Using MC_GetPhaseCurrentAmplitudeMotor1() function, wrong current

[DTort.1]

Hi, I use B-G431B-ESC1 board and Motor SDK 5.Y.3 and am trying to determine the phase current for my application. I am calling the function

I_Phase = MC_GetPhaseCurrentAmplitudeMotor1 ();

but when the motor is running and the current is almost zero, the function returns a large value. About 2600 units. Could it have something to do with the polarization scheme. In the B-G431B-ESC1 board, the polarization voltage is 2.057V, which is more than 1.65V.

I also noticed that 2.057V is 2553 units in a 12-bit ADC, although this may not be necessary and the problem lies elsewhere.

In any case, I don't understand why I get a number around 2600 at almost zero real current.

I have attached a link to a similar question, but there is no answer there.

https://community.st.com/s/question/0D53W0000081i95SAA/calculation-of-the-motor-current-using-the-motor-control-sdk-function-mcgetphasecurrentamplitudemotor1

[cedric H]

Hello @DTort.1​ ,

With the release SDK5.Y release, comes the new motor Pilot tool that allows you to log the data coming from the FOC algo without any loss.

If you want to check the current, you can plot the current waveform.

In the motor Pilot you will find a "Graph and Record" frame, if you click on the "Graph" button a high frequency plot window will pop up, and from there you can select to plot Ia and Ib, (you can also record them if you want to do post processing).

As 0 Amp means 2553 units, I am not surprised that "almost zero real current" leads to "around 2600".

Regards

Cedric

[DTort.1]

It turns out that the "Motor pilot" application returns a completely correct zero current value in s16A units, but the MC_GetPhaseCurrentAmplitudeMotor1(); behaves strangely. I would agree that this is a polarization of 2.057V, but the function should return not a 12-bit ADC value, but a 16-bit value, that is, there is some processing up to the s16A format.

Here's from the description:

/ **

* @brief returns the amplitude of the phase current injected in Motor 1

*

* The returned amplitude (0-to-peak) is expressed in s16A unit. To convert it to amperes, use the following formula:

*

* @f [

* I_ {Amps} = \ frac {I_ {s16A} \ times V_ {dd}} {65536 \ times R_ {shunt} \ times A_ {op}}

* @f]

*

* /

To be honest, I haven’t climbed into the code of this function yet, and I don’t really want to. Perhaps there is another way to get the correct current value inside my application? Any signed number format would have tripled me.

[DTort.1]

And one more thing that I do not understand when measuring current is why the measured current is so large, about 15000 units, although the power is just over 100W. My application is designed for 28 Amperes of maximum current and it seemed to me that the number 15000 should correspond exactly to 28A if I indicated it in the engine settings. This number in file drive_parameters.h

#define IQMAX 15246

It really depends on the set current of the motor and it will be really true about 15000 for a current of 28 Amperes according to the formula given in the previous message, but I am sure that in reality I do not see this power even close.

Explanation to get a high current at zero speed I am blocking the motor. And I get about 114 watts, which is about 3-4 amperes.

The second moment when the current rises I start to get a Bad CRC Hader error and this knocks down the control through the motor pilot.

[DTort.1]

Oh, I found exactly the same question with exactly the same results as mine. There is also no answer in this thread, but the main question is not even why the current function does not work quite correctly, but why the current is generally measured incorrectly, and a large current leads to data transmission failure.

https://community.st.com/s/question/0D53W00001CNP6mSAH/problem-with-current-reading-in-stm-mc-5y3-evspin32g4-and-bldc-and-abi-encoder

[cedric H · Accepted solution]

Hello @DTort.1​ ,

We found an issue in the implementation of the MC_GetPhaseCurrentAmplitudeMotor1().

We wrongly use the HW cordic module to compute the sqrt of the modulus, but as specified in the data sheet, the result is Ok for input value greater than 0.027 only.

We are preparing a 5.Y.4 version and we will replace the call to the cordic sqrt to a call to the cordic modulus function which is defined between [-1,1].

Thanks for reporting the issue

Cedric