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Automate turning of manual rotary switch?

Discussion in 'Electronic Design' started by markp, Mar 31, 2010.

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  1. markp

    markp Guest

    Got a little problem. I've been asked to come up with something that turns a
    manual 3 way rotary switch from one position to another and back again under
    software control. The switch is one of those bulky things with a pointer
    style knob. They basically want to control a piece of equipment without
    modifying it in any way.

    Does anyone know of a switch actuator manufacturer? I'm looking for
    something I can place over the switch with a motor or solenoid to turn the
    switch (about 45 degrees per switch position).


  2. markp

    markp Guest

    Thanks for that. That's something like what I'm looking for, except I don't
    need the wafers. However, the switch has mechanical stop positions on it
    that the motor would have to overcome, so may need a bit more gearing.

  3. Jon Kirwan

    Jon Kirwan Guest

    I don't.

    But you reminded me of a remove control TV I used to have. It
    used a few metal bars that were "hammered" when you pushed a
    mechanical button. (Basically, tuning forks.) These emitted
    a precise audible tone which was decoded and used by the TV
    set to cause the tuner to move left or right (you had one
    control for each.) The TV could be set while sitting in a
    rocking chair from across the room! The knob was _also_
    designed as a standard manual tuner so the remote wasn't
    required, at all. You could, of course, use the TV as a
    "normal" TV of the time where you had to get up and switch
    channels by hand.

    Of course, manufacturers soon got rid of the manual tuner
    assemblies and digitized these and then began using buttons
    and remote controls which, today, seem more necessary than
    mere convenience, anymore.

    Interesting question.

  4. markp

    markp Guest

    Indeed, but this is a one-off so a ahnd crafted solution would be OK (as
    long as it's reliable!)
    Ideally leave the knob on, but could remove it and replace it. I'm not sure
    whether the knob is press-fitted or screwed.
    No, there will be a little control box with a manual override so all done in
    Good question. There are some existing fixings I could attach to, I'm sure
    they could accept a few holes drilled in the panel that has the switch.
    This might be quite a tough mechanical challenge, as you say it sounds easy
    but the devil is in the details. The switch has click positions, I may need
    a way to sense it has seated properly in a particular position. Also I don't
    want this to ever sit just off centre of a position in case the switch
    contacts arc.

  5. markp

    markp Guest

    Tuning forks to control tellies! Nice. I shan't be using that method though
  6. markp

    markp Guest

    I think the stepper may have to be geared to get the torque needed to move
    the switch into the click positions. Technically with a stepper you
    shouldn't need a position ecoder as long as it is calibrated for position,
    however lose a step or two and you're in trouble, so I think you're right
    that an absolute position sensor would be needed.

    What might work is stepping to one of the nominal click positions then
    releasing (I assume this allows the shaft to freely rotate?). Assuming the
    gearing isn't too large the switch may then self align, similar to your
    springy coupling suggestion.

    Another interesting possibility is having some kind of torque sensor on the
    shaft itself. By measuring the torque as the switch clicks into place it
    might be possible to reliably detect it. Can you get torque sensors like

  7. I would use a large RC servo and couple that to the shaft or knob.
  8. Yup, here's one for $119 that has 250 oz-in of torque at 4.8V
  9. markp

    markp Guest

    Thanks Phil. Wasn't aware that servos like this had good torque. I'll look
    into that.

  10. markp

    markp Guest

  11. ehsjr

    ehsjr Guest

    Replace the knob with a gear. Drive that gear with a gear attached
    to a motor's shaft. A cam on the driven gear can activate a micro
    switch for the stop positions. You'll need a gear motor already
    reduced in speed to keep the driven gear relatively small - maybe
    something like cat # DCM-276 ($9.00) or DCM-245 ($13.75) or
    DCM-110 ($3.50) at Allelectronics.
    The first two mentioned might provide enough torque to turn the
    switch with direct coupling and no additional gearing advantage.

    It's a kludge, but a kludge is what they're asking for, whether
    they are aware of it or not. The response will be slow at 5 or
    6 rpm with the first two motors and 1:1 gearing or direct coupling,
    in the neighborhood of 1/2 second. With the DCM-110, it depends on
    the gearing you use. Without knowing the torque requirements
    it's a guess whether any of the motors will work for you.

  12. hamilton

    hamilton Guest

    If this is a one off project, you can get some old 8-track drives and
    remove the ratchet, paw and solenoid.

    Once you look it over, you'll understand how it works.

  13. E

    E Guest

    Perhaps proportional ball valve actuator, the kind you can put on a valve
    and control with 0-10V
  14. Jon Kirwan

    Jon Kirwan Guest

    hehe. No batteries to replace. Can get tuning forks from
    the store if you need them laying about the house. Might
    even be able to learn to whistle it out without one, which
    means you are never looking around for the darned remote any
    more. It does have its advantages. ;)

  15. John Nagle

    John Nagle Guest

    For this application, I'd suggest an R/C servo intended for boat
    rudders. Those are geared down, slow, and not too expensive.
    Here's an example, for $37:

    There are plenty of resources on the Web on how to drive an R/C
    servo from a computer, so I'll let you find that

    John Nagle
  16. markp

    markp Guest

    Thanks John. I actually haven't seem the real switch yet, but I know
    sometimes these things can require quite a bit of torque to overcome the
    click position. I think I'll need to measure it first so I can choose an
    appropriate R/C servo.

  17. JosephKK

    JosephKK Guest

    Sure, but what you measure is torque reaction on the motor housing.
  18. markp

    markp Guest

    Just about as ugly as it needs to be, this thing is out of site in a control
    room and there's plent of room as it's an open rack. What they don't want is
    to modify any of the control logic or wiring as they have a maitenance
    contract with a third party. I want to come up with a solution that's
    completely non-intrusive that simply automates the otherwise manually
    controlled on/off switch.

  19. tm

    tm Guest

    If all you want to do is switch it on and off, why not just leave
    it on and switch the power to it with a relay?

  20. hamilton

    hamilton Guest

    How far apart are the positions ?

    90 Degrees, 30 Degrees, 120 Degrees

    This looks like its going to be a Real Rube Goldberg type of operation. :)

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