Quote:
Originally Posted by EricH
Vector attack! Assume you have a mecanum drive arranged this way:
(front)
/// \\\
\\\ /// roller pattern, 45-degree rollers.
Now, going forwards, the vectors will go:
\/
/\ with the arrows pointing towards the front. The sideways components cancel, producing:
^^
^^
Now, for sideways:
Vectors will go
.^...v
..\.../
../...\
v....^
(45 degree angles; v's are arrows)
The front-to-back cancel (obviously), so you should get the same speed sideways in theory. The problem is that when the robot is going forwards, the rollers "jam" better than when two wheels are going opposite directions.
Also, I have noticed this on several mecanum robots. Can it be compensated for? Sure. Is it easy to program a mecanum in the first place? No. I think the current controller is adequate for the job.
For some empirical evidence: My team once decided to try out mecanum drive. So we put one on a Kitbot. We tested it against an old 4WD robot. Speed forwards was about equal (once the mecanum was revved up a bit), even with relatively slick rollers. With the mecanum going sideways, though, the 4WD would win every time. Before the mecanum got to where we wanted it, the 4WD was there, every time.
(And, if you really want to make sure you translate sideways as fast as you go forwards, try an omni-drive.)
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I've read your argument several times and it still doesn't make sense to me. Unless your wheels are messed up in some way or the transmission is off that behavior should not be present. In AM Mecanum wheels, I can definitely see this happening due to over-tightening the roller bolts. On a more Airtrax-style wheel, I can also imagine this happening for the same reason (depending on how the rollers are attached to the wheel). However, in both of those cases the speed difference is a result of reducing the effectiveness of the bearings due to user error--not something intrinsic to mecanum designs. Of course, I imagine many mecanum robots experience something like the aforementioned problems, but I don't think the difference would be major enough to driving sideways to be a poor strategic choice. In fact, in the situation you describe it appears that the mecanum robot's problems are not caused by the team's choice of a mecanum drive, but more due to inadequate programming or driver practice--essentially, the drivers were not comfortable controlling a holonomic robot.