Gantry or Stepper issue?


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  1. #1
    Member 5Carbides's Avatar
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    Default Gantry or Stepper issue?

    See video: https://sendvid.com/i5f1ho8k

    Running around 170 IPM per LinuxCNC. Acceleration is set very low. It seems that the gantry randomly binds (have cleaned the screws and bearings) while traveling down the length. Where on the y axis it binds and which side binds is inconsistent. Is this a stepper motor or software issue or a mechanical issue?

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  2. #2
    Member Sterob's Avatar
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    Default Re: Gantry or Stepper issue?

    Whats the pitch of your ballscrews?
    What RPM are your steppers doing at 170 IPM?
    Do you have dual ballscrews?
    Whats the torque of your steppers?
    Whats the Inductance of your steppers?
    What voltage are your running your steppers at?



  3. #3
    Member 5Carbides's Avatar
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    Default Re: Gantry or Stepper issue?

    I am new to all of this so bear with me. I purchased this already built Joe's CNC 4x4 Hybrid which is functioning, but it has opportunity for improvement. :-D

    - Whats the pitch of your ballscrews?
    + 1/2"-8 8 start acme screws

    - What RPM are your steppers doing at 170 IPM?.
    + Steps per revolution are 200. Not sure how to check rpm at a given IPM. I am using LinuxCNC | MESA 5i25 + 7i76 | HobbyCNC Chopper Driver (rev1)

    - Do you have dual ballscrews?
    + Yes one screw and stepper on each side.

    - Whats the torque of your steppers?
    +They are Bipolar NEMA 23s with 425 oz-in of torque (per specs, not personal measurement. Do I need to?)

    - Whats the Inductance of your steppers? I
    +I believe they are wired as Bi-Polar Serial (Orange and brown connected | Blue and yellow wires connected) which per manufacture documentation the inductance would be 12.8 mH/Phase (+-20%)

    - What voltage are your running your steppers at?
    + 4.2 volts. 3amp from the Hobby CNC board



  4. #4

    Default Re: Gantry or Stepper issue?

    Hi,

    +I believe they are wired as Bi-Polar Serial (Orange and brown connected | Blue and yellow wires connected) which per manufacture documentation the inductance would be 12.8 mH/Phase (+-20%)
    I believe that one or both of your steppers is losing steps. With an inductance of 12.8mH I'm surprised they just don't stall.

    Most first time buyers buy on the basis of holding torque, 425oz.in is good....right? What they don't know is that inductance is a very good predictor of the speed that a stepper
    can go before is loses enough torque that it misses steps. I have posted on a number of occasions about the NECESSITYof low inductance for steppers as axis motors.

    I would guess that your steppers will have less than 5% of their 425oz.in at 1000 rpm.

    If buying 23 size steppers look for 1-2mH, 1 mH preferred and reject anything over 2mH.

    Craig



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    Member 5Carbides's Avatar
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    Default Re: Gantry or Stepper issue?

    According to the Hobby CNC documentation on this stepper, if I wire it BiPolar Parallel then the inductance is 3.2 mH/Phase (+-20%).

    I have attached the specs I am going off of. Again the inductance values mentioned are all per documentation, none of them per my own measuring.

    Attached Files Attached Files


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    Member peteeng's Avatar
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    Default Re: Gantry or Stepper issue?

    Hi 5C
    1) are the two axis slaved? or is one driver running 2 axes?
    2) 4V is very small to run steppers need more like 36V plus
    3) 4250mm/min or 170inch/min is this cutting or a rapid?
    4) if the spacing of the gantry column bearings is small the gantry can walk and jam or if the gantry is mounted not very stiff it can walk and jam. If you disconnect the drives and push one side it should slide easily. If it judders (stick slip) or jams then you need to address the mechanics first.

    cheers Peter



  7. #7

    Default Re: Gantry or Stepper issue?

    Hi,
    you very definitely MUST wire in Bipolar Parallel. Even then 3.2mH is higher than I'd recommend but by no means the worst out there in stepper land.

    One way to counteract the negative effect of inductance is to have as higher driving voltage as you can get. What is the max voltage spec of your drivers?
    Have you got the matching (high voltage) power supply, and if not......why not?

    Craig



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    Member 5Carbides's Avatar
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    Default Re: Gantry or Stepper issue?

    Quote Originally Posted by peteeng View Post
    Hi 5C
    1) are the two axis slaved? or is one driver running 2 axes?
    2) 4V is very small to run steppers need more like 36V plus
    3) 4250mm/min or 170inch/min is this cutting or a rapid?
    4) if the spacing of the gantry column bearings is small the gantry can walk and jam or if the gantry is mounted not very stiff it can walk and jam. If you disconnect the drives and push one side it should slide easily. If it judders (stick slip) or jams then you need to address the mechanics first.

    cheers Peter
    1. One stepper is running on the board's Y axis and the other on the A axis. Picture of board attached. "A" axis is on the far right, and Y is about mid photo.
    2. I am likely wrong. The Transformer is putting out 36 volts. 4.2 volts was a number the previous owner of the CNC put in their documentation. How would I get a correct measurement, Red and Black wires that go to the stepper? And do so while it is moving the axis?
    3. Jogging.
    4. Gantry is a good 68.5" across the length and each side's carriage is about 9" wide. Each carriage has 1 anti-backlash nut. See attached photos.

