Mecanum Differential Drive

[efoote868]

The annulus would have to be machined for this special purpose, but I think if it had teeth on the outside (3rd image down) it would make powering it much easier than needing 3 concentric shafts.

[MichaelBick]

Lots of misinformation in this thread.

The only team that I have heard of using shifters for better acceleration is 33. Everybody else uses their second speed as a "pushing speed". 16fps has clear advantages. If you have a higher top speed then you can get to your destination faster. They way we got around the acceleration problem was by adding more motors(something you have already done). Also, our low gear is traction limited(if it wasn't teams like 254 with 6 CIMs would blow their main every time they hit another robot)

Locking mecanums does not improve forward traction. To improve forward traction on a mecanum you would have to change the roller material.

Also, planetaries are extremely heavy(especially ones big/strong enough for drive). For all the weight of the planetary, you might as well put in another shifter(which would also be easier to fabricate because you could put it COTS).

This doesn't even start to mention the fact that you are removing all the benefits of mecanum. Mecanums are easy and relatively light. With this, you are making it heavy and extremely hard to manufacture. You might as well run a swerve.

[Oblarg]

Originally Posted by MichaelBick

The only team that I have heard of using shifters for better acceleration is 33. Everybody else uses their second speed as a "pushing speed". 16fps has clear advantages. If you have a higher top speed then you can get to your destination faster. They way we got around the acceleration problem was by adding more motors(something you have already done). Also, our low gear is traction limited(if it wasn't teams like 254 with 6 CIMs would blow their main every time they hit another robot)

This is true.

Locking mecanums does not improve forward traction. To improve forward traction on a mecanum you would have to change the roller material.

This is false; if you draw a free-body diagram for the rollers in contact with a ground on a Mecanum drive train, you'll see that the effective coefficient of friction compared to a traction wheel of the same material is reduced by sqrt(2)/2, because the friction force on each roller acts parallel to its axis. That is to say, if you apply the same force in the forward/backward direction to a mecanum drive and a traction drive, the induced friction force on the rollers needed to keep the robot stationary is greater on the mecanum drive by a factor of sqrt(2), since the friction force from each roller is pointing at a 45-degree angle (it must, to keep the rollers on opposite sides from spinning).

Also, planetaries are extremely heavy(especially ones big/strong enough for drive). For all the weight of the planetary, you might as well put in another shifter(which would also be easier to fabricate because you could put it COTS).

This doesn't even start to mention the fact that you are removing all the benefits of mecanum. Mecanums are easy and relatively light. With this, you are making it heavy and extremely hard to manufacture. You might as well run a swerve.

This is dead-on. Large increases in weight and complexity must yield large benefits to be justified. For the amount of added weight/complexity in this design, there are a number of other proven designs with much greater added benefits.

[AdamHeard]

Originally Posted by Oblarg

This is false; if you draw a free-body diagram for the rollers in contact with a ground on a Mecanum drive train, you'll see that the effective coefficient of friction compared to a traction wheel of the same material is reduced by sqrt(2)/2, because the friction force on each roller acts parallel to its axis. That is to say, if you apply the same force in the forward/backward direction to a mecanum drive and a traction drive, the induced friction force on the rollers needed to keep the robot stationary is greater on the mecanum drive by a factor of sqrt(2), since the friction force from each roller is pointing at a 45-degree angle (it must, to keep the rollers on opposite sides from spinning).

He was talking about locking the rollers so they can't rotate, in which case traction is equivalent.

[Oblarg]

Originally Posted by AdamHeard

He was talking about locking the rollers so they can't rotate, in which case traction is equivalent.

He said that "locking the rollers on a mecanum drive does not increase forward traction." I took that to mean, "does not increase forward traction c.f. a mecanum drive with un-locked rollers," which is not true for the reasons I outlined; locking the rollers gives you an immediate traction boost by a factor of sqrt(2).

[efoote868]

Originally Posted by MichaelBick

16fps has clear advantages. If you have a higher top speed then you can get to your destination faster.

In FRC, 16fps for a single speed has no advantages to 12fps for a single speed. By the time you accelerate to the top speed, you're already clear across the field.

Originally Posted by MichaelBick

They way we got around the acceleration problem was by adding more motors(something you have already done). Also, our low gear is traction limited(if it wasn't teams like 254 with 6 CIMs would blow their main every time they hit another robot)

You can't use 6 cims in present mecanum drives.

