Field Oriented Control (FOC)

FOC sensor modes, HFI, observer types, field weakening, and key configuration parameters for VESC firmware.

Field Oriented Control (FOC) is the default and most capable motor control algorithm in VESC firmware. It controls the d-axis and q-axis currents independently in a rotating reference frame, enabling smooth torque, high efficiency, and precise speed and position control.

Sensor modes

FOC sensor mode is configured via mc_foc_sensor_mode in datatypes.h. The default is FOC_SENSOR_MODE_SENSORLESS.

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

The flux observer estimates rotor position from stator voltages and currents. No position sensor is required. The motor coasts through open-loop at low speeds until the observer locks in, controlled by MCCONF_FOC_OPENLOOP_RPM (default: 1500 ERPM) and the open-loop hysteresis/timing parameters.

FOC_SENSOR_MODE_ENCODER

Uses an incremental (ABI) encoder for rotor position. Requires encoder offset calibration. Ratio and inversion are set by MCCONF_FOC_ENCODER_RATIO (default: 7.0) and MCCONF_FOC_ENCODER_INVERTED (default: false).

FOC_SENSOR_MODE_ENCODER_AB

Encoder AB mode — uses only the A and B signals (no index pulse). Useful when the index channel is unavailable.

FOC_SENSOR_MODE_HALL

Hall effect sensors provide 60-degree resolution commutation. Sensor angles are stored in the FOC hall table (MCCONF_FOC_HALL_TAB_0 through MCCONF_FOC_HALL_TAB_7). Hall interpolation is disabled below MCCONF_FOC_HALL_INTERP_ERPM (default: 500 ERPM) to avoid noise at low speeds.

FOC_SENSOR_MODE_HFI

High-Frequency Injection. A high-frequency voltage is superimposed on the motor to detect rotor position from inductance anisotropy. Intended for salient-pole motors (e.g., IPM motors). See the HFI section below.

FOC_SENSOR_MODE_HFI_START

Sensorless mode that uses HFI only during startup and low-speed operation to resolve initial rotor position, then hands off to the sensorless observer. Avoids open-loop coasting on startup.

FOC_SENSOR_MODE_HFI_V2

Revised HFI algorithm (V2) with a correction gain (MCCONF_FOC_HFI_GAIN, default: 0.3) and max error truncation (MCCONF_FOC_HFI_MAX_ERR, default: 0.30).

FOC_SENSOR_MODE_HFI_V3

HFI V3 — an incremental variation of the V2 algorithm.

FOC_SENSOR_MODE_HFI_V4

HFI V4 — further refined HFI variant.

FOC_SENSOR_MODE_HFI_V5

HFI V5 — latest HFI variant with additional hysteresis control via MCCONF_FOC_HFI_HYST (default: 0.0).

HFI (High-Frequency Injection)

HFI detects rotor position at standstill and low speeds by injecting a high-frequency test voltage and measuring the resulting current response. It requires a motor with significant inductance anisotropy (Ld ≠ Lq).

HFI key parameters

Parameter	Default	Description
MCCONF_FOC_HFI_VOLTAGE_START	20 V	Injection voltage during ambiguity resolution at startup
MCCONF_FOC_HFI_VOLTAGE_RUN	4 V	Injection voltage during normal HFI tracking
MCCONF_FOC_HFI_VOLTAGE_MAX	6 V	Injection voltage at maximum motor current
MCCONF_FOC_HFI_SAMPLES	HFI_SAMPLES_16	Samples per electrical revolution (8, 16, or 32)
MCCONF_FOC_HFI_START_SAMPLES	5	Samples used at startup to resolve pole ambiguity
MCCONF_FOC_SL_ERPM_HFI	3000 ERPM	ERPM above which the observer takes over from HFI
MCCONF_FOC_HFI_RESET_ERPM	500 ERPM	Reset HFI state when falling below this ERPM
MCCONF_FOC_HFI_OBS_OVR_SEC	0.001 s	Continue using observer for this long after entering HFI speed range
MCCONF_FOC_HFI_GAIN	0.3	Correction gain for HFI V2
MCCONF_FOC_HFI_MAX_ERR	0.30	Truncate HFI error above this magnitude
MCCONF_FOC_HFI_HYST	0.0	Sense vector offset hysteresis for HFI V5

HFI ambiguity resolution mode

Because inductance anisotropy is 180° ambiguous, HFI requires a separate step to resolve the correct pole. The ambiguity mode is set by mc_foc_hfi_amb_mode:

typedef enum {
    FOC_AMB_MODE_SIX_VECTOR = 0,
    FOC_AMB_MODE_D_SINGLE_PULSE,
    FOC_AMB_MODE_D_DOUBLE_PULSE
} mc_foc_hfi_amb_mode;

Mode	Description
FOC_AMB_MODE_SIX_VECTOR	Applies voltage vectors in six directions and selects the strongest response. Default.
FOC_AMB_MODE_D_SINGLE_PULSE	Applies a single d-axis pulse.
FOC_AMB_MODE_D_DOUBLE_PULSE	Applies two d-axis pulses with opposite polarity for more reliable detection.

