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Original question:

MCF8316A: Questions regarding datasheet

MCF8316A: Output resolution

Hideki Hayashi1
Expert 8400 points
Part Number: MCF8316A

Hi team,

I was asking about output PWM resolution in the original thread. But I was not able to report customer's concern correctly.

What customer wants to do is to adjust <1rpm in output rotation in maximum rotation <2000 rpm. Input resolution is 13 bit so customer feels good as input (2000 / 8192 = 0.24 rpm). However, regarding output PWM, they doesn't satisfy my explanation.

I explained as below.

Fig 7-13 shows how speed command apply to output PWM.

PWM -> PWM Duty: 13bit from E.C.

PWM Duty -> DUTY CMD: Linear (= 13 bit) from Fig 7-15

DUTY CMD -> SPEED REF: Linear but threshold is 8 bit value (= 13 bit or 8 bit?) from Fig 7-16

SPEED REF -> DUTY OUT: Depends on FOC algorism, so I don't figure out about resolution from Fig7-29

Could you please advise how to answer to customer for proceed our solution to customer?

Best regards,

Hayashi

  • 0
    TI__Mastermind 24415 points

    Hayashi,

    Is the customer asking for the motor speed accuracy? If the customer wants to know the speed accuracy, Speed loop accuracy in MCF8316A is 3% with internal clock and 1% with external clock reference.

    Regards,

    Vishnu

  • 0
    TI__Mastermind 24415 points

    Let me check with the team on the output resolution and get back to you. 

  • +1 in reply to Vishnu Balaraj
    TI__Mastermind 24415 points

    Output speed is 15 bit register.

  • 0 in reply to Vishnu Balaraj
    TI__Expert 8400 points

    Hi Vishnu

    I understand output speed information is stored in 15 bit register. I would like to ask how 13 bit PWM input is degraded while internal conversion.

    Other than that, you mentioned speed loop accuracy is 3% with internal clock. If it is true in any situation, the resolution of output speed will be wasted since 3% variation is about 5 bit (100/3 ≈ 2^5) (For example, customer wants 1500rpm and 1502rpm in another case. If 1500rpm setting could vary between 1455~1545, high resolution adjustment is wasted.)

    I would like to ask whether it is a max value or typical at first, Then, if max, what is the considered situation and what is typical value?

    FYI, customer's motor is 8-poles, 12 slots.

    Best regards,

    Hayashi

  • 0 in reply to Hideki Hayashi1
    TI__Mastermind 24415 points

    Hideki,

    I didn't quite understand your question. Output speed has better resolution than the input PWM resolution which is good. I don't think the 13 bit PWM is degraded. Can you please add more details to your question?

    Output speed resolution is motor parameter dependent. We have seen motor speed accuracy to be <1% with internal clock for certain motors. So, customers should tune their motor and check for the speed accuracy. 

    Regards,

    Vishnu

  • 0 in reply to Vishnu Balaraj
    TI__Expert 8400 points

    Hi Vishnu,

    I agree what you mentioned a little but I'm not clear as well...

    It is difficult to explain, but I will show you an example. For example, if you can only operate on two significant digits, but you get an output of three or more digits, that value is not really controllable.
    If the input only accepts steps of 01,02,,,10,11,,,20,,,99, but the output is 10.102 or something like that, surely the 0.102 part is uncontrollable and an error from the actual expected output. Of course it could be 10.000, this is good, but output PWM duty changes in every pulse, so there might be difference. And also DUTY CMD -> SPEED REF conversion uses 8 bit value as the threshold.

    If you do not understand or agree, that is no problem.
    If the speed accuracy depends on the motor and tuning as well, we have already delivered an EVM to them, so I will ask them to consider customer's evaluation at EVM so that they can make a decision.

    Best regards,

    Hayashi

  • 0 in reply to Hideki Hayashi1
    TI__Mastermind 24415 points

    Let me get back to you next week. 

  • +1 in reply to Hideki Hayashi1
    TI__Mastermind 24415 points

    Hideki,

    Speed accuracy depends on motor parameters and tuning. I would recommend the customer to refer to the tuning guide and let us know if they see any issues in meeting the speed accuracy requirements.

    Regards,

    Vishnu