Subject:
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Re: Competitive MINDSTORMS: Now in a store near you!
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Newsgroups:
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lugnet.robotics
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Date:
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Fri, 20 Aug 2004 12:24:49 GMT
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1213 times
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In lugnet.robotics, David Perdue wrote:
> 1:15 and 1:75 are, in fact, the gear ratios... there are
> actually only two motors applying torque into the
> drivetrain... wheel radius is 81.6 x 15
Hmm. The way I see it, pushing power is limited (in an ideal world!) by one
of two factors: applied torque (a function of motors and gearing), and the
frictional force between the wheels and the surface (controled by the weight of
the 'bot and the coefficient of friction). There's little point maximizing
applied torque beyond the point where frictional contact of the wheels fails, so
ideally (there's that word again) you'd like them to be equal. In mathmatical
terms (forgive me):
F_push = Torque_applied / r <-- applied torque limit
F_push = mu m g <-- frictional limit
Setting these two equal should give us the minimum torque needed to just match
the frictional force that actually provides a forward "push":
Torque_applied = mu m g r
where mu = coefficient of friction
m = mass of 'bot
g = acceleration of gravity
r = tire radius
So, for instance, for a 3 lb (1.36 kg) sumo bot, assuming *all* that weight is
carried on the driving wheels, you can get an idea of the minimum torque
required to break the wheels free; assume mu = 1 (high friction, although under
some circumstances there are LEGO wheels for which mu > 1. Honest.) and r = 4.1
cm, and you need at least (1)(1.36)(9.81)(4.1) = 54.7 N cm (see, there's a
reason I stayed in SI units the whole time... besides allowing certain
rtlToronto folk to follow, that is ;-), these are the same units I know the
motor torques in). That's the bare minimum that needs to be applied. I usually
aim at this point or above, as there are a number of things that can reduce the
delivered torque: due to low batteries, frictional losses in the geartrain,
friction in the wheel axles (this can be minimized with good axle support, but
there's a lot of weight riding on them), variations in motors.
Also note that in terms of maximizing pushing power, skid plates (or
non-driven wheels) should carry the minimum of weight possible: any weight not
carried by the driven wheels reduces the maximum frictional force they can
exert.
> I'm not sure about the 900 Ncm wheel axle torque...
The way I got that was to consider that when stalled, the standard gear motor
delivers about 6 N cm of torque (less, about 4.5 N cm, if two are connected to
one current-limited output port). Take 6 N cm and gear it up by a factor of 75,
and each drive axle is at 450 N cm delivered, or a sum total of 900 N cm for the
'bot as a whole. Even at 15-to-1, delivered torque (again, assuming no losses)
should be 180 N cm, enough to break the Mindstorms motorcycle tires free even if
the 'bot weighed in at 3.3 kg (over 7 pounds).
> > Any chance you could bring ZR1 or any of the other 'bots
> > from your book to Sumo 6 in Ann Arbor?
>
> I would like to go... I'm not 100% sure that I can attend,
> but I'll see what I can do.
I hope you can make it - last year (my first event) was a wonderful time for
everybody, AFOL & more properly aged kids alike.
> I'd love to bring Gargantuan-Bot Version BL58 with me to the
> event, but I think that one might be too big under the rule
> set. :)
What's it weigh? Sumo 6 is a 2 lb limit (rtlToronto folks, that's 909 grams)
and must fit upon start-up inside a 12" by 12" square (30 cm, guys). Come on
down!
http://www.24tooth.com/sumobot6/
--
Brian Davis
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Message has 2 Replies: ![](/news/x.gif) | | Re: Competitive MINDSTORMS: Now in a store near you!
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| (...) Wow! That's a lot of math. I see now where the 900 N cm comes into the picture. (...) Unfortunately, BL58 does weigh over 2 lbs, but as Brian suggested, I could bring it anyway and we could have several rounds with it just for fun. Thanks, (...) (20 years ago, 20-Aug-04, to lugnet.robotics)
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Message is in Reply To:
![](/news/x.gif) | | Re: Competitive MINDSTORMS: Now in a store near you!
