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,

    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

    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

    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

    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

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

    Phil Hobbs Guest

    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:
    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

    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

    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

    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:
    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

    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

    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, Jan 9, 2014
    #14
  15. ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

    Always with the optimism, eh?


    Best regards,
    Spehro Pefhany
     
    Spehro Pefhany, Jan 9, 2014
    #15
  16. <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.
     
    George Herold, Jan 9, 2014
    #16
  17. Phil Hobbs

    Tim Williams Guest

    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
     
    Tim Williams, Jan 9, 2014
    #17
  18. Phil Hobbs

    Bill Sloman Guest

    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.
    It often does. I'm very glad to hear it.
    There's nothing quite like asking the right question.
     
    Bill Sloman, Jan 9, 2014
    #18
  19. Phil Hobbs

    Phil Hobbs Guest

    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

    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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