    Attached Thumbnails Attached Thumbnails Gantry or Stepper issue?-screen-shot-2020-02-08-10-13-a   Gantry or Stepper issue?-screen-shot-2020-02-08-10-40-a   Gantry or Stepper issue?-screen-shot-2020-02-08-11-21-a  
    Last edited by 5Carbides; 02-09-2020 at 11:08 AM.


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    Default Re: Gantry or Stepper issue?

    Quote Originally Posted by joeavaerage View Post
    Hi,
    you very definitely MUST wire in Bipolar Parallel. Even then 3.2mH is higher than I'd recommend but by no means the worst out there in stepper land.

    One way to counteract the negative effect of inductance is to have as higher driving voltage as you can get. What is the max voltage spec of your drivers?
    Have you got the matching (high voltage) power supply, and if not......why not?

    Craig
    HobbyCNC documentation seems to imply the Serial wiring.
    https://hobbycnc.com/wp-content/uplo...t-Contents.pdf

    Also see attached photo (cannot find original document).

    Attached Thumbnails Attached Thumbnails Gantry or Stepper issue?-screen-shot-2020-02-08-11-47-a  


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    Default Re: Gantry or Stepper issue?

    Hi 5C - assuming wiring correct as it is moving, look at the bearing. You say one bearing per side? This is unlikely. If it is one bearing per side how long are they? This maybe the answer. The bearing length is not long enough for stable travel. Especially if the drive nuts are worn. Can you post better picture of bearings?

    I looked at the video and it seems the gantry gets to full velocity then stalls. But it does stall initially. Seems as if the accel is OK as it does accelerate and even out but try backing the accel down further. Where it stalls is this where it is meant to slow down? Decelerations are just as bad as accelerations. Peter

    from your read out does it get to the intended speed?

    Afterthought - by "jogging" do you mean using the remote control? I say this because some systems do this because when under manual control there is no velocity planner involved. So do the same path but under program control and see what happens. My guess is that you will get a different response as then the velocity planner will be involved better.

    Last edited by peteeng; 02-09-2020 at 01:25 AM.


  11. #11

    Default Re: Gantry or Stepper issue?

    Hi,

    HobbyCNC documentation seems to imply the Serial wiring.
    As you wish, at 12.5mH inductance don't expect your motors to run above 100, maybe 150 rpm.

    Craig



  12. #12
    Community Moderator ger21's Avatar
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    Default Re: Gantry or Stepper issue?

    Steps per revolution are 200
    You are running in full step mode. If those drives can be change to 1/8 microstepping, you should do that.

    Gerry

    UCCNC 2017 Screenset
    http://www.thecncwoodworker.com/2017.html

    Mach3 2010 Screenset
    http://www.thecncwoodworker.com/2010.html

    JointCAM - CNC Dovetails & Box Joints
    http://www.g-forcecnc.com/jointcam.html

    (Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)


  13. #13
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    Default Re: Gantry or Stepper issue?

    CORRECTION: One anti-backlash nut per carriage. There are four v-groove bearings, two on top and two on bottom.


    Peteeng -
    Yes I it is getting to its intended speed.

    It is stalling at a point where it should still be going full speed.

    By jogging I mean I am moving the gantry via keyboard in manual mode with the spindle off and no contact between the bit and work surface or material.


    joeavaerage -
    I would not say that i wish 12.5mH or Serial wiring, just saying the documentation implies it. If there is a better way, I want to do it or at least know the limitations of what I have.

    The whole Hobby CNC Chopper board + Mesa 7i76 daughtercard + Messa 5i25 PCI seems like a lot of hoopla and opportunity for errors to be introduced.


    ger21,
    Yes the controller supports 1/8 micro stepping. There is always a possibility that the documentation I am reading for the Steps Per Revolution is wrong. I received that number from the 2nd owner's (who was not the builder) documentation.



  14. #14

    Default Re: Gantry or Stepper issue?

    Hi,

    If there is a better way, I want to do it or at least know the limitations of what I have.
    The better way is to run them in parallel, or indeed any wiring method that reduces the inductance. You need to reduce it so the steppers
    will run at speed, at the moment I would expect them to start losing steps at as low as 100rpm, which if I understand your description,
    fits the symptoms.

    Craig



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    Default Re: Gantry or Stepper issue?

