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ST MC workbench Current sense

chriswalck9
Associate
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 2.0

Vout(polorization) of 0.0V

When 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
2 REPLIES 2
Gigi
ST Employee
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...
Lead II

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