Simulation Troubleshooting

Can’t test your robot code in simulation? This guide helps you fix common simulation problems quickly.

Quick Test: Is Simulation Working?

Run this command:

# In VS Code terminal
./gradlew simulateJava

What should happen:

Robot simulation window opens.
SimGUI shows field.
Robot appears on field.
No red error messages.

If you see errors: Follow the troubleshooting steps below.

Problem: Simulation Won’t Start

Symptom: “Command failed” or “Build failed”

Quick fixes to try:

1. Check Java version:

java -version

You should see: Java 17 or 21

If not, install correct Java version.
Update JAVA_HOME environment variable.

2. Clean and rebuild:

./gradlew clean
./gradlew build
./gradlew simulateJava

3. Check Gradle wrapper:

./gradlew --version

Should show Gradle 8.x or higher

If not, reimport Gradle project in VS Code.

Symptom: “Class not found” errors

The problem: Java can’t find your classes

Quick fixes:

1. Check package names:

// Wrong: File is in src/main/java/frc/robot
package com.example.robot;  // Wrong package!

// Right:
package frc.robot;  // Matches directory!

2. Check imports:

// Make sure you import what you use
import frc.robot.subsystems.Drivetrain;
import frc.robot.commands.ExampleCommand;

3. Rebuild project:

./gradlew clean build

Symptom: “NoSuchMethodError”

The problem: Method doesn’t exist or signature doesn’t match

Quick fixes:

1. Check method signature:

// Calling code
drivetrain.drive(1.0, 0.5);  // 2 arguments

// Method signature
public void drive(double speed) { ... }  // Only 1 argument!

// Fix: Make signatures match
public void drive(double speed, double rotation) { ... }

2. Check WPILib version:

// In build.gradle
dependencies {
    implementation "edu.wpi.first.wpilibj:wpilibj-java:2024.+"  // Make sure versions match
}

Problem: Simulation Crashes

Symptom: Simulation window opens, then crashes

Common causes and fixes:

1. Graphics driver issues:

Windows: Update graphics drivers

# Check for Windows updates
# Or visit NVIDIA/AMD website for latest drivers

Linux: Install required packages

sudo apt-get update
sudo apt-get install libgl1-mesa-glx libxext-dev libxrender-dev libxtst-dev libxrandr-dev

macOS: Update macOS and XQuartz

2. Memory issues:

Increase Gradle memory:

// In gradle.properties
org.gradle.jvmargs=-Xmx4096m  // Increase from default

3. Null pointer exceptions:

Check for uninitialized objects:

// Wrong:
private Drivetrain drivetrain;

public Robot() {
    drivetrain.drive(0, 0, 0);  // Crashes! drivetrain is null
}

// Right:
private final Drivetrain drivetrain = new Drivetrain();

public Robot() {
    // Now this works!
}

Problem: Robot Doesn’t Move in Simulation

Symptom: Simulation runs, but robot stays still

Debug steps:

1. Check if commands are scheduled:

@Override
public void execute() {
    System.out.println("Command running!");  // Check console
    drivetrain.drive(x, y, rot);
}

If you don’t see “Command running!”, command isn’t scheduled.

2. Check subsystem periodic:

@Override
public void periodic() {
    System.out.println("Drivetrain periodic: " + speed);
    motor.set(speed);
}

If you don’t see this, subsystem isn’t running.

3. Check simulation initialization:

// In Drivetrain subsystem
public Drivetrain() {
    // For simulation, you MUST use simulation IO
    if (RobotBase.isSimulation()) {
        gyroIO = new GyroIOSim();  // Simulation IO
    } else {
        gyroIO = new GyroIOTalonFX();  // Real hardware
    }
}

Problem: Simulation behaves differently than real robot

Symptom: “It works in sim but not on real robot!”

Common causes:

1. Physics not realistic enough:

Adjust simulation physics:

// In simulation IO
@Override
public void periodic() {
    // Add realistic delays
    velocity = velocity * 0.95 + targetVelocity * 0.05;

    // Add noise
    velocity += (Math.random() - 0.5) * 0.01;
}

2. Timing differences:

Simulation runs faster than real time!

// Don't assume real-time
@Override
public void periodic() {
    double now = Timer.getFPGATimestamp();
    double dt = now - lastTime;
    lastTime = now;

    // Use dt for calculations
    position += velocity * dt;
}

3. Hardware differences:

Real hardware has delays sim doesn’t:

// Add artificial delay in sim
public void setVelocity(double velocity) {
    if (RobotBase.isSimulation()) {
        // Simulate CAN bus delay
        Timer.delay(0.01);  // 10ms delay
    }
    motor.setVelocity(velocity);
}

Problem: Field visualization doesn’t work

Symptom: SimