Proportional Control (the P in PID)

Stop snapping at full power. Slow down as you approach the target.

The big idea

Imagine you're trying to park a car exactly 10 feet from a wall. If you drive at full speed until 10 feet then slam the brakes, you'll skid past the line. That's bang-bang control. Instead: the closer you get, the slower you drive. That's proportional control.

Error

error = target − current

If your goal is 1000 motor degrees and you've traveled 200, error = 800. Far from the goal → high error → drive fast. As you approach, error shrinks → motor power shrinks → smooth landing.

P-controlled turn

// Turn to a target heading using only the P term
void turnTo(double targetHeading) {
  const double kP = 0.6;   // tune this

  while (true) {
    double error = targetHeading - Gyro.rotation(degrees);
    if (fabs(error) < 1.0) break;             // close enough

    double power = kP * error;                 // proportional
    if (power >  60) power =  60;              // clamp so we don't fly
    if (power < -60) power = -60;

    LeftMotor.spin(forward,  power, percent);
    RightMotor.spin(reverse, power, percent);
    wait(20, msec);
  }
  LeftMotor.stop();
  RightMotor.stop();
}

Tuning kP

• kP too low, robot creeps toward the target and may never reach it.
• kP too high, robot overshoots, oscillates back and forth.
• kP just right, quick approach, gentle landing.

Start with kP = 0.1, double it until you see overshoot, then back off slightly. This is the same tuning process pros use.

Try it

Write a P-controlled driveTo(distanceInches) that uses the average of the two motor encoders. Test with different kP values. What happens at kP = 5?

Curated references

• videoVEX IQ Proportional Control (full walkthrough)
• videoVEX Robotics PID Control: An Introduction
• readCautionTape: VEX IQ Robot Drive Straight (proportional with gyro)

void turnTo(double targetHeading) {
  double kP = 5.0;

  while (true) {
    double error = targetHeading - Gyro.rotation(degrees);
    if (fabs(error) < 1.0) break;

    double power = kP * error;
    LeftMotor.spin(forward,  power, percent);
    RightMotor.spin(reverse, power, percent);
    wait(20, msec);
  }
  LeftMotor.stop();
  RightMotor.stop();
}

void turnTo(double targetHeading) {
  double kP = 0.6;

  while (true) {
    double error = targetHeading - Gyro.rotation(degrees);
    if (error < 1.0) break;

    double power = kP * error;
    LeftMotor.spin(forward,  power, percent);
    RightMotor.spin(reverse, power, percent);
    wait(20, msec);
  }
  LeftMotor.stop();
  RightMotor.stop();
}