foc-sysid

Raspberry Pi data capture and frequency-domain analysis for a bare-metal STM32 FOC servo drive.

Companion firmware: stm32-servo-drive

What it does

• Captures 32-byte telemetry frames from STM32F411 over SPI at ~10 kHz
• Triggers the STM32 sysid sequence through a GPIO handshake
• Logs timestamped CSV data including encoder position, d/q currents, voltage commands, and chirp frequency
• Computes the current-loop plant Bode plot using Welch CSD estimation
• Extracts R, L, and corner frequency from the measured frequency response
• Overlays the confirmed plant model H(s) = 1 / (Ls + R) with phase-margin analysis
• Captures and plots closed-loop q-axis current step responses

Current results — Kollmorgen AKM11E

Parameter	Measured	Datasheet (line-to-line)	Method
R	3.55 Ω	3.10 Ω	Bode DC magnitude
L	2.47 mH	2.04 mH	Bode -3 dB, fc = 229 Hz
fc	229 Hz	241 Hz	Bode -3 dB from flat response

Measured R and L are slightly higher than the datasheet values, consistent with additional series resistance and inductance from the long motor cable used in the test setup.

Closed-loop current response

The current-loop PI gains were calculated from the measured first-order RL plant and implemented on the STM32 at 20 kHz.

The plot below shows the commanded and measured q-axis current for positive and negative 0.5 A steps, along with the PI voltage output and measured phase currents.

The measured q-axis current tracks the command with little steady-state error. The rotor was mechanically locked during this test.

Hardware

• Raspberry Pi 5
• STM32F411RE Nucleo-64 — bare metal, no HAL
• Texas Instruments DRV8353RS-EVM gate driver
• 7 mΩ low-side shunt resistors with CSA gain of 10 V/V
• Kollmorgen AKM11E BLDC motor
• 3 pole pairs
• 8192-count RS-422 encoder
• SPI0 at 4 MHz with telemetry capture at ~10 kHz
• GPIO3 to STM32 PC3 active-low trigger

Build

mkdir build
cd build
cmake ..
make -j4
./drive

Analysis

Generate the measured current-plant Bode plot:

python3 py-script/bode_plot.py drive_data/sysid_log.csv

Generate the closed-loop current step-response plot:

python3 py-script/step.py drive_data/sysid_log.csv

The scripts save:

bode_plot.png
step_response.png

Copy the generated images into docs/ before committing:

cp bode_plot.png docs/bode_plot.png
cp step_response.png docs/step_response.png

Roadmap

• Current-loop plant sysid — R, L, and fc confirmed
• Bode plot with Welch CSD, coherence, and phase-margin analysis
• Current-loop PI closure
• Closed-loop q-axis current step response
• Validate orthogonal current measurement and transform conventions
• Velocity-loop plant sysid — J, B, and Kt
• Velocity-loop closure
• Position-loop closure