Hall rotary encoder tempco--wisdom?

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

  1. 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.


    [%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.

    --
    ********************************************************************
    Paul E. Bennett IEng MIET.....<email://>
    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..
    ********************************************************************
     
    Paul E Bennett, Jan 9, 2014
    #21
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  2. Phil Hobbs

    Guest

    On Thursday, January 9, 2014 3:50:05 AM UTC-5, Jan Panteltje wrote:
    > On a sunny day (Wed, 08 Jan 2014 16:40:06 -0500) it happened Phil Hobbs wrote:


    > >Hi, all,
    > >
    > >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.

    >
    > Right, large gear box, lot of play.
    >
    > >It used a pot for the encoder, of course.

    >
    > No gear box?
    >
    > >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.

    >
    > 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
     
    , Jan 9, 2014
    #22
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  3. On Thu, 09 Jan 2014 00:50:42 -0500, the renowned Phil Hobbs
    <> wrote:

    >
    >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
    --
    "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
    #23
  4. On Wednesday, January 8, 2014 11:28:12 PM UTC-5, Tim Williams wrote:
    > "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!

    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.
    >
    >
    >
    > Tim
    >
    >
    >
    > --
    >
    > Seven Transistor Labs
    >
    > Electrical Engineering Consultation
    >
    > Website: http://seventransistorlabs.com
     
    George Herold, Jan 9, 2014
    #24
  5. Den torsdag den 9. januar 2014 18.01.01 UTC+1 skrev Tim Wescott:
    > 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

    >
    >
    >
    > 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
     
    Lasse Langwadt Christensen, Jan 9, 2014
    #25
  6. On Thu, 09 Jan 2014 15:41:32 -0500, Phil Hobbs
    <> wrote:

    >
    >Interesting. Cryogenic or ambient?
    >
    >Cheers
    >
    >Phil Hobbs


    Ambient.
     
    Spehro Pefhany, Jan 9, 2014
    #26
  7. Phil Hobbs

    Phil Hobbs Guest

    On 1/9/2014 4:37 PM, Tim Wescott wrote:
    > On Thu, 09 Jan 2014 15:39:28 -0500, Phil Hobbs wrote:
    >
    >> On 1/9/2014 12:01 PM, Tim Wescott wrote:
    >>> 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
    >>>
    >>> 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.
    >>>
    >>>

    >> Interestingly the software fix seems to have got rid of the issue,
    >> hurrah.
    >>
    >> Cheers
    >>
    >> 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
     
    Phil Hobbs, Jan 9, 2014
    #27
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