Torque control using FOC currents

This torque control mode allows you to the true torque control of a BLDC motor and it requires current sensing in order to do it. The user sets the target current \(I_d\) to the FOC algorithm calculates the necessary phase voltages \(u_a\) ,\(u_b\) and \(u_c\) in order to maintain it by measuring the phase currents (\(i_a\), \(i_b\) and \(i_c\)) and the rotor angle \(a\). This mode is enabled by:

// FOC current torque control mode
motor.torque_controller = TorqueControlType::foc_current;

How does it work exactly

BLDC motors Stepper motors

The FOC current torque control algorithm reads the phase currents of the Stepper motor (\(i_a\) and \(i_b\)). The phase currents are transformed into the currents \(i_\alpha\) and \(i_\beta\) using the Inverse Clarke transform. For example if the phase currents \(i_a\) and \(i_b\) are measured:

\[i_\alpha = i_a, \quad i_\beta = i_a\frac{1}{\sqrt{3}} + i_b\frac{2}{\sqrt{3}}\]

Using the current rotor angle \(a\) and the currents \(i_\alpha\) and \(i_\beta\) the FOC algorithm calculates the \(i_d\) and \(i_q\) components of the current using the inverse Park transform.

\[i_q = i_\beta \cos(a) - i_\alpha \sin(a), \quad i_d = i_\alpha \cos(a) + i_\beta \sin(a)\]

Using the target current value \(I_d\) (d as desired) and the measured currents \(i_q\) and \(i_d\), the PID controllers for each axis calculate the appropriate voltages \(U_q\) and \(U_d\) to be set to the motor, to maintain \(i_q\)=\(I_d\) and \(i_d\)=0.

\[U_q = \text{PID}_q(I_d - i_q), \quad U_d = \text{PID}_d(0-i_d)\]

Finally using the Park+Clarke (or SpaceVector) transformation the FOC algorithm sets the appropriate \(u_a\), \(u_b\) and \(u_c\) voltages to the motor. By measuring the phase currents this torque control algorithm ensures that these voltages generate the appropriate currents and magnetic force in the motor rotor with exactly 90 degree offset from its permanent magnetic field, which guarantees maximal torque, this is called commutation.

The torque generated in the motor is proportional (with the torque constant \(K_t\)) to the q-axis current \(i_q\). So for any target motor torque \(\tau\) the target current \(I_d\) can be calculated as:

\[I_d = \frac{\tau}{K_t}\]

Configuration parameters

In order to make this loop run smoothly the user needs to configure the PID controller parameters of the PID_current_q and Low pass filter LPF_current_q time constant.

// Q axis
// PID parameters - default 
motor.PID_current_q.P = 5;                       // 3    - Arduino UNO/MEGA
motor.PID_current_q.I = 1000;                    // 300  - Arduino UNO/MEGA
motor.PID_current_q.D = 0;
motor.PID_current_q.limit = motor.voltage_limit; 
motor.PID_current_q.ramp = 1e6;                  // 1000 - Arduino UNO/MEGA
// Low pass filtering - default 
LPF_current_q.Tf= 0.005;                         // 0.01 - Arduino UNO/MEGA

// D axis
// PID parameters - default 
motor.PID_current_d.P = 5;                       // 3    - Arduino UNO/MEGA
motor.PID_current_d.I = 1000;                    // 300  - Arduino UNO/MEGA
motor.PID_current_d.D = 0;
motor.PID_current_d.limit = motor.voltage_limit; 
motor.PID_current_d.ramp = 1e6;                  // 1000 - Arduino UNO/MEGA
// Low pass filtering - default 
LPF_current_d.Tf= 0.005;                         // 0.01 - Arduino UNO/MEGA

Torque control example code

BLDC motors Stepper motors

A simple example of the FOC current based torque control using Inline current sensor and setting the target value by serial command interface.

