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Hall rotary encoder tempco--wisdom?

P

Phil Hobbs

Jan 1, 1970
0
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.

One thing I noticed is that the Hall effect shaft encoders are quite
unstable with temperature, something like 3 arc min per kelvin. Since
the grating only has to rotate about 8 degrees for the whole
measurement, that winds up being, like, 3 nm/K, which is _horrible_.
(They're US Digital type MAE3-P12-125-500-7-1: 12-bit PWM.)

The RC airplane servo that I used in the proto appeared much more stable
on the medium scale motion, although it didn't do small steps very
repeatably. It used a pot for the encoder, of course.

Right now we're working on putting a temperature controller on the
encoder, mostly by thermally grounding the leads to a controlled plate,
since the case is plastic and there's no contact between the encoder
body and the shaft.

Is the tempco usually this bad?

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.

Thanks

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
 
D

Don Y

Jan 1, 1970
0
Hi Phil,

One thing I noticed is that the Hall effect shaft encoders are quite
unstable with temperature, something like 3 arc min per kelvin. Since
the grating only has to rotate about 8 degrees for the whole
measurement, that winds up being, like, 3 nm/K, which is _horrible_.
(They're US Digital type MAE3-P12-125-500-7-1: 12-bit PWM.)

The RC airplane servo that I used in the proto appeared much more stable
on the medium scale motion, although it didn't do small steps very
repeatably. It used a pot for the encoder, of course.

Right now we're working on putting a temperature controller on the
encoder, mostly by thermally grounding the leads to a controlled plate,
since the case is plastic and there's no contact between the encoder
body and the shaft.

Is the tempco usually this bad?

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.

Can you add mechanical gain, instead? Is the motion always "one way"
(or, can it be made to be so?)
 
P

Phil Hobbs

Jan 1, 1970
0
Hi Phil,



Can you add mechanical gain, instead? Is the motion always "one way"
(or, can it be made to be so?)

It's not a backlash issue--it really is the encoder tempco. (Happens
both forward and reverse.)

The next rev will have gears and a preload spring, or possibly a sine
bar drive with a shaft encoder on the lead screw, but we need to get
this one working to make a clinical trial date.

Thanks

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
 
W

whit3rd

Jan 1, 1970
0
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 noticed is that the Hall effect shaft encoders are quite
unstable with temperature, something like 3 arc min per kelvin.

Maybe that shouldn't be surprising; Hall effect sensing starts with (usually) a few
dozen microvolts, and thermal and other drifts aren't swamped by that signal.

I'm not pleased with most shaft encoder designs, in fact. My preference is
for stepper/loaded geartrain with an optointerrupter so I can get a step 0 indication.
That has faults too (but one COULD imagine optical lever variants); I just recalibrated
every measurement run against a standard.

Have you considered ovenizing the shaft encoder, yet? It sounds icky, but it
might be a good quick fix.
 
P

Phil Hobbs

Jan 1, 1970
0
Maybe that shouldn't be surprising; Hall effect sensing starts with (usually) a few
dozen microvolts, and thermal and other drifts aren't swamped by that signal.

I'm not pleased with most shaft encoder designs, in fact. My preference is
for stepper/loaded geartrain with an optointerrupter so I can get a step 0 indication.
That has faults too (but one COULD imagine optical lever variants); I just recalibrated
every measurement run against a standard.

Have you considered ovenizing the shaft encoder, yet? It sounds icky, but it
might be a good quick fix.

Yup, that's underway, as I mentioned, thanks.

As soon as I found out that the encoder used Hall sensors, which are
notoriously drifty, I fished out the cold spray and had at it.

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
 
D

Don Y

Jan 1, 1970
0
It's not a backlash issue--it really is the encoder tempco. (Happens
both forward and reverse.)

No. My point was that (additional) mechanical gain can be a win -- but
only if it isn't offset by the loss of precision that backlash in the
geartrain can introduce.
The next rev will have gears and a preload spring, or possibly a sine
bar drive with a shaft encoder on the lead screw, but we need to get
this one working to make a clinical trial date.

Good luck!
 
P

Phil Hobbs

Jan 1, 1970
0
Why not an optical encoder, with lots of lines?

Too big and expensive. This one is only about 12 mm long by 12 mm
diameter, and costs about $15.

It's in what looks like an LC plastic housing, which may well be the
origin of much of the problem. Do you folks ever use these things?

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
 
L

Lasse Langwadt Christensen

Jan 1, 1970
0
Den onsdag den 8. januar 2014 22.56.58 UTC+1 skrev Phil Hobbs:
Too big and expensive. This one is only about 12 mm long by 12 mm

diameter, and costs about $15.



It's in what looks like an LC plastic housing, which may well be the

origin of much of the problem. Do you folks ever use these things?

the datasheet says mechanical Angular Accuracy <=.5deg@25C, <=0.9deg@-40C to 125C

how do you sample pwm?

-Lasse
 
P

Phil Hobbs

Jan 1, 1970
0
Could be that a Hall encoder doesn't have a lot of actual lines, and
they are interpolating. The Hall TC could mess that up.
Internally it's a Hall version of a synchro or maybe a resolver. The
differential-transformer ones are wonderful, but much bigger and more
expensive.

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
 
P

Phil Hobbs

Jan 1, 1970
0
Den onsdag den 8. januar 2014 22.56.58 UTC+1 skrev Phil Hobbs:
the datasheet says mechanical Angular Accuracy <=.5deg@25C,
<=0.9deg@-40C to 125C how do you sample pwm?

-Lasse

It actually drifts 0.7 degrees with a 15 degree temperature increase, so
I sort of doubt that last spec.

The PWM is sampled with a high speed capture input.

