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Unread 11-04-2014, 16:50
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Re: Swerve Drive vs Mecanum Wheel drive?

Quote:
Originally Posted by XaulZan11 View Post
I agree 100%. My point was to provide an example of how teams can maximize their drivetrain. I wasn't providing an overall assessment of omni drives.

Compared to a team like 469 that can plow through teams to get to their smaller sweet spot near the goal, 33 needs the large sweet spot as they cannot depend on getting to a specific location and staying there under defense. 33 was smart enough to understand this and put a lot of time and effort in ensuring their shooter had a huge sweet spot and could shoot fade aways or while being pushed sideways. The same applies to human player loading: 33 cannot depend on getting right next to the human player (and staying there) so they went with the largest possible catch radius. The biggest weakness to an omni drive, the inability to resist being pushed, was down played by 33 because they can still accomplish the game's task without needing to be at one small, specific location.
I see, you're completely right.
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Unread 11-04-2014, 17:26
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Re: Swerve Drive vs Mecanum Wheel drive?

The lag time on getting a swerve drive to start strafing is insignificant. It's 1/2 of a second to 1/8 of a second.(shortest path algorithms help a lot) The drive motors are already ramping up at this point. By the time any other omni directional drive gets to speed the swerve will be their the extra acceleration that a swerve has strafing over mechanum is amazing.

On another note if you have a swerve drive, who spends their time pushing a west coast drive around. Most good teams with swerve know that the whole goal of a pushing match is to waste time. You can role out of pushing matches really easily. We have some really cool code that changes our center of rotation making doing this really easy. If done right you can use the other robots pushing power to help you accelerate past their top speed. (if you have a higher top speed)
If a swerve is playing defense just turn your wheels perpendicular to their thrust, Pushing a robot sideways is much much harder then trying to push one head on. With a swerve their is no "foward" making pushing agenst it is hard if the drivers know how the thing works well. If you do try pushing a swerve side ways the lifting motion on the wheels you are next to will cause the outhes to dig in to the carpet making the while thing much more difficult.
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Unread 11-04-2014, 19:06
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Re: Swerve Drive vs Mecanum Wheel drive?

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Originally Posted by fox46 View Post
I don't follow you...
You mention the fact that you have easily plowed through holonomic drivetrains as the proof that ALL holonomic drivetrains will be easy to plow through. I’m sure you have plowed through another bot with tank drive too, so why doesn’t it follow that ALL tank drivetrains will be easy to plow through?
My point is you are overstating your point to the degree of being misinformation. tank is better at pushing than mec or swerve, but let’s not make someone who is looking to get information think that any team with a thrown together tank will be able to easily push around any holonomic drive.


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Originally Posted by fox46 View Post
How many blue banners have been won with skidsteer and how many with Omni/mechanum?
I was not arguing which is better. I was making the point that I think it is bad advice to say:
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Originally Posted by fox46 View Post
don't even bother with Omni if you aren't using a gyro and accelerometer to take some of the work away from the driver
and was providing evidence to support my claim that you gave bad advice. I myself would advise to try mecanum, and also try to get sensor feedback to make it better, but that’s different advice.

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Originally Posted by fox46 View Post
Tipping to two wheels is inherent in every design! This is what allows for the wonders of wheelies! This includes cars during acceleration, ATVs, bobcats- most any wheeled or tracked vehicle exerting a force above the level of the surface it is driving on transfers weight to the rear. I would think this is common knowledge.
I am not disregarding weight transfer, I am trying to make you see the difference between your 2 statements:

1:
Quote:
Originally Posted by fox46 View Post
If you have four motors, one driving each wheel of a swerve drive, as you reduce the normal force on your front two wheels during a pushing contest, then YES you are reducing the tractive power of your machine as the front wheels and hence two of your drive motors lose their effectiveness.
2:
Quote:
Originally Posted by fox46 View Post
in a shoving match, typically you are only using half your available drive power
I have no problem with statement 1. If you had originally stated it this way you would have been the first to mention that in this thread and it would have been helpful, and we could have talked about torque limited vs traction limited
statement 2 means that 2 wheels are off the ground. You are saying a typical pushing match ends up with 2 swerve wheels off the ground, and I find that to be misleading

Last edited by lcoreyl : 11-04-2014 at 19:10. Reason: forgot a quote
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Unread 12-04-2014, 00:24
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Re: Swerve Drive vs Mecanum Wheel drive?

From what I have seen mechanum has 3 problems.

1: Almost every time I see a team with mechanum I see the having issues with being pushed around and playing bad defense because they can be easily moved.

2: Drifting, I guess drifting can be good sometimes but when you are drifting every time you do a turn at high speed then I can see it being a problem.

