Subject:
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Re: Using un`Lego sensors
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Newsgroups:
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lugnet.robotics
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Date:
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Wed, 30 Jan 2002 03:09:52 GMT
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Viewed:
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637 times
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> I have two small rotative (circular) potenciometer (variable resistance)
> from some old headphones, that I'm thinking of using as an angle sensor. It
> won't measure 360º but it'll be close. I only don't know how accurate will
> be (or if it'll even work) and what range of raw values will have.
For what it's worth, there is another type of high-accuracy
potentiometer which is used primarily in instrumentation. They are
usually used with vernier dials to set accurate values. Their
advantage is that, unlike most basic, common potentiometers which have
a full end-to-end range of motion of about 270 degrees, these
instrumentation variants rotate 10 complete times (at least the ones I
have - others may even do more). That's 3600 degrees of rotation with
a resistive element that is designed for high-accuracy and linearity.
This would be a great advantage for something like a turntable that
may be required to several times in one direction (a crane comes to
mind) but which you wouldn't want to rotate too many times in one
direction for fear of tangling wires. From it's centre position these
would allow five rotations in either direction before you'd have to
start moving in the other direction again. This still doesn't work as
a substitute for a true rotation sensor, but it's far better than the
cheap Radio-Shack potentiometers you can get. They also move very
smoothly, BTW.
There are three serious disadvantages: They actually use a straight
resistive element (as opposed to a curved one), the wiper of which is
moved by an internal worm gear. It's the coarsness of the worm gear
thread as well as the gear's length that determines the number of
turns end-to-end. If you visuallize this, you will see that, unlike a
standard pot which wraps the resistive element around the shaft (and
hence limits it's angular length to 360 degrees less the space
required for terminals - or about 270 dgrees), these instrumentation
pots have an element that starts at the base of the shaft and extends
away. In some cases, these can get quite long and this may be a
problem. The second and third problems are that they aren't easy to
come by (generally) and when you do find them, they are relatively
expensive.
Hint: If you can't find new one's (preferred, since modern ones tend
to be smaller), you might be able to locate a dealer of used
electronics or scientific equipment and try to find an old instrument
of one kind or another. If they are working, they will likely be very
expensive and you would be better ordering the potentiometers on-line.
In most cases, these dealers will have a view junkers that they
couldn't fix and you might be able to pick one up that has two or four
of them for $5. That's how I came by all of mine.
Related Hint: Community Colleges and Universities sometimes have
closets full of outdated equipment. This was the source for at least
one of the instruments I gutted.
Matthias Jetleb
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Message has 1 Reply:
 | | Re: Using un`Lego sensors
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| (...) The trouble with this (I suppose) is that the precision of the A-to-D convertor in the RCX is going sharply limit the angular precision when that number range is 'stretched' over ten revolutions. I've seen the devices you talk about though - (...) (22 years ago, 29-Jan-02, to lugnet.robotics)
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Message is in Reply To:
| | RE: Using un`Lego sensors
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| Mike Reddy said: (...) hmmm... I did a search once, a "long" time ago, only to find DIY schematics. It's time to do a search again :). Maybe I'll find some already-built'n'ready-to-sell distance sensors "shop". (...) I have two small rotative (...) (22 years ago, 28-Jan-02, to lugnet.robotics)
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