Current measurement

Christopher Flesher

I'm trying to measure the current from the InstaSpin board. I'm getting a total current measurement Is_A as follows:

  // read Id and Iq vectors in amps
  gMotorVars.Id_A = _IQmpy(CTRL_getId_in_pu(ctrlHandle), _IQ(USER_IQ_FULL_SCALE_CURRENT_A));
  gMotorVars.Iq_A = _IQmpy(CTRL_getIq_in_pu(ctrlHandle), _IQ(USER_IQ_FULL_SCALE_CURRENT_A));

  // calculate vector Is in amps
  gMotorVars.Is_A = _IQsqrt(_IQmpy(gMotorVars.Id_A, gMotorVars.Id_A) + _IQmpy(gMotorVars.Iq_A, gMotorVars.Iq_A));

I am converting the Is_A IQ value by right shifting 24 bits.

I have a current sensor connected that is measuring input current to the board and there is some discrepancy. There seems to be a second order polynomial fit between the Is_A and measured values. I am looking for some insight into what the units of Is_A are and why the second order fit?

over 10 years ago

0 ChrisClearman over 10 years ago

TI__Guru** 119510 points

the currents - like many of the variables - are in IQ24 format

this thread may be of interest to you
e2e.ti.com/.../1441564

0 Christopher Flesher over 10 years ago in reply to ChrisClearman

Prodigy 115 points

Thanks, that helped a lot. I am calculating the voltage (in mV) and current (in mA) as follows:

  uint16_t voltage = gMotorVars.VdcBus_kV/16.777216;
  int16_t rpm = gMotorVars.Speed_krpm/16777.216;
  int16_t torque = gMotorVars.Torque_Nm/16777.216;
  float_t input_power_sf = _IQtoF(gAdcData.dcBus) * (float_t)(0.5 * USER_IQ_FULL_SCALE_VOLTAGE_V * USER_IQ_FULL_SCALE_CURRENT_A);
  float_t input_power = input_power_sf * _IQtoF(
      _IQmpy(gPwmData.Tabc.value[0], gAdcData.I.value[0]) +
      _IQmpy(gPwmData.Tabc.value[1], gAdcData.I.value[1]) +
      _IQmpy(gPwmData.Tabc.value[2], gAdcData.I.value[2]));
  uint16_t current = input_power/((float_t)voltage)*1.0e6;

Now the measured current seems pretty close to the reported but maybe there is still an offset by ~0.1 - 0.2A. I haven't looked into this very hard yet so not sure if there is still a discrepancy or not.

0 Richard Cooke over 10 years ago in reply to Christopher Flesher

Expert 1530 points

Christopher,

This is probably a dumb question but where does the 16777.216 factor come from in the RPM calculation?

Thanks,

Richard C

0 Christopher Flesher over 10 years ago in reply to Richard Cooke

Prodigy 115 points

It's not a dumb question, my implementation is probably dumb. I was digging through the GUI code and found they were right shifting by 24 bits and multiplying by 1000.0 to convert several of the gMotorVars into displayed values. (1<<24)/1000.0 = 16777.216. I think the way you are supposed to do this is to use the _IQtoF function somehow.

0 Andrew Buckin over 9 years ago in reply to Christopher Flesher

Expert 2130 points

Hi ChrisClearman. is It peak to peak current?

0 ChrisClearman over 9 years ago in reply to Andrew Buckin

TI__Guru** 119510 points

is what peak to peak current?

USER_ADC_FULL_SCALE_CURRENT_A is peak-peak

in the original post he is looking at Is = sqrt (Iq^2 + Id^2), which is the current in the phase of the motor, but some of this is recirculating in other phases. You can't relate it to the bus current.

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Current measurement