Encoders overview

How position sensors integrate with FOC motor control in VESC firmware.

Field-Oriented Control (FOC) requires accurate rotor position information to apply current in the optimal direction. The VESC firmware supports several strategies for obtaining this position, ranging from pure estimation (sensorless) to dedicated hardware encoders.

Why position sensing matters for FOC

FOC decomposes stator current into a torque-producing component (Iq) and a flux-producing component (Id). Accurate alignment of these components with the rotor flux requires knowing the electrical angle of the rotor at all times.

• Low-speed performance: Back-EMF observers struggle at near-zero speed. A physical sensor eliminates this limitation.
• Startup torque: Sensored modes can produce full torque from standstill without the alignment pulses needed in sensorless operation.
• Position control: Applications such as servo drives require absolute or incremental position feedback.

Sensor modes (mc_foc_sensor_mode)

The active sensor strategy is selected through the FOC_SENSOR_MODE parameter, defined in datatypes.h:

typedef enum {
    FOC_SENSOR_MODE_SENSORLESS = 0,
    FOC_SENSOR_MODE_ENCODER,
    FOC_SENSOR_MODE_HALL,
    FOC_SENSOR_MODE_HFI,
    FOC_SENSOR_MODE_HFI_START,
    FOC_SENSOR_MODE_HFI_V2,
    FOC_SENSOR_MODE_HFI_V3,
    FOC_SENSOR_MODE_HFI_V4,
    FOC_SENSOR_MODE_HFI_V5,
    FOC_SENSOR_MODE_ENCODER_AB
} mc_foc_sensor_mode;

FOC_SENSOR_MODE_SENSORLESS

Uses a back-EMF observer (Ortega or other configured observer) to estimate rotor position. No external hardware required. Performance degrades at very low speeds.

FOC_SENSOR_MODE_ENCODER

Uses a connected absolute or incremental encoder (ABI, AS504x, AS5x47U, BiSS-C, MA782, MT6816, SinCos, TLE5012, TS5700N8501). Requires an offset calibration run to align the encoder zero with the motor’s electrical zero.

FOC_SENSOR_MODE_ENCODER_AB

Variant of encoder mode that uses only the A and B quadrature channels without requiring an index pulse. Useful when the index line is unavailable.

FOC_SENSOR_MODE_HALL

Uses three digital Hall-effect sensors embedded in the motor. Provides 6-step commutation-quality position (60-degree resolution per electrical cycle). Suitable for motors with built-in Hall sensors.

FOC_SENSOR_MODE_HFI

High-Frequency Injection. Injects a small HF voltage signal and measures the resulting current anisotropy to estimate rotor position without an external sensor. Effective at zero and low speed on motors with saliency (e.g., IPM motors).

FOC_SENSOR_MODE_HFI_START

Combines HFI at standstill and low speed with a sensorless back-EMF observer at higher speeds. Provides smooth sensorless operation across the full speed range.

HFI variants (V2–V5)

Successive revisions of the HFI algorithm with improved noise rejection, startup behaviour, and compatibility with different motor geometries. Select the version that works best with your motor through empirical testing.

Configuring the encoder in VESC Tool

1. Connect your VESC

   Open VESC Tool and connect to the controller over USB or Bluetooth.

2. Open Motor Configuration

   Navigate to Motor Configuration → FOC → General.

3. Select a sensor mode

   Set the Sensor Mode drop-down to the mode matching your hardware (for example, Encoder for SPI/ABI encoders, Hall Sensors for Hall-effect sensors).

4. Run the detection wizard

   For encoder modes, click Detect Encoder (or the equivalent wizard button). The firmware will spin the motor slowly to measure the encoder offset angle and store it in the configuration.

5. Write configuration

   Click Write Motor Configuration to persist the settings to flash.

The encoder offset must be re-detected any time you mechanically reposition the encoder relative to the motor shaft.

Encoder API

The encoder subsystem exposes a unified API regardless of the underlying sensor type:

// encoder.h
bool  encoder_init(volatile mc_configuration *conf);
void  encoder_deinit(void);
float encoder_read_deg(void);             // Electrical angle, 0–360°
float encoder_read_deg_multiturn(void);   // Cumulative angle (absolute encoders)
bool  encoder_index_found(void);          // ABI index pulse detected
float encoder_get_error_rate(void);       // Communication error rate (0.0–1.0)
void  encoder_reset_errors(void);

The active encoder type is reported by encoder_is_configured(), which returns an encoder_type_t value from encoder_datatype.h.