Mecanum Differential Drive

[efoote868]

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?

[mman1506]

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

[Oblarg]

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.

[efoote868]

Originally Posted by Oblarg

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.

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.

[Oblarg]

Originally Posted by efoote868

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.

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.

[efoote868]

Originally Posted by Oblarg

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.

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.

Originally Posted by Oblarg

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.

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.

[Oblarg]

Originally Posted by efoote868

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.

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.

[magnets]

Originally Posted by efoote868

In this setup, the annulus, the planets, and the sun gear are all utilized, instead of the traditional fixed annulus.

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.

[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.