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Automatic Motor Characterisation with characteriseMotor()

The characteriseMotor() function in SimpleFOClibrary provides an automated way to measure critical motor parameters: phase resistance and phase inductance (both D-axis and Q-axis). This guide explains how to use it effectively.

What Parameters Does characteriseMotor() Measure?

Phase Resistance

  • The DC resistance of motor windings (in Ohms)
  • Measured by: Applying small amplitude high-frequency voltage and measuring current response
  • Essential for: Current control, torque estimation, and FOC algorithms

Phase Inductance (D and Q axis)

  • The inductive reactance of motor windings (in Henry)
  • D-axis inductance (\(L_d\)): Inductance along the direct axis (aligned with magnets)
  • Q-axis inductance (\(L_q\)): Inductance along the quadrature axis (perpendicular to magnets)
  • Important for: High-speed operation, lag compensation, and advanced control
  • Note: D and Q axis inductances may differ in some motor types

Requirements

To use characteriseMotor(), you must have:

  1. Current sensor set up and linked to the motor:

  2. Driver initialized and linked to the motor

  3. Motor initialized (but NOT in control mode)

  4. Serial connection for output monitoring (optional but recommended)
Important: The motor must remain still during characterisation. Ensure the motor shaft cannot move freely, or hold it steady by hand.

Basic Usage 

#include <SimpleFOC.h>

// Your motor, driver, and current sensor setup here
// ...

void setup() {
  Serial.begin(115200);
  
  // Initialize driver, motor, and current sensor
  driver.init();
  motor.linkDriver(&driver);
  current_sense.init();
  motor.linkCurrentSense(&current_sense);
  motor.init();
  
  // Run characterisation with 2.0V test voltage
  motor.characteriseMotor(2.0f);
  
  // Motor parameters are now automatically set
}

void loop() {
  // Your control code here
}

Function Signature 

void characteriseMotor(float test_voltage);

Parameters:

  • test_voltage (float): The voltage amplitude used during measurement, in Volts
    • Should be high enough to produce measurable current (typically 1-5V)
    • Should NOT be so high as to damage the motor
    • Start with 2-3V for most hobby motors
    • Use lower values (0.5-1V) for high-KV drone motors
    • Use higher values (3-5V) for low-KV gimbal motors

Understanding the Output

The function prints detailed results to the serial monitor:

MOT: Init
MOT: Enable driver.
MOT: Measuring phase to phase resistance, keep motor still...
MOT: Estimated phase to phase resistance: 5.94
MOT: Measuring inductance, keep motor still...
MOT: Inductance measurement complete!
MOT: Measured D-inductance in mH: 0.50
MOT: Measured Q-inductance in mH: 0.59

Interpreting the Results

Parameter Units Typical Range Notes
Phase resistance Ω 0.05 - 50 Gimbal: 3-10Ω, Drone: 0.05-1Ω, Stepper: 1-50Ω
D-inductance mH 0.1 - 10 Usually measured value
Q-inductance mH 0.1 - 10 May differ from D-inductance depending on motor type

Accessing Measured Values

After characteriseMotor() completes, the measured parameters are automatically stored in the motor object:

// After characteriseMotor() has run:

// Access phase resistance (already stored)
float R = motor.phase_resistance;

// Access inductance values (v2.4.0+)
float L_d = motor.axis_inductance.d;  // D-axis inductance
float L_q = motor.axis_inductance.q;  // Q-axis inductance

Serial.print("Resistance: ");
Serial.print(R);
Serial.println(" Ohms");

Serial.print("D-axis inductance: ");
Serial.print(L_d * 1000);  // Convert H to mH
Serial.println(" mH");

Serial.print("Q-axis inductance: ");
Serial.print(L_q * 1000);  // Convert H to mH
Serial.println(" mH");
Version Note: Starting from v2.4.0, SimpleFOClibrary measures both D-axis and Q-axis inductance separately. Earlier versions measure only a single inductance value.

