Hall rotary encoder tempco--wisdom?

Discussion in 'Electronic Design' started by Phil Hobbs, Jan 8, 2014.

  1. Phil Hobbs

    Phil Hobbs Guest

    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
     
    Phil Hobbs, Jan 8, 2014
    #1
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  2. Phil Hobbs

    Don Y Guest

    Hi Phil,

    On 1/8/2014 2:40 PM, Phil Hobbs wrote:

    > 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?)
     
    Don Y, Jan 8, 2014
    #2
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  3. Phil Hobbs

    Phil Hobbs Guest

    On 1/8/2014 4:44 PM, Don Y wrote:
    > Hi Phil,
    >
    > On 1/8/2014 2:40 PM, Phil Hobbs wrote:
    >
    >> 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?)


    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
     
    Phil Hobbs, Jan 8, 2014
    #3
  4. Phil Hobbs

    whit3rd Guest

    On Wednesday, January 8, 2014 1:40:06 PM UTC-8, Phil Hobbs wrote:

    > 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.
     
    whit3rd, Jan 8, 2014
    #4
  5. Phil Hobbs

    Phil Hobbs Guest

    On 1/8/2014 4:51 PM, whit3rd wrote:
    > On Wednesday, January 8, 2014 1:40:06 PM UTC-8, Phil Hobbs wrote:
    >
    >> 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.
    >


    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
     
    Phil Hobbs, Jan 8, 2014
    #5
  6. Phil Hobbs

    Don Y Guest

    On 1/8/2014 2:47 PM, Phil Hobbs wrote:
    > On 1/8/2014 4:44 PM, Don Y wrote:
    >> Hi Phil,
    >>
    >> On 1/8/2014 2:40 PM, Phil Hobbs wrote:
    >>
    >>> 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?)

    >
    > 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!
     
    Don Y, Jan 8, 2014
    #6
  7. Phil Hobbs

    Phil Hobbs Guest

    On 1/8/2014 4:54 PM, John Larkin wrote:
    > On Wed, 08 Jan 2014 16:40:06 -0500, Phil Hobbs
    > <> wrote:
    >
    >> 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

    >
    > 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
     
    Phil Hobbs, Jan 8, 2014
    #7
  8. Den onsdag den 8. januar 2014 22.56.58 UTC+1 skrev Phil Hobbs:
    > On 1/8/2014 4:54 PM, John Larkin wrote:
    >
    > > On Wed, 08 Jan 2014 16:40:06 -0500, Phil Hobbs

    >
    > > <> wrote:

    >
    > >

    >
    > >> 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

    >
    > >

    >
    > > 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?
    >


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

    how do you sample pwm?

    -Lasse
     
    Lasse Langwadt Christensen, Jan 8, 2014
    #8
  9. Phil Hobbs

    Phil Hobbs Guest

    On 1/8/2014 5:08 PM, John Larkin wrote:
    > On Wed, 08 Jan 2014 16:56:58 -0500, Phil Hobbs
    > <> wrote:
    >
    >> On 1/8/2014 4:54 PM, John Larkin wrote:
    >>> On Wed, 08 Jan 2014 16:40:06 -0500, Phil Hobbs
    >>> <> wrote:
    >>>
    >>>> 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
    >>>
    >>> 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

    >
    > 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
     
    Phil Hobbs, Jan 8, 2014
    #9
  10. Phil Hobbs

    Phil Hobbs Guest

    On 1/8/2014 5:09 PM, Lasse Langwadt Christensen wrote:
    > Den onsdag den 8. januar 2014 22.56.58 UTC+1 skrev Phil Hobbs:
    >> On 1/8/2014 4:54 PM, John Larkin wrote:
    >>
    >>> On Wed, 08 Jan 2014 16:40:06 -0500, Phil Hobbs

    >>
    >>> <> wrote:

    >>
    >>>> 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
    >>>> myspectrometer. 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

    >>
    >>>

    >>
    >>> 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?
    >>

    > 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
     
    Phil Hobbs, Jan 8, 2014
    #10
  11. Phil Hobbs

    Phil Hobbs Guest

    On 1/8/2014 5:25 PM, John Larkin wrote:
    > On Wed, 08 Jan 2014 17:16:43 -0500, Phil Hobbs
    > <> wrote:
    >
    >> On 1/8/2014 5:09 PM, Lasse Langwadt Christensen wrote:
    >>> Den onsdag den 8. januar 2014 22.56.58 UTC+1 skrev Phil Hobbs:
    >>>> On 1/8/2014 4:54 PM, John Larkin wrote:
    >>>>
    >>>>> On Wed, 08 Jan 2014 16:40:06 -0500, Phil Hobbs
    >>>>
    >>>>> <> wrote:
    >>>>
    >>>>>> 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
    >>>>>> myspectrometer. 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
    >>>>
    >>>>>
    >>>>
    >>>>> 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?
    >>>>
    >>> 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

