive may be up against are all always well done. People seem to also judge said mecanum poorly based on pushing matches, why? Whoever said a mecanum was for pushing?

The advantage is strafing and maneuverability for a team that wants more dynamic offensive play and less predictable maneuvering. If you want that plus pushing power then you go for swerve if you have time, money, people for that. If your strategy doesn't need any of that then tank.

And for all you who denounce mecanum without ever driving one in fall 2011 before our team had started its' first season we were allowed to drive anther teams robot from that year in an off season competition. Despite no field-centric or PID nonsense we absolutely were able to push and deflect other robots, TANK drive robots. So in the right circumstances a rookie driver and overall FIRST rookie has actually done what you are all arguing about.

Re: Swerve Drive vs Mecanum Wheel drive?
 

Jake, it seems we've been talking past each other. None of my posts have been about the "tank pushing mecanum" hot-button issue. I've intentionally stayed out of that debate.

Ever since my post #83, I've been responding specifically to the following statement in your post #81:

I think we now agree that the above statement is false as it stands. Gearing is not the only factor which makes a difference.

The reason this is important is because not being aware of (or not paying due attention to) the factors¹ that affect the driveability (including traction) of a mecanum is a common pitfall for teams using mecanum.


¹Here's a list of some of them: http://www.chiefdelphi.com/forums/sh....php?p=1373392

Re: Swerve Drive vs Mecanum Wheel drive?
 

One problem I see in this conversation is the OP specifically asked for input from people who had direct experience with swerve AND mecanum drives, and the majority of posts here are primarily assumptions, generalizations, and misinformation.
Often from people who can't even spell mecanum correctly.
I hope the OP is able to sift through the clutter and pick out the reasoned, evidence-based data and ignore the baseless, rumor-based opinions.

Re: Swerve Drive vs Mecanum Wheel drive?
 

I think there are very few teams that have used both Swerve and Mecanum, and it is probably a good idea to get info from teams that have used one or the other.

Re: Swerve Drive vs Mecanum Wheel drive?
 

I know I haven't built or driven a swerve but in the comparisons people are making I want to make it clear that the generalization that pushing tank with mecanum is not viable is incorrect. By that metric it is not universally always as inferior to swerve as many seem to think. And I want to make it clear I know this from actually haven driven one in a competition against other tank drive robots.

Re: Swerve Drive vs Mecanum Wheel drive?
 

..

Re: Swerve Drive vs Mecanum Wheel drive?
 

Re: Swerve Drive vs Mecanum Wheel drive?
 

I've offered my views on both systems vs tank drive despite having intimate experience with each and was told I was "wrong". So you guys are on your own here. I can't be bothered to argue with people on the internet anymore.

Everything posted on an internet forum should be treated as opinion. Some has more value than others based on the credentials of the individual posting. Typically, what users define as "fact" tends to be what most people agree on but that doesn't mean it's correct. The OP should realize this and when sifting through these discussions, use the statements brought forth to form their own conclusions.

Re: Swerve Drive vs Mecanum Wheel drive?
 

Well what's the point of comparing a good tank drive to a bad mecanum drive? Of course the better drivetrain will always come out on top. That's not even a question.

Nearly half the people in this thread have claimed that. My post is not dismissing mecanum as bad, it's pointing out the truth that mecanum drivetrains are not good for pushing which multiple people in this thread claimed otherwise.

My bad, I should have been more clear in my first post. The point I was trying to get across is that both a good, and a bad mecanum drive will lose pushing matches against a well built tank or swerve drive. Re-reading my post I realize that was probably not the way it was interpreted.

I will agree that there absolutely are differences in the pushing power between the best and the worst mecanum drives. However, I also assumed from the beginning that we were comparing drivetrains that used the same COTS parts because it's pretty obvious that two drivetrains made using different COTS parts will be different. I figured that was a given since it really applies to every drivetrain, not just mecanum.

Re: Swerve Drive vs Mecanum Wheel drive?
 

I was not going to get involved with this thread, but because my team has done both, I figured I should probably share my opinion.

In 2012, at about week 3, my team decided to use the wild swerve module from AndyMark. It was quite a bit of work to implement, and the performance was lackluster. But ultimately, it was part of why teams 1717 and 469 picked us for their alliance on Newton. So in hindsight, it was probably worth it.

In 2013, my team tried to do a custom independent swerve drive, but was not prepared enough so we ended up changing to mecanum before it was too late. I was way more disappointed with the performance. We barely had any ability to strafe because the rollers had too much friction, our top speed and pushing force were diminished by the inefficiency of each gearbox, and the drive train was heavy because we used gearboxes with steel gears that were made to take two cims.

This year we made the jump to custom independent swerves. I could barely be any happier with a drive train. The swerves are light (about 7 lbs each), somewhat affordable ($220 each), reliable (knock on wood because we've had no failures yet), and highly maneuverable. I will admit that they were a lot of work, but I truthfully believe that they did not have any negative effects on the rest of the robot.

In conclusion, I would strongly recommend that any team with CNC machining capabilities spend their off-season research time on swerve drive rather than mecamum. I realize that many people disagree with me on this, but this is my honest opinion.:D

Re: Swerve Drive vs Mecanum Wheel drive?
 

Our team is in its 11th year (I think). We've used swerve for 2 years, mecanum for 3 years and tank drive the other years.

Our mecanum was very well done, almost as well as it can be done w/o adding locking wheels or brakes of some kind. We had no trouble with the average tank drive opponent in qualifying matches. We just spun off them or juked them - often it was embarrassing. But late in eliminations rounds tank drive robots with good drivers assigned to specifically stop us could really slow us down. But we stuck with mecanum for a goofy reason I reckon, the students loved to drive the thing. It is a lot of fun.

Our swerve drive worked pretty well the first year (lunacy) but not so well the next. We had some steering problems but it was a design issue (windows motors, not enough torque etc). We fixed it eventually. We loved swerve also - it is also fun to drive. But swerve involves a lot of relatively complex machining. I disagree that the software is complex - the math is pretty straight forward. We stopped doing swerve because one of our mentors (and the best machinist) got sick the next year.

For the last several years we've tried chain and belt driven single and two-speed tank drives. They work well and are relatively simple to design and build. It takes us a single afternoon to hammer out the design. It is very strong and reliable (except for changing out the occasional chain or belt).

So what advice can I give you? Do NOT try swerve w/o a foolproof off-season design working. You could kill a build season schedule (for most teams). If you have a good swerve you will be more maneuvrable than a tank drive. Is it worth the weight and complexity? ... depends on the game. A properly geared two-speed 6-motor tank will still push you around a little (not like a mecanum though) because you will probably be limited to 4 motors and a single gear.

Finally persons on CD and in competitions are too hard on mecanum drives. They can be better than most people think. And as Ether points out (in dozens of other threads) the mathematics and operation behind mecanum is almost universally misunderstood. For example ours could park on a bridge (but not push someone else up the bridge), fly over bumps and play passable defense against tanks (if we caught them on the corner and spun them around rather than push them). But the prejudice against mecanum is not w/o some basis and there are MANY teams that will NEVER pick a mecanum alliance partner.

HTH