    It looks like the drivers are unipolar which means there are no serial/parallel options available.
    (and the inductance is 3.2 mH)

    Is it possible that your maximum axis velocities are set too high?

    EDIT I just looked at the video and it does not appear to be a velocity related stall, so the maximum velocity is probably OK

    If you are using full steps, as Ger21 suggested, change to the highest microstep ratio available

    Also make sure that the step timings are quite long (10 usec would not hurt, never set them to the drives minimums)

    If you post your hal/ini files I can check if they look sensible.

    Last edited by PCW_MESA; 02-09-2020 at 11:17 PM.


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    Default Re: Gantry or Stepper issue?

    Agreed on the HobbyCNC Pro V1 being Unipolar only. After spending 2 hours researching online, I just read your response. I was confused by the documentation online because HobbyCNC's wiring diagram appears to match the BiPolar Serial wiring in the stepper's (23-305-DS8A) schematic. However the HobbyCNC Pro V1 is clearly Unipolar as it is written directly on the PCB. This also clearly highlights the previous owner documented this machine wrong as they stated it was Bi-Polar.

    In the video I am attempting 170 IPM which is regularly binding on the Y axis. I can "reliably" do 138 IPM with only an occasional bind. Since these speeds are only while jogging the gantry, I have not lost any pieces as a result.


    CORRECTIONS
    - Unipolar
    - Current: 3 amps (per 23-305-DS8A-Specs.pdf)
    - Resistance: 1.4 (+-10%)/Phase (per 23-305-DS8A-Specs.pdf)
    - Inductance: 3.2 (+-20%)/Phase (per 23-305-DS8A-Specs.pdf)
    - Holding TogQue: 215 N.cm Min (per 23-305-DS8A-Specs.pdf)


    Huge thank you to everyone so far! You have helped me identify incorrect documentation, motivate me to chase down the correct information, and that will help me more effectively setup LinuxCNC (or maybe Mach3 [I kind of like it better])



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    Default Re: Gantry or Stepper issue?

    I'm by no means an expert but:
    You have some very high quality motion control hardware (Mesa 7i76) and you have Peter from Mesa (PCW_MESA) already offering to help but you have not posted your hal and in in files as he asked. You will be taking a step backwards if you go to Mach 3.

    But in my view the way you have put your system together after the 7i76 motion control board is just crippling your system and I suspect your motors are just crying out for more voltage ad if you heed their cries for help, all your problems will go away. I have some 8 wire small Nema34's on my gantry machine (with a 48 volt power supply). In my experiments as I built my machine anything other than bipolar only parallel wiring just did not cut it. But when you adopt that wiring scheme, you need 4.2 amps per motor to drive them properly and your drives just cannot deliver that amount of current.

    So i think to persevere with those motors, you really need to upgrade your system so that you can feed them 4.2 amps and preferably at 80 volts.
    So that will not be a cheap exercise as it will require upgrading your power supply and your stepper controllers.
    If you do that, just be aware that the 7i76 can step at up to 10 Mhz but I keep mine around 100 kHz max (at around 750 rpm) so there is plenty of scope to increase your microstepping beyond what controllers are capable of.

    Alternatively, replace/downgrade your stepper motors so they are within the specs of your power supply and drivers.

    Rod Webster
    www.vmn.com.au


  18. #18
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    Default Re: Gantry or Stepper issue?

    I meant no disrespect or lack of appreciation towards Peter from Mesa (PCW_MESA). I just did not have access to those files at the time I was posting (2am). I should have noted I intended to provide the files later. :-)

    Mesa Hal and Mesa Ini files attached.

    Attached Files Attached Files


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    Default Re: Gantry or Stepper issue?

    Incredibly helpful everyone, thank you.

    Resolution to Gantry binding:
    - The current being sent to the Y axis steppers was different. The CNCHobby board was sending about a half an amp different to one of the steppers due to a potentiometer for that stepper on the board being mistuned.
    - Once I re-dialed in the same amperage being send to both of the steppers (around 2.5 amps), binging on the Y axis is gone.
    - I am now achieving 180 IPM on the Y axis according to the LinuxCNC via DRO screen.

    Long Term Solution:
    - New stepper driver boards that can do bi-polar parallel and push out 3 amps
    - Larger power supply. ~48 volts and 9-10amps to better power these steppers.



  20. #20
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    Default Re: Gantry or Stepper issue?

    Long term solution is now in place. CNC is running better than every on StepperOnline DM542T digital stepper drivers and a 48 volt/480 watt (yeah could be a bit larger on the amps) power supply. Nema 23 steppers are wired bipolar parallel. MESA 5i25 + 7i76 are feeding the stepper drivers. I am can jog at 400 IPM (anymore seems too fast) and have cut at 120 IPM. Beyond no more binding of any axises, I am loving the performance improvements.

    Thank you everyone for your help!



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