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  1. #21
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    Default Re: Ballscrew mapping with an addon DRO

    Quote Originally Posted by PCW_MESA View Post
    LinuxCNC does support closing the loop of a stepper motor or step/dir controlled
    servo motor with a linear scale.

    Basically what you do is set the step generation hardware
    (or software for parallel port systems) to run in velocity mode
    rather than position mode and then setup a PID component
    in HAL to close the position loop with feedback from the scale.

    Very low backlash is a requirement for loop stability.
    Would this allow us to keep the same drivers/ motors?
    It would be great to be able to add some encoders and linear scales to greatly amplify the accuracy and repeatability of this machine. How can it be achieved in PP?



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    Default Re: Ballscrew mapping with an addon DRO

    Ball mapping or not, dovetail ways and a machine that doesn't remain temperature stable will make it difficult to obtain that accuracy. And wtf are you building that requires tool and die precision, is that for real or are you just thinking it needs to be that tight. Just wondering, I'm a retired as a tool and die maker and that precision is not real common in normal machining products.

    RAD. Yes those are my initials. Idea, design, build, use. It never ends.
    PCNC1100 Series II, w/S3 upgrade, PDB, ATC & 4th's, PCNC1100 Series II, ATC, 4th


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    Default Re: Ballscrew mapping with an addon DRO

    Although this solution didn't exist back in 2015 when Dave Lawrence asked this question, you can now get a dual encoder step/dir drive for Mach3 that takes linear glass scales as the second encoder input.
    By choosing a motor encoder with a higher effective resolution than the glass scales, you can achieve positional accuracy within 1/2 an encoder pulse on the glass scales. This eliminates errors due to the machine bed thermal expansion and accuracy comes down to the quality and thermal expansion of the glass scales.
    This means you can use C7 rolled ball screws and add glass scales to eliminate all pitch and thermal errors. The ball screw (or belt, rack etc) basically becomes a low backlash transport device rather than a precision positioning device.
    Another advantage is you can now home the axis using the glass scales index pulse.
    David
    Machdrives



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    Default Re: Ballscrew mapping with an addon DRO

    Quote Originally Posted by machdrives View Post
    Although this solution didn't exist back in 2015 when Dave Lawrence asked this question, you can now get a dual encoder step/dir drive for Mach3 that takes linear glass scales as the second encoder input.
    By choosing a motor encoder with a higher effective resolution than the glass scales, you can achieve positional accuracy within 1/2 an encoder pulse on the glass scales. This eliminates errors due to the machine bed thermal expansion and accuracy comes down to the quality and thermal expansion of the glass scales.
    This means you can use C7 rolled ball screws and add glass scales to eliminate all pitch and thermal errors. The ball screw (or belt, rack etc) basically becomes a low backlash transport device rather than a precision positioning device.
    Another advantage is you can now home the axis using the glass scales index pulse.
    David
    Machdrives
    Yeah, I took a look at your product and am kinda blown away. This has been a Holy Grail of sorts to eliminate all error and have linear encoders just like the very high end machines. Extremely impressive! The Tormach uses steppers and I believe your drives only use DC servos, correct?



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    Default Re: Ballscrew mapping with an addon DRO

    Quote Originally Posted by SwampDonkey View Post
    Yeah, I took a look at your product and am kinda blown away. This has been a Holy Grail of sorts to eliminate all error and have linear encoders just like the very high end machines. Extremely impressive! The Tormach uses steppers and I believe your drives only use DC servos, correct?
    Hi SwampDonkey,
    Yes you are correct, our dual encoder drive only works with brushed DC servo motors. You would need to swap out the stepper motors. Do you think there would be good demand for a stepper drive with glass scale encoder input? As we currently only do servo products we don't get to hear a lot from stepper users. If you or anyone has any input on this it would be much appreciated. What is the typical stepper size on the Tormach and similar machines in NEMA size and oz-in torque?
    David
    Machdrives



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    Default Re: Ballscrew mapping with an addon DRO

    Quote Originally Posted by machdrives View Post
    Hi SwampDonkey,
    Yes you are correct, our dual encoder drive only works with brushed DC servo motors. You would need to swap out the stepper motors. Do you think there would be good demand for a stepper drive with glass scale encoder input? As we currently only do servo products we don't get to hear a lot from stepper users. If you or anyone has any input on this it would be much appreciated. What is the typical stepper size on the Tormach and similar machines in NEMA size and oz-in torque?
    David
    Machdrives
    Hi David,
    I believe the Tormach uses 840 oz/in 3 phase steppers for all 3 axis. The 3 phase steppers are definitely an improvement. A good stepper version with ratings of 20- 80V and 2-15A would be great. I'd snap up 3 in a hurry.
    I would love to do a DC servo retrofit using both encoder inputs and PathPilot. I already have glass scales on all 3 axis.

    Thanks,
    Marty



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    Default Re: Ballscrew mapping with an addon DRO

    Quote Originally Posted by SwampDonkey View Post
    Hi David,
    I believe the Tormach uses 840 oz/in 3 phase steppers for all 3 axis. The 3 phase steppers are definitely an improvement. A good stepper version with ratings of 20- 80V and 2-15A would be great. I'd snap up 3 in a hurry.
    I would love to do a DC servo retrofit using both encoder inputs and PathPilot. I already have glass scales on all 3 axis.

    Thanks,
    Marty
    Hi Marty
    Thanks for the information. Am I correct that you have a Tormach 1100 with generic Leadshine 3ND883 drives? I see these are 80V with a current rating of 8.3A peak and 5.9A RMS. Also the popular Gecko steppers seem to be in the 7-8A range. Is there any reason you specified 15A? The 7-8A is a sweet spot design wise and increasing current would increase cost.
    Thanks
    David
    Machdrives



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    Default Re: Ballscrew mapping with an addon DRO

    Quote Originally Posted by SwampDonkey View Post
    Hi David,
    I believe the Tormach uses 840 oz/in 3 phase steppers for all 3 axis.
    That is incorrect. The Z axis stepper has more holding torque than the otherwise matching X and Y axis stepper motors. You can even see that from the physical construction of the motors - the Z axis motor is longer than the X or Y axis motors (ignoring the Z axis brake).

    The above is only correct for a Series 3 PCNC 1100. A Series 1 or 3 PCNC 1100 has 2X the holding torque in the Z axis relative to the X and Y axes.



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    Default Re: Ballscrew mapping with an addon DRO

    Quote Originally Posted by SwampDonkey View Post
    Would this allow us to keep the same drivers/ motors?
    It would be great to be able to add some encoders and linear scales to greatly amplify the accuracy and repeatability of this machine. How can it be achieved in PP?
    This can be done (and has been done for years) with LinuxCNC using standard step drives
    and linear scales. As I mentioned before, it requires near 0 backlash for good results.

    One nice feature of this is that you can now measure the actual realtime dynamic performance
    of the motion system (sometimes this is a double edged sword as people either try to
    tweak things endlessly or are disappointed when errors due to things like stiction are actually visible)

    PP is basically LinuxCNC under the hood so this is all done by modifying the hal file
    so that the stepgen position feedback comes from the linear encoders rather than
    from the stepgens step count. This also means that you need 3 quadrature encoder inputs
    This could be added to the second 5I25 connector with the right firmware and BOB.

    If the linear scales have an index signal, this will also allow very precise homing



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Ballscrew mapping with an addon DRO

Ballscrew mapping with an addon DRO