Usability of Encoder on DC Motor,


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  1. #1
    Member Joey2point0's Avatar
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    Default Usability of Encoder on DC Motor,

    Doing a retrofit of D&M 5 Lathe, adding Acorn CNC controller. It has the stock 90 vdc motor.

    Is it worth it to add a rotary encoder? Wondering if this will allow me to do threading?

    Also, any encoder recommendations would be appreciated.

    Thanks, Joey

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    Default Re: Usability of Encoder on DC Motor,

    definitely need an encoder to do proper threading. Normally I'd say on the spindle not the motor but from the looks of it you have a toothed belt drive so slipping is not likely - and if it does it will be obvious!
    The CUI's AMT programmable encoders (models 102 and 103) have been working for me, but might be tricky to install on your setup - needs a small shaft to go over. Readily available from lots of sources (I use Digikey).
    Trying to reuse the slotted disk will probably be more trouble than its worth.



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    Default Re: Usability of Encoder on DC Motor,

    I bought these from robotdigg for really cheap.
    https://www.robotdigg.com/product/13...per-revolution

    it comes with either a hollow or solid shaft. They also sell higher resolution encoders if needed.



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    Default Re: Usability of Encoder on DC Motor,

    The encoder will have to be on the shaft of the spindle and will need to have an Index signal. The index is what tells the controller where to start the threads.

    An encoder with an index and an A signal will work best. The index pulse tells the controller where the spindle zero degrees is and the A signal will produce a number of pulses for each revolution. The more pulses that A sends the more accurately the spindle RPM can be calculated by the controller. Thus making better threads.

    Here are several encoders.
    https://www.automationdirect.com/adc...otary_encoders



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    Default Re: Usability of Encoder on DC Motor,

    However, depending on the controller used, it could be possible to swamp the control with spindle pulses. Check your control's manual for a recommendation on the number of pulses per revolution of the spindle.



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    Default Re: Usability of Encoder on DC Motor,

    Decided to go with this since I've read many people with Acorn boards have had success with it and acorn recommends 1000 ppr minimum

    https://www.automationdirect.com/adc.../trda-2e2500vd

    Acorn also requires 1:1 spindle to encoder ratio, with the limited space I'm working with and what looks to be an inability to mount a timing pulley directly to the spindle I'll essentially be duplicating the gearing ratio between the motor and spindle but it will turn the encoder instead.

    Ill be 3D printing all the pulleys and mounting brackets to get the alignment and tension correct then making some aluminum ones as a permanent solution.



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    Default Re: Usability of Encoder on DC Motor,

    Quote Originally Posted by Joey2point0 View Post
    Doing a retrofit of D&M 5 Lathe, adding Acorn CNC controller. It has the stock 90 vdc motor.

    Is it worth it to add a rotary encoder? Wondering if this will allow me to do threading?

    Also, any encoder recommendations would be appreciated.

    Thanks, Joey
    The Encoder would be geared to the spindle the motor is unimportant

    Tt seems like you have a lot of replies but nobody though to see what is suitable for your control

    Attached Thumbnails Attached Thumbnails Usability of Encoder on DC Motor,-acorn-encoder-requirments-png  
    Mactec54


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    Default Re: Usability of Encoder on DC Motor,

    Acorn also requires 1:1 spindle to encoder ratio
    If you have a 10 PPR encoder geared 1:10 to the spindle it has an encoder resolution of 100. If use this value, it should work. I guess what acorn means is that the resolution should be an integer number.

    I have made an 8 PPR encoder on the spindle back using 2 proximity switches, 8 magnets for the synchronization and 1 magnet for an index pulse. That works fine, even on my low torque variable speed lathes. The synchronization error overall is less than 0.05 mm and about 0.01 mm during the final shallow passes.

    If your spindle speed is reduced by gears, the spindle speed won't change very much during threading and you only need a few synchronization pulse to produce a good quality thread.



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    Default Re: Usability of Encoder on DC Motor,

    Quote Originally Posted by hfjbuis View Post
    If you have a 10 PPR encoder geared 1:10 to the spindle it has an encoder resolution of 100. If use this value, it should work. I guess what acorn means is that the resolution should be an integer number.

    I have made an 8 PPR encoder on the spindle back using 2 proximity switches, 8 magnets for the synchronization and 1 magnet for an index pulse. That works fine, even on my low torque variable speed lathes. The synchronization error overall is less than 0.05 mm and about 0.01 mm during the final shallow passes.

    If your spindle speed is reduced by gears, the spindle speed won't change very much during threading and you only need a few synchronization pulse to produce a good quality thread.
    I'm not sure what your point is his Control has a requirement, and a low count Encoder like you describe is a waste of time, for anything to do with machining, the more PPR the reaction and smoothness is what counts

    Mactec54


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    Default Re: Usability of Encoder on DC Motor,

    Here is what I plan on doing, the motor/encoder ratio is the same as the motor/spindle ratio. I don't see why this wouldn't work, though I'm open to suggestions as this is my first time doing anything of the sorts.

    PS for mocking up purposes everything is 3D printed from PLA but once i settle on a configuration ill be printing them solidly out of Nylon

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    Default Re: Usability of Encoder on DC Motor,

    and a low count Encoder like you describe is a waste of time, for anything to do with machining, the more PPR the reaction and smoothness is what counts
    You would think that... (for that matter, I used to think that)... however, I've seen YouTube videos of people with converted mini-lathes doing threading with just one pulse per spindle revolution. They then put a standard nut on the threaded part; and it fit. I'm guessing that the spindle drive motor (and associated inertia) kept the RPM from deviating enough to ruin the part. It's remarkable what some people accomplish with rather modest means...

    However, if I had the computational grunt in the control's processor; I'd obviously prefer more pulses-per-revolution than just one, though... the theoretical point of diminishing returns will depend on the thread being cut, machine resolution & accuracy, and what type of algorithm the control is using to drive the motors (non-isochronous step & direction; or isochronous PID update cycle; and what kind of math it's using internally).





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    Default Re: Usability of Encoder on DC Motor,

    'm not sure what your point is his Control has a requirement, and a low count Encoder like you describe is a waste of time, for anything to do with machining, the more PPR the reaction and smoothness is what counts
    @mactec54 I am sorry if I wasn't clear
    • Not the gearing of the encoder is important but the final pulses per spindle revolution is what counts
    • By design, threads have a lot of clearance. Trying to get this few hundreds of a mm clearance within 0.001 mm accurate will not make a better thread.
    • Even so, according to the measured accuracy, even using this low resolution encoder, the result is accurate, not a waist of time but a learning experience.
    • I agree on the X or Z axis, a decent encoder resolution is a must because it is used for positioning.



    By the way, for accurate threading, meaning even the last winding (deceleration of the Z-axis) is in spec, I use a stepper on the spindle.



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