Subject:
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Re: designing new NXT parts (propeller and motor) using rapid prototyping
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Newsgroups:
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lugnet.robotics.nxt
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Date:
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Sun, 30 Dec 2007 21:06:47 GMT
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Viewed:
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29246 times
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In lugnet.robotics.nxt, Saskia Bakker wrote:
> I have been exploring the use of rapid prototyping
> technology in building new parts for the NXT.
Nice looking parts! One of those times I wish I had a access to such equipment.
> Read my results on
> www.student.tue.nl/h/s.bakker/lego.pdf
A nice idea, and well-written. There are a few interesting points that I thought
of while reading through it.
First, you mention the NXT limits the total output current to 180 mA. I think
this might be in error. For instance on Philo's excellent page on the NXT motor,
he mentions that the NXT can support it at 15 N•cm torque, corresponding to a
current of about 500 mA, and it can easily exceed this to at least 800 mA for a
short time (before thermal protection engages, not due to a limitation the
driver in the NXT).
Second, in your analysis you make a lot of the Reynolds # on the thrust of a
propeller, but then never try to find the function for the thrust coefficient. I
understand why (it's a pain, and very non-linear), but you might mention that
the inclusion of a duct can change this significantly (there's a reason LEGO
ducts around the prop in the underwater motor, and it's not safety... try
measuring the thrust with & without that duct).
The prop speed is also not limited by the current provided - you can always gear
up or gear down. It's the total power delivered that is limited (yeah, it's a
subtle difference, but it can be an important one).
Finally, I suspect the reason your axle is breaking has less to do with the
speed or even total thrust, and a lot more to do with the initial acceleration
of the prop. It may be a small prop, but upon motor start up all that mechanical
power is going into overcoming the rotational inertia of the blade, and that's
not insignificant.
You might also try to directly measure the thrust provided by the propeller. For
instance, if you can keep the thrust directed while the prop is mounted on the
end of a pendulum, then at some point the thrust from the prop will balance (in
a vector way) the component of gravity trying to restore the pendulum to its
lowest position. Form that (& the mass of the pendulum & a little bit of
algebra), you can directly measure the thrust.
--
Brian Davis
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