3: From what I have heard from people on my team, (maybe true maybe not) they cannot go as fast because their speed is limited.

the third problem may not exist, but I think it does because I trust my teammates.
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Unread 12-04-2014, 00:38
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Re: Swerve Drive vs Mecanum Wheel drive?

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Originally Posted by kuraikou View Post
3: From what I have heard from people on my team, (maybe true maybe not) they cannot go as fast because their speed is limited.

the third problem may not exist, but I think it does because I trust my teammates.
The third one is both true and false :) The speed is equal to the speed of any other wheel of the same diameter when traveling forwards/reverse, but the speed when just strafing is only sqrt(2)/2 times the maximum forward speed (I believe... its definitely less than 1). Tank doesn't have this option in the first place, and swerve has 100% of its forward speed when traveling sideways.
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Unread 12-04-2014, 01:07
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Re: Swerve Drive vs Mecanum Wheel drive?

A lot of misinformation in this thread.

Mecanum drivetrains have a fraction of the traction that skid steers does. This really isn't debatable. There is no difference in traction between a good and a bad mecanum drive, they have the same 4 wheels touching the ground, if they are static there is nothing that makes them any different. The only difference is in gearing but that doesn't matter when the robot is sitting still.

Most drivetrains in FRC (especially with the advent of the 6 cim drivetrain) are traction limited, not limited by the power output of the motors. Our drivetrain is geared for ~24 fps in high gear and it's still traction limited.

A well designed swerve drive can have just as much pushing power as any tank drive assuming it uses the same tread material.

The maneuverability of mecanum or omni wheel drivetrains can be very advantageous for offense, however it's been proven time and again that they really can't do anything on defense. After watching over 400 matches just this season I can't even count the number of times I've seen a mecanum bot just get brushed aside by a tank or swerve drive as if they weren't even there.

Robots rarely have their wheels lifted off the ground. Nearly all pushing matches have both robots with all 4 wheels planted on the ground. Despite their weight shifting to the back wheels their force applied on the ground is relatively constant and it marginally affects a robots traction.

Mecanum wheels never behave the same as traction wheels. The rollers on mecanums make it so there is always a slight loss in efficiency since those rollers will always spin slightly unless you are going perfectly perpendicular to their direction of rotation. Also the rollers have a relatively smooth surface so even if the wheels are traveling perpendicular to direction of the rollers they still slip and cause a loss in efficiency.

It's not any easier to push a robot (on traction wheels) backwards than is to push it sideways assuming that it has enough power to at least keep it's wheels from spinning. It's the same amount of friction in every direction as long as the wheels are locked.

Do not take this to mean that mecanums are bad. Just stop saying that they can do things that they can't.
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Unread 12-04-2014, 05:28
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Re: Swerve Drive vs Mecanum Wheel drive?

Here are two somewhat minor advantages to mecanum drives vs tank drives that I have never seen discussed.

1. I have never ever seen a mecanum robot tip over because they braked too fast, but I have seen numerous tank drive bots do this. Given, this can be corrected fairly simply with some ramp-up code, but most teams don't have anything like this in their code.

2. It is not possible to (for lack of a better word) pin a good mecanum bot in the center of the field with nothing around. Almost every tank drive I have seen gets trapped when a robot side loads them. They can still spin in circles, and maybe manage to break free this way, but a good mecanum robot can spin off quickly in this situation.

Value these things as you will, but keep them in mind.
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Unread 12-04-2014, 09:09
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Re: Swerve Drive vs Mecanum Wheel drive?

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Originally Posted by themccannman View Post
There is no difference in traction between a good and a bad mecanum drive
There is a world of difference in traction between a good and a bad mecanum drive. Proper design and craftsmanship are crucial to traction for a mecanum.

Quote:
The rollers on mecanums ... will always spin slightly unless you are going perfectly perpendicular to their direction of rotation.
When the vehicle is going at a 45 degree diagonal (i.e. perfectly perpendicular to the direction of rotation of the rollers), the rollers do spin.

The only time the rollers do not spin1 is when going forward and backward, is which case the vehicle is not going perfectly perpendicular to the direction of rotation of the rollers.


Quote:
It's the same amount of friction in every direction as long as the wheels are locked.
This is not true if the coefficient of friction of the wheels is different in different directions.


1In theory, with no roller axial free play and a non-compliant floor surface.
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Unread 12-04-2014, 09:12
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Re: Swerve Drive vs Mecanum Wheel drive?

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Originally Posted by ekapalka View Post
the speed when just strafing is only sqrt(2)/2 times the maximum forward speed
The theoretical strafing speed is the same as the forward/reverse speed. There is no sqrt(2) factor.

The reason most mecanums have noticeably slower strafing speed is due to roller friction, roller axial free play, and floor surface compliance.


