Re: Source Sealed Potentiometers?

On 9 Oct 2006 12:15:19 -0700, Robert Adsett <sub2@xxxxxxxxxxxxxxxxxxxxx> wrote:
Frnak McKenney wrote:
--snip--
http://www.taosinc.com/downloads/pdf/encoderdesign4b.pdf
http://www.taosinc.com/downloads/pdf/IOSDN1.pdf
--snip--
I'm also having trouble translating the "tracked" approach out of
the optical domain and into the magnetic... um, "field" (oooog!)
to satisfy the OP's concerns regarding outdoor use. It's not that I
think that this translation _couldn't_ be done, it's that I get
stuck at the part where I picture _my_ rather clumsy fingers trying
(for the third time) to glue 256 magnets of two different widths
around the circumference of the OP's platform in the correct
pattern.

Hm. What if I wrapped _one_ flexible magnetic strip around the
platform's rim and then covered it with a carefully punched strip of
metal? Will enough of the strip's magnetic field get through so
that a linear Hall Effect sensor can tell the difference between a
narrow and a wide slit? I suspect I can print-and-paste-and-punch a
pattern more reliably than I can glue individual itsy-bitsy
magnets... I need to think about this.

Reverse the set up a bit, put the magnet on the sensor and just leave
the punched pattern on the table. That's essentially how gear tooth
detecting hall sensors work, at least some of them incorporate the
magnet into the sensor assembly. Building an array of them to achieve
the desired resolution I'll leave to you ;)


Reverse the set up a bit, put the magnet on the sensor and just
leave the punched pattern on the table. That's essentially how
gear tooth detecting hall sensors work, at least some of them
incorporate the magnet into the sensor assembly.

Robert,

By "leave the punched pattern on the table" did you mean not use it?
Or that I should attach my oh-so-laboriously punched metal strip to
the rim of the platform ("table")?

I've run across descriptions of Hall Effect geartooth detectors, but
I never took the time to look at the datasheets. I assumed that
they were looking for a change in the magnetic field caused by the
presence (or absence) of a tooth. It feels like one could build an
absolute encoder with this type of detector by using an irregularly-
toothed "gear", that is, by putting gear teeth at appropriate
positions around the rim of John Mianowski's (the OP's) platform,
but I'd have to do a bit of research on the detectors before I
started building one.

Your posting started me thinking (not necessarily a good thing when
I have RealWork(tm) to get done <grin>). There are a number of
fascinating things about the approach TAOS describes:

1) It's separable. On the one hand there's the pattern and
variations on that, and on the other there are the sensors
themselves. You could implement this absolute position
detection scheme with an in-line 128-element optical sensor as
TAOS did, but you could equally well use Hall Effect magnetic
sensors, capacitive sensors, or even crusty ol' mechanical
MicroSwitches.

Imagine, for example, JohnM's platform with a variable-depth cam
pattern around the rim, and a set of those cam-driven
MicroSwitches (the ones with with rollers at the end of the
switching lever) arranged around the perimeter.

2) It's robust. If someone twirls the encoder while the encoder
is powered down, the new resting position will be reported
accurately when the system powers back up.

3) It's extensible, at least up to the limits of sensor resolution
and placement. 256 divisions not enough? Just get narrower
sensors and print more bits/bars.

4) Sensor placement is flexible. TAOS used a single in-line
128-sensor array with all the sensor elements clumped together,
but one could also arrange individual sensors at roughly
equidistant points around the perimeter of the platform or in
other locations as long as one suitably adjusted the encoder
pattern. This is a GoodThing(tm), since mounting six or eight
of even the smallest MicroSwitches so that their rollers are
only 1-2mm apart could get a bit tricky. <grin!>

If I were going to implement an absolute position reporting system
based on TAOS's approach, I'd want to pay particular attention to
two things: "transition" points, and the properties of truncated
MLSes.

Transition points, those "points between", are always tricky. One
of the reason Gray Codes are so handy for absolute encoders is that
even if the encoder wheel happens to be midway between "official"
positions, at most one bit will be affected. The Maximal Length
(bit-)Sequences used by TAOS, on the other hand, will almost always
have more than one sensor changing value as the encoder moves from
one "official" position and another, so multiple bits will be
"uncertain" at the same time. TAOS's 128 sensors solve this problem
with redundancy -- multiple sensors per encoder "bit" -- but
implementations based on other sensors need to worry about what
happens at transition points.

The other "open" topic is whether, if we need to know a rotation
angle to within 2 degrees, we can find a 180-bit MLS pattern. It
would be really _handy_ to have our encoder pattern "bits" sitting
exactly at 2 degree points around the rim of the platform whose
position we're monitoring; an angle reported out in increments of
(360/180) degrees is much easier to work with than one reported in
units of (360/256) degrees.

Unfortunately, the only references I've found so far assume that one
only cares about MLS sequences with lengths of ((2^N)-1), that is,
one less than a power of two. I'd be _really_ cautious about
trusting that I could chop off (say) the last (255-180) or 75 bits
from the end of a 255-bit MLS pattern, join the head to the tail,
and still have an MLS pattern, but there might be another method for
generating a 180-bit MLS.

... Building an
array of them to achieve the desired resolution I'll leave to
you ;)

Aw... you're _too_ kind! <grin!>

Oh, and just to stir the pot a bit more, in my search for an array
of Hall Effect sensors I ran across the Honeywell HMC1512 magnetic
sensor:

Honeywell Linear / Angular / Rotary Displacement Sensors
http://www.ssec.honeywell.com/magnetic/datasheets/
hmc1501-1512.pdf

AppNote AN-211: APPLICATIONS OF MAGNETIC POSITION SENSORS
http://www.ssec.honeywell.com/magnetic/datasheets/an211.pdf

The HMC1512 reports the _angle_ direction of a magnetic field with a
reported range of -90 to +90 degrees and a resolution of less than
5/100 of a degree! Not bad for a sensor chip that Digi-Key sells
for $6.45, and a pair of them and a couple of magnets might be all
that's needed to satisfy JohnM's requirements (assuming he can
solder an SOIC-8 package <grin!>).

Ah, well... It's been fun, but I have to go do some RealWork(tm).
Sigh.


Frank McKenney, McKenney Associates
Richmond, Virginia / (804) 320-4887
Munged E-mail: frank uscore mckenney ayut minds pring dawt cahm (y'all)
--
The joy of research must be found in doing, for every other
harvest is uncertain. -- Theobald Smith
--
.