Quote:
Originally Posted by JesseK
1.) More wheels can support more weight and still have omni-movement in a non-FRC application
2.) More wheels = more traction in a dynamic environment where actual amount of surface contact is unpredictable. Why? More wheels = higher probability that you will get a better mate of carpet and rubber at any given contact point. Tonight I'll try to find the link where some Japanese researchers showed this relationship when determining tread design for shoes that wouldn't slip when going up/down carpeted stairs. Essentially, the shorter the carpet and finer the threads the less this is an issue but it could explain the anecdotal evidence some teams claim they've seen.
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I was viewing this through the limited lens of FRC, and there are situations where having more wheels/greater contact area could improve traction. In an FRC game (at least like every one we've seen so far), the benefit would be minimal in terms of traction, and quite possibly harmful in terms of "pushing power."
A middle set of driven wheels could be beneficial for tasks such as a ramp and stair climbing, but on a planar surface I question the advantage it gives.
Quote:
Originally Posted by JesseK
Just thought about something after reading a couple of posts. In the few instances where the robot is trying to move forward, the middle omni wheels would essentially give the drive train 100% torque moving forward, and 71% torque moving sideways.
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Assuming equal coefficients of friction and even weight distribution (neither of which are likely to be true in the real world), wouldn't forward/backward motion actually utilize 88.4% of the total torque? The three sets of omni-wheels would have a total of 60% of the weight and (theoretically) 100% efficiency in that direction, while the mecanums would hold 40% of the weight and have 71% efficiency.
In perpendicular (strafing) motion, it would (theoretically) have only 28.4% of the total torque, as the omni-wheels (which hold 60% of the bots weight) are only acting in the forward/reverse direction.
Quote:
Originally Posted by JesseK
Furthermore (and I just had this thought....), if your bot tries to push on the diagonal, (which should have the code only driving two motors at full speed), you only have 0.5 * 0.71 = 35% of the available total torque with the other two wheels acting as castors. This situation is probably where most people get their eye-witness accounts of 'mecanums can't push'.
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On a standard mecanum you'll have 50% of total torque. 2 wheels operating at 100%, and 2 wheels operating at 0% (casters).
In this drive (once again assuming equal coefficients of friction and even weight distribution), 20% of the robots weight will be acting at 100% (driven mecanums), 40% will be operating at 71% (four driven omni-wheels operating at 45º), and 40% will be operating at 0% (undriven omnis and mecanums). That theoretically means you're effectively using 48.4% of your torque. Or just under what a standard mecanum would be using.
Quote:
Originally Posted by JesseK
384 & 1086 seemed to have massively powerful mecanums in 2007 & 2008, and what I've gathered from them is it's all about gearing and how you try to push 'bots around. If your bot is square to the bot you're trying to push, so long as your bot is geared properly there shouldn't be a problem in pushing the other bot. Traction (as defined by the ability to convert torque into forward movement, which takes the roller angle into account)) is still a matter of what the rubber rollers are made out of and not the angle of the rollers.
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I think you've hit a good note, though from a slightly different perspective. In general, I've found, that a robots anecdotal "pushing power" has every bit as much, if not more, to do with how it's driven than the mechanical features of the bot. Many of the 8-wheel defensive monsters out there seem to have more pushing power than a 4-wheeled bot because they drive more aggressively, and actively use their momentum and defensive driving techniques to their advantage.