DC motors

Discussion in 'General Electronics Chat' started by shredability, Jan 16, 2014.

  1. shredability

    Thread Starter Member

    Apr 1, 2012
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    I'm building a small desktop CNC designed for milling PCB's, I need small DC motors w/ encoders (NEMA17 mounting style)

    [​IMG]

    I need something like this, but a DC motor instead of stepper motors. I simply just can't get the accuracy from step motors. Brushed motors preferred & they need to be less then $40. I was hoping someone might know about something about this.
     
  2. MaxHeadRoom

    Expert

    Jul 18, 2013
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    If you are not getting the accuracy you need is this due to the gearing or resolution of the steps to distance, or maybe mechanical deficiencies?
    DC motors are fairly plentiful but you need servo grade with a feedback element of some kind and a drive/controller that has PID control.
    Brushless DC are becoming more popular and replacing the DC brushed.
    Max.
     
  3. shredability

    Thread Starter Member

    Apr 1, 2012
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    I'll be building the feedback system and writing my own PID libraries using the encoders and driving the motors with an L298 H-bridge. I haven't tried with stepper motors yet but typically they're only 1.8deg/step and I'm not a fan of the open-loop factor, especially if the system may be loaded
     
  4. Papabravo

    Expert

    Feb 24, 2006
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    I doubt you will find the motors you want for that price. Last time I bought servo motors for a CNC machine the price was north of $2500.00, with servo amps that took ±10V input and output ±300V.

    Stepper motors can be half stepped and drive an ACME leadscrew with an anti-backlash nut to achieve resolutions on the order a mil. As long as you ramp the velocity up and down through the mechanical resonance and stay away from the electrical resonance you can engrave expensive silver anniversary trays without worry.
     
  5. MaxHeadRoom

    Expert

    Jul 18, 2013
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    If you want more details on this application you are using, have you looked at the CNCzone site?
    Max.
     
  6. strantor

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    Oct 3, 2010
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    They can be "micro-stepped" too, down to 1/16 reliably. I've heard some going to 1/32 and reporting no issues, but I don't trust that. Some drivers support all the way down to 1/256 microstepping, but that's probably laughably unreliable.
     
  7. MaxHeadRoom

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    strantor likes this.
  8. atferrari

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  9. strantor

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  10. Papabravo

    Expert

    Feb 24, 2006
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    Microstepping is different than half stepping. In half stepping on a 4 phase motor you alternate 2 phases on / one phase on. In micro stepping you are actually playing with discrete levels rather than just a binary on-off.

    So half stepping a 1.8° stepper motor with a lead screw pitch of 2.5 turns per inch gives 1000 steps per inch or .001" per step.
     
  11. shredability

    Thread Starter Member

    Apr 1, 2012
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    You can half step a motor with only two phases

    [​IMG]

    Where C = -A & D = -B

    Something like this would be ideal, just w/ the n17 face plate. Something like this must exist.

    http://www.pololu.com/product/1442
     
  12. MaxHeadRoom

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    Jul 18, 2013
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    If you have the motors sized right and the right drive, and you eliminate mechanical inaccuracy, then it the only thing left is the linear resolution equal to one step.
    What is it, and is this resolution accurate enough?
    Max.
     
  13. GopherT

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    Nov 23, 2012
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    What is the application and what accuracy (resolution) are you looking for?
     
  14. shredability

    Thread Starter Member

    Apr 1, 2012
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    I'm building a cnc machine, but the real purpose of the project is a motion controller. Its a DC brushed servo controller, so steppers are out of the question.
     
  15. MaxHeadRoom

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    Jul 18, 2013
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    So is this a controller such as an intelligent drive where the PID loop is closed in the drive, or does the loop go back to a trajectory planner/controller?
    Max.
     
  16. GetDeviceInfo

    Senior Member

    Jun 7, 2009
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    Try holding a brushed dc motor still. Nice thing about steppers is that they can be held at torque. Regardless of your loop's capabilities, your resolution is only as good as your commutation, typically much worse than a stepper.
     
  17. MaxHeadRoom

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    I generally use 2500p/rev encoders, with the x4 factor, this translates to 10,000counts/turn, I can position to 1 pulse, with both DC brushed and BLDC.
    This is using Galil motion cards with torque mode drives.
    Max.
     
  18. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    Yeah, I'm one of the guys who designs microstepping drivers. ;)

    A microstepping driver can function at 1/256th stepping, which was actually the last one I designed.

    The two main problems are; the motor's step accuracy which can vary from step to step.

    And of course the main problem is the load deflection, as the position of a stepper motor depends on load torque. It is held in place by magnetic "springs" as as load torque is applied the rotor deflects further and further from the original position. If deflection becomes large enough the motor will "jump" to the next pole position which is generally 1/50th of a rotation (on a 200 step/rev motor).

    -----------------------
    To Shredability; A size 17 stepper can be bought as 400 step/rev (0.9 degree) and be driven very well with 16th microstepping giving positioning ability to 1/6400th of a rotation (plus or minus a couple of microsteps depending on load torques).

    They can be very reliable if you don't exceed load torque which comes down to good machine design.

    Total cost about $17 for a motor and maybe $20-$30 for a decent microstepping driver IC module. Cheaper if you buy a PCB that will drive 3 or 4 motors.

    If you choose to go with a DC motor and encoder, firstly you won't get a motor with encoder for $40, not even close. Your brushless 2-phase driver will be expensive and a hassle to design. Your encoder won't give 6400 steps per rev. And even if you paid extra for a fine resolution encoder like that it would be very difficult to get it to hold that position anyway.

    So even after much greater cost and enormous effort you probably won't get as good a performance as a cheap stepper setup anyway.

    Just go with steppers and avoid stupidly high load torques and fault torques. :)
     
  19. MaxHeadRoom

    Expert

    Jul 18, 2013
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    I don't quite understand that? I can hold position to much finer resolution than that, and in fact industrial encoders now go up to 100,000 counts/turn.
    Max.
     
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