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Unread 12-04-2014, 14:22
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Re: Swerve Drive vs Mecanum Wheel drive?

Quote:
Originally Posted by Ether View Post
There is a world of difference in traction between a good and a bad mecanum drive. Proper design and craftsmanship are crucial to traction for a mecanum.
I don't understand how the same 4 wheels touching the ground in a static state could possibly have different amounts of traction.

Quote:
When the vehicle is going at a 45 degree diagonal (i.e. perfectly perpendicular to the direction of rotation of the rollers), the rollers do spin.
How can a free wheel be forced to spin when going perfectly perpendicular to it's direction of rotation?

Quote:
The only time the rollers do not spin1 is when going forward and backward, is which case the vehicle is not going perfectly perpendicular to the direction of rotation of the rollers.
In a standard mecanum drive set up if you drive the robot straight forward the rollers on all 4 wheels will slip slightly due to the fact the ground is applying force at a 45 degree angle to their direction of rotation.


Quote:
This is not true if the coefficient of friction of the wheels is different in different directions.
In the case of normal traction wheels commonly used in FRC I think it's pretty safe to say the only difference in traction would be the slight change in the profile of your contacting surfaces depending on the direction of travel. Aside from that the materials on our wheels probably have a pretty uniform friction in every direction assuming the same profile.

I'm not trying to stir up an argument, I'd just like to see something the supports these claims.
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Unread 12-04-2014, 15:04
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Re: Swerve Drive vs Mecanum Wheel drive?

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Originally Posted by themccannman View Post
I don't understand how the same 4 wheels touching the ground in a static state could possibly have different amounts of traction.
We were discussing a "good" and a "bad" mecanum drive. Who said the wheels on the "good" one were the same part number as those on the "bad" one?

And even if they were the same part number, who said they were all assembled and adjusted with the same care and attention to detail?

Differing amounts of friction in the rollers (due to wheel design, manufacturing tolerances, and care in assembly and adjustment) affects the wheel-to-floor traction of the wheels. A detailed explanation of the physics can be found here.


Quote:
How can a free wheel be forced to spin when going perfectly perpendicular to it's direction of rotation?
We were talking about the rollers, not the wheels. The rollers need to spin in order for the bot to translate at 45 degrees.


Quote:
In a standard mecanum drive set up if you drive the robot straight forward the rollers on all 4 wheels will slip slightly due to the fact the ground is applying force at a 45 degree angle to their direction of rotation.
Read Note 1 at the bottom of the post to which you were responding.


Quote:
In the case of normal traction wheels commonly used in FRC I think it's pretty safe to say the only difference in traction would be the slight change in the profile of your contacting surfaces depending on the direction of travel.
It sounds like you are saying that the tread pattern on commonly-used FRC wheels results in isotropic friction. Was that your intent?


Quote:
I'm not trying to stir up an argument
Understood, and appreciated.


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Unread 12-04-2014, 15:57
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Re: Swerve Drive vs Mecanum Wheel drive?

Quote:
Originally Posted by Ether View Post
We were discussing a "good" and a "bad" mecanum drive. Who said the wheels on the "good" one were the same part number as those on the "bad" one? And even if they were the same part number, who said they were all assembled and adjusted with the same care and attention to detail?
Well that's what I was assuming; that both teams had the same wheels and simply assembled their drivetrains differently (e.g. different tolerances, different gearing, etc...). Aside from some play in the mounting of the wheels they shouldn't behave any differently. I think we're debating different situations.

Quote:
Differing amounts of friction in the rollers (due to wheel design, manufacturing tolerances, and care in assembly and adjustment) affects the wheel-to-floor traction of the wheels. A detailed explanation of the physics can be found here.
I was assuming that the wheels didn't have differing amounts of internal friction in the rollers or on the axles. If they did then I would definitely agree with you that that would make a difference.
Quote:
We were talking about the rollers, not the wheels. The rollers need to spin in order for the bot to translate at 45 degrees.
I'm not talking about the motion of the bot, just the motion of the wheels. Mecanum wheels can obviously only spin in two directions. Both of which are applying force at a 45 degree angle to the rollers. In practice it's impossible to make your robot move while keeping the rollers on all 4 mecanum wheels static, which is why I pointed that out. You will always have a slight loss in efficiency.

Quote:
Read Note 1 at the bottom of the post to which you were responding.
I'm making all of my statements assuming this is done in practice, not in theory.

Quote:
It sounds like you are saying that the tread pattern on commonly-used FRC wheels results in isotropic friction. Was that your intent?
Yes. Blue nitrile tread(what I have the most experience with and what is generally accepted to be the best tread material) interacting with carpet will have a relatively isotropic interaction resulting in fairly uniform friction in all directions assuming the same surface area and profile in the direction of motion each time.


