Tuning Motion Magic PID

Follow these steps to tune the Motion Magic PID for your robot:

Prerequisites

  • Use a fresh battery to ensure consistent performance during tuning.
  • Ensure you are connected to the robot via its Wi-Fi.
  • Open the ConstMotion.java or ConstRotors.java file to adjust the PID constants.
  • Verify the gear ratio is correct (sensor to mechanism ratio in configuration).
  • Verify the direction is correct before beginning tuning.
  • Make sure soft limits are in Units Rotations (not raw encoder units).

For Arm Mechanisms:

  • Deploy should be a positive value (e.g., moving the arm outward/upward should increase position).
  • Use the closed loop sign setting in the motor configuration to invert feedback polarity if needed (ensures feedback matches motor direction).
  • Verify the forward limit is in the correct direction before tuning to prevent damage.

Steps

  1. Set Gravity Type
    Configure the gravity compensation type in ConstMotion.java or ConstRotors.java based on your mechanism:

    • Elevator_Static: For vertical lifts (elevators, lifts)
    • Arm_Cosine: For rotating arms (pivoting mechanisms)
    MOTOR_NAME_CONFIG.Slot0.GravityType = GravityTypeValue.Elevator_Static; // or Arm_Cosine

  2. Use Phoenix Tuner for Initial Tuning

    • Open Phoenix Tuner and connect to your robot.
    • Navigate to the motor controller you're tuning.
    • Use Phoenix Tuner's built-in PID tuning interface to adjust values in real-time:
    • Start with all PID constants set to 0.
    • Tune kS (Static Gain) first - increase until the mechanism begins to move slightly.
      > Note: Keep this value as low as possible while ensuring the mechanism moves.
    • Tune kG (Gravity Compensation) - increase until the mechanism holds position without drifting.
      > Note: If the gear ratio is high enough, kG might just be 0 as the mechanism can hold itself.
    • Tune kP (Proportional Gain) - increase until the mechanism reaches setpoint without overshooting.
    • Fine-tune the values in Phoenix Tuner until you achieve desired performance.

  3. Transfer Values to Constants File
    Once you have tuned the PID values in Phoenix Tuner:

    • Copy the final tuned values from Phoenix Tuner.
    • Update the corresponding constants in ConstMotion.java or ConstRotors.java:
    // Feedforward
MOTOR_NAME_CONFIG.Slot0.kS = 0.25;    // Value from Phoenix Tuner
MOTOR_NAME_CONFIG.Slot0.kG = 0.30;    // Value from Phoenix Tuner
// PID
MOTOR_NAME_CONFIG.Slot0.kP = 10.0;    // Value from Phoenix Tuner
    • Deploy the code with the new constants to verify the mechanism still performs as expected.

For Flywheel Mechanisms

Flywheels use velocity control rather than position control. The key constants to tune are kS and kV.

  1. Set kS
    Tune kS (Static Gain) the same way as other mechanisms — increase it until the flywheel just begins to spin.

    Note: Keep this value as low as possible while still overcoming static friction.

  2. Set kV
    Tune kV (Velocity Gain) so that, with only feedforward active (no PID), the flywheel reaches slightly below your lowest velocity setpoint.
    This ensures the closed-loop controller (kP) only needs to make small corrections upward, avoiding overshoot.

    Tip: Test at your lowest expected RPM setpoint and increase kV until the flywheel gets close but does not exceed it.

  3. Transfer Values to Constants File
    Once tuned, copy the values into your constants file:

// Feedforward
MOTOR_NAME_CONFIG.Slot0.kS = 0.25;    // Value from Phoenix Tuner
MOTOR_NAME_CONFIG.Slot0.kV = 0.12;    // Value from Phoenix Tuner (reach slightly below lowest setpoint)
// PID (fine-tune if needed)
MOTOR_NAME_CONFIG.Slot0.kP = 1.0;     // Value from Phoenix Tuner
  • Deploy the code with the new constants to verify the flywheel performs as expected at all setpoints.

Additional Notes

  • If you encounter issues with feedforward, you may need to revisit and refine your feedforward calculations.