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  1. #1
    DenJon's Avatar
    Lives in tacoma, United States. Last Activity: 4 Weeks Ago Has been a member for 0-1 years. Has a total post count of 3.
    I Ďm new to CNC building.
    I would like to use a DRO / linear encoder for controlling the axisí on my first DIY router build for a closed loop system. My build so far consists of a steel frame with THK linear rails. Iím leaning towards the Mach4 CNC software. I plan to use NEMA 23 stepper motors.
    I really donít want to use motors with built in rotary encoders. I have heard that you can use linear scales in place of the encoders on the motors.
    Iíve heard good things about doing it this way:
    backlash free from the get go, linear scales read the actual table position, easily configured, connect the motor to controller, you can tie the motor encoder to the drive as normal but reads from the linear scales. more failsafe if I break a motor shaft or there is inaccuracy in the ballscrew
    And bad things:
    impossible to do any kind of feedback loop, instability cause by too much delay between the motor and carriage.
    blah blah blah. Too much conflicting info and technical stuff to sort through.
    Iím on a budget and was wondering if I could make these work? https://www.ebay.com/itm/800MM-Linea...l/233243351657
    Just in case the link doesnít work they are

    Linear Scale For Milling Lathe Machine Linear Glass Scale Double Seal
    Specs:
    Voltage: 5V DC
    Current: 50mA
    Travel Length: 800mm
    Grating Pitch: 0.02mm (50LP/min)
    Resolution: 5μm (0.005mm) 0.0002"
    Accuracy: Ī0.005 mm or 0.0002" (at 20 deg C)
    Response Speed: 60m/min or 40inch/sec
    Cable Length: 3m
    Sealing: Double seal
    IP Class: 53
    Product Dimention(LxWxH): 94 x 2 x 5cm (37.01" x 0.79" x 1.97")

    If they will work will they just connect to the encoder input on the motor drive and off I go or is there more to it? Do I need some type of motor control board or what?
    Any help would be greatly appreciated.

  2. #2
    I was recently told that you can include linear scales in the loop using LinuxCNC, but as far as I know it isn't possible with Mach 4.

    I don't think it would work just connecting the encoder output to the input of the motor driver because the motor driver probably doesn't understand anything about backlash since there isn't any between the motor and the usual rotary encoder. Also the control dynamics of the motor are quite different to the table, which has much more mass and more friction. You don't want to be making a compound cut when the table can't keep up with what the controller thinks it should be doing.

    Don't overthink things. I can't see why for your first DIY CNC router you don't just use ordinary stepper motors, especially if you are using ballscrews already. The screws are likely to be at least as accurate as your build, and will have low enough backlash that you can just use the inbuilt compensation routine. Modern stepper drivers are pretty cheap and work very well.

    If you manage to get a stepper motor to break its shaft I'd be VERY surprised!!!

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  4. #3
    Like John says you'll be better off not bothering because of all the complications it's going to through up for very little gain.
    But I'm going to slightly disagree with him, mostly because I've got a lot of experience with Closed-loop steppers so I know how much better they are than a standard setup and say just buy some Closed-loop steppers like the Lichuan drives/Motors here. Feed them 65Vac and watch them fly.
    By the time you have bought Linear scales and messed around you will have ended up paying the same or more and will most likely have a system that performs no better or worse.!

    https://www.aliexpress.com/item/3279...61803421wQM0nz

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  6. #4
    DenJon's Avatar
    Lives in tacoma, United States. Last Activity: 4 Weeks Ago Has been a member for 0-1 years. Has a total post count of 3.
    Thank you John and Jazzcnc for your replies.
    My original thought was to go with steppers with built in rotary encoders. but then during my research I found the following YouTube video
    https://www.youtube.com/watch?v=b5JD2-hlvtw&t=172s

    And several articles on linearrmotiontips.com

    "In contrast, using a linear encoder means that the controls track the X-Y position of the drill over the raw material. Here, the linear encoder accounts for thermal expansion through measurements of linear displacement, so controls can compensate for induced errors."

