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The driver at a glance

STEPFOC is a field-oriented controller for bipolar stepper motors. It sits on the back of a NEMA 17 motor, reads the rotor position from a magnet on the motor shaft, and drives the two coils with sinusoidal currents the way a servo drive does, instead of the fixed step currents of a conventional stepper driver. The result is a stepper that runs quietly and efficiently, never loses steps, reports its position, and can be commanded in position, speed, current (torque) or impedance.

The STEPFOC driver, 39 × 39 mm, with the encoder chip in the middle of the board.
Figure 1.1: The STEPFOC driver, 39 × 39 mm, with the encoder chip in the middle of the board.

What is on the board

Part What it does
STM32F103CB microcontroller runs the control loops at 6250 Hz and the CAN and UART interfaces
14-bit magnetic encoder (MT6816) reads the position of the magnet on the motor shaft: 16384 ticks per revolution
H-bridge driver with inline current sensing drives the two coils with up to 2.8 A and measures the phase currents
16 Kbit EEPROM keeps the configuration and the calibration over a power cycle
Two CAN connectors and two power connectors daisy-chain several drivers on one bus and one supply
CAN termination switch the 120 Ω resistor the first and the last node need
UART connector (3.3 V) the serial terminal: every setting, calibration, tuning
Thermistor connector an NTC on the motor for temperature protection
ADDITIONAL2 pin an input or output you can use: HALL mode homes against a sensor wired here
JTAG pads (1.27 mm) the programming adapter, for installing the bootloader or for development
Status LED normal operation, calibration or error, see below

The connectors and their pinout are in Connectors and pinout.

What the firmware does

The firmware closes three nested loops on the motor: a current (torque) loop, a velocity loop around it and a position loop around that, plus an impedance (PD) loop that makes the motor behave like a spring and damper. A command picks the loop: a current setpoint, a speed, a position or a PD setpoint. Open-loop stepping, a voltage-torque mode and a homing mode against a sensor exist beside them; Control modes describes each.

The driver protects itself and the motor: it stops on a supply voltage outside the limits, on an encoder fault (no magnet), on a motor over-temperature (with the thermistor), on an E-stop command and on a watchdog that runs out. The status LED and #Error tell which; see Errors and recovery. It does not monitor its driver stage: a fault there is not reported.

How it is controlled

  • UART, a 3.3 V serial port at 256000 baud: human-readable commands such as #P 16384, and the only way to run the calibration. See Over UART.
  • CAN, 1 Mbit/s, up to 14 drivers on one bus (node IDs 0 to 13): compact frames for real-time control from a PC with the CANvas adapter, a microcontroller or a robot control box. See Over CAN.
  • Python, the Spectral-BLDC library on top of CAN. See With Python.
  • Motor GUI, in the browser, over serial or CAN: the usual tool for setup, testing and tuning. See Motor GUI.

The status LED

Pattern Meaning
three short flashes and a pause normal operation: no error, calibrated
fast flashing, about ten times a second the calibration routine is running
on, not flashing error mode: at least one error is active, or the motor is not calibrated

The LED can be turned off with #LED 0.

Normal operation Calibration Error mode
Three short flashes and a pause Fast flashing during calibration On, steady

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