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DRV8825 Questions

Arian Aghababaie
Prodigy 70 points
Other Parts Discussed in Thread: DRV8825

Hello,

I was looking at page http://www.pololu.com/catalog/product/2133, and I have a couple of question related to the use of the driver.

For my application, I need to drive a stepping motor at 60RPM with a torque of 2.5Nm. I have selected a stepping motor that has a max permissible torque of 3Nm and the rated current per phase is 0.6A. The stepper motor is a NEMA 16 with a 14:1 planetary gearbox and so the step angle is 0.131degrees.

My question is how can I calculate what RPM the DRV8825 stepper motor driver can drive the stepper motor at?

I've read the TI datasheet for the DRV8825 which says that the maximum step frequency is 250kHz I assume this is for the highest micro-stepping resolution at 1/32 and with the maximum driver output current of 2.5A. 

Is there a reference table or simple formula I can use to determine the maximum step frequency at lower micro-stepping resolutions and lower driver output currents?

Is the relationship between micro-stepping and torque linear? As I have read that the mirco-stepping reduces torque?

  • 0
    TI__Guru 55615 points

    Arian,

    The 250kHz rating refers to how fast you can articulate the STEP input.  It is nothing more than that and applies to all micro-stepping modes.  You would divide this number by the micro-stepping mode to get full steps that are possible. 

    For example, 250kHz/32 = 7812.5 full steps per second.  From my calculations, that is enough to get you up to 60RPM since you basically need to issue 2748 steps to make 1 complete rotation. 

    There is a table in the datasheet that shows the regulated current for each step in the table.  As you can see if you plot it out, the incremental torque per step decreases with the higher levels of micro-stepping.  In full step mode, you always have full current in the winding.  In half step mode, you have a fraction of this current in one of the steps and 1/4 step even cuts this down further.  This impacts your holding torque. 

    The value of micro-stepping is lower mechanical noise and resonance problems (steps skipping) can be more easily avoided. 

  • 0 in reply to Ryan Kehr
    Prodigy 70 points

    Hi Ryan, 

    Thanks for the reply, has been very useful. 

    A further question -  on the data sheet it's assumed that the current for full step is at 70% rather than 100%. Is this an accepted loss in the system or an RMS value that results in the signal?

    Thanks again, 

    Arian

  • 0 in reply to Arian Aghababaie
    TI__Guru 55615 points

    Arian,

    The logic is setup in the DRV8825 to chop at 70% of the current threshold as determined by Vref/Rsense equation in the datasheet.  Knowing this, you can adjust the maximum current in the winding to the value you desire. 

     

  • 0 in reply to Ryan Kehr
    Prodigy 130 points

    Hey !  
                I want to design a stepper motor drive which can run stepper motor upto 8000 rpm and more. i decided to use drv8825 and drv 8829 driver ic's . Can anyone tell me how to start start and are these driver ic's are capable to run a stepper motor above 8000 rpm .


    thank you...!!!