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
 
P

Phil Hobbs

Jan 1, 1970
0
Are you ratiometric on frequency? The spec says that the frequency can
drift a bunch with temperature.
Ah, good catch, thanks. I asked the SW guy, who said he's just
measuring the pulse width! (All in LabView, of course.)

One of the many reasons that I hate LabView is that it's ridiculously
hard to debug compared with procedural code--it's as bad as debugging a
spreadsheet.

We'll probably ovenize it anyway--it's cheap insurance.

Cheers

Phil


--
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
 
L

Lasse Langwadt Christensen

Jan 1, 1970
0
Den onsdag den 8. januar 2014 23.24.03 UTC+1 skrev John Larkin:
Yeah, it looks like maybe two analog elements, not a bunch of lines

like an optical encoder. I can see how the case, the magnets, the Hall

sensors would all have bad TCs.



It's, basically, interpolating 2 bits up to 12.

it like a resolver, trig on two voltages

I wonder if it works better at certain angles such as multiple of 45deg

-Lasse
 
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.



One thing I noticed is that the Hall effect shaft encoders are quite

unstable with temperature, something like 3 arc min per kelvin. Since

the grating only has to rotate about 8 degrees for the whole

measurement, that winds up being, like, 3 nm/K, which is _horrible_.

(They're US Digital type MAE3-P12-125-500-7-1: 12-bit PWM.)



The RC airplane servo that I used in the proto appeared much more stable

on the medium scale motion, although it didn't do small steps very

repeatably. It used a pot for the encoder, of course.



Right now we're working on putting a temperature controller on the

encoder, mostly by thermally grounding the leads to a controlled plate,

since the case is plastic and there's no contact between the encoder

body and the shaft.



Is the tempco usually this bad?



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.



Thanks



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

It's not supposed to budge out of spec'd performance limits over -40oC to +125oC operating temperature range.
http://www.usdigital.com/products/encoders/absolute/rotary/kit/MAE3
Not quite sure how it affects your final display, but it sounds like someone was doing more dreaming than thinking when they designed in just exactly how the encoder made things work.
 
B

Bill Sloman

Jan 1, 1970
0
Too big and expensive. This one is only about 12 mm long by 12 mm diameter, and costs about $15.

HP used to do reasonably compact and tolerably cheap optical encoders and optical encoder kits, aimed at front panel controls.

Farnell has three like this from Avago

http://au.element14.com/avago-technologies/heds-9730-a50/encoder-rotary-500ppr-2ch/dp/1654865

http://www.avagotech.com/pages/en/m..._encoders/transmissive_encoders/heds-9730q50/

It's twice the size you want, not cheap and needs a code-wheel with lines on it to make it work. I've tried to take a look at the data-sheet, but can't get it to download.
 
S

Spehro Pefhany

Jan 1, 1970
0
The quoted spec on duty cycle error vs temperature is about 10 times
better than we're measuring, assuming that the drift is a straight line,
and that "0.9 degrees" means +-0.45 degrees. If it's quadratic, the
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Always with the optimism, eh?


Best regards,
Spehro Pefhany
 
G

George Herold

Jan 1, 1970
0
<snipping ('cos I'm lazy)>
Grin, (I hope it's OK if a laugh at your pain a bit.) Everything magnetic has a tempco. This has nothing to do with your question, but I dunked a ferrite bead into LN2 today, and measured the resistance from one end to the other. I was looking for a phase transition (which I didn't see), but the resistance went from 35 ohms to 50k.

George H.
 
T

Tim Williams

Jan 1, 1970
0
George Herold said:
Grin, (I hope it's OK if a laugh at your pain a bit.) Everything
magnetic
has a tempco. This has nothing to do with your question, but I dunked a
ferrite bead into LN2 today, and measured the resistance from one end to
the other. I was looking for a phase transition (which I didn't see),
but
the resistance went from 35 ohms to 50k.

Well, it would be a semiconductor!

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

Tim
 
B

Bill Sloman

Jan 1, 1970
0
Thanks. The highest-resolution one is 2048 counts per rev, and we're already hurting a bit at 4096, since the actual measurement range is only 8 degrees.

If you are going to have to make your own code-wheel anyway, and since you are only working over 8 degrees, it would be easy enough - in theory - to work with a 16 degree segment out of a larger "code-wheel" and treat the segment you look at as a 2048 segment out of an up to 46,080 counts per rev theoretical code wheel. The Avago screed talks about a "code strip" and bodging your mechanical set-up so that your 8 degree range moves the code-strip a useful distance isn't impossible, though I can see all sorts of argumentsfor not doing it.

The best - as you point out below - is that you don't actually have to.
However, good news: with the software fixed so it measures duty cycle, cold spray is _completely_unable_ to move the indicated angle--the same pulse width variation that made a 7-degree change yesterday gives less than 1 count (0.09 degrees) today. So it looks like measuring the right thing fixed the problem more or less completely.

It often does. I'm very glad to hear it.
From looking like complete crap, these encoders now come up smelling of roses. Amazing the buried treasure you can find in other people's software. (Sometimes mine too, of course.)

There's nothing quite like asking the right question.
 
P

Phil Hobbs

Jan 1, 1970
0
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Always with the optimism, eh?


Best regards,
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.

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

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
 
P

Phil Hobbs

Jan 1, 1970
0
<snipping ('cos I'm lazy)> Grin, (I hope it's OK if a laugh at your
pain a bit.) Everything magnetic has a tempco. This has nothing to
do with your question, but I dunked a ferrite bead into LN2 today,
and measured the resistance from one end to the other. I was looking
for a phase transition (which I didn't see), but the resistance went
from 35 ohms to 50k.

It's okay, I'm used to it by now.

The tempco of the magnet is taken out to leading order by combining the
four Hall elements--when you compute ratios, the field ideally cancels out.

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
 
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