#include <SimpleFOC.h>

// BLDC motor & driver instance
BLDCMotor motor = BLDCMotor(11);
BLDCDriver3PWM driver = BLDCDriver3PWM(9, 5, 6, 8);

// encoder instance
Encoder encoder = Encoder(2, 3, 500);
// channel A and B callbacks
void doA(){encoder.handleA();}
void doB(){encoder.handleB();}

// current sensor
InlineCurrentSense current_sense = InlineCurrentSense(0.01, 50.0, A0, A2);

// instantiate the commander
Commander command = Commander(Serial);
void doTarget(char* cmd) { command.scalar(&motor.target, cmd); }

void setup() { 
  
  // initialize encoder sensor hardware
  encoder.init();
  encoder.enableInterrupts(doA, doB); 
  // link the motor to the sensor
  motor.linkSensor(&encoder);

  // driver config
  // power supply voltage [V]
  driver.voltage_power_supply = 12;
  driver.init();
  // link driver
  motor.linkDriver(&driver);
  // link the driver to the current sense
  current_sense.linkDriver(&driver);

  // current sense init hardware
  current_sense.init();
  // link the current sense to the motor
  motor.linkCurrentSense(&current_sense);

  // set torque mode:
  motor.torque_controller = TorqueControlType::foc_current; 
  // set motion control loop to be used
  motor.controller = MotionControlType::torque;

  // foc current control parameters (Arduino UNO/Mega)
  motor.PID_current_q.P = 5;
  motor.PID_current_q.I= 300;
  motor.PID_current_d.P= 5;
  motor.PID_current_d.I = 300;
  motor.LPF_current_q.Tf = 0.01; 
  motor.LPF_current_d.Tf = 0.01; 

  // use monitoring with serial 
  Serial.begin(115200);
  // comment out if not needed
  motor.useMonitoring(Serial);

  // initialize motor
  motor.init();
  // align sensor and start FOC
  motor.initFOC();

  // add target command T
  command.add('T', doTarget, "target current");

  Serial.println(F("Motor ready."));
  Serial.println(F("Set the target current using serial terminal:"));
  _delay(1000);
}

void loop() {

  // main FOC algorithm function
  motor.loopFOC();

  // Motion control function
  motor.move();

  // user communication
  command.run();
}

#include <SimpleFOC.h>

// Stepper motor & driver instance
StepperMotor motor = StepperMotor(50);
StepperDriver2PWM driver = StepperDriver2PWM(9, 5, 6, 8);

// encoder instance
Encoder encoder = Encoder(2, 3, 500);
// channel A and B callbacks
void doA(){encoder.handleA();}
void doB(){encoder.handleB();}

// current sensor
InlineCurrentSense current_sense = InlineCurrentSense(0.01, 50.0, A0, A2);

// instantiate the commander
Commander command = Commander(Serial);
void doTarget(char* cmd) { command.scalar(&motor.target, cmd); }

void setup() { 
  
  // initialize encoder sensor hardware
  encoder.init();
  encoder.enableInterrupts(doA, doB); 
  // link the motor to the sensor
  motor.linkSensor(&encoder);

  // driver config
  // power supply voltage [V]
  driver.voltage_power_supply = 12;
  driver.init();
  // link driver
  motor.linkDriver(&driver);
  // link the driver to the current sense
  current_sense.linkDriver(&driver);

  // current sense init hardware
  current_sense.init();
  // link the current sense to the motor
  motor.linkCurrentSense(&current_sense);

  // set torque mode:
  motor.torque_controller = TorqueControlType::foc_current; 
  // set motion control loop to be used
  motor.controller = MotionControlType::torque;

  // foc current control parameters (Arduino UNO/Mega)
  motor.PID_current_q.P = 5;
  motor.PID_current_q.I= 300;
  motor.PID_current_d.P= 5;
  motor.PID_current_d.I = 300;
  motor.LPF_current_q.Tf = 0.01; 
  motor.LPF_current_d.Tf = 0.01; 

  // use monitoring with serial 
  Serial.begin(115200);
  // comment out if not needed
  motor.useMonitoring(Serial);

  // initialize motor
  motor.init();
  // align sensor and start FOC
  motor.initFOC();

  // add target command T
  command.add('T', doTarget, "target current");

  Serial.println(F("Motor ready."));
  Serial.println(F("Set the target current using serial terminal:"));
  _delay(1000);
}

void loop() {

  // main FOC algorithm function
  motor.loopFOC();

  // Motion control function
  motor.move();

  // user communication
  command.run();
}