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DRV8811EVM Question

Mike Tucker
Prodigy 90 points
Other Parts Discussed in Thread: DRV8811

I have been experimenting with the DRV8811EVM.  I have a question about the Current Control parameter on the GUI.  I have the control set at 1345 (displayed number when you drag the pointer on the screen).  My 12 VDC power supply has an ammeter on it and sometimes the draw is about 1/2 amp when the motor is operating and other times it is much less (barely shows on the ammeter).  The stepper motor always seems to be operating consistently although I see two considerably different current draws.  This same behavior occurs sometimes when the motor is at rest--sometimes the ammeter shows about 1/2 amp and sometimes it barely registers.  It is almost as if the ammeter is intermittent because the pointer still says 1345.  I have tested the torque for both conditions (1/2 amp reading and low current reading) and there appears to be no difference in motor performance.  When the ammeter is reading about 1/2 amp and the motor is at rest, the current drops slightly when the motor is running.  I have touched the DRV8811 chip and its temperature "feels" about the same for both cases (not hot at all).  Can someone educate me on this?

I am using full-step mode, decay control is set to 0, Pulses Per Second is set to 700, and I am using the "Move Steps" function.  Power supply is 12VDC.  My stepper motor is an Applied Motion # HT17-275 http://www.applied-motion.com/products/stepper-motors/ht17-275 connected in the "bipolar series" configuration.

Thanks in advance.

  • 0
    TI__Mastermind 26065 points

    Hi Mike

    Sounds strange. if you found motor torque no difference at both normal current reading and bare current reading, maybe there is something related to you ammeter sample rate and display refresh rate. It is possible because the power supply current will have big change for temporary during the motor running and stop as the VM cap always be charged and discharged greatly. 

    Instead of power supply current, you can check the motor winding current waveform with current probe. 

    Unless the driver is reset, disable or sleep, the current will always excited. If you can found much holding torque in motor, there should be holding current on the motor phase.

    Besides, there will be another scenario of different current level. If the motor is driver by micro stepping like 1/8, the winding current will be sinusoidally regulated. There will also be zero current time or MAX current time for each phase.

    Thanks.

    Wilson

    Motor Application Team

  • 0 in reply to Wilson Zuo
    TI__Guru 55655 points

    Mike,

    In full step, your current waveform should look similar to below.  Notice it is bi-directional current. 

    This is showing 1 phase and the step input is shown in channel 1.  The other phase is 90 degrees out of phase.