    >
    > 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
     
    Phil Hobbs, Jan 8, 2014
    #11
  12. Den onsdag den 8. januar 2014 23.24.03 UTC+1 skrev John Larkin:
    > On Wed, 08 Jan 2014 17:09:58 -0500, Phil Hobbs
    >
    > <> wrote:
    >
    >
    >
    > >On 1/8/2014 5:08 PM, John Larkin wrote:

    >
    > >> On Wed, 08 Jan 2014 16:56:58 -0500, Phil Hobbs

    >
    > >> <> wrote:

    >
    > >>

    >
    > >>> On 1/8/2014 4:54 PM, John Larkin wrote:

    >
    > >>>> On Wed, 08 Jan 2014 16:40:06 -0500, Phil Hobbs

    >
    > >>>> <> wrote:

    >
    > >>>>

    >
    > >>>>> 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

    >
    > >>>>

    >
    > >>>> 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

    >
    > >>

    >
    > >> 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

    >
    >
    >
    > 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
     
    Lasse Langwadt Christensen, Jan 8, 2014
    #12
  13. Phil Hobbs

    Guest

    On Wednesday, January 8, 2014 4:40:06 PM UTC-5, Phil Hobbs wrote:
    > 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.
     
    , Jan 9, 2014
    #13
  14. Phil Hobbs

    Bill Sloman Guest

    On Thursday, 9 January 2014 08:56:58 UTC+11, Phil Hobbs wrote:
    > On 1/8/2014 4:54 PM, John Larkin wrote:
    > > On Wed, 08 Jan 2014 16:40:06 -0500, Phil Hobbs
    > > <> wrote:
    > >
    > >> 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 andbodge in a pot if we have to, but it'll limit the instrument lifetime fairly severely.

    > >
    > > 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.


    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.

    --
    Bill Sloman, Sydney
     
    Bill Sloman, Jan 9, 2014
    #14
  15. On Wed, 08 Jan 2014 19:05:13 -0500, the renowned Phil Hobbs
    <> wrote:

    >
    >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
    --
    "it's the network..." "The Journey is the reward"
    Info for manufacturers: http://www.trexon.com
    Embedded software/hardware/analog Info for designers: http://www.speff.com
     
    Spehro Pefhany, Jan 9, 2014
    #15
  16. On Wednesday, January 8, 2014 4:40:06 PM UTC-5, Phil Hobbs wrote:
    > Hi, all,
    >

    <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.

    > 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
     
    George Herold, Jan 9, 2014
    #16
  17. Phil Hobbs

    Tim Williams Guest

    "George Herold" <> wrote in message
    news:...
    > 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

    --
    Seven Transistor Labs
    Electrical Engineering Consultation
    Website: http://seventransistorlabs.com
     
    Tim Williams, Jan 9, 2014
    #17
  18. Phil Hobbs

    Bill Sloman Guest

    On Thursday, 9 January 2014 11:41:09 UTC+11, Phil Hobbs wrote:
    > On 1/8/2014 7:07 PM, Bill Sloman wrote:
    > > On Thursday, 9 January 2014 08:56:58 UTC+11, Phil Hobbs wrote:
    > >> On 1/8/2014 4:54 PM, John Larkin wrote:
    > >>> On Wed, 08 Jan 2014 16:40:06 -0500, Phil Hobbs
    > >>> <> wrote:

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


    <snip>

    > > 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, butcan't get it to download.

    >
    > 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.

    --
    Bill Sloman, Sydney
     
    Bill Sloman, Jan 9, 2014
    #18
  19. Phil Hobbs

    Phil Hobbs Guest

    On 1/8/2014 9:05 PM, Spehro Pefhany wrote:
    > On Wed, 08 Jan 2014 19:05:13 -0500, the renowned Phil Hobbs
    > <> wrote:
    >
    >>
    >> 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
    >


    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
     
    Phil Hobbs, Jan 9, 2014
    #19
  20. Phil Hobbs

    Phil Hobbs Guest

    On 1/8/2014 9:06 PM, George Herold wrote:
    > On Wednesday, January 8, 2014 4:40:06 PM UTC-5, Phil Hobbs wrote:
    >> Hi, all,
    >>

    > <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
     
    Phil Hobbs, Jan 9, 2014
    #20
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