Wire draw encoder

Discussion in 'General Electronics Chat' started by cmartinez, Aug 13, 2016.

  1. cmartinez

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    I need an encoder of the wire-draw type for a particular application. It's main advantage is that it's not as expensive as a long, magnetic (or optical) linear encoder, although it's not as precise either.
    Anyway, I don't need that much precision. About 0.1 mm will do, which is something I believe can be attained with a wire draw encoder. But the big objection here, is that those encoders are being sold on the market for far too much I'm willing to pay. I have resources, and I can easily design and fabricate all the components needed for this small project.

    I'm planning on using one of these as the encoding component. It's of the optical type, can easily work at up to 100 kHz, and it's got 5,000 divisions per revolution. Working the numbers, I'd need to wind the wire (never mind its gauge at this moment) around a drum measuring about 6-1/4" in diameter to get what I want.
    To round things down a bit, I think I'll go for a 4.7" drum instead. The material would be easy to find and rectify to its desired diameter, and I'd be getting a resolution of about 0.075 mm per encoder pulse.

    At this stage, my question is mechanical (and I'm a mechanical engineer... as embarrassing as it is to admit ... but I'm no fool, and I'd rather ask for help in matters on which I have no experience and try to save myself some painfully spent time and money).
    The drum must travel back and forth along its axis, so as to properly wind and unwind the wire without it wrapping around itself. For this to happen, I plan to install a fine-pitch nut on the drum's axis, and the axis will have a matching screw (which will be of a special design and fabrication). I do not plan to use the screw as the axis, of course, but the axis will be a split shaft at which ends will be installed a couple of ball bearings.
    I promise I'll post the final design when it's done, and also share whatever circuitry is needed to properly adapt and transmit the encoder signal over a given wire length.... maybe this will even qualify in the completed projects collection forum.

    This encoder will be working on a machine with a length of about 6.5m (a little over 21'), and I plan to use a pitch of about 40 TPI for the screw and nut responsible for moving the drum back and forth along its axis. So, for 6,500 mm of thin-gauge wire to be wrapped around a 119 mm drum, around 18 turns would be needed. And 18 turns of a nut on a threaded rod with 40 TPI would result in travel of 0.450". That's pretty neat and compact... just the way I like it. Of course, the wire would have to have a maximum diameter of 0.025" (1/40th of an inch) for this to work, but I think that that thickness will be more than enough for my purposes.

    Now, back to the original question, which is this:
    • The machine will be performing a couple of hundred of cycles back and forth its total length per day. Of what material should I fabricate the threaded rod and the matching nut that will make the drum travel back and forth along its axis?

    I was thinking about using D2 steel for the 40 TPI threaded rod, which will have an o.d. of 0.750". And bronze for the nut, which will have a width of at least 1/2".

    @MaxHeadRoom, I think this question is right up your alley, what do you think?
     
  2. MaxHeadRoom

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    I have never used that particular encoder but I once did a project that used four screw jacks to raise railway box cars so in order to sync the four jacks I used a similar type of mechanism on each jack, the unit used a rotary encoder and was constructed along the same lines as a retractable tape measure IOW when extending it winds the spring mechanism up and when retracting winds pulls the wire back on the drum.
    These were off the shelf mechanisms and I think you can still buy them in some form, if this is what you have in mind.
    I think they are called encoder wire draw mechanisms.
    Not sure of the mechanism you have in mind, but what about fine pitch rack and pinion?
    Max.
     
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  3. cmartinez

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    Wouldn't work the way I want to. The application is for 6.5m and I've used 22-pitch racks for that in the past. They normally have a ±1.5mm accuracy in that sort of lengths. I think that a draw wire encoder could do much better (up to ±0.02mm, from what I understand)

    Anyway, I'll keep posting and let you all know how it's going. Thanks for the input!
     
  4. jpanhalt

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    Just some thoughts:
    1) What will your cable be made of? A tolerance of ±0.004" (0.1 mm)/252" is asking a lot. Will there be a correction for temperature changes? If it is wire rope, it will stretch with use, so I would include a turnbuckle/tension spring device to maintain tension.
    2) I suspect the cable will lay in a single layer for just 18 turns without moving the drum on its axis. Might be worth a try.
    3) If #2 doesn't work, could you thread the surface of the drum to force a single layer? Having the drum move in its axis may well introduce more than 0.1 mm of play.
    4) Your choice of materials for the lead screw and nut sound fine to me, if you use that setup.

