Lesson 3: Subsystems

Why Use Subsystems?

Think of each subsystem as a self-contained box that manages a specific robot mechanism.

• Keeps code organized by robot functions (eg. drivetrain, intake)
• Improve readability
• Makes testing easier

CTRE Talon FX Motor

• Smart motor controller with integrated encoder (can translate mechanical motion into code representing an angle, position, speed, etc)
• Supports advanced features
  • Built in PID controls
  • Motion profiling
  • Current limiting

Key Talon FX Features

• Tells the motor to spin forward/backward at x percent power (Motor output power is in between -1.0 to 1.0): .Set(ControlMode::PercentOutput, value)
• Returns motor's encoder position: .GetSelectedSensorPosition()
• Tunes motor's built-in PID controllers: .Config_kP(...), .Config_kI(...), .Config_kD(...), .Config_kF(...)
• .SetNeutralMode(Brake/Coast)

Overview

For each subsystem (eg. Wrist), you may see:

Each plays a different role to make code clean, testable, and adaptable.

Why This Structure?

• Makes it easier to switch between real robot and simulation
• Supports hardware abstraction
• Enables unit testing without needing the robot
• Encourages clean separation of logic

File 1 - Wrist.java

aka the main logic of the subsystem

• Owns an instance of WristIO
• Receives sensor data from WristIO
• Tells WristIO what motor outputs to apply
• Holds PID loops, state machines, etc.

Think of it as the "brain of the wrist"

File 2 - WristIO.java

aka the Java interface file

• Defines the contract between the logic and the hardware
• Includes methods like updateInputs(), setVoltage(), etc. that must be provided by the real/sim classes.

public interface WristIO {
  public default void updateInputs(WristIOInputs inputs) {}
  public default void setVoltage(Voltage voltage) {}
  public default void setBrakeMode(boolean enable) {}
  /** Reset the motor encoder to the specified position. */
  public default void resetPosition(Angle position) {}
}

• It ensures that implementations like WristIOTalonFX or WristIOSim will work when using Wrist.java

Implementation example:

public class WristTalonFX implements WristIO {
  @Override
  public void setVoltage(Voltage voltage) {
    motor.setControl(voltageRequest.withOutput(voltage.in(Volts)));
  }
  // more implementation...
}

No matter the type of implementation, follow this interface

File 3 - WristIOTalonFX.java

aka real hardware implementation

• Implements WristIO using a TalonFX motor
• Talks to real motor controllers
• Updates sensor readings and sends voltage outputs

Plug in this file for real-world operation

File 4 - WristIOSim.java

aka simulation implementation

• Mocks sensor values and simulates movement
• Lets you run and test Wrist.java without a robot
• Implements WristIO using WPILib's sim features

Swap this in when testing in the simulation

How They Work Together

Wrist.java

↕ (uses)

WristIO.java (interface)

↕ (implemented by)

WristIOTalonFX.java ← for real robot

WristIOSim.java ← for sim/testing

Choose which implementation to use based on which real/sim build mode you want to run

How to "Plug In" The Implementation

• Inside RobotContainer, create your subsystem like this:

// For real robot
Wrist wrist = new Wrist(new WristIOTalonFX());
// For sim
Wrist wrist = new Wrist(new WristIOSim());

When to Use This Pattern

• If the subsystem uses motors/sensors
• Systems that want to be simulated
• Subsystems that's hardware might change later