PWM controlled motor current measurement

Hello Pololu People,

Wondering if someone here may have experience with measuring PWM controlled motor current? We are using the the 50:1 Metal Gearmotor 37Dx70L mm 12V with 64 CPR Encoder (Helical Pinion) - Pololu item #: 4753. The motor is coupled with the Dual TB9051FTG Motor Driver Shield for Arduino - Pololu item #: 2520. I have been evaluating the ACS711EX Current Sensor Carrier -15.5A to +15.5A ( Pololu item #: 2452), and able to measure static, non-PWM, current accurately. However, with PWM controlled motor operation, the measured current is erratic given the obvious nature of PWM.

I see that a RC low-pass filter may be necessary to average the PWM controlled current. Perhaps this may be further added to the ACS711EX Current Sensor VOUT? I noticed that the ACS714 Current Sensor Carrier -5A to +5A (Pololu item #: 1185) mentions a RC low-pass filter already built into the design, perhaps this may be an option to consider? Wondering what you may recommend for a PWM controlled motor current sense solution? Is there a hardware solution available or is extra circuit building required? If possible, I would rather buy a device which incorporates all features rather than custom circuit making, especially as we may be offering many PWM motor controlled systems in the future, so a lower part count is best.


Hello, Sean.

Even if you use an RC filter for noise reduction, I would still expect that monitoring the current draw of a motor controlled by PWM will typically involve some amount of averaging current readings with your microcontroller to get useful measurements, though it might depend on your PWM frequency and the characteristics of your motor. As you mentioned, some of our current sensor carriers, like the ACS714 or the ACHS-7121, already have an RC filter as part of their circuit, and those boards also have pins where you could add a capacitor to adjust the board’s bandwidth by adding more capacitance to the filter. For others, like the ACS711EX, you would have to add that yourself. If you do not know much about how RC filters affect signals, this “RC Circuit” Wikipedia article might be a good place to start.

- Patrick

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