Mecanum Differential Drive
 

I've been toying around with an idea for a while, and I'd like to share it with the FRC community, and maybe get others to improve on it or (if they feel like a challenge :) ) implement it.

My idea combines some of the present implementations of 6 wheel tank drive along with the transmission in the Toyota Prius, to improve upon some of the perceived weaknesses in mecanum drives - namely motor distribution and shifting.

I'll try to describe it as best I can here, although if it doesn't make sense a crude paint drawing may be in order.

First, start with the standard shifting tank drive. Each side has an independent shifting transmission which directly powers two or more wheels. When driven together, the robot can move forward and backward. When driven against one another, the robot can spin clockwise and counter clockwise. This accounts for 2/3rds of the motion in mecanum drives.

Second, take the planetary gearbox used in the Prius. It takes an input from a gas motor as well as an input from an electrical motor, and combines the two into power for the wheels. Now, instead of using the shifting transmission to directly drive the wheels in the mecanum drive, use the output of the shifting transmission as the first input of a planetary gearbox. Use another motor as the second input to the planetary gearbox.

Since each wheel has an independent motor, the left/right motion of the robot can be controlled by spinning the auxillary motors against each other.

Foreseeable issues include the problem of back-driving the motor at each wheel. This will have to be designed out mechanically with a brake or worm gear. There will also be an issue mechanically with weight, complexity, as well as the number of motors required for this design. Finally, controlling this will take lots of thought and optimization.

Another observation that I've had while thinking about this is that each wheel doesn't require a planetary gearbox, just one on each side. While this will allow the robot to move sideways with less complexity, it doesn't allow all motors to push together for a higher top speed.


Thoughts?

Re: Mecanum Differential Drive
 

Dude you forgot encoders, it would win champs if it had encoders.

Re: Mecanum Differential Drive
 

Motor distribution and shifting aren't really the primary drawbacks of a mecanum drive; it's the decrease in traction that gets you. There's little purpose in shifting to a lower-gear with a mecanum drive train when your pushing ability is traction-limited.

As for your idea, I'm having a hard time visualizing what exactly it is you're suggesting, or how it'd result in a noted improvement over the standard one motor and one gearbox per wheel.

Re: Mecanum Differential Drive
 

One could incorporate locking mecanum wheels to gain more traction, but the limitation this addresses is top speed / acceleration time to top speed. I haven't heard of a mecanum setup pushing 16 feet per second or higher in FIRST, which seems to be a fairly regular occurrence with tank drive.



Output of shifting gearbox A is used as one of the inputs to planetary gearboxes B and C. The other input to B and C are independent motors. The output of the planetary gearbox goes to each wheel.
In this setup, the annulus, the planets, and the sun gear are all utilized, instead of the traditional fixed annulus.

Re: Mecanum Differential Drive
 

Acceleration in FRC (with the exception of 2009's wonky drive surface/wheels) has never been traction-limited, so this is purely a function of the gear ratio people are using on their drives. There's nothing about a shifter which would provide any significant benefit to a mecanum bot; a well-designed mecanum drive is geared for high-speed at all times, because the ability to push is something you've already compromised on.

Your setup adds two gearboxes and two motors to the assembly, but I don't see how you get a significant benefit in return. When moving forward, you'd still have two CIMS pushing each side (as the auxiliary motors won't be doing anything, if I understand correctly), so you don't have any more effective, power-wise, than the standard setup.

Re: Mecanum Differential Drive
 

A well designed two speed gearbox has a significant advantage in acceleration. DC Motor power is on a curve, and the well designed gearbox will keep the motor close to peak power. Without a shifting gearbox, 16 fps in FRC provides nearly no advantage to 12 fps or 8 fps.

No, the auxiliary motors would be used in this setup as well. In the picture presented, 8 motors would be driving this robot when it is going full forward. Alternatively, 1 motor could be in gearbox A, which would allow for 6 motors driving full forward in mecanum drive.

The alternative configuration is that planetary gearbox C does not have an auxiliary motor, which would mean that only 4 motors are driving forward.

Re: Mecanum Differential Drive
 

In that case, I think you'd almost certainly do better to just plop the extra motors on the gearboxes in a standard configuration.

The gear ratio spread on most shifters (usually ~2.5:1 for a small spread) is such that a mecanum in the low gear will have little use if the high gear is in a reasonable range.

Shifters are useful for tank drives because they allow the same drive train to have high mobility and high pushing power. A mecanum drive will never have the latter to begin with. I've seen many drives set for ~16 f/s without use of a shifter; it's not a requirement to run at that speed effectively. For what it's worth, this thread is the first time I've ever seen any criticisms leveled at mecanum purely regarding mobility - if you build it right, you have a very agile robot.

If you're dissatisfied with mecanum as an omnidirectional solution and are willing to cope with a moderate increase in complexity, octanum and/or nonadrive are the direction to look in; these give you the full benefit of gear shifting, because you do not have low-traction wheels in contact with the ground in your lower gear.

