Mecanums: Let's Settle This

[StimpNZ]

One factor when turning is the higher mass of the wheels, this means they have a higher rotational inertia, so take longer to accelerate, this is also why your gears were getting damaged. When using mecanums, make sure you have enough torque to accelerate them quickly or use some kind of ramp code

[The_VEX_Raptors]

@StimpNZ

One factor when turning is the higher mass of the wheels, this means they have a higher rotational inertia, so take longer to accelerate, this is also why your gears were getting damaged. When using mecanums, make sure you have enough torque to accelerate them quickly or use some kind of ramp code

The higher mass also has advantages, such as increased downforce and traction.

[StimpNZ]

@The VEX Raptors

The higher mass also has advantages, such as increased downforce and traction.

Down force and traction come from the mass whole of the robot and can be changed by weighing down the chassis, rather than putting on mecanum wheels just to get it, if you want better traction (and don’t need to strafe) a heavier chassis with omni’s is better than one with mecanums.

[Oliver_W]

@StimpNZ

One factor when turning is the higher mass of the wheels, this means they have a higher rotational inertia, so take longer to accelerate, this is also why your gears were getting damaged. When using mecanums, make sure you have enough torque to accelerate them quickly or use some kind of ramp code

The rotational inertia of the wheels is negligible when you compare it to the other things the motor has to do and is capable of doing.

If we assume that the moment of inertia of a mecanum wheel is the same as that of a thin ring of diameter 4" (this is a conservative estimate) then:

I = m*r^2

Then the energy required to accelerate it to 100 rpm, say, is:

E = 1/2(I*ω^2)

E = 1/2(mr^2ω^2)

m = 0.41 lbs = 0.186 kg
r = 2" = 0.0508 m
ω = 100 rpm = 3.33(pi) rad/s

E = 0.0263 J

This is a very small amount of energy. To illustrate:

Peak power of a 393: 3.98 W

0.0263/3.98 = 0.0066s

So it takes 0.0066s for a 393 at full power output (50 rpm) to provide the same amount of energy as is required to accelerate a mecanum wheel from rest to 100 rpm. (This isn’t supposed to be representative of something that could happen in real life because you would need a CVT etc, it’s just meant to be indicative of the relative scale of the numbers involved).

The angular acceleration of the wheels isn’t likely to be a major factor in how fast the robot accelerates or whether you break parts.

[banditofernando]

@dontworryaboutit

Were the wheels oriented the correct way? The rollers on top of the wheels should be pointing toward the center of the robot. If they aren’t, turning won’t really work. When turning, you shouldn’t have any less force with Mecanum wheels than with omni wheels.

Lol yes they were. But when we switched to Omnis, we also chained the motors, so that also helped.

[dontworryaboutit]

@banditofernando

Lol yes they were. But when we switched to Omnis, we also chained the motors, so that also helped.

ok you never know

That’s one of the reasons why I like chaining all the wheels together so much: the force is distributed evenly among the motors.

[chris1]

@The VEX Raptors

Of course they roll when the robot strafes. My question is, do they roll when the robot is driving forward or backward?

No, they don’t.

For anyone that still doesn’t believe me, try it yourself, like this:

Get a couple of mecanum wheels, one with the rollers one way and one with the rollers the other way. Put both on to a long shaft. This will simulate both wheels moving at the same speed in the same direction on a normal robot, just as they do when you drive forwards.
Use some shaft collars as well to stop the wheels sliding along the shaft.

  (////)
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    |
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    |
  (\\\\)

Now, try turning the shaft while the wheels are on the floor. You should now be able to tell that the rollers effectively lock against each other.

The only way one roller can roll is if the roller on the opposite wheel is actually dragged sideways along the floor.

By simply playing around with the parts in this way, you should be able to satisfy yourself that when driving straight forwards or backwards, and providing you are not skidding your wheels, there is simply no way that the mecanum rollers can roll.

[The_VEX_Raptors]

@StimpNZ

Down force and traction come from the mass whole of the robot and can be changed by weighing down the chassis, rather than putting on mecanum wheels just to get it, if you want better traction (and don’t need to strafe) a heavier chassis with omni’s is better than one with mecanums.

But we DO plan to strafe. We figure that the little extra downforce and standard chassis shape that you get with a mecanum drive make it better for our design than an X-drive or H-drive. We’re not worried much about sacks getting caught, since we’ve been planning on having a low chassis anyway.

[dontworryaboutit]

@Vex Mundi

The rotational inertia of the wheels is negligible when you compare it to the other things the motor has to do and is capable of doing.

If we assume that the moment of inertia of a mecanum wheel is the same as that of a thin ring of diameter 4" (this is a conservative estimate) then:

I = m*r^2

Then the energy required to accelerate it to 100 rpm, say, is:

E = 1/2(I*ω^2)

E = 1/2(mr^2ω^2)

m = 0.41 lbs = 0.186 kg
r = 2" = 0.0508 m
ω = 100 rpm = 3.33(pi) rad/s

E = 0.0263 J

And if you look at the translational kinetic energy of the wheel:
v = 100rev/min * pi*4in/rev * 1m/39.4in * 1min/60s = .53m/s

KE = .5*.186kg*(.53m/s)^2 = .026J

Notice how it’s the same (it’s because Vex Mundi treated the wheel like a ring when doing the rotational energy).

However, at .41lb each, they do add up to quite a bit of weight (1.64lb for 4). That’s about 10% of the weight of a lot of robots, but could be upwards of 20% the weight of a small robot. Compare that to omni wheels, which are .232lb each (.928lb for 4), and you’ve got a decision to make.

[1039B]

@dontworryaboutit

And if you look at the translational kinetic energy of the wheel:
v = 100rev/min * pi*4in/rev * 1m/39.4in * 1min/60s = .53m/s

KE = .5*.186kg*(.53m/s)^2 = .026J

Notice how it’s the same (it’s because Vex Mundi treated the wheel like a ring when doing the rotational energy).

However, at .41lb each, they do add up to quite a bit of weight (1.64lb for 4). That’s about 10% of the weight of a lot of robots, but could be upwards of 20% the weight of a small robot. Compare that to omni wheels, which are .232lb each (.928lb for 4), and you’ve got a decision to make.

Hes got a point…

[Stanley_Shi_2R]

@chris@aura

No, they don’t.

For anyone that still doesn’t believe me, try it yourself, like this:

Get a couple of mecanum wheels, one with the rollers one way and one with the rollers the other way. Put both on to a long shaft. This will simulate both wheels moving at the same speed in the same direction on a normal robot, just as they do when you drive forwards.
Use some shaft collars as well to stop the wheels sliding along the shaft.

  (////)
    |
    |
    |
    |
  (\\\\)

Now, try turning the shaft while the wheels are on the floor. You should now be able to tell that the rollers effectively lock against each other.

The only way one roller can roll is if the roller on the opposite wheel is actually dragged sideways along the floor.

By simply playing around with the parts in this way, you should be able to satisfy yourself that when driving straight forwards or backwards, and providing you are not skidding your wheels, there is simply no way that the mecanum rollers can roll.

I do agree with Chris, since the rollers on the mecanum wheel are there to allow strafing by rolling, if the rollers were moving when the robot was moving foward, the robot would move sideways as well.

[mykiscool]

We’ve had mecanums on our robots in the past and one con of mecanums is that you can not have it geared for speed as much because it takes more force to move the robot sideways with the mecanums. This will result in less speed when going strait forward than a robot without mechanum wheels.