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  1. #1
    Member
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    Feb 2020
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    Understanding relationship between pulse per revolution and RPM

    Hello everyone,

    I'm new to CNC and stepper motor world and I would like to see if I have understood correctly the concept of microstepping and pulse per revolution.

    If a stepper motor has a step angle of 1.8 degrees it will take 360 degree/1.8 degree = 200 steps to make a full revolution (360 degrees rotation).
    Now we say that 200 step per revolution is the variable SPR (step per revolution).

    The microstepping driver has the following table for the pulse per rev settings (those that you select with the switches) and one pulse is a microstep, a subdivision of a full step.

    We call "pulse per revolution" as PPR which is equal to microstep per revolution; we also call "microstep for each step" as MfeS.

    ( Mfes = PPR / SPR )

    PPR MfeS
    -------------------

    Default(200) 1
    800 4
    1000 5
    1600 8
    2000 10
    3200 16
    4000 20
    5000 25
    6400 32
    8000 40
    12800 64
    20000 100
    25600 128
    40000 200
    51200 256


    My driver has a max frequency (the pulse it can generate in a second) of 200KHz. Now we say that 200000 pulse for each second is the variable PPS and we notice that each pulse is a microstep.
    We call "revolution per second" as RPS (RPS = PPS / PPR).

    So we have the following table:

    PPR RPS RPM
    --------------------------

    Default(200) 1000 60000
    800 250 15000
    1000 200 12000
    1600 125 7500
    2000 100 6000
    3200 62.5 3750
    4000 50 3000
    5000 40 2400
    6400 31.25 1875
    8000 25 1500
    12800 15.625 937.5
    20000 10 600
    25600 7.8125 468.75
    40000 5 300
    51200 3.90625 234.375

    These calculations are done without considering any error, anything is totally ideal.

    Now my questions are:

    1) Is this all correct?

    2) In practice I've noticed that even with PPR set to 200 I'm nowhere near reaching 60000 RPM. Is the driver that actually limits the maximum frequency when I select a low PPR value to avoid mechanical issues or is the Software (MACH3 for example) that limits the maximum frequency?

    2Bis) Always with PPR set to 200 my motor should make 1 step (1.8 degree of rotation) for each 1/200000 of a second to keep up with the pulse generation frequency. Is this even possible? If not how do I calculate the actual maximum steps per second that the motor can do?

    3) When I set a feed rate in Mach3 the software is actually setting the correct frequency to reach those RPM that give the mm per minute required?

    4) I have read that torque change based on the PPR that I set but my datasheet only give the torque curve for 200 PPR. Is there a way to calculate (via matlab or something similar) the torque curve for each PPR setting? If I ask to the manufacturer to give me this torque curves is likely thai I will respond (manufacturer is STEPPERONLINE)?


    Thanks.

    P.S. Sorry for parsing I don't know why but any multiple space is automatically converted in a single space.

  2. #2

    Re: Understanding relationship between pulse per revolution and RPM

    Consider that on a good day under ideal conditions, the stepper motor is going to max out at about 900 RPM reliably. This is because the torque drops off rapidly with an increase in RPM. Above that, the stepper may not even have enough torque to turn itself, let alone do any useful work.

    Your item #3 above. Yes, that is correct. If you set the motor for 1/2 stepping, that will give you 400 steps per revolution. This is normally the best performance setting, but try some other settings to see if you get better performance.
    Jim Dawson
    Sandy, Oregon, USA

  3. #3
    Community Moderator ger21's Avatar
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    34698

    Re: Understanding relationship between pulse per revolution and RPM

    Consider that on a good day under ideal conditions, the stepper motor is going to max out at about 900 RPM reliably
    Not necessarily true.

    Depends greatly on the size of the motor, it's inductance, and power supply voltage. I've heard of people with small mills getting a reliable 2500rpm from steppers. But the bigger the motor, the slower it's going to go.


    2) In practice I've noticed that even with PPR set to 200 I'm nowhere near reaching 60000 RPM. Is the driver that actually limits the maximum frequency when I select a low PPR value to avoid mechanical issues or is the Software (MACH3 for example) that limits the maximum frequency?

    The Kernel frequency in Mach3 will limit the maximum pulses, and the Velocity setting will also limit the motor speed.

    4) I have read that torque change based on the PPR that I set but my datasheet only give the torque curve for 200 PPR. Is there a way to calculate (via matlab or something similar) the torque curve for each PPR setting? If I ask to the manufacturer to give me this torque curves is likely thai I will respond (manufacturer is STEPPERONLINE)?
    Torque curves are meaningless unless it's the exact same motor, drive, and power supply voltage. And almost all I've ever seen are at full step or half step.
    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)

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