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In AN3206 "PMSM vector control..." how can u_ab be {0, 0} and u_dq {non_zero, 0} in align_start state?

Question asked by Peter Dufault on Jun 25, 2014
Latest reply on Jun 26, 2014 by Peter Dufault

This is a follow up to my previous question about scaling.  After a day of looking into this I'm still confused, but I'm pretty sure it's scaling.  I have a non-zero value for u_dq.d but a zero value for u_ab.alpha when in the align_start state and the PWM stays at 50%.

  • phase_current_range_ma is 3125 (-3.125A=0V, 0A=2.5Vref, 3.125A=5V)
  • dc_bus_voltage_range_mv is 25000 (0V=2.5Vref, 25V=5V)
  • All ADCS are set to sign-extend.
  • ASAC bit shift is set to shift left by 10.

When I go into align_start I set i_dq_desired.d to about 100mA.  My current rail is 3.125A, the value I see for i_dq_desired is d=262136 i=0 (262136/(0x7FFFFF/3.125)=.098).  So that is OK.

The value I see for u_dq is the same (262136).  I'm confused about that, since looking in the eTPU code the comments for the PMSMVC_PID_OFF case say:

/* u_dq.d = i_dq_desired.d / 4; */

/* u_dq.q = i_dq_desired.q / 4; */

so I expect u_dq.d to be 262136/4=65534.  Is the /4 to convert from 1.23 to 3.21 format?

In either case, I have a non-zero u_dq.d.  In the PMSMVC_PID_OFF case the eTPU code goes directly to PMSMVC_INV_PARK_TRANSFORMATION.  This does:

         MC_TRAN_PARK_INV(u_ab, u_dq, sin_cos);

        /* u_ab values contain quarters of the actual values (format 3.21) */

Since we're stationary MC_TRAN_PARK_INV should be a form of identity transform.  The position is zero, theta = *p_qd_pc * encoder_scale is zero as well, and the output should be the input value times 1 in fract24 1.23 format.  Is that correct?  I assume the call converted it back to 3.21 format.


In desperation to see something happen I disabled reading the bus rail and set it to zero, assuming the ripple eliminator would see the values above the bus rail and return +/- 1.  Sure enough, when I go into the align_start state it slams to a fixed position.  So something is screwy with my units.

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