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| (...) 1:15 and 1:75 are, in fact, the gear ratios. I achieved these ratios not by the rotations--for instance, an 8t gear turning a 24t would result in a 3:1 ratio--but by counting the teeth--for instance, an 8t gear turning a 24t gear would result (...) (20 years ago, 20-Aug-04, to lugnet.robotics)
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29 Messages in This Thread: ![Competitive MINDSTORMS: Now in a store near you! -David Perdue (17-Aug-04 to lugnet.announce, lugnet.robotics)](/news/x.gif) ![](/news/46.gif) ![Re: Competitive MINDSTORMS: Now in a store near you! -Steve Hassenplug (19-Aug-04 to lugnet.robotics)](/news/x.gif) ![](/news/246.gif) ![Re: Competitive MINDSTORMS: Now in a store near you! -David Perdue (19-Aug-04 to lugnet.robotics)](/news/x.gif) ![](/news/246.gif) ![Re: Competitive MINDSTORMS: Now in a store near you! -Steve Hassenplug (19-Aug-04 to lugnet.robotics)](/news/x.gif) ![](/news/46.gif) ![Re: Competitive MINDSTORMS: Now in a store near you! -David Perdue (20-Aug-04 to lugnet.robotics)](/news/x.gif) ![](/news/46.gif) ![Re: Competitive MINDSTORMS: try again... -Steve Hassenplug (20-Aug-04 to lugnet.robotics)](/news/x.gif) ![](/news/46.gif) ![Re: Competitive MINDSTORMS: try again... -David Perdue (20-Aug-04 to lugnet.robotics)](/news/x.gif) ![](/news/46.gif) ![Re: Competitive MINDSTORMS: try again... -Steve Hassenplug (20-Aug-04 to lugnet.robotics)](/news/x.gif) ![](/news/46.gif) ![Re: Competitive MINDSTORMS: try again... -David Perdue (20-Aug-04 to lugnet.robotics)](/news/x.gif) ![](/news/46.gif) ![Re: Competitive MINDSTORMS: try again... -Steve Hassenplug (21-Aug-04 to lugnet.robotics)](/news/x.gif) ![](/news/46.gif) ![Re: Competitive MINDSTORMS: try again... -David Perdue (21-Aug-04 to lugnet.robotics)](/news/x.gif) ![](/news/46.gif) ![Re: Competitive MINDSTORMS: try again... -Steve Hassenplug (21-Aug-04 to lugnet.robotics)](/news/x.gif) ![](/news/46.gif) ![Re: Competitive MINDSTORMS: try again... -Chio Siong Soh (24-Aug-04 to lugnet.robotics)](/news/x.gif) ![](/news/46.gif) ![Re: Competitive MINDSTORMS: try again... -Steve Hassenplug (24-Aug-04 to lugnet.robotics)](/news/x.gif)
![](/news/x.gif) ![](/news/28.gif) ![](/news/x.gif) ![](/news/68.gif) ![Re: Competitive MINDSTORMS: Now in a store near you! -Brian Davis (19-Aug-04 to lugnet.robotics)](/news/x.gif) ![](/news/46.gif) ![Re: Competitive MINDSTORMS: Now in a store near you! -David Perdue (20-Aug-04 to lugnet.robotics)](/news/x.gif) ![](/news/246.gif) ![You are here](/news/here.gif) ![](/news/246.gif) ![Re: Competitive MINDSTORMS: Now in a store near you! -Joe Strout (20-Aug-04 to lugnet.robotics)](/news/x.gif) ![](/news/46.gif) ![Re: Competitive MINDSTORMS: Now in a store near you! -David Perdue (20-Aug-04 to lugnet.robotics)](/news/x.gif)
![](/news/x.gif) ![](/news/28.gif) ![](/news/x.gif) ![](/news/28.gif) ![](/news/x.gif) ![](/news/68.gif) ![Re: Competitive MINDSTORMS: Now in a store near you! -David Perdue (20-Aug-04 to lugnet.robotics)](/news/x.gif)
![](/news/x.gif) ![](/news/28.gif) ![](/news/x.gif) ![](/news/x.gif) ![](/news/68.gif) ![Re: Competitive MINDSTORMS - at Sumo-bot 6? -Steve Hassenplug (28-Aug-04 to lugnet.robotics)](/news/x.gif) ![](/news/246.gif) ![Re: Competitive MINDSTORMS - at Sumo-bot 6? -Jon Gilchrist (28-Aug-04 to lugnet.robotics)](/news/x.gif)
![](/news/x.gif) ![](/news/28.gif) ![](/news/x.gif) ![](/news/x.gif) ![](/news/68.gif) ![Re: Competitive MINDSTORMS - at Sumo-bot 6? -David Perdue (1-Sep-04 to lugnet.robotics)](/news/x.gif) ![](/news/46.gif) ![Re: Competitive MINDSTORMS - at Sumo-bot 6? -Steve Hassenplug (7-Sep-04 to lugnet.robotics)](/news/x.gif) ![](/news/46.gif) ![Re: Competitive MINDSTORMS - NOT at Sumo-bot 6? -Steve Hassenplug (14-Sep-04 to lugnet.robotics)](/news/x.gif)
![](/news/x.gif) ![](/news/x.gif) ![](/news/68.gif) ![Re: Competitive MINDSTORMS: Now in a store near you! -Brian Davis (22-Aug-04 to lugnet.robotics)](/news/x.gif) ![](/news/46.gif) ![Re: Competitive MINDSTORMS: Now in a store near you! -David Perdue (23-Aug-04 to lugnet.robotics)](/news/x.gif) ![](/news/46.gif) ![Re: Competitive MINDSTORMS: Now in a store near you! -Brian Davis (23-Aug-04 to lugnet.robotics)](/news/x.gif) ![](/news/46.gif) ![Re: Competitive MINDSTORMS: Now in a store near you! -David Perdue (23-Aug-04 to lugnet.robotics)](/news/x.gif)
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