    John
     
  5. markdem

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    Jul 31, 2013
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    Just throwing it out there but last time I needed position over a long distance I just used a single wire from the moving part, around a pulley that has a encoder, back past the moving part, around another pulley with encoder (crosscheck) then back to the moving part. Benefit here is that you don't have a spool of cable, don't need to think about the change in diameter of the spool or trying to spool in a single layer, expansion of the wire can be cancelled out if you have 2 encoders and best of all, easy to make yourself.
    Not sure if you can use this setup, just my .02.

    Have fun.
     
    Last edited: Aug 14, 2016
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  6. irventulsa

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    I have been an electrical/mechanical design engineer as well as R&D and New product development and upgrades with over 12 years in wire handling/manufacturing and extrusion. I would recommend IGUS.COM as they have tools which you can put in all your information and needs and they will tell you exactly what you need in their model builder.
    Unfortunately you have given quite a bit of information but not what is really important, ie. how and what type of control (PLC) are you using ? are you wanting digital or analog ? what is you experience in programing ? is this a pid application or an open loop ? what other components are you using as far as d/c motor, VFD, inverted etc.. and is it an re-spooling application or extrusion. what is your budget as it's really open to interpretation, 200.00 may be nothing to one person as 2000.00 may be nothing to another.
    If you are just trying to tinker around or just proof of concept you could get by rather cheaply by doing a teardown of a wire winder off Amazon and repurpose it for your needs, I hope this helps but without all the information above it is just speculation on your need's. I would use IGUS as mentioned above if you have the time because you can either use their model builder and build it for yourself or have a completed piece in your hands in a few weeks.
     
  7. MaxHeadRoom

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    I think I misunderstood the application intent, Is it a wire wind-unwind drum type of application but you say you are not using the screw-acme nut as the drum axle?
    Max.
     
  8. MaxHeadRoom

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    Another good source of hardware and mechanical design is the Misumi catalogue.
    Although I think they have split them up now as the single one was Huge.
    Max.
     
  9. WBahn

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    What is your budget? How much is your time worth? What is your estimate for the total amount of your time it will take to design, fabricate, assemble, and test what you need (assuming no mistakes anywhere along the way)?
     
  10. cmartinez

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    I plan to use either music wire, or perhaps stainless steel wire. 22g (0.64mm, 0.025" diam) should do the trick.

    Yeah, I could do that, but then the wire would not be able to travel back and forth in a straight line

    A 0.1mm play along its axis would have a negligible effect on its drawn length.

    Thanks, your opinion is always thoroughly appreciated!


    That sounds like an interesting idea, but somehow I cannot bring myself to trust a mechanism which is operated by friction only. In fact, the current encoder I'm using is attached to a hardened steel roller (about 26mm in diam) that rolls along a 1-1/2" chromed and hardened hydraulic steel shaft. No matter how much pressure I apply to the roller (using a spring) for it to firmly push against the shaft, the thing slips a bit (in the order of couple of mm) along the 6.5m travel.

    I'm using a 8051 MCU on a PCB of my own design that I've used very successfully in the past to monitor this exact type of encoder.

    Digital is always the best way to go for me. I try to avoid analog unless it's strictly necessary.

    Very extensive, both on PCs and the MCU I'm using. Also, I'm quite familiar with the IGUS line of products. In fact, I might throw one of their plastic (derlin) bushing in my design. Thanks for your suggestion.


    See diagram below.

    Those are very valid questions, Bahn. But like I said, I'm pretty sure I know what I'm facing here, and have a good estimate of how much work and component cost it will take me to accomplish what I want. Plus, I failed to mention that I'm planning tu build at least 4 units. That alone should diminish the cost.

