Subject:
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Re: Newbie needs Help (diff sensor)
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Newsgroups:
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lugnet.robotics
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Date:
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Wed, 7 Jun 2006 16:42:07 GMT
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Viewed:
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4745 times
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In lugnet.robotics, steve <sjbaker1@airmail.net> wrote:
> [snip description of "tangent board" test to measure static friction]
>
> The problem is that this is measuring the static friction...
As you note in your mini-cooper example, *if* you can keep the wheels from
"spinning out", but instead are always in rolling contact with the ground,
static friction is what's important... and generally, the static coefficient of
friction is larger than the dyanmic coefficient of friction. Ideally, to get the
maximum tension in the rope, you do *not* want your tires to break free. This is
why I brought up this example.
> To measure the dynamic friction you can use the same locked-up
> wheel rig and just pull it along flat ground with a spring
> balance. Drag it along at a reasonably constant speed where it
> isn't jerking along.
That's not very easy in practice, and you need to make sure the rope is
pulled parallel to the floor. I've always wanted to use the RCX and a sensor to
do this automaticly, but the results "by hand" have always been so poor I've
never bothered.
> Furthermore, realise that the high school physics equations for
> sliption say that the frictional force is proportional to the
> weight - but NOT related to the contact area. People find that
> counter-intuitive - but it's true.
It's wonderfully true in the world of high-school physics, but the real world
can be significantly different. Since the behavior of the rubber depends on the
pressure exerted, as well as the dynamic physics of what happens as the tire
repeatedly sticks and slips (both are still occuring), this is another one of
those places where I suggest you do some empirical tests, and not worry too much
about what the physicist say (and for the record, I *am* one of them... I have
great fun in my college level classes showing the students where the textbook
solution fails miserably in the realy world).
> Tank tracks are useless for the same reason - they increase
> the contact area
The problem is, the LEGO tank treads really don't - even if you include a
bunch of idler wheels like on a real tank, take a look at how much of the tread
actually contacts the surface and you'll see what I mean.
> Stiffer rubber is better...
All other things being equal, perhaps... but how "hard" or "soft" the rubber
is plays a major role in the coefficient of friction. "Stiff" tires often have
lower coefficients of friction (both static and dynamic) on certain surfaces...
which brings up another factor that hasn't been mentioned (too much), the fact
that different tires have different coefficients of friction on different
surfaces... and the one that's good on surface A may not be the best on surface
B.
> and you'll need to be aware that stictional forces ARE
> proportional to the contact area...
OK, I'm confused. Why would one of these forces be proportional to the
surface area, and the other not? In both cases, the frictional force is just the
normal force (usually the weight) times the coeffecient of friction. Period.
Surface area (in "textbook" physics) just doesn't enter into it.
> MAYBE even consider tank tracks IF you support the bottom
> of the track with a bunch of little idler wheels.
Only on the right kind of carpet, or maybe mud.
--
Brian Davis
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Message has 2 Replies:
 | | Re: Newbie needs Help (diff sensor)
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| (...) hmm. This sounds like a good project... (...) All things being equal, I'd put my money on the robot with ten spinning wheels, over a couple (or even ten) stationary ones. I'd also mechanically connect all the motors together, so they drive a (...) (18 years ago, 7-Jun-06, to lugnet.robotics)
| | | Re: Newbie needs Help (diff sensor)
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| (...) Hmmm - that's unfortunate...but if the 'right' strategy is to go with a non-slipping robot, the measurement of sliption is really a lot less important. (...) Yeah - so many of those basic mechanics equations are only approximations - yet they (...) (18 years ago, 7-Jun-06, to lugnet.robotics)
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Message is in Reply To:
| | Re: Newbie needs Help (diff sensor)
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| (...) The problem is that this is measuring the static friction ("stiction"). That's the wrong thing to measure if your strategy is to jam all the motors full on and progress forwards (we hope!) with all wheels spinning. To measure the dynamic (...) (18 years ago, 7-Jun-06, to lugnet.robotics)
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