Example Code

BLDC motors Stepper motors Hybrid Stepper motors

#include <SimpleFOC.h>

// BLDC motor
BLDCMotor motor = BLDCMotor(11);
BLDCDriver3PWM driver = BLDCDriver3PWM(6, 10, 5, 8);

InlineCurrentSense current_sense = InlineCurrentSense(185.0f, A0, A2);

void setup() {
  // Serial for output
  Serial.begin(115200);
  
  // Enable debug output
  SimpleFOCDebug::enable(&Serial);
  
  // Configure driver
  driver.voltage_power_supply = 20;  // 20V power supply
  driver.init();
  motor.linkDriver(&driver);
  
  // Link and initialize current sensor
  current_sense.linkDriver(&driver);
  current_sense.init();
  motor.linkCurrentSense(&current_sense);
  
  // Initialize motor
  motor.init();
  
  // Run characterisation with 3.5V test voltage
  motor.characteriseMotor(3.5f);
}

void loop() {
  // Your control code here
  delay(1000);
}

#include <SimpleFOC.h>

// Stepper motor
StepperMotor motor = StepperMotor(50);
StepperDriver2PWM driver = StepperDriver2PWM(6, 10, 5, 8);

InlineCurrentSense current_sense = InlineCurrentSense(50.0f, A0, A2);

void setup() {
  // Serial for output
  Serial.begin(115200);
  
  // Enable debug output
  SimpleFOCDebug::enable(&Serial);
  
  // Configure driver
  driver.voltage_power_supply = 20;  // 20V power supply
  driver.init();
  motor.linkDriver(&driver);
  
  // Link and initialize current sensor
  current_sense.linkDriver(&driver);
  current_sense.init();
  motor.linkCurrentSense(&current_sense);
  
  // Initialize motor
  motor.init();
  
  // Run characterisation with 3.5V test voltage
  motor.characteriseMotor(3.5f);
}

void loop() {
  // Your control code here
  delay(1000);
}

#include <SimpleFOC.h>

// Hybrid Stepper motor
HybridStepperMotor motor = HybridStepperMotor(50);
BLDCDriver3PWM driver = BLDCDriver3PWM(6, 10, 5, 8);

InlineCurrentSense current_sense = InlineCurrentSense(50.0f, A0, A2);

void setup() {
  // Serial for output
  Serial.begin(115200);
  
  // Enable debug output
  SimpleFOCDebug::enable(&Serial);
  
  // Configure driver
  driver.voltage_power_supply = 20;  // 20V power supply
  driver.init();
  motor.linkDriver(&driver);
  
  // Link and initialize current sensor
  current_sense.linkDriver(&driver);
  current_sense.init();
  motor.linkCurrentSense(&current_sense);
  
  // Initialize motor
  motor.init();
  
  // Run characterisation with 3.5V test voltage
  motor.characteriseMotor(3.5f);
}

void loop() {
  // Your control code here
  delay(1000);
}

Tips for Successful Characterisation

1. Choose the Right Test Voltage

  • Too low: Current measurement may be noisy or at the edge of sensor accuracy
  • Too high: May stress the motor or saturate the current sensor
  • Rule of thumb: Start with 2-3V and adjust based on measurement quality

2. Keep the Motor Still

  • Any movement will corrupt the inductance measurement
  • The motor is stationary during the entire process

3. Ensure Good Current Sensing

  • Verify current sensor is properly calibrated
  • Check that current sensor bandwidth is sufficient (typically >5 kHz recommended)
  • Ensure solid electrical connections

4. Multiple Measurements

  • If results seem inconsistent, run characteriseMotor() multiple times
  • Results should be repeatable within ±5%
  • If variation is large, check mechanical coupling and sensor stability

Troubleshooting

“Measurement Failed” or No Output

  • Verify current sensor is working (motor.current_sense is not null)
  • Check serial connection and baud rate (typically 115200)
  • Ensure power supply is adequate for test voltage
  • Verify driver and motor initialisation completed successfully

Results Seem Wrong

  • Phase resistance should typically be 0.05Ω to 50Ω
  • If much higher or lower, check mechanical connection to motor
  • Check the gains and shunt values of the current sensor
  • D and Q inductances should be similar for isotropic motors, but may differ for salient-pole motors
  • Compare with datasheet values if available

High Variability in Measurements

  • Ensure motor is completely stationary
  • Check for vibration from power supply or mechanical coupling
  • Verify current sensor hasn’t saturated
  • Try different test voltage values