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Steppers as rotary sensors

Discussion in 'Electronic Basics' started by Iman Habib, Oct 2, 2003.

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  1. Iman Habib

    Iman Habib Guest

    Hi guys.

    I've been thinking a bit about steppers as incremental rotary sensor today.

    I guess its pretty simple to use them as sensors by just continuously
    normalizing the signal and sampling voltage change (looking for highs and/or lows)
    But that would only work well if the shaft moved at acceptable speed.
    I think that to slow movements would not induce any worthwhile signal.

    So my question is.

    Would it not be possible to instead induce a small (small enough so the stepper
    wont try to move) current through the coil, sample the signal strength that comes
    back when the field collapses and calculate the position of the stepper between
    the "teethes". Or are there any strange irregularities in stepper coils that would
    prohibit me from effectively doing this or some other problems that might occur?

    Of course there would be somewhat of a problem if the stepper shaft
    was moving at the same time so I guess this kind of measurement would
    only work at standstill or when the shaft is moving very slowly (right?).

    Anyone out there that has links to sites related to this idea or has any
    personal experience regarding this. I cant seem to find any websites
    covering this matter.

    regards
    /iman
     
  2. default

    default Guest

    I don't think you have to worry about slow speed operation. I used a
    stepper to generate a signal for another stepper to use as its drive
    (using steppers as a "selsyn" system - turn one shaft the driven shaft
    turns the same amount at a remote location - with two identical
    steppers or a ratio with different step angles)

    Anyhow I used an LM324 to sense the coils on the pickup stepper and
    couldn't turn the signal stepper so slow that the remote wouldn't
    follow.

    I'll post my circuit on the alt.binaries.schematics.electronic if the
    posting gods will allow it. Use the part on the "input side" to get
    your sensing output.

    It'll take an hour to scan and post it.
     
  3. Mike Harding

    Mike Harding Guest

    And there's a lot of us! You have probably posted this message
    to a couple of million people - maybe more?

    Was it really necessary to post to:
    alt.engineering.electrical,aus.electronics,sci.electronics.components,sci.electronics.basics,sci.electronics.design
    ?

    Why not try just one group to start with?

    Mike Harding
     
  4. Bill Sloman

    Bill Sloman Guest

    About the only way I can see you getting useful position information
    out of a stationary stepper motor would be by measuring the
    inductances of the two sets of coils.

    You could do this with very low AC currents, so it wouldn't generate
    any torque. You couldn't use particularly high frequencies - the coils
    are going to go self-resonant not too far above the maximum stepping
    rate, and the iron path losses will become crippling in the same sort
    of range - but you should be able to work something out.

    There are motor-like parts designed for this sort of job, called
    "synchros"

    http://www.incon.com/products/prs/1292-ks.htm

    shows an example. Analog Devices used to sell integrated circuits
    specifically designed for decoding the outputs of these devices, but
    I've not had any occasion to check on the status of these parts in
    recent years.
     
  5. Alan

    Alan Guest


    Even at slow speed as in single step, there is a breakaway function
    and snapping to the new position. Don't really expect you could do it
    that it couldn't be sensed, short of having a 3 foot long lever on the
    shaft so that you could move it VERY slowly accurately against it's
    internal magnetic field.

    Also, use a high voltage stepper. A 24 V stepper turned will put
    something out even if you turn it very slowly. Should be very hard to
    get a non-detect.

    Alan
     
  6. Rod Speed

    Rod Speed Guest

    In your dreams...
    It does make a lot of sense to do that.
    Because its much more efficient to include that lot instead.
     
  7. Jim Thompson

    Jim Thompson Guest

    It really puzzles me when people get so bent out of shape at
    cross-posting....

    ALL the experts don't hang out on a single group.

    A *good* news-reader, such as Agent, handles cross-posting with grace,
    only showing you the post ONCE.

    What's your problem? Using an MShit program as a reader... maybe
    Outhouse Excuse?

    ...Jim Thompson
     
  8. Rod Speed

    Rod Speed Guest

    Just the usual mindless ranting about any change.
    Or automatically marking it as read in all
    the groups its posted to when its first read.
    Ear to ear dog shit, basically.
    It handles cross posting just as elegantly as Agent.

    And doesnt turn you into a rabid bigot to boot.;
     
  9. Mike Harding

    Mike Harding Guest

    Read the header Jim.

    http://www.uwasa.fi/~ts/http/crospost.html

    Mike Harding
     
  10. Rod Speed

    Rod Speed Guest

  11. For the OP perhaps. My ISP won't post my reply, bounces it back
    to me, yelling that crossposting is not allowed to >3 groups.
    So I have to remove a couple of newsgroups, to post it.

    Of course, people can change provider, can change newsreaders,
    can move from dial-up accounts to cable/dsl etc.

    In the meantime, it's not a such bad idea to limit crossposting.

    But I also see people grab a stack of 15 tissues at McDonalds(tm).
    Efficient to them, no doubt, and free, so why bother.

    Jim Thomson has the same attitude, always posts to 4 newsgroups.
    Crossposts when the temperature in Arizona changes. It charactarizes
    the person.
     
  12. Mike Harding

    Mike Harding Guest

    Illiterate as well as stupid!

    Mike Harding
     
  13. John G

    John G Guest

    I do not always agree with Rod and sometimes he is a bit intolerant but
    could you please explain what is wrong with "Cross Posting".
    It is better than multiple posting and that is explained in YOUR reference
    http://www.uwasa.fi/~ts/http/crospost.html

    Even Outlook Express handles cross posts quite reasonably.
    Wot's Your Real Problem?
     
