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Torque and Micro-Stepping.

Discussion in 'Helpful Tools' started by GrayUK, May 16, 2017.

  1. GrayUK

    GrayUK Master
    Staff Member Moderator Builder

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    Supplied by:- Bob R. Copied & Transferred by:- GrayUK

    I've seen this article before, but most people don't fully understand what it means. You DO NOT lose torque or accuracy by increasing the number of steps...

    The article says "the INCREMENTAL torque per microstep drops off drastically".

    Well of course it does! But that doesn't mean the torque per degree has changed. In fact it doesn't. Let's try to understand this better...

    If you apply a steady current to each of the two motor windings and there is no load on the shaft, the motor will turn until it finds a resting position based on the magnitudes and polarity of the currents applied. At this position there is ZERO torque. If you try to turn the shaft by hand it will move slightly off of the resting position and fight you. It's now producing torque because it's not in it's happy position. So how much torque is it generating? Well that depends on how far you pushed it off its resting position. The greater the angle the more it's fighting to push it back. For small angles the torque is roughly proportional to the angle. If the angle becomes too great it will break free. At that point you've hit the holding torque limit.

    Table 1 tells you the amount of torque produced for various angles - the angles resulting from being 1 step off from the resting position. Well obviously if you use 256 microstepping and then apply 1 pulse, the new angle is tiny so the off angle torque is tiny.

    Let's say you are currently using 8-step microstepping, and lets say you apply 1 pulse. The incremental torque will be a certain amount.

    Now let's say you change to 256-step microstepping. But instead of applying 1 pulse you apply 32. Why 32? Because that will try to move the shaft to the same new position as a single step in the 8-step example. Take a guess how much torque will be produced. It's exactly the same as 1 pulse using the 8-step microstepping!!! No torque loss!

    If you had applied only 1 pulse with 256-step microstepping then you would expect the torque to be approximately 1/32 that of the 8-step example. Look in table 1 at the 8-step and it reads 19.51%, and for 256 it reads 0.61%. The ratio of the 2 numbers is 19.51/0.61= about 32 which agrees with what we expect.

    Also realise when you increase the stepping on your system (let's say from 8 to 256) the micro-controller that pulses the motor driver will increase the number of pulses it sends by the same factor - in this case by 32. Since it increases the number of pulses sent then in the end there isn't any torque lost. You simply have more resolution.

    There is a problem however with increasing the number of pulses that has nothing to do with torque. The stepper driver you are using has an upper limit on how quickly it can receive consecutive pulses. Additionally the micro-controller outputting the pulses has an upper limit as well. These upper limits will determine the maximum rotational speed of your motor.

    If you've increased the microstepping in your system and noticed a reduction in the maximum speed, it's got nothing to do with torque, but rather the upper frequency limit of your stepper system, which could be the driver itself, or the micro-controller that's pulsing it depending on your system.The software I use to control my stepper board is Grbl Panel and unfortunately it doesn't warn that you've exceeded the frequency limit on your system.

    Here's another argument that the absolute (not incremental) torque doesn't change. If you think the torque drops with an increase in microstepping, then if there were a way to increase the stepping to be extremely high (like infinite) then there would be zero torque. There is a way to simulate infinite microstepping. Toss the motor driver and instead connect the two winding to two current regulated power supplies that you can adjust to any value you want. So you have infinite resolution. Does that mean the torque goes to zero? Of course not! For example if you connect a fixed current of 1 amp to each winding I can guarantee there will be torque if you try to rotate the shaft by hand. Yet this is equivalent to infinite microstepping since you have infinite resolution.

    There is no loss of torque by increasing the microstepping. Period. So crank up the microstepping and enjoy the smoother ride!

    If you find your maximum velocity is now lower than you require, then either reduce the microstepping or get a better stepper controller board which can handle the higher frequency requirement.
     

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