Quote:
Understood, and appreciated.
I think this debate is a largely a difference in interpretation of the conditions under which these tests are done. I'm assuming that two teams with either a bad or good mecanum drive assembled their wheels and axles identically. Also I'm comparing mecanum specifically to well built 6 wheel WCD on 1-2" blue nitrile traction wheels.

Hope that clears things up.
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Unread 12-04-2014, 17:11
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Re: Swerve Drive vs Mecanum Wheel drive?


Jake,

I agree that if you assume there are no differences between "good" and "bad" mecanum bots then there will be no differences between them. I just think that's a bad assumption, in practice.

When you said "The rollers on mecanums ... will always spin slightly unless you are going perfectly perpendicular to their direction of rotation" I read that to mean that the only time the rollers on a mec wheel on a mec vehicle would not spin is when said wheel was going in a direction perfectly perpendicular to the direction of rotation of the roller presently touching the ground (i.e., going in the direction of that roller's axis). If that is what you meant, it is not correct. If that is not what you meant, could you please clarify what you did mean?

In practice it is possible to build a mecanum vehicle which, when operated in the forward (or reverse) direction on a hard surface, will exhibit virtually no motion of the rollers. In other words, spinning rollers in the forward/reverse direction is not an inherent property of mecanum wheels: it is a mostly due to roller axial free play and operating on a compliant surface.


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Unread 13-04-2014, 16:47
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Re: Swerve Drive vs Mecanum Wheel drive?

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Originally Posted by lcoreyl View Post
zero appealing aspects? How about the tiny fraction of time to design and implement compared to the other holonomic drives (some with “unappealing” rollers) which receive your blessing? I’m not sure how giving such a strong opinion without mentioning any characteristics or tradeoffs is useful.


by this logic, if robot A has 6WD and bulldozes robot B with 6WD, then robot A has just proven their own drivetrain sucks. There’s more to it than this.

Yes, sensors make driving mecanum easier, but you can be successful without them. For example, a mentor from 2996, Cougars gone wired, told me their mecanum drive encoders and gyro were not working in Utah. They still have a shiny blue banner they brought home, however.

How about discussing the actual tradeoffs of swerve vs. tank tractive capabilities instead of saying tipping to 2 wheels is somehow inherent in swerves design? “typically” you are using only half power? Really?


You make me want to create a verb form of hyperbole.

Never built or driven a swerve, but wouldn’t lag depend on how it’s built and programmed? Are you saying it’s impossible to build one with acceptably small lag? In watching swerve used in competitions, I can’t say I’ve noticed lag as a “painful” problem. Hopefully someone with good swerve experience can speak on this.


As a driver from 2996, I would have to say that I personally don't like the robot auto correcting for me, It slows me down. I prefer no sensors during driver control because at least for me, it isn't as responsive in a controlling sense. Accurate, yes. Responsively accurate, no. Just my opinion for the way I drive.
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Unread 13-04-2014, 21:33
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Re: Swerve Drive vs Mecanum Wheel drive?

Quote:
Originally Posted by Ether View Post

Jake,

I agree that if you assume there are no differences between "good" and "bad" mecanum bots then there will be no differences between them. I just think that's a bad assumption, in practice.
I'm not assuming there's no difference, I'm assuming that every team has the capability to at least assemble their wheels correctly.

Quote:
When you said "The rollers on mecanums ... will always spin slightly unless you are going perfectly perpendicular to their direction of rotation" I read that to mean that the only time the rollers on a mec wheel on a mec vehicle would not spin is when said wheel was going in a direction perfectly perpendicular to the direction of rotation of the roller presently touching the ground (i.e., going in the direction of that roller's axis). If that is what you meant, it is not correct. If that is not what you meant, could you please clarify what you did mean?
That is exactly what I meant. When a wheel is traveling on a surface perfectly perpendicular to it's axis of rotation there will be no net force causing it to spin. Since a mecanum wheel only has two directions (one axis) of rotation and neither of those directions is perfectly perpendicular to the axis of rotation of the rollers; it is not possible to drive a mecanum wheel on a surface with friction in a manner that will cause a net zero force on it's rollers in their axis of rotation.

Quote:
In practice it is possible to build a mecanum vehicle which, when operated in the forward (or reverse) direction on a hard surface, will exhibit virtually no motion of the rollers. In other words, spinning rollers in the forward/reverse direction is not an inherent property of mecanum wheels: it is a mostly due to roller axial free play and operating on a compliant surface.
We're not talking about a hard surface, we're talking about robots on a soft carpet. Even so however, I don't buy that. I don't think that you can build a vehicle on mecanum wheels that will exhibit near-zero roller motion under hard acceleration forwards or backwards. With very slow acceleration of course it's possible have very little roller spin, however with the acceleration we see in FRC, I don't believe that.
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