  14. Rod Speed

    Rod Speed Guest

    Even you should be able to bullshit your way out of your
    predicament better than that pathetic effort, fuckwit.
     
  15. default

    default Guest

    (snip)
    You will never get "absolute" position information (turn it on and it
    tells you how many degrees the shaft is relative to "north") but it is
    easy to get relative information. With my synchro, the position was
    very accurately reflected in the drive motor. As long as no one turned
    the drive motor deliberately, it would show just what the driven motor
    was doing.

    There's all kinds of shaft encoders, the simplest is just a series of
    lines on a disc to pick up rotational speed, a stepper could easily
    handle that task. A stepper just as easily goes one further, it puts
    out two phases so it can tell which direction the shaft is turning in.
    (or, more correctly, if the direction has changed).

    If one is a hobbyist and cost is a consideration, or just fooling
    around with an idea, steppers are interesting devices. Where else can
    you get an off the shelf alternator that puts out a few watts at slow
    rotational speeds, for instance? Priced a full fledged synchro system
    recently? Even surplus 400 cycle aircraft synchros aren't used much
    these days, shipboard 120/60 cycle ones are practically museum pieces.
    I built my stepper synchro for less than $10. Solve a problem
    inexpensively and get to tinker with it is how I relax - Working on
    someone else's 8 million dollar project and I buy better and more
    costly hardware.
     
  16. default

    default Guest

    You're right. My first attempt was to input the stepper to a
    darlington transistor with no amplification - the stepper had to turn
    at some speed before the 1.2 volts the darlington needed was met.

    Second attempt was to bias the darlington's just shy of conduction -
    worked but as ambient temperature increased, the darlingtons would
    work in the linear range and self heat.

    Third attempt was with an op amp, and it is so sensitive that it is
    hard to imagine anything moving so slowly that it wouldn't catch it.
    And like you said, the natural cogging action of the stepper will
    cause it to jump ahead when the magnetic field between steps is
    overcome (assuming there's just a little slop in the linkage between
    the turning shaft and the stepper shaft).

    Turning it by hand with a knob on the shaft, I couldn't move it so
    slowly that the op amp wouldn't detect it. Geological events might
    move slowly enough that a stepper won't detect them - for that we have
    lasers and mirrors.
     
  17. It's not just a matter of newsreaders. Cross-posting is usually the
    result of someone who can't be bothered to figure out the most appropriate
    newsgroup, so they shotgun across a bunch of somewhat related ones:
    alt.engineering.electrical,
    aus.electronics,
    sci.electronics.components,
    sci.electronics.basics,
    sci.electronics.design

    I can say outright that if it's posted to basics, it doesn't belong
    in design, or vice versa. If you are a beginner, then you aren't ready
    for design, and if you can design, then you've gone past the basics.

    And then, let's post to a two other hierarchies, just because they exist.
    While I can see someone post to the Australian newsgroup, because they live
    there and are among familiar people, I don't see the point if they are posting
    to some other newsgroup. And if the original poster is not in Australia,
    then the only reason they've posted there is because they saw the "electronic"
    in the newsgroup name and decided to post there. That happens a lot with
    cross-posting, people search for pertinent newsgroups using a keyword,
    and then don't bother checking the newsgroups before they post.

    And while I have no idea what alt.engineering.electrical is supposed to
    be for, electrical invokes an image of motors and light switches and
    things that Tesla, Edison, and Westinghouse worked with. A stepper is
    nominally an electrical rather than electronic item, but I'd unless someone
    is discussing making stepper motors, I'd place them in the electronic realm
    since they virtually all the time require

    Another key point against cross-posting is that it can often result
    in unrelated issues, such as this, because someone has jumped into
    some existing groups at the same time. People who don't normally talk
    together are suddenly interacting, but without the interaction of
    other things to temper the discussion. So if a discussion is kept to
    one newsgroup, if someone says something that I'd consider outrageous,
    I might think twice about saying something because I know some common
    points that I have with that poster, and maybe even I realize that
    I'll have to live with that person in the newsgroup in the future.
    That's not the case when suddenly "people from other newsgroups" are
    in the familiar newsgroup. This guy is an idiot for cross-posting.
    The only real thing I know about him is that he's cross-posted; I
    don't even know which, if any, of the newsgroups he posted to
    is the one he regularly hangs out in.

    Michael
     
  18. default

    default Guest

    Hi Mike
    How do you feel about off topic rants posted to five newsgroups?

    Just joking . . .
     
  19. daestrom

    daestrom Guest

    Well, it does mention that 'should never be used indiscrimenately.' and
    'Avoid cross-posting to groups that are branches of the same sub-hierarchy,
    that is don't cross-post to adjacent newsgroups.' Like, oh, maybe
    'sci.electronics.components, sci.electronics.basics, and
    sci.electronics.design.'

    But as cross-posts go, this thread is pretty 'nice'. It started
    electrical/electronic in nature and is limited to groups that have that
    frame of mind.

    The really nasty ones are political posts to science and engineering groups
    like the recent rash of them by 'Your Special Friend' and his related names.
    Or the obvious virus trolls and porno spam. But hey, I'm a big boy, I can
    ignore them. Even with MS Outlook Express, it isn't very hard to do.

    daestrom
     
  20. Rod Speed

    Rod Speed Guest

    Yes, but that clearly wasnt done with the original post.
    Thats a stupid proscription.
    Yep, it clearly is doing what that FAQ recommends.
    Completely trivial, actually.
     
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