Ambiguity current and threshold are set by MCCONF_FOC_HFI_AMB_CURRENT (default: 60.0 A) and MCCONF_FOC_HFI_AMB_TRES (default: 15).

Position observer

For sensorless operation, the flux observer estimates rotor position. The observer type is selected by mc_foc_observer_type:

typedef enum {
    FOC_OBSERVER_ORTEGA_ORIGINAL = 0,
    FOC_OBSERVER_MXLEMMING,
    FOC_OBSERVER_ORTEGA_LAMBDA_COMP,
    FOC_OBSERVER_MXLEMMING_LAMBDA_COMP,
    FOC_OBSERVER_MXV,
    FOC_OBSERVER_MXV_LAMBDA_COMP,
    FOC_OBSERVER_MXV_LAMBDA_COMP_LIN,
} mc_foc_observer_type;

The default observer is FOC_OBSERVER_MXLEMMING_LAMBDA_COMP. Observer gain is set by MCCONF_FOC_OBSERVER_GAIN (default: 9×10⁷; suggested starting value: 600 / L). At low duty cycles the gain is scaled down by MCCONF_FOC_OBSERVER_GAIN_SLOW (default: 0.05).

Hybrid sensor / sensorless

When using Hall or encoder feedback, you can configure the firmware to blend in the observer at higher speeds:

Parameter	Default	Description
MCCONF_FOC_SL_ERPM_START	2500 ERPM	Below this, only the sensor is used
MCCONF_FOC_SL_ERPM	3500 ERPM	Above this, only the observer is used

Between MCCONF_FOC_SL_ERPM_START and MCCONF_FOC_SL_ERPM the two sources are blended linearly.

Open-loop startup (sensorless)

In sensorless mode the motor runs open-loop at low speeds before the observer has locked in. Key parameters:

Parameter	Default	Description
MCCONF_FOC_OPENLOOP_RPM	1500 ERPM	Open-loop speed target
MCCONF_FOC_OPENLOOP_RPM_LOW	0.0	Fraction of OPENLOOP_RPM at minimum motor current
MCCONF_FOC_SL_OPENLOOP_HYST	0.1 s	Time below minimum RPM before entering open-loop
MCCONF_FOC_SL_OPENLOOP_TIME	0.05 s	Time to remain in open-loop after ramping up
MCCONF_FOC_SL_OPENLOOP_T_LOCK	0.0 s	Time to lock rotor in place at start of open-loop sequence
MCCONF_FOC_SL_OPENLOOP_T_RAMP	0.1 s	Ramp duration from 0 to open-loop RPM
MCCONF_FOC_SL_OPENLOOP_BOOST_Q	0.0 A	Q-axis current boost during open-loop
MCCONF_FOC_SL_OPENLOOP_MAX_Q	-1.0 A	Q-axis current maximum during open-loop (-1 = disabled)

Current controller

The FOC current controller (inner PI loop) operates on the d and q axes:

Parameter	Default	Description
MCCONF_FOC_CURRENT_KP	0.03	Proportional gain
MCCONF_FOC_CURRENT_KI	50.0	Integral gain
MCCONF_FOC_F_ZV	25000 Hz	Zero-vector switching frequency
MCCONF_FOC_DT_US	0.12 µs	Dead-time compensation

Current controller decoupling

typedef enum {
    FOC_CC_DECOUPLING_DISABLED = 0,
    FOC_CC_DECOUPLING_CROSS,
    FOC_CC_DECOUPLING_BEMF,
    FOC_CC_DECOUPLING_CROSS_BEMF
} mc_foc_cc_decoupling_mode;

The default is FOC_CC_DECOUPLING_DISABLED. Cross-coupling and BEMF feedforward can improve dynamic response at high speeds.

Motor parameters

The observer and current controller use these motor-specific parameters:

Parameter	Default	Description
MCCONF_FOC_MOTOR_L	7 µH	Phase inductance
MCCONF_FOC_MOTOR_R	0.015 Ω	Phase resistance
MCCONF_FOC_MOTOR_FLUX_LINKAGE	0.00245 Wb	Flux linkage
MCCONF_FOC_MOTOR_LD_LQ_DIFF	0.0 H	Difference between d and q inductance (for MTPA)

Use VESC Tool’s motor measurement wizard to automatically detect resistance, inductance, and flux linkage for your specific motor.

