Encoders for EMC


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  1. #1
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    Default Encoders for EMC

    Hi,

    I'm collecting information on hard-wear for EMC. (I am in the process a very slow process of CNCing a 1944 Colchester lathe!) What I need to know that I couldn't find a clear novices answer to was what kind of encoder set up do I need on the spindle to do threading. My project will be running steppers so no encoders. I have a mechanical bent, but am electronically challenged!

    Matthew

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    Member samco's Avatar
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    if your going to use the printer port to read the encoder back - then I would look for a low count one. Somewhere around 100 line. Remember you need to have an index pulse also - so find an encoder with A,B and Z channels.


    To be safe - you should only count 1/2 of your base period. So if your base period is a 50us (most computers have no problem with this). This is (1/50us)/2 = 10khz. so at 100 line - that would give you (10khz/(100line*4edges))*60 = 1500 rpm max threading. 50us is really safe - most computers will do much better. So say if you had a 250line encoder and could do a base period of 20us

    (1/20us)/2=25khz max counting rate

    (25khz/(250line*4edges))*60=1500rpm. (it just happend to come out the same )

    sam



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    Default 100 line encoder

    Sam

    First of all thank you for your rapid reply. I was thinking about making a 100 slot disk and use a photo switch. I had the intention of running two discs one with a single or double 90° apart slot for position. 100 slots is quite easy for me to make using my home made dividing head I could also gear it up with a toothed belt to take the 100 slot disk up to 250 slot per revolution, leaving the single or double wheel directly on the shaft.

    Thoughts, thanks



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    acondit is doing something similar..

    http://www.cnczone.com/forums/showth...t=37656&page=4
    notice the single index 'hole' in the outer edge. (you only need one)
    He is going to use 3 sensors - a,b and index (z).

    sam



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    Having more than one pulse per rev allows emc to track the spindle more accuratly. You can do some pretty cool stuff... (more than just threads ) emc will blend many spindles synced motions togather.. For things like this



    (like a fusee in a clock.)
    (an emc developer.. read about it here http://jmkasunich.dyndns.org/cgi-bin/blosxom )
    He is going to make a video at some point... I can't wait.

    You could even do rigit tapping if you can reverse the spindle under emc control.

    sam



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    Default Gearing up

    If I gear up a disk, I then need a separate one to have a one per RPM. Is 100 pulses/rev sufficient? I have no idea about any of this!

    Matthew



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    Member samco's Avatar
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    You cannot have more than one index pulse per rev. So if you do gear up the encoder disk - keep the index separate.

    I think that a 100 line encoder will work just fine. This guy converted an emco lathe - it has a 100 line encoder on it.

    http://www.lerneaenhydra.net/index.p...d=15&Itemid=28

    sam



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    Default 100count

    Oooops! I didn't read the first line closely enough! you answered my question about the 100 pulses. I presume that this is an adjustable parameter. I'm going to make my own disk.

    [QUOTE=samco;420662]if your going to use the printer port to read the encoder back - then I would look for a low count one. Somewhere around 100 line. Remember you need to have an index pulse also - so find an encoder with A,B and Z channels.



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