ST MC workbench Current sense

[chriswalck9]

Posted on September 24, 2015 at 19:54

Hi,

I am using ST MC Workbench to design a BLDC FOC motor driver, and am running into a problem.  I am using three shunt resistors to sense current (one per channel) and am trying to use ''embedded PGA'' as an amplification method.Here are my pertaining specs-Nominal current of 25A-Processor nominal voltage of 3.3V-I am using 0.04 ohm shunt resistor.  (actually a 0.001 ohm shunt resistor with external amplification gain of 40 such that the circuit output resembles a 0.04 ohm shunt)-Internal gain of 2-overall network gain of 2.0Vout(polorization) of 0.0VWhen I click ''generation'' I get the following error message.''Nominal Current Higher than maximum supported by sensing network (20.625A)''I am confused by this because at 25A, the voltage across a 0.04 shunt should be 1V. Then with a 2.0 PGA gain, the ADC's would see 2V at my nominal current.  This shouldn't be a problem because the embedded PGA is a rail-to-rail amplifier and should be able to operate with this voltage range.  The suggested max sensing range of 20.625 suggests a max PGA input voltage of 0.825V (1.65V at the ADC, i.e. half of its range at 3.3V supply)I'm just wondering, am I missing some setting in the software that's only allowing use of half the ADC voltage?  Does anyone know why MC Workbench may not like this configuration?thanks,Chris

[Gigi]

Posted on October 05, 2015 at 17:51

Ciao Chris

The formula is not just I * Rshunt * Gain This because the current network need to sample also the negative current and so an offset of Vdd/2 (1.65V) is added at the input of the ADC. Take a look at the current feedback network to understand how it works. The WB is calucalting the maxium readable current according the current feedback topology. Ciao Gigi

[Laurent Ca...]

The question has been moved from the "Motor Control Hardware" section to the "STM32 Motor Control" section (the question is about the STM32 MC SDK). 

Best regards