PLL speed estimator

A phase-locked loop (PLL) tracks the rotor angle from the observer output:

Parameter	Default	Description
MCCONF_FOC_PLL_KP	2000.0	PLL proportional gain
MCCONF_FOC_PLL_KI	30000.0	PLL integral gain

Field weakening

Field weakening extends operating speed above the rated RPM by injecting negative d-axis current:

Parameter	Default	Description
MCCONF_FOC_FW_CURRENT_MAX	0.0 A	Maximum field weakening current (0 = disabled)
MCCONF_FOC_FW_DUTY_START	0.8	Duty cycle fraction at which field weakening begins
MCCONF_FOC_FW_RAMP_TIME	0.0 s	Ramp time for field weakening current
MCCONF_FOC_FW_Q_CURRENT_FACTOR	0.05	Fraction of FW current fed to the q-axis to decelerate when setpoint is 0

Field weakening increases motor and controller temperatures. Ensure adequate thermal headroom before enabling it.

MTPA (Maximum Torque Per Amp)

MTPA exploits d/q inductance asymmetry to maximize torque output at a given current. It is configured by MCCONF_FOC_MTPA_MODE and requires MCCONF_FOC_MOTOR_LD_LQ_DIFF to be non-zero.

FOC functions (mcpwm_foc)

The mcpwm_foc module implements the FOC algorithm. You typically call it through mc_interface, but the direct API is available:

// Initialization
void mcpwm_foc_init(mc_configuration *conf_m1, mc_configuration *conf_m2);
void mcpwm_foc_deinit(void);
bool mcpwm_foc_init_done(void);
void mcpwm_foc_set_configuration(mc_configuration *configuration);

// Control
void mcpwm_foc_set_duty(float dutyCycle);
void mcpwm_foc_set_duty_noramp(float dutyCycle);
void mcpwm_foc_set_pid_speed(float rpm);
void mcpwm_foc_set_pid_pos(float pos);
void mcpwm_foc_set_current(float current);
void mcpwm_foc_set_brake_current(float current);
void mcpwm_foc_set_handbrake(float current);
void mcpwm_foc_set_openloop_current(float current, float rpm);
void mcpwm_foc_set_openloop_phase(float current, float phase);
void mcpwm_foc_set_openloop_duty(float dutyCycle, float rpm);
void mcpwm_foc_set_openloop_duty_phase(float dutyCycle, float phase);
void mcpwm_foc_set_fw_override(float current);  // override field weakening current
void mcpwm_foc_release_motor(void);
void mcpwm_foc_stop_pwm(bool is_second_motor);

// State
mc_state mcpwm_foc_get_state(void);
mc_control_mode mcpwm_foc_control_mode(void);
bool mcpwm_foc_is_dccal_done(void);

// Speed and position
float mcpwm_foc_get_rpm(void);
float mcpwm_foc_get_rpm_fast(void);
float mcpwm_foc_get_rpm_faster(void);
float mcpwm_foc_get_phase(void);             // estimated rotor phase (degrees)
float mcpwm_foc_get_phase_observer(void);    // observer phase estimate
float mcpwm_foc_get_phase_bemf(void);        // BEMF-derived phase
float mcpwm_foc_get_phase_encoder(void);     // encoder phase
float mcpwm_foc_get_phase_hall(void);        // hall-derived phase

// dq-frame values
float mcpwm_foc_get_id(void);               // d-axis current
float mcpwm_foc_get_iq(void);               // q-axis current
float mcpwm_foc_get_id_set(void);
float mcpwm_foc_get_iq_set(void);
float mcpwm_foc_get_vd(void);               // d-axis voltage
float mcpwm_foc_get_vq(void);               // q-axis voltage

// Observer state
float mcpwm_foc_get_est_lambda(void);       // estimated flux linkage
float mcpwm_foc_get_est_res(void);          // estimated stator resistance
float mcpwm_foc_get_est_ind(void);          // estimated inductance

// Motor measurement utilities
int mcpwm_foc_measure_resistance(float current, int samples, bool stop_after, float *resistance);
int mcpwm_foc_measure_inductance(float duty, int samples, float *curr, float *ld_lq_diff, float *inductance);
int mcpwm_foc_measure_inductance_current(float curr_goal, int samples, float *curr, float *ld_lq_diff, float *inductance);
int mcpwm_foc_encoder_detect(float current, bool print, float *offset, float *ratio, bool *inverted);

Sample modes

FOC sampling behavior is configurable:

// When to sample voltages for the control update
typedef enum {
    FOC_CONTROL_SAMPLE_MODE_V0 = 0,
    FOC_CONTROL_SAMPLE_MODE_V0_V7,
    FOC_CONTROL_SAMPLE_MODE_V0_V7_INTERPOL
} mc_foc_control_sample_mode;

// Which current sensor readings to use
typedef enum {
    FOC_CURRENT_SAMPLE_MODE_LONGEST_ZERO = 0,
    FOC_CURRENT_SAMPLE_MODE_ALL_SENSORS,
    FOC_CURRENT_SAMPLE_MODE_HIGH_CURRENT,
    FOC_CURRENT_SAMPLE_MODE_BEST_SENSOR  // experimental
} mc_foc_current_sample_mode;

Defaults are FOC_CONTROL_SAMPLE_MODE_V0 and FOC_CURRENT_SAMPLE_MODE_LONGEST_ZERO.