Mecanum Drive control

[TylerHarmon]

Originally Posted by Ether

Hi Tyler,

Thank you for taking the time to respond to my question.

Some of the things you wrote are still a bit ambiguous to me. I think it would be enlightening to discuss it further. Would you be interested in doing that? I don't want to pester you if you don't want to do that.

Sure, if you want to discuss it further we can. My main point is that the rollers on mecanum wheels...
... freely spin, which allows a robot to strafe, but results in inconsistent data when used with an encoder.

[AriMindell]

Originally Posted by TylerHarmon

Sure, if you want to discuss it further we can. My main point is that the rollers on mecanum wheels...
... freely spin, which allows a robot to strafe, but results in inconsistent data when used with an encoder.

Ether posted the following code for determining robot position based on the wheel velocities of mecanum wheels in another thread:

FWD = r*(w1+w2+w3+w4)/4

STR = r*(w1-w2+w3-w4)/4

Wv  = (1/k)*(w1+w2-w3-w4)/4

r is wheel radius
k is |trackwidth/2| + |wheelbase/2|
w1,w2,w3,w4 are FL,BL,BR,&FR wheel speeds in rads/sec
Wv is robot clockwise rotation rate in radians per second

The implicit assumption is that the velocity vector applied to the robot by a mecanum wheel spinning at constant velocity is constant.

are you arguing that the velocity vector created by a mecanum wheel spinning at a constant velocity is not constant or predictable, therefore that equation provides an unreliable estimate of robot position?

[TylerHarmon]

Originally Posted by AriMindell

Ether posted the following code for determining robot position based on the wheel velocities of mecanum wheels in another thread:

FWD = r*(w1+w2+w3+w4)/4

STR = r*(w1-w2+w3-w4)/4

Wv  = (1/k)*(w1+w2-w3-w4)/4

r is wheel radius
k is |trackwidth/2| + |wheelbase/2|
w1,w2,w3,w4 are FL,BL,BR,&FR wheel speeds in rads/sec
Wv is robot clockwise rotation rate in radians per second

The implicit assumption is that the velocity vector applied to the robot by a mecanum wheel spinning at constant velocity is constant.

are you arguing that the velocity vector created by a mecanum wheel spinning at a constant velocity is not constant or predictable, therefore that equation provides an unreliable estimate of robot position?

I understand all of this logic, but it just doesn't work in practice. You simply cannot achieve the precision of tank drive with mecanum wheels because the rollers on mecanum wheels sometimes spin and sometimes do not. In other words, there is a large amount of slop in the system that cannot be removed with programming.

If you show me an example of you consistently driving a set distance and/or turning a certain amount without a gyroscope, only with encoders then I will accept that mecanums can be well controlled with encoders. However, based on the mechanical nature of mecanum wheels, such motion cannot practically be controlled.

[TylerHarmon]

Yes, that is what I'm saying. The force vectors are not reliable in practice, only in theory. The variable "rolling" of the rollers causes this effect. Because those force vectors change (not only in magnitude, but also in direction) you simply cannot achieve precise motion control of a robot running on mecanums.