Subject:
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Re: Received my RIS 2.0 - the two motors run at different speed! :-(((
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Newsgroups:
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lugnet.robotics
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Date:
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Sat, 11 Jan 2003 16:36:48 GMT
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Original-From:
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Steve Baker <SJBAKER1@AIRMAILsaynotospam.NET>
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Viewed:
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712 times
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Stephane Simard wrote:
> I opened the box of my first Mindstorms set yesterday, and it came with two
> motors that are slightly different from the usual motors I have, they are
> much lighter, have notches, and sound differently than normal geared down
> motors (such as those that came with the 8479 barcode truck set and 8735
> power pack).
Yes - they've recently changed the design slightly - you should check the
mailing list archives - there has been a lot of discussion about this.
> Anyway, they both work, but they sound differently from each other. I built
> the most basic robot possible, the roverbot, and when it's supposed to run
> on a straight line, it actually slightly veers towards right! It appears
> that the "noisier" of my new motors is faster than the other.
Motors don't all run at the same speed - that's a given - and you have to
design your robot in full knowledge that this is the case.
> Is this good enough of a reason to call Lego and get two motors who run at
> the same speed?
Well - it's a matter of degree. If one runs half the speed of the other - then
it's obviously faulty - send it back. If one runs 10% slower than the other,
then that's just how it is.
Somewhere between 10% slower and half the speed is the point where you'd
have to regard the thing as 'broken'.
I have a dozen supposedly identical motors - and there is an 11% spread
between their speeds. (Admittedly, these are the older geared motors -
I don't have numbers for the newer ones).
> I don't mind if they ask me to send them back to them.
> In the past I asked them for a new motor because the one that came with my
> 8457 Power Puller set was too slow and weak, and they sent me a new one free
> of charge. But that was with the "older" geared down motor. I kinda feel bad
> about it since the RIS 2.0 motors work fine; they just turn at slightly
> different speeds.
Yeah - well - I think "slightly" sounds like something you'll have to live
with. You could quite easily find that when you send the slower one back,
the new one you'd get in return would be faster than the one you didn't
send back.
I looked into this in some detail with the OLDER geared motor:
http://www.sjbaker.org/steve/lego/motor_speed.html
...mainly to dispel the myth that these motors run faster when
running clockwise than anticlockwise. Of course it could be
that the new motors *do* run faster in one direction than the
other - which might explain the speed difference in your case.
I don't think so though.
I also found that if I assembled a reasonably complex gear train
with a particular motor - measured the speed - then disassembled
it, put it back together again in EXACTLY the same way - I could
measure an 8% difference in speed between the two runs!
There is also a difference in speed between a motor that's been
running for a while (and is therefore getting hot inside) and
one that's been stopped for ten minutes. This means that if
your robot spins on the spot for a while, it may not drive
straight when you start off again.
That means that no matter *HOW* good the match between your
two motors, you'll still end up with a robot that won't drive
especially straight.
Advice that I've heard for making robots that DO go straight:
1) Buy 50 motors - measure their speeds - label them accordinly
and always use 'carefully matched pairs'. I think this sucks
as a solution - for reasons explained above - but some people
do it and it certainly must help.
2) Build an 'adder/subtractor' drive in which one motor drives
both wheels forwards and the other motor drives them in
opposite directions.
3) Use a rotation sensor on each motor to detect their speeds
and use the power level control and 'float' command to compensate
for the difference in their speeds. This also lets you measure
how far your robot has moved or turned...which is quite important
in many robotic applications. If your motors are *WAY* different
in performance, this won't work well...but you can deal with a
10% difference using this technique.
4) Build a robot that has steering - so again, one motor drives
the robot forwards and the other one steers...there are a variety
of clever mechanisms (like a Killough platform) that do this in
interesting ways that avoid the mismatched-motor phenomenon.
Of course - even with these precautions, you'll get *some* error - the
friction between tyres and ground, slight unevenness in the floor, etc, etc.
I think that figuring out ways to steer straight with somewhat mismatched
motors is just a part of good robot design - so unless they are grossly
mismatched, you should suck it up and work on your robot design.
---------------------------- Steve Baker -------------------------
HomeEmail: <sjbaker1@airmail.net> WorkEmail: <sjbaker@link.com>
HomePage : http://www.sjbaker.org
Projects : http://plib.sf.net http://tuxaqfh.sf.net
http://tuxkart.sf.net http://prettypoly.sf.net
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