Subject:
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Re: Rotational Sensor & Gearing Down
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Newsgroups:
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lugnet.robotics
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Date:
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Wed, 24 Jan 2001 16:15:06 GMT
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Original-From:
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Steve Baker <sjbaker1@airmail.net[NoSpam]>
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Reply-To:
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sjbaker1@airmail+IHateSpam+.net
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Viewed:
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1443 times
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Rebel Transcanner wrote:
>
> "Bob Fay" <rfay@we.mediaone.net> wrote in message
> news:G7MKLo.2tB@lugnet.com...
>
> > Rotation sensors will lose counts randomly in spite of the firmware.
>
> I'll back that! It's totally hit-and-miss. I've done several tests on
> different gearings (some running the thing off a micromotor...) and no
> matter what, they *always* loose count. I'm really dissapointed with them,
> and I've hardly used them for this exact reason. :(
OK - time for the rigerous scientific approach here (well - OK - the
half-assed semi-scientific Steve Baker method :-)...
I'm typing this email as I run the tests...so right now, I don't know
the answers...
Test 1:
Connect two rotation sensors to a motor, run for 10 seconds:
Sensor 1 Sensor 2
792 792
790 790
788 788
784 784
Well, at standard 'new Mindstorms' motor speeds, EITHER no counts
were lost at all over four ten second runs *or* the lossage is in
the system software and happens identically for both motors.
Test 2:
I did one ten minute run - and the two sensors returned identical
counts at the end.
Test 3:
We can test for the theory that when the system firmware 'glitches',
it always drops counts on BOTH sensors. If that were true then
if I run two sensors at different speeds one will drop a different
number of counts from the other.
OK - so I'll change the setup so that one rotation sensor is
on the motor shaft - and the other on another shaft that's
geared down 5 times.
Sensor 1 Sensor 2 Sensor2 * 5
778 156 780
774 155 775
772 154 770
771 154 770
Yep - this is fine - since we are using integer math, sensor2 * 5
is always within 5 of sensor1 - that's as good as it gets.
Conclusion: At ~50 to ~300 RPM, we don't lose counts.
Test 4:
It's known that when the shaft speed is *too* high, the
software will eventually lose counts for sure. So, let's
speed up one shaft and keep the other at the 'known good'
speed of the Lego motor.
I'm swapping the gears to we are geared UP 5 times...
Sensor 1 Sensor 2 Sensor2 / 5
752 2517 503
749 2494 498
749 2499 499
748 2480 496
Woah!! BIG TIME Lossage. It's pretty clear that the rotation
sensor doesn't work at 1,400 RPM without losing data.The
good news is that it's pretty consistent in that is loses about
one count in three. This would still be useful in some
applications...but I don't recommend it.
Test 5:
So, I wonder about the slow speed lossage (which is a pretty well
known 'phenomenon'), let's gear down by 25:1 and run again:
Sensor 1 Sensor 2 Sensor2 * 25
790 32 800
785 32 800
784 29 725 <===== Oops!
779 31 775
...so yes - at SLOW speeds (say 12 RPM) we lost at least
one count over ~40 seconds of testing.
Conclusions:
~~~~~~~~~~~~
You can run your sensors directly on the motor shafts of
the standard Mindstorms motor - and you can run it 5x
slower than that and still not lose counts. However, don't
run 25x slower or 5x faster than the motor or you *WILL*
lose counts.
Try to stick to rates around 50 to 300 RPM - the range may
be a little higher - but we know that <12 RPM or >1400 RPM
are definitely bad news.
Future Work:
~~~~~~~~~~~~
Someone (not me!) should probably figure out a bunch of gear
trains and motor loadings to check the actual limits - and
run the tests over MUCH longer periods than 10 seconds.
I also wonder if it's possible to make the thing lose more
counts by giving the firmware more work to do. It I run
6 threads all doing lots of sensor and motor work - and run
the light sensor at the same time - and play lots of beeps
on the speaker...would that load the firmware down enough
to make it drop counts at all speeds?
I'd *hope* not. If I were designing the RCX, I'd make the
rotation sensor generate high priority interrupts so that
sensors would be accurate no matter what...but who knows?
OK - so do we have any more long-established 'myths' to bust?
--
Steve Baker HomeEmail: <sjbaker1@airmail.net>
WorkEmail: <sjbaker@link.com>
HomePage : http://web2.airmail.net/sjbaker1
Projects : http://plib.sourceforge.net
http://tuxaqfh.sourceforge.net
http://tuxkart.sourceforge.net
http://prettypoly.sourceforge.net
http://freeglut.sourceforge.net
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Message has 2 Replies:
 | | Re: Rotational Sensor & Gearing Down
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| (...) But I don't think you did what we reported IS the problem... Did you run the motor FORWARD, then REVERSE so we can see the missing positions? That's where the errors come in. Dean -- Coin-Op's For Sale!: (URL) Lego Workshop: (URL) Lego Club: (...) (24 years ago, 25-Jan-01, to lugnet.robotics)
| | | Re: Rotational Sensor & Gearing Down
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| "Steve Baker" <lego-robotics@crynwr.com> wrote in message news:3A6EFF8A.416F3D...ail.net... (...) Nice post, but you only ran the sensors counting up for x number of seconds. Try *reversing*. Then watch what happens... (IE, bring a sensor back home. (...) (24 years ago, 26-Jan-01, to lugnet.robotics)
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Message is in Reply To:
| | Re: Rotational Sensor & Gearing Down
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| "Bob Fay" <rfay@we.mediaone.net> wrote in message news:G7MKLo.2tB@lugnet.com... (...) I'll back that! It's totally hit-and-miss. I've done several tests on different gearings (some running the thing off a micromotor...) and no matter what, they (...) (24 years ago, 23-Jan-01, to lugnet.robotics)
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