    Here's my idea in its most basic form:

    Capture.JPG

    This assembly will be mounted to is a cart that runs along 6.5m rails. One of the two wires (shown in green) will be attached to one end of the rails, and the other one to the other end, but through an adjustable spring. Depending on the direction of motion, on of the wires will wind around the drum, while the other unwinds, and vice-versa. This arrangement allows the preloading of both wires to a desired constant tension, while also saving me the hassle of having to design a spring return mechanism for the thing. A plus will be that it will rid the device of any radial play on its axis.

    Not shown in the diagram are the ball bearings for the drum, nor the way that the threaded rod will be mounted in such way as to not rotate while the drum does.... those last two points will be the real challenge here.
    As for temperature changes, I'm not really worried about them, because all the materials involved (which are all steel) will be handled at the same temperature that the machine will be working at.

    Thank you all for your interest in this little project of mine, and the support you've shown to this point.
     
    Last edited: Aug 15, 2016
  11. MaxHeadRoom

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    Usually steel to steel is never used for a encoder roller, there are several makes out there that use this technique using a friction type roller material and have found to be very accurate and high repeatablility.
    One use used that I am familiar with is a flying shear for manuf. of portable building sides.
    Max.
     
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  12. cmartinez

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    You may be on to something. A few months ago I designed a small device to measure the length of a flat of stainless steel being unwound from a coil. The mechanism is extremely simple, with a hardened cylindrical D2 roller (26 mm diam, 30 mm long) making full contact with the SS strip. It works beautifully, with an accuracy of less than 0.2mm in lengths larger than 7m. And it's highly reliable. Not a single problem shown in over 6 months, with hundreds of cycles per day.

    In this case, the roller is not rolling against a flat piece of metal, but rather a round shaft. So instead of a line of contact between them, it has a point of contact. Maybe that's why it's prone to slipping.
     
    Last edited: Aug 15, 2016
  13. cmartinez

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    @markdem, would it be too much to ask for you to draw a diagram so I can visualize what you've just described? I want to make sure that I understood you correctly.
     
  14. jpanhalt

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    I envisioned something like this. The cart will move in a straight line.
    upload_2016-8-15_1-53-49.png
    John
     
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  15. cmartinez

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    Yes, that would in fact be much easier to implement than my original idea.

    But the system of pulleys you've scketched relies on friction to work, and can also slip. I'm assuming that the wire is wound at least one turn around the drum?

    I must mention though, that the machine resets its position every cycle by means of an inductive sensor both acting as a limit switch and a start position indicator. So maybe this design could be viable.

    I'll have to think about it.
     
  16. jpanhalt

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    The wire wire would be is affixed to the drum. I would not trust just a turn or two since the tiniest amount of slipping may accumulate. Of course, you could re-zero it periodically and use the "tuning dial" design suggested by markdem.

    John
     
  17. markdem

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    Jul 31, 2013
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    I was just about to start drawing but then noticed John did the hard work for me. (Thanks John).
    The only difference in my setup is that there is no "drum" part. The encoders are connected to the pulleys. This will give you plenty to contact from wire to pulley.
    Also note that my pulleys made out of plastic (no idea what kind) that did not allow the wire to slip.

    The other thing I had was springs pushing the pulleys apart. This provide tension and looked after stretching. I used two encoders as they are cheap and I could then cross check the movements. If one encoder moved without the other I knew something no-so-good was going on.

    If you are still worried about slip, you could always use a timing belt like this: http://www.ebay.com/itm/Geeetech-Ti...803987?hash=item33bd181453:g:IxIAAOxyOlhS2MNL
    As it is toothed it will not allow slipping. 2 of these http://www.ebay.com/itm/2pcs-20T-2G...hash=item2a6a5ad8c8:m:m2rfq6y1Yui7TzeMJR_VYDA and a few springs and you got solid setup.

    Have fun
     
    Last edited: Aug 15, 2016
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  18. markdem

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    I have thought about it, but still can't work it out. How would you affix the wire to the drum and allow it to turn?
     
  19. jpanhalt

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    Attach a wire to a drum, then wind the ends in opposite directions to give the movement needed. As one unwinds, the other winds, and visa versa.

    John
     
  20. markdem

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    Ahh, now I see it. You would still have the same issue as the initial setup as you would need to make sure that the wire winds in one layer or the diameter would change.
     
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