    "Linear encoders boost accuracy by correcting errors downstream of mechanical linkages.

    Linear encoders track axis position without intermediate mechanical elements. The encoders even measure transfer errors from mechanical linkages (such as rotary-to-linear mechanical devices), which helps controls correct for errors originating from the machine. Thus, this feedback lets controls account for all mechanics in position-control loops."

    It appears that I can get regular stepper motors with the encoder or ones without the encoder and a linear encoder for about the same price or slightly higher.
    So am I missing something here? Though this is my first DIY router build I would also like to do it up right so that it will be my last build. I'm not set on the software I would just like something mainstream with a not to complex learning curve.

  7. #5
    With a fully closed loop system then you are entering into another level of complexity which I wouldn't recommend to a new user. But also I certainly wouldn't go through the painful and expensive learning process your going to have go thru learning Linux CNC (mach3/4 cannot close the loop without special hardware) just to use a stepper system.

    To get the best from closed-loop you need AC or DC Servo motors which can react fast enough to the tiny fractional positional commands being fired at it and hold the position. Steppers cannot do this very well.


    Also just let me point this out about a Fully closed-loop System that some new users don't realize.
    If the machine goes too far and overshoots or undershoots then the Fully closed-loop system knows and the controller brings it back into the correct place which an Open-loop system doesn't.!! . . . BUT it's still lost the position and gone too far so cut into an area you probably didn't want with the same result that it's potentially damaged or ruined the workpiece..!!
    This is no different to a stepper system losing steps. The main difference is that the controller's Fully closed-loop system won't let it get out of control and keep the error tiny or fault and constantly watching and adjusting the following error and when it gets too large throws a fault. A properly set up closed-loop Servo system will keep these following errors to tiny values which in the grand scheme mean nothing and you won't get faults.
    But going back to steppers and not reacting fast enough then it's a lot of effort for not much gain to tune a system so it will be stable and not throw constant following errors because the steppers cannot keep up.
    Whereas a properly setup and tuned Closed-loop at the drive stepper system will give you just the same protection by watching the position matches the commanded position and correcting if it doesn't, it just doesn't tell the upper controller anything about it. Also, it will throw a fault if the following error gets too great just the same.

    But the bottom line is Stepper or Servo open or Closed-loop system if not correctly set up and tuned correctly will wreck the job. The only difference usually is that with a servo system you won't know until much later because it didn't stall, likewise with a poorly setup stepper system that doesn't stall but drops missed steps you won't know until later either.

    My advice is to save your self a world of pain and just use a purpose-built closed-loop stepper system which is tuned conservatively because they do work more than good enough to match the limits of a stepper driven machine. After this then you enter a whole new and to be honest, expensive world of AC/DC servo motors and the related hardware needed to get the best from them.

    Also just to point something out in that video that he's VERY WRONG about is that the parallel port is certainly NOT the best and most reliable way or choice.!! . . . The USB and Esp the USB Smooth stepper was or is rubbish and unreliable. Ethernet is far superior to both of them and much faster, more stable and capable of handling servos or steppers easily.

    Hope this helps and if it helps any I've built well in excess of 100 stepper driven machines both open and closed-loop ranging from small to 10x5 machines and they all worked perfectly fine because it's how they are set up and the quality of components used along with build quality that matters most.
    Build a sloppy machine and no closed-loop or servo system in the world will make it any better, in fact, it will make it worse, because they don't tolerate sloppily built machines.!!

  8. #6
    DenJon's Avatar
    Lives in tacoma, United States. Last Activity: 4 Weeks Ago Has been a member for 0-1 years. Has a total post count of 3.
    Wow JAZZCNC thank you for all of your effort, you are a wealth of information. I really appreciate the time.
    I just can't grasp the concept of how what ever is reading the info from the encoder knows weather it's coming from a rotary or a linear encoder. If the motor makes one revolution and the rotary encoder translates that it has moved 5 mm's or if it did one revolution and the linear encoder translates that it moved 5 mm's why should that make any difference? I can't seem to grasp why the encoder type would make a difference.

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