Originally Posted by MichaelBick

Also, planetaries are extremely heavy(especially ones big/strong enough for drive). For all the weight of the planetary, you might as well put in another shifter(which would also be easier to fabricate because you could put it COTS).

Controlling a robot through 4 independent shifts on a mecanum drive would be difficult.

Originally Posted by MichaelBick

This doesn't even start to mention the fact that you are removing all the benefits of mecanum. Mecanums are easy and relatively light. With this, you are making it heavy and extremely hard to manufacture. You might as well run a swerve.

8 years ago if I told you that in the future, mecanum wheels would be available for purchase to every team at a reasonable cost, you would have called me crazy.

[EricH]

Not all innovation leads to acceptance. Just ask Dean.


There IS one drivetrain with mecanums that COULD have 6 CIMs applied, though not during strafe maneuvers. 1322 built a 6-wheel mecanum back in 2010 (2008 version). If you powered the central omniwheel driveshaft (and one of the two wheels on it), you could apply a 5th and 6th CIM. I leave how to do that up to the reader.

[MichaelBick]

Originally Posted by efoote868

In FRC, 16fps for a single speed has no advantages to 12fps for a single speed. By the time you accelerate to the top speed, you're already clear across the field.

You can't use 6 cims in present mecanum drives.

Your calculations are totally dependent on weight. Acceleration on a 90lb robot is fine. Just look at 254 to see very sucessful drivetrains that have been geared fast(16-18 fps).

While you can't run a 6 CIM mecanum, you are ignoring the fact that at this weight and complexity you might as well make an octocanum or swerve.

Controlling a robot through 4 independent shifts on a mecanum drive would be difficult.

Teams do it all the time on butterfly. Mecanum is no different.

8 years ago if I told you that in the future, mecanum wheels would be available for purchase to every team at a reasonable cost, you would have called me crazy.

Honestly I have no idea what will be available in 8 years, so it isn't worth speculating about.

[efoote868]

Originally Posted by MichaelBick

While you can't run a 6 CIM mecanum, you are ignoring the fact that at this weight and complexity you might as well make an octocanum or swerve.

I'm not ignoring that, I even stated it as a problem in the OP.

[Oblarg]

Originally Posted by MichaelBick

Teams do it all the time on butterfly. Mecanum is no different.

Actually, thinking along these lines, has any team tried a 4-shifter mecanum with locking rollers? Seems to me it might be a nice alternative to octanum; it'd offer pretty much identical functionality with comparable complexity/weight.

[MichaelBick]

Originally Posted by Oblarg

Actually, thinking along these lines, has any team tried a 4-shifter mecanum with locking rollers? Seems to me it might be a nice alternative to octanum; it'd offer pretty much identical functionality with comparable complexity/weight.

Complexity and weight isn't comparable whatsoever. Look at 3928 butterfly and 973's offseason butterfly. Substitute in mecanums and you have a octocanum. Their drives are significantly less resources than a WCD even and almost as light.

[Oblarg]

Originally Posted by MichaelBick

Complexity and weight isn't comparable whatsoever. Look at 3928 butterfly and 973's offseason butterfly. Substitute in mecanums and you have a octocanum. Their drives are significantly less resources than a WCD even and almost as light.

Shifters aren't that much heavier than normal toughbox nanos (~3-4lbs extra, total), and a locking mecanum system doesn't add much weight either if you build it intelligently.

In terms of moving parts, you've got four extra cylinders on a locking mecanum system, yes, but you remove the need for an extra belt, axle, and pulleys.

With the probable advent of 4'' mecanums from VexPro (which one could run straight from a CIM with direct-mounted gears and no gearbox), octanum does get quite a bit lighter, admittedly.

[BBray_T1296]

What about a swerve drive with mecanum wheels? haha nevermind...

[Oblarg]

Originally Posted by BBray_T1296

What about a swerve drive with mecanum wheels? haha nevermind...

Nah, that's not good enough, you've gotta put octanum modules on the swerve modules.

With locking plates on the mecanums, of course.

[BBray_T1296]

Originally Posted by Oblarg

Nah, that's not good enough, you've gotta put octanum modules on the swerve modules.