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DRV8308EVM: Closed loop speed control

Part Number: DRV8308EVM
Other Parts Discussed in Thread: DRV8308,

For close loop control, DRV8308 EVM GUI, section 3.2 Quick Guide to Spin With Speed Control was followed. The following problem was found

  1. In the instruction On All Registers tab, I am not able to select the SPDMODE bits to 00 (FREQ) Mode. When I set it to FREQ mode it sets to PWM mode by itself.
  2. The speed control is not working. The RPM does not remain consistent upon giving load. 
  3. The instruction provided in the last line of Section 3.1 was found untrue.

Please guide how can we operate the BLY171S-24-4000 (provided with EVM) in close loop control mode.

  • Hello Tejas,

    FYI - The DRV8308 closed loop operation is disabled in PWM and REGISTER mode.

    It sounds like SPDMODE is not being properly set to FREQ mode. Can you describe in more detail how the DRV8308 is setting back to PWM mode by itself?

    Thanks,

    Matt

  • The tab 3 has SPDMODE selection option. I am trying to select 0-FREQ. It is automatically going back to 1-PWM as shown in the image.

  • EVM application does not works. However this link works in  "Use HALL_U to generate FG" only. Other FGSEL is not working. dev.ti.com/.../

  • Hi Tejas,

    Have you updated GUI Composer to the latest version, and can you confirm what the GUI says in the black bar at the bottom left of the screen?

    Can you also send a picture of the EVM so I can check all of the jumper configurations?

    Thanks,

    Matt

  • Hi Matt,

    Thanks for the reply. I am having the issue with close loop speed control at 1500 RPM with BLY171S-24-4000 and 750RPM with my motor. I can't adjust the Speed of either motor. It has been fixed to a specific value. Why so? How can I change speed and let Driver operate in constant speed control close loop mode?

  • Hi Tejas,

    You can you send me all of your settings so I can confirm? 

    I have seen that too low SPDGAIN results in the motor not being able to ramp to target speed.

    Thanks,

    Matt

  • Please see the attached settings

  • Hi Tejas,

    Reviewing the tuning guide section in the EVM user guide (LINK) gives me the following insights:

    • The SPEED register should be set to 4000. This is the open-loop gain during spin-up, before LOCKn goes Low. It simply must be high enough in order to reach the commanded RPM.
    • MOD120  should be set to 3970.
    • LOOPGAIN - This gain primarily affects max RPM, max torque, and startup speed overshoot. If it’s too low, the motor won’t spin up to the commanded RPM, or the torque capability may be limited. If it’s too high, motor speed will initially overshoot

    Thanks,

    Matt

  • Thanks Matt, We tried above. Turns out the Max CLK Frequency from MSP430 is 100Hz. we connected external CLK Generator, we could achieve the RPM. Is there any way, we can exceed this limitation on EVM?

  • Hello Tejas,

    Sorry for the delay in response here. 

    The EVM firmware is fixed, and I don't think the MSP430 on board will have the horsepower to exceed 100Hz with the duty cycle granularity we are going for.

    You would have to tweak the DRV8308EVM firmware (MSP430) to make any changes to this limit,

    Thanks,

    Matt