AD2S1210 resolution setting

Hello

             Customer would like to meet 16000rpm ( 266.6rps). Do you have any suggestion or comments for 12bit or 14 bit resolution that more suitable for this specification?  It looks max rps of 12 bit and 14 bit should meet 266.6 rps. Thank you. 

 

Xtal:8.192Mhz

Pole pair: 4

Speed: 16000rpm (266.6rps)

       

BR

Patrick

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  • +1
    •  Analog Employees 
    on Nov 30, 2020 7:33 PM 2 months ago in reply to patrickchen@morrihan.com

    Patrick,

    Can you confirm what the settings for the encoder interface so that I can do some math to see if we aren't running into what I would characterize as a quantization error related to the master clock?

    To confirm lets take a case where we see the issue, like at 60000 RPM.

    Can you please confirm your use conditions,  from what I recall.

    • Resolution (POSITION/VELOCITY REGISTERS): 12-Bit
    • Master Clock:  10.24 MHZ
    • When we state 60000 RPM we mean electrical revolutions, i.e. the input to the RDC, not the rotation rate of the mechanical system.

    Thanks 


    Sean

  • Hello Sean

              Yes. The condition above is correct. But RPM is 64000 RPM.  We check with customer and they modify encoder resolution from 12bit to 10 bit and they get better signal quality that has less variation. But if run 64000 RPM, customer will read 16000 RPM for 1/4 time RPM via encoder interface. Do you have any comments for it?

                Do we have any encoder resolution versus Max RPM (RDC input) relationship formula or doc.?  Thank you. 

    BR

    Patrick

  • +1
    •  Analog Employees 
    on Dec 1, 2020 6:30 PM 2 months ago in reply to patrickchen@morrihan.com
    But if run 64000 RPM, customer will read 16000 RPM for 1/4 time RPM via encoder interface. Do you have any comments for it?

    By reducing the resolution to 10-bits the effective weight of each pulse transition( angular position change per edge)  increases by a factor of 4.  Thus for a given unit of time the number of pulses measured *4 should be equal to the same measurement range observed at 12-bit resolution.   Thus the customers reading of 16000 RPM in 10-bit mode is in fact still 64000 RPM.   

          Do we have any encoder resolution versus Max RPM (RDC input) relationship formula or doc.?  Thank you. 

    BR

    The incremental encoder is a function of position in that we observe a single edge transition (on either A or B) for every 360/2^RES degrees; for short we'll refer to the distance between sequential pulse transitions as R, for the rotational displacement.  Thus the pulse width of either A or B should be equal to 2*R/time and would be representative of the angular rotation velocity.  Note that the digital is limited to change at a rate that is a function of the master clock, thus the time resolution is 1/MCLKIN which will be in the range of 122ns.  Thus depending on the rate of rotation an error in the pulse width of up to about 66ns could be observed in the velocity.

    So utilizing 64000 RPM and 10.24MHZ (48.8ns)

    64000 RPM ~= 1066 RPS = 272,896 pulses per second (at 10-bits) which should be approximately 1.8us pulse width.  Thus with a time resolution error of 48.8ns would be about 2.7% maximum.  It then makes sense that at 12-bit the same error would result in about 10% (4*2.7 = 10.1%) as the period of the pulses at 12 bits would be 1/4 that of 10-bits.

    So in the end my suspicion of the "quantization" effect of the master clock at high frequency in the end was in fact accurate.   

  • Hello Sean

           Customer test 10 and 12 bit Encoder resolution @ 64000RPM. But the 12 bit resolution error about 10% and the duty cycle unbalance.

    So customer can’t use 12 bit @ 64000RPM, right ? Could you math method to figure out  what’s the max speed support by encoder resolution ?

           And how to calculate 48.8ns time resolution error by 10.24MHz CLKIN?  How to get 272,896 pulses per second (at 10-bits) for 1066 RPS ? 

    Thank you. 

    Encoder A signal Resolution 12bit

    Encoder A signal Resolution 10bit

    BR

    Patrick

                  

  • +1
    •  Analog Employees 
    on Dec 4, 2020 12:56 PM 2 months ago in reply to patrickchen@morrihan.com

    Patrick,

    Based on my computations the encoder interface maximum rate will be limited by the internal tracking rate limit and thus at 10 bits and 12 bits respectively the maximum velocity would be 187,500 rpm and 75000 rpm.

    Sean