Hall rotary encoder tempco--wisdom?

[Paul E Bennett]

Hi, all,

I don't have much to contribute to the LCR MOSFET argument except
possibly some ethological insight:
<http://electrooptical.net/papers/TerritorialityInTheWhiteRhinoceros.pdf>
. But I digress.

I'm on a trip to SoCal to debug the pre-production models of my
spectrometer. The SNR is pretty good, way over 60 dB (which is good for
a SWIR spectrometer).

Looks like the last remaining problem is that the spectrum has a way of
sort of swimming around a bit, i.e. the gross shape stays roughly the
same, and the small scale noise is low, but there are small systematic
variations on scales of 1/10 to 1/2 of the scan range, where multiple
spectra don't quite line up with each other.
[%X]

Any additional wisdom on these things? We can rip out the encoder and
bodge in a pot if we have to, but it'll limit the instrument lifetime
fairly severely.

There are a couple of options for sensing shaft rotation. As someone else
already mentioned the optical encoder I will confine my response to
suggesting that you could use a Synchro/Resolver to monitor the shaft. These
can be quite accurate over fairly wide temperature regimes. The decoding of
resolver signals (Sin/Cos) is not that difficult and there are chips from
Analog Devices that will provide a digital output from the signals. You can
even get rate of rotation informtion from them.

--
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Paul E. Bennett IEng MIET.....<email://[email protected]>
Forth based HIDECS Consultancy.............<http://www.hidecs.co.uk>
Mob: +44 (0)7811-639972
Tel: +44 (0)1235-510979
Going Forth Safely ..... EBA. www.electric-boat-association.org.uk..
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[[email protected]]

On a sunny day (Wed, 08 Jan 2014 16:40:06 -0500) it happened Phil Hobbs wrote:

Right, large gear box, lot of play.


No gear box?


Stepper motor, worm drive. constructon with springs that eliminates any play.
Would take a re-design.
You need help from a mechanical wizzard.

John already solved it--the encoder frequency isn't guaranteed stable,
and the software was measuring pulse width instead of duty cycle.

Cool gadgets.

Cheers,
James Arthur

 

[Spehro Pefhany]

Well, it just says 0.5 degrees at room temperature and 0.9 degrees over
-40 to +125. That can be read several ways.

The most negative way is usually the intended one. When you're writing
a spec you have to make your product look better than the competition,
and they're doing it too.

In any case, it looks like it's pretty stable with temperature now.

Cheers

Phil Hobbs

That's great! I hate things that "almost work".

By the way, getting a real environmental chamber might not be a bad
idea now that you've got the room for it (and 3-phase power?). You can
discover a lot of interesting stuff by logging gigs of data and
running over the whole temperature range- even on supposedly mature
designs.

I was recently debugging someone else's design and discovered an
interesting temperature-dependent instability at would never have been
seen otherwise. It went bonkers repeatably at two distinct
temperatures (with the temp moving in either direction). Can't discuss
the cause or cure, but that was a strange one.



Best regards,
Spehro Pefhany

 

[George Herold]

Well, it would be a semiconductor!

Hi Tim,
Yeah I was going to plot it up, log (R) vs 1/T.. but I've got a new 'puter at work and I've yet to get my old copy of Origin installed... But maybe later today/

That would be MnZn, no? I wonder if NiZn freezes out more, or completely..

Well I don't know exactly what it is. Pieces that I inhereted from elsewhere.
There's a nice article "Low Temperature Transformation in Ferrites", L. Bickford, Rev. mod. Physics, January 1953 pg75. Where he looks at magnetite and see's a nice phase transition at ~120 K. (change in R, magnetization and peak in specific heat.) Kinda showy.

Hmm, I doubt ferrites are pure enough to be controlled for dopant level
impurities, and defects (they're polycrystalline besides). There might be
residual resistance, at least until very low temperatures.

No idea if anyone's tried doping ferrites as semiconductors. Mixed
oxidation state materials tend to be very difficult to control (e.g., ZnO,
which I believe is almost always P-type due to oxygen deficiency), so
magnetic transistors (magistors? spinistors?) are unlikely, at least by
direct equivalence (electron/hole junctions and so on).

OK I don't know about the semi conductor nature. I looked at some other beads and found that the conduction wasn't a bulk effect. But looked more like percolation... resistance was not proportinal to the length.

George H.

 

[Lasse Langwadt Christensen]

Den torsdag den 9. januar 2014 18.01.01 UTC+1 skrev Tim Wescott:

AH HA!



I was wondering how in hell you could have a 5 degree variation over 100

K -- that would have you falling off the lines of a real encoder.



Now I know...



I would go looking for rotary LVDTs. When that failed, I'd be mightily

tempted to roll my own. Yes, you'd need some signal processing somewhere

to decode the LVDT, but if you've got enough capacity on a microcontroller

it can all be done with good accuracy in software.



With the right magnetic design, you could probably even make a restricted-

range LVDT with high sensitivity over your 8-degree span, at the you-

don't-care cost of crappy performance outside of that.



Failing that, if you have room for a long moment arm consider a couple of

magnets on sticks attached to your shaft, and a couple of linear hall

effect sensors on a board nominally centered on the magnets. Then read

the outputs, do some figgerin' and have an answer.

with more than one magnet that could quickly get complicated, permanent
magnets are ~2000ppm/C


-Lasse

 

[Spehro Pefhany]

Interesting. Cryogenic or ambient?

Cheers

Phil Hobbs

Ambient.

 

[Phil Hobbs]

It's reports like that which lead managers to ask the software engineers
to please increase the speed of light by changing the code.

In this case, it was just measuring the right thing. Which is good,
because the clinical trial starts in a week.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics

160 North State Road #203
Briarcliff Manor NY 10510

hobbs at electrooptical dot net
http://electrooptical.net