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DRV8301-69M-KIT: Output data translation

Terry Harrison
Intellectual 600 points
Part Number: DRV8301-69M-KIT
Other Parts Discussed in Thread: MOTORWARE

I am not clear on using the IQ library but if someone can give me a checkpoint hopefully it will clear it up. I am running the Instaspin FOC and one of the values that is measured in the motor calibration is the series resistance of the BLDC motor I am using. The value comes in for most of the motors at about 0.01479 ohms.  I take this value and:

myvalue = _IQ(gMotorVars.Rs_Ohm)

Which gives me a hex value of 0x0003c322. 

I take this value and put it in an array

mtuResponse[12] = (uint16_t)(offsetRs >> 24) & 0xFF;
mtuResponse[13] = (uint16_t)(offsetRs >> 16) & 0xFF;
mtuResponse[14] = (uint16_t)(offsetRs >> 8) & 0xFF;
mtuResponse[15] = (uint16_t)(offsetRs) & 0xFF;

I then send this out on the serial port to my host laptop. 

The default in the motorware is IQ(24) which means the resolution is 0.000000060

I take the 0x0003c322, convert to decimal and then multiply by 0.000000060 to arrive at a readable display value for our test set. It is very close to the value I can read on the Code Composer Studio display but it is consistently higher value. If I right click in CCS and ask it to display the value by translating the IQ value it is almost perfectly the same value so I am guessing I am missing something in the conversion. Can someone point me to what I am doing wrong?

If I am trying to make this hard for getting values out of the serial port I would love to know a better way also.

Thanks

  • 0
    Intellectual 600 points

    Thanks for the link but it did not help much. I had looked through that document and have been able to use the IQ library within the TI code. My issue is that I need to get values out of the TI device via a serial link and be able to read them on a display computer. I understand that the global default is for IQ24 so the range is between -128 and 127.999 999 940 with resolution of 0.000 000 060. As I noted above, I am reading the value as a float, converting to the IQ value and then to split that value into bytes that I can send out the serial port, I am doing the shift operations as noted above. All of that is working. On the display end I am converting the 4 bytes back to a 32 bit hex value, converting to decimal and then multiplying that value by the resolution to obtain a decimal value that is human readable on a display. The value is very close, but it seems to be consistently higher, and I am looking to find out why. As a calibration as I explained above, I look at the values displayed in the CCS vars window and by switching the numeric format displayed I can see the float value and the IQ value and using the same IQ value in my display laptop yields a higher float value. My question is simply how do you manually translate an IQ value without being able to use the TI libraries such as in a test setup like I am describing. Is the roundoff buildup to blame - it seems a bit high if so - or is there some other way to obtain a translated IQ value for example in a spreadsheet?

    Alternatively, is there some better way to send values out the serial port that are measured as float values in the TI code? Breaking a hex number into bytes and sending it via serial is fairly standard I think which is why I did this technique. Any help would be greatly appreciated.

    Thanks,

    Terry

  • 0 in reply to Terry Harrison
    TI__Guru** 109810 points

    Seems like this is not related to the IQmath and the example labs of motorWare. You just need to know the data type of the variables, and convert the 32-bit IQ or float vairables to a data format per your own serial communication protocol. You may try to multiply a scalar value to convert the small/large value for sending the data out the serial port.