Re: Mecanum Differential Drive
 

How do you plan on doing this?

This is similar to how any automatic transmission works, but I don't see a way that you could manufacture these parts.

Re: Mecanum Differential Drive
 

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.

Re: Mecanum Differential Drive
 

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.

Re: Mecanum Differential Drive
 

This is true.

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).

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.

Re: Mecanum Differential Drive
 

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

Re: Mecanum Differential Drive
 

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).

Re: Mecanum Differential Drive
 

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.

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

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.

Re: Mecanum Differential Drive
 

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.

Re: Mecanum Differential Drive
 

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.


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

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

Re: Mecanum Differential Drive
 

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

Re: Mecanum Differential Drive
 

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.

Re: Mecanum Differential Drive
 

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.

Re: Mecanum Differential Drive
 

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.

Re: Mecanum Differential Drive
 

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

Re: Mecanum Differential Drive
 

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

With locking plates on the mecanums, of course.

Re: Mecanum Differential Drive
 

Re: Mecanum Differential Drive
 

Care to expand?

Re: Mecanum Differential Drive
 

They both use the same basic design:

https://scontent-b-lax.xx.fbcdn.net/...29116488_n.jpg
https://scontent-a-lax.xx.fbcdn.net/...54889069_n.jpg

Essentially they have gotten butterfly down to the bare minimum causing it to be extremely easy to make(easier than a WCD) and just a bit heavier.

Re: Mecanum Differential Drive
 

With current COTS hardware, it's very hard to just swap out those omni-wheels with mecanum wheels without any changes; the 12t pinion on the 72t gear direct-mounted to the wheel relies on a 4'' wheel to work.

Incidentally, 4464 is making a slightly-modified version (we've made some material/layout changes to accommodate our specific machining capabilities) of that second type of module (actually, we cribbed most of our design choices from RoboWranglers - they had one of their modules on display at Worlds this year) as an offseason project.

Re: Mecanum Differential Drive
 

Our variant of this was trivially different (biggest difference was using 2" tall tube versus 3") and I can confirm it was easier to manufacture than our WCDs. There just aren't a lot of parts.

Re: Mecanum Differential Drive
 

The omni wheel is 6" dia and the traction 4" dia. I don't see the problem

Re: Mecanum Differential Drive
 

I am quite sure that omni wheel is 4'', with a 72-tooth VexPro gear on it; vexpro does not sell gears that are that size proportional to a 6'' omni.

Re: Mecanum Differential Drive
 

VexPro doesn't sell a traction wheel smaller than 4" either

Re: Mecanum Differential Drive
 

But VexPro is going to unveil its new products soon, which should include this tiny devil

Re: Mecanum Differential Drive
 

Yeah, I know, I mentioned it earlier. That should make octanum quite a bit easier to realize.

It is very easy to make your own small traction wheels out of delrin and plaction tread; I've seen several teams that did this.

Re: Mecanum Differential Drive
 

Are they really quicker/easier to make? At this point you essentially do a COTS everything WCD, whereas the butterfly, you have to make the plates for the wheels and mill the tubing. I'd also expect that WCDs are a lot harder to get wrong than butterfly drives, at least for most teams.

Of course, basically all drives in FRC (except possibly swerve) are to the point where they work quickly and well 90% of the time. It's funny to think just a few years ago just driving around the field was considered something to be proud of.

Re: Mecanum Differential Drive
 

It was a loaded statement I suppose, as we haven't run a WCD with a COTS gearbox yet. That certainly brings it to be more comparable.

If we do WCD again next year, we'll be sticking with our bearing blocks. If we also purchased COTs blocks it makes the drives pretty comparable.

With our router in house the butterfly side plates are very fast to make.

Re: Mecanum Differential Drive
 

Sorry buddy, but it's 4" Omni w/ a 64T gear and a custom plastic wheel at ~ 2.4" Tread diameter. :(

For the sake of completeness, the pulley ratio from omni to traction is 18:30.

Re: Mecanum Differential Drive
 

Oh, it's a 64t? Do the 72t gears have ground clearance problems, or did you just want it to run faster? I ask because I had been previously advised that 12t pinion on a 72t gear with a 4'' wheel ends up at 16 fps and works well.

Fair enough, I'll do the same.

Re: Mecanum Differential Drive
 

I believe, but am not certain, that 3928 ran 12:72 (15.5 fps free speed).

We ran 12:64 (17.4 fps free speed) because we like things faster in California.

The extra ground clearance is a bonus. Even with the 64T a lot of crud was sucked into the gears.

Re: Mecanum Differential Drive
 

Cool, thanks for the info.

4464 currently has 12:72 for our offseason butterfly design, which is basically that but on a standard c-base frame and with delrin side-plates (the only CNC machining we have ready access to is a lasercutter at UMD). Might consider changing to 12:64 in later iterations if the ground clearance is a problem.

Re: Mecanum Differential Drive
 

We had no issues with the gears being that close to the ground, for reference

And 973 was driving underweight which helps a fair amount.

Also Californians are weird.

-Aren