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DRV8711EVM: Stepper motor control application help

Kenneth Nwokolo
Prodigy 30 points
Part Number: DRV8711EVM
Other Parts Discussed in Thread: DRV8711,

HI, 

I am doing some motor control development and are using the DRV8711 but are running into some challenges and are looking for some help with our development process. 

Out setup is a DRV8711EVM but we are using an external MCU to control it. The pins we have connected to our DRV8711EVM are CLK, MISO, MOSI, CS, STP, SLEEP and DIR. Both boards share a common GND The remaining pins are connected in the default state. we are using a 2.8A 1.1 Ohm NEMA 23 style stepper motor (5718L-01P). we are operating in indexer mode and our register configuration is as listed below, the only difference is that we are setting our gain to 10 and Torque to 86

We were having some unexpected behaviors so we decided to change out testing approach by reducing our step resolution to a full step and try and resolve our problems. We have a few issues but for the sake of the thread we will mainly be discussing one, Vibrations while moving. 

In the first video labeled Video 1 we were attempting to move the motor a 1/4 turn (90 deg) per second. Our frequency to the indexer was 50Hz. We then changed the speed during to 1/2 turn per sec (100Hz) and then finally 1 turn per Sec (200Hz). As you can see in the video attached the motor makes audible noise and vibrates while it is being run.

Q1: Can you provide some insight into this phenomenon and what we can do to eliminate this? 

As for the 2nd video we kept the settings the same and changed the step resolution to 256 at 50Khz. As you can see the motor vibrated a lot and kept jumping during motion

Q2.  Can you provide some insight into this phenomenon and what we can do to eliminate this? 

Thank you in advance for advice on this issue, we are happy to provide any more information as needed. We have custom boards designed based on the DRV8711 but we will like to use the EValuation board to eliminate the chances of an error in our PCB Design

Best

afam

  • 0
    TI__Mastermind 23421 points

    Hey Afam,

    Do you have an oscilloscope with a current probe you can use to measure one phase of the stepper motor?  This would help us debug what is going on a lot.  Captures similar to those in the app note below would be best. I highly recommend reading through this as well:  https://www.planetanalog.com/tips-and-tricks-for-testing-and-debugging-stepper-motors/ 

    Also look through the app note "DRV8711 Decay Mode Setting Optimization"

    Have you tried lowering the TORQUE setting?  Sometimes in stepper motor more power is not better, as it can over-saturate the motor coils and cause distortion.  Try a lower value of it.  

    FYI the motor will likely be loud in full-step mode no matter what.  I recommend 1/8 microstep or more (1/16 1/32 etc) for best results.  See How to Reduce Audible Noise in Stepper Motors for an in-depth analysis about your question.  

    Best,

    Jacob

  • 0 in reply to Jacob Thompson
    Prodigy 30 points

    Hi Jacob, 

    Yes i have a current probe. The screenshot below is the current probe on the B- line of the Stepper motor. This reading was obtained with the following register values 

    8d39
    8056
    8032
    8100
    8510
    8a02
    8000
    8020

    My Input frequency was 3kHz and the current waveform output was shown below. I observed vibrations on the motor in an almost "Clicking" sound at 1/128

    I then reduced the torque from 86 to 50 and repeated the experiment exactly as before. My Current output looked like this now. The vibration and clicking sounds remained and i also heard an audible hissing noise.

    Finally i reduced the step resolution to a full step as in my initial post and ran the motor at 100Hz and my output waveform looked like this 

     

    to answer the questions you posed, We played around with the decay mode settings but were not able to significantly improve the output waveform. I will review the links you have shared to better educate myself of the debugging steps

  • 0 in reply to Kenneth Nwokolo
    TI__Mastermind 23421 points

    Hey Kenneth,

    Those waveforms look about perfect to me.  I believe what you're hearing is normal for a stepper motor with typical control modes like the DRV8711 has.  

    I highly recommend looking into our latest stepper motor driver the DRV8462 with Automatic Microstepping and Silent Step features.  Either of those features should result in super quiet motor operation even at Full-Step.  It also has Auto-Torque which can reduce your current when not under load.  Check out this video about it - https://www.ti.com/video/6321397814112

    Best,

    Jacob

  • 0 in reply to Jacob Thompson
    Genius 3700 points

    Hi Kenneth,

    Jacob already has given you most of the necessary information about problem possible causes and the ways of dealing with them. I will try to add something based on my experience with DRV8711.

    Looking at your motor torque curve I would expect the motor should work in microstep mode up to around 200rpm at VM=24VDC and up to around 400 rpm at VM=48VDC with no major problems.

    First video shows normal work of motor in full step at low speed below low speed resonance.

    Second video shows probably motor working at one of motor low speed resonance frequencies. There a few ways to limit vibrations and noise at low speed resonance frequencies:

    - use microstep mode - 1/8 step or higher,

    - in microstep mode peak driver current should not exceed motor nominal current, you can use peak equal to 1,41x motor nominal current if you need higher torque but at the cost of higher vibrations.

    - try different decay settings (in my case Auto mixed decay was the best in most cases)

    - try to load motor shaft,

    - use viscous dampers,

    - try to replace motor with another one.

    First I would use DRV8711EVM board together with program provided by TI and try to find proper settings and then transfer them to your MCU program.

    I would try to do the following steps:

    - set microstep mode,

    - set current to around 50% of motor nominal current, you can set gain to 20, 

    - check with current probe if motor current value is close to set value at low speed, 

    - increase slowly motor speed from zero to around 300 rpm for VM=24V or 600 rpm for VM=48V and note down any speed when vibrations occur and motor max speed if it stalls.

    - try to play with driver settings and use tricks I mentioned above to limit vibrations and noise at low speed resonances.

    - if motor works OK and you need more torque increase gradually driver current up to motor nominal current and observe motor behavior. 

     

    Regards,

    Grzegorz