Subject:
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Re: Maximum sample rate (turn rate) for rotation sensor
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Newsgroups:
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lugnet.robotics.rcx
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Date:
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Sat, 21 Jul 2001 01:51:13 GMT
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Viewed:
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1537 times
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When using the standard firmware, sensors are sampled every 3 ms. The
rotation sensor uses a quadrature encoder with a 1:4 gearing between the
axle and the encoder, so there are 16 "ticks" per revolution of the
axle. The decoding routine in the software needs to see every
successive tick, so with 16 ticks per revolution and 3ms per tick you
get 1250 revolutions per minute.
Note that this assumes everything is absolutely perfect...if one of
those ticks happens just after the sensor is sampled, then you'll miss
the next three ticks as well. There is probably some jitter in the
actual timing of the sensor sampling, and some variability in the
rotation speed. So I'd expect a few missed rotations as you get close
to 1250 rpm.
In my own tests, I didn't have any accurate way to measure the real
rotation, so I couldn't verify the complete accuracy of the sensor. All
I could do was see if the sensor was at least in the right ballpark.
For example, take something at about 350 rpm, then do a 1:3 gearing
bringing it to approximately 1050 rpm, then let the rotation sensor
watch it for 10 seconds and see if the measured rotation corresponded to
somewhere near 1050. These weren't exact tests, but at least showed it
was close. If I used 1:5 gearing (1750 rpm), then the sensor wasn't
even close.
Other people have reported the sensor missing counts under various
circumstances. Perhaps there are other factors that occasionally cause
it to miss some ticks in the 500-1250 rpm range.
Dave Baum
In article <GGryIH.IG@lugnet.com>, "G.W. Lucas" <gwlucas@connix.com>
wrote:
> Does anyone know the correct maximum sampling rate
> for the Lego rotation sensor? This morning I noticed
> that Dave Baum gives a value of 1250 RPM in his
> "Definitive Guide" (page 29), but looking at
>
> http://guide.lugnet.com/set/9756
>
> I see a value of 500 RPM. Which is correct?
>
> Thanks.
>
> gary
--
reply to: dbaum at enteract dot com
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Message is in Reply To:
 | | Maximum sample rate (turn rate) for rotation sensor
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| Does anyone know the correct maximum sampling rate for the Lego rotation sensor? This morning I noticed that Dave Baum gives a value of 1250 RPM in his "Definitive Guide" (page 29), but looking at (URL) see a value of 500 RPM. Which is correct? (...) (23 years ago, 20-Jul-01, to lugnet.robotics.rcx)
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