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Encoder selection and User parameters

Tamil Kadir Rajavel
Intellectual 945 points

Hi,

I would like to run a PMSM with 48 poles at 2500 RPM, which is 1kHz electrical frequency. Due to mechanical reasons and for the need of high precision, we are considering an 18 bit absolute encoder, which has a maximum repetition rate of 4kHz and a fixed latency of 250us plus communication latency.

I have three questions:

1. What would be the optimum ISR, CTRL and SPEED ticks/frequency for operating the motor at 1kHz electrical frequency? 

2. Is 4kHz position update rate enough to operate at 1kHz electrical frequency? 

3. Is there a way in InstaSPIN to compensate for the encoder latency?

I would really appreciate your comments.

Thanks,
Tamil

  • 0
    Genius 5885 points

    Hi,

    1. for proper torque control in order for minimal torque ripple I would go with 20 kHz or higher 

    2. no really

    3. at 1kHz electrical frequency you also have to compensate for control loop latency.

    Do you really need 18 bit encoder? This gives you 0.0013 ° mechanical accuracy or 0.0329 ° electrical accuracy.

    1° accuracy electrical is usually enough for control of the machine (torque, speed and position)

    Regards, Mitja

  • 0 in reply to Mitja Nemec
    Intellectual 945 points

    Hi Mitja,

    Thank you very much for your comments. I was wondering if you could clarify the following:

    1. Did you mean a 20kHz or higher CTRL frequency?

    2. What do you think must be a reasonable position update rate for 1kHz electrical frequency operation?

    3. Does InstaSPIN have provision for compensating for these latencies? 

    We were considering 18 bit encoders for being able to use more aggressive control gains and to achieve better accuracy in position control, which is another mode we want this same setup to operate in. I think we might have to look for faster encoders with lower resolution.

    Thanks,
    Tamil

  • 0 in reply to Tamil Kadir Rajavel
    Genius 5885 points

    1. Yup. Control frequency should be 20 kHz or higher. Obviously PWM frequency should be integer multiple of this value

    2. 20 kHz or higher. For this kind of applications I prefer absolute encoders which also have quadrature output. You use absolute information only at initialization to properly offset values from QEP. At run-time, you only use QEP, which gives you angle information without significant delay. I've also seen prototypes of absolute encoders, which had only quadrature interface. Absolute information was sent at power on sequence, when the sensor would generate proper quadrature signal sequence to offset the QEP unit.

    At such high frequencies you have to put some attention to quadrature signal physical limits (interface circuitry, slew rate, ...).

    3. I do not know as I haven't used InstaSPIN yet.

    If you really require such high performance sensing, I would go with two step solution. They are common using resolvers.

  • 0 in reply to Mitja Nemec
    Intellectual 945 points

    Thank you very much for your inputs Mitja, I really appreciate it.

    -Tamil