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Unread 17-05-2014, 23:28
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Re: Coaxial Swerve Derivation with Paired Modules

Quote:
Originally Posted by Oblarg View Post
I'd naively think that when you're slipping the wheels you wouldn't have much of an effect from vehicle speed at all; in the reference frame of the wheel, the only thing vehicle speed does is decrease the effective rotational speed of the wheel (almost negligibly at those speeds, at that). It's not clear what effect that would actually have on the force generated by the wheel.
As I stated in the previous post, the force exerted on the wheel by kinetic friction with the carpet is modeled to be constant (independent of relative slip speed) when the wheel is slipping.

But the acceleration of the vehicle depends on the net external force on the vehicle, not just the kinetic friction force of the carpet acting on the wheel.

Krv allows you to account for that. A robot with a large "shot-blocking shield" for example could generate windage force. The carpet could generate speed-dependent force to plow through it.

I'm not claiming that 0.5 is a "typical" value for Krv. AFAIK, nobody yet has empirical data to establish a typical value. But if there is a vehicle-speed-dependent force resisting the robot motion, Krv is there in an attempt to model it. The model is a teaching tool - it allows you to see what effect a vehicle-speed-dependent resisting force has on robot acceleration.

I'd be willing to bet that if someone ever runs some precision tests of robot acceleration with wheels slipping over a sufficient speed range, it will show a decrease in acceleration with speed as shown in the plot (i.e., Krv>0).

I've attached a plot showing the effect of changing Krv to 0 (notice the flat line when the wheels are slipping), and of changing μs to 2 (not realistic I know, but it shows what the 6CIM would do if not traction limited).



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