(Needs proofreading! Written by: Keshav)

CTRE Motors

This page covers CTRE Talon FX motor controllers using Phoenix 6 and WPILib Java. On our robots, Talon FX controllers are commonly integrated into Kraken x60 motors.

Installing Phoenix 6

Install Phoenix 6 with the WPILib Dependency Manager and commit its vendor JSON. Use Phoenix Tuner to set CAN IDs, update firmware, test devices, and inspect faults.

Phoenix 6 Java packages begin with:

com.ctre.phoenix6

Constructing a Talon FX

private final TalonFX motor = new TalonFX(10);

For a named CAN bus:

private final TalonFX motor =
    new TalonFX(10, "rio");

private final TalonFX swerveMotor =
    new TalonFX(11, "swerve");

The ID and bus must match Phoenix Tuner X.

Configuration

Use a TalonFXConfiguration to describe the complete desired state:

TalonFXConfiguration config =
    new TalonFXConfiguration();

config.MotorOutput.Inverted =
    InvertedValue.CounterClockwise_Positive;
config.MotorOutput.NeutralMode =
    NeutralModeValue.Brake;

config.CurrentLimits.SupplyCurrentLimit = 40.0;
config.CurrentLimits.SupplyCurrentLimitEnable = true;

motor.getConfigurator().apply(config);

A new full configuration contains factory-default values for fields we did not change. Applying it establishes a repeatable baseline without a separate factory-default call.

Mechanism Units

The Talon FX integrated sensor natively measures rotor rotations and rotations per second. SensorToMechanismRatio lets device status signals and closed-loop requests use mechanism rotations.

For a 100:1 reduction:

config.Feedback.SensorToMechanismRatio = 100.0;

After applying it, a position of 0.25 represents one quarter of a mechanism rotation, not 0.25 motor rotations.

Control Requests

Phoenix 6 represents each control mode with a reusable request object.

Voltage

import com.ctre.phoenix6.controls.VoltageOut;

private final VoltageOut voltageRequest =
    new VoltageOut(0.0);

public void setVoltage(double volts) {
    motor.setControl(
        voltageRequest.withOutput(volts)
    );
}

Voltage requests are preferred when applying calculated feedforward or when repeatable response across battery voltage matters.

Stopping

motor.stopMotor();

Phoenix also provides neutral and static-brake requests for specific behaviors. Understand the difference between sending zero, neutral output, brake neutral mode, and an active static-brake request before using them.

Onboard Closed-Loop Control

Configuring Gains

config.Slot0.kP = kP;
config.Slot0.kI = 0.0;
config.Slot0.kD = kD;
config.Slot0.kS = kS;
config.Slot0.kV = kV;
config.Slot0.kA = kA;
config.Slot0.kG = kG;

Phoenix supports multiple gain slots for mechanisms that need different configurations. Configure the correct gravity type and static-feedforward sign behavior for the mechanism and request type.

Position

import com.ctre.phoenix6.controls.PositionVoltage;

private final PositionVoltage positionRequest =
    new PositionVoltage(0.0).withSlot(0);

public void setPositionRotations(double rotations) {
    motor.setControl(
        positionRequest.withPosition(rotations)
    );
}

Velocity

import com.ctre.phoenix6.controls.VelocityVoltage;

private final VelocityVoltage velocityRequest =
    new VelocityVoltage(0.0).withSlot(0);

public void setVelocityRps(double rotationsPerSecond) {
    motor.setControl(
        velocityRequest.withVelocity(
            rotationsPerSecond
        )
    );
}

Current Limits and Output Limits

Phoenix distinguishes supply current, drawn from the battery, from stator current, flowing through motor windings. They protect different parts of the system and affect behavior differently.

Also configure:

Peak forward and reverse voltage
Hardware and software limits
Closed-loop output limits
Neutral mode
Ramp rates when needed

Current limiting is not a substitute for preventing a mechanism from repeatedly hitting a hard stop.

Faults and Diagnostics

Phoenix exposes active and sticky fault signals. Useful diagnostics include:

Supply voltage
Motor voltage
Supply and stator current
Device temperature
Position and velocity
Closed-loop reference and error
Reset-during-enable faults
Hardware, undervoltage, and communication faults

Use Phoenix Tuner X self-test and logs alongside robot telemetry.

The 8726 Control Handbook