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TMCS1101: Field oriented control. Current sensor selection

giorgio rancilio
Prodigy 10 points
Part Number: TMCS1101
Other Parts Discussed in Thread: INA240

Hallo,

I am selecting the current sensor for a FOC of SMPM.

The PWM frequency is 32kHz. DClink voltage 48V. The current is sampled each 32.25 uS.  The output frequency up to 500Hz.

I am evaluating the TMCS1101.

In FOC the sampling instant must be accurate and sit in the middle of the two commutation to assure precision.
I am wondering if this chip, which update its output each 4us, can impact on the current reading introducing subharmonics or some other effects.

Which is your opinion?

Thank you

  • 0
    TI__Mastermind 20315 points

    Giorgio, 

    The question here would be the time the transient has to work itself out vs the time that the sample is captured. Given that the step of the TMCS1101 as shown is about 8us, and your motor is spinning at around 500Hz, I would think that the output would have more than enough time to settle inside the sampling window to achieve an accurate sample. If this is of concern, you could also have a look at the INA240, which allows for inline current sensing with its PWM rejection feature. 

  • 0 in reply to Carolus Andrews
    Prodigy 10 points

    Andrews,
    thank you for your answer.
    The output frequency is 500Hz but the switching frequency is 32kHz. I have to update the current reading for the control at this frequency.
    Based on the motor the current can reach a slope of 0.2A/4us.
    I see two issues:
    one is due to chip which update its output based on its last 4us. The output values is shifting according two un-synchronized time-bases.
    the second one is when my on-off time goes below 8us. In this case I am going to loose the information about this period.
    Because all the sampling theory of this control is based to the synchronized sampling and I had experienced in the past sub harmonics due to sampling error I am afraid about this. On the other side I am attracted by the other features.

    Did you use it in this kind of applications?

  • 0 in reply to giorgio rancilio
    TI__Mastermind 30460 points

    Hello Giorgio,

    Today is a holiday in the US.  We will look into this and get back to you.

  • 0 in reply to giorgio rancilio
    TI__Mastermind 20315 points

    Giorgio, 

    I understand what you are saying now. We do indeed use these sensors in these types of applications, and I have found something that should help with the setup here. We actually have a TI design demonstrating the capability of the device family here, with the PWM provided by TIDA-00915, which is capable of up tp 32kHz through 85C.

  • 0 in reply to Carolus Andrews
    Prodigy 10 points

    thank you for your answer.
    I am going to test it.

    In the meantime I found an article which explain my fear. Its title is "Optimized Sigma-Delta Modulated Current Measurement for Motor Control" on https://www.bodospower.com/ jan 2022.