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TMS320F28027F: Instaspin FOC current sensing

Part Number: TMS320F28027F
Other Parts Discussed in Thread: TMS320F28027, DRV8305

HI,

I'm using instaspin FOC labs along with TI EVMs for TMS320F28027 and DRV8305. This is working well and I'm building a custom version, but due to shortages the DRV8305 I have is the version with 5V internal regulator.

This changes the DC offset on the current sense amplifiers output. With the EVM the original dc bias on the amplifier output was 1.65V (middle of the full scale voltage of 3.3V), but now the dc bias will be at 1.25V.

Some questions from this:

  1. This should not be an issue since the estimators compensate the ADC measurements using the measured DC bias right?
  2. Full scale current should also remain the same since that is peak to peak right?
  3. Since the dc bias will not be centered, the measurable current in one direction will have a smaller range within the 3.3V scale. Why is it needed to measure current in both directions?
  4. Most of the time current will be flowing from motor phase to ground. Is it ok then to have less measurable range for currents flowing from ground to the motor phase (if any at all)?

  

Thanks,
David

  • This should not be an issue since the estimators compensate the ADC measurements using the measured DC bias right?

    Yes, but the valid sampling range will be reduced.

    Full scale current should also remain the same since that is peak to peak right?

    Not peak to peak. It's still equal to 3.3V/Gain/Rs.

    Since the dc bias will not be centered, the measurable current in one direction will have a smaller range within the 3.3V scale. Why is it needed to measure current in both directions?

    The phase current is bidirectional with negative value, so need the bias offset to convert the current sampling value to 0.0V~3.3V for the ADC input of the controller.

    Most of the time current will be flowing from motor phase to ground. Is it ok then to have less measurable range for currents flowing from ground to the motor phase (if any at all)?

    The phase current is bidirectional, so the best bias offset should be half of the maximum ADC input (3.3V) for achieving the highest resolution.

  • Thanks, I'll select Rs such that I can still measure the peak current either negative or positive.