Field-Oriented BLDC Motor Controller with Custom Inline Current Sensing

We built a field-oriented controller for a brushless DC motor as a two-person course project, writing the full pipeline in C on a bare-metal STM32 Nucleo F411RE: Clarke/Park transforms, cascaded PI position and current loops, and software space-vector PWM. We evaluated the Arduino-based SimpleFOC library early on and decided against it over timing reliability, which meant owning every layer ourselves.

Current sensing

Our DRV8313 driver board gave us no access to the low-side current path, so current had to be measured on the high side, where a differential signal of a few millivolts rides on a 12 V common-mode swing. This is widely considered the hardest sensing topology to get right. We designed and breadboarded a three-stage front end for it:

1. An instrumentation amplifier to reject the common-mode voltage
2. A combined gain and low-pass filter stage, with cutoffs around 5 kHz and 10 kHz
3. A buffered level shifter to center the signal in the STM32 ADC’s 0–3.3 V range

We also made the 0.1 Ω shunt resistors ourselves, trimming wire length under a 1 A test current and reading the drop with a DMM in voltage mode. (We worked out beforehand that voltage mode gave roughly 10× better measurement accuracy than resistance mode.) Folding the wire in half before coiling it cancels most of the self-inductance.

For what it’s worth, the high-side design wasn’t the original plan. An earlier low-cost driver board caught fire during testing from poor heat dissipation, and working through the replacement DRV8313 board’s schematic to understand why low-side sensing was inaccessible is what forced the harder topology.

Control architecture

Current sampling is synchronized to the PWM carrier: TIM1 triggers injected ADC conversions at 20 kHz, timed to the midpoint of each PWM cycle so measurements land at a repeatable point during switching. The inner current loop runs at 20 kHz with PI controllers regulating Id and Iq, holding Id at zero. A 500 Hz outer position loop adds speed damping and integral action.

The real-time loop uses a sine lookup table with interpolation instead of library trig. Telemetry streams over UART as CSV into MATLAB scripts we wrote for loop tuning and diagnostics.

Calibration and characterization

The motor was undocumented, so we measured what we needed: 7.25 Ω phase resistance (Wye model) and 7 pole pairs, found by current-injection lock-in testing. The current-sense channels were calibrated against a bench supply using a least-squares fit of ADC counts to known currents, and the AS5600 magnetic encoder got a full-rotation lookup-table correction plus IIR filtering on the angle measurements.

Results

Compared against an encoder-only baseline on the same hardware, the current-sensing controller held a 200 mL load at 9.5 in versus 7.5 in, a measurable improvement in holding stiffness.