Subsystems

Overview

A Subsystem represents physical hardware - motors, sensors, and configuration. Each subsystem should:

• Own and configure its hardware
• Enforce safety limits
• Expose intention-level commands (like setPosition(), not “set motor voltage”)
• Report telemetry for logging and dashboards
• Define a default command for idle behavior

Design Checklist

• Hardware setup and configuration
• Built-in limits (current, soft, neutral mode)
• Telemetry (positions, currents, state)
• No blocking or sleeping threads
• Private raw controls; public “intent” methods
• Clean default command behavior

Example – Arm Subsystem

public class ArmSubsystem extends SubsystemBase {
  private final TalonFX motor = new TalonFX(31, "rio");
  private final MotionMagicDutyCycle motionMagic = new MotionMagicDutyCycle(0);
  private double goalPosition = 0.0;

  public ArmSubsystem() {
    // Configure limits, PID slots, neutral mode, etc.
    setDefaultCommand(hold());
  }

  public double getPosition() {
    return motor.getPosition().getValueAsDouble();
  }

  public boolean atTarget() {
    return Math.abs(getPosition() - goalPosition) < 0.5;
  }

  /** Hold position when idle */
  public Command hold() {
    return Commands.run(() ->
        motor.setControl(motionMagic.withPosition(goalPosition)), this)
        .withName("Arm Hold");
  }

  /** Move to a new target position */
  public Command setPosition(DoubleSupplier pos) {
    return Commands.run(() -> {
      goalPosition = pos.getAsDouble();
      motor.setControl(motionMagic.withPosition(goalPosition));
    }, this).withName("Arm SetPosition");
  }

  /** Percent control (manual mode) */
  public Command percent(DoubleSupplier speed) {
    return Commands.run(() ->
        motor.setControl(new DutyCycleOut(speed.getAsDouble())), this)
        .finallyDo(i -> motor.stopMotor())
        .withName("Arm Percent");
  }
}

Best Practices

• Keep raw motor calls private. Commands should only interact through intent-level methods.
• Each subsystem should define its own commands — don’t mix logic into other subsystems.
• Default commands should reflect idle state (e.g., hold(), stop(), or coast()).
• Telemetry helps with debugging: log states, goals, and errors.
• Prefer using Commands.run() and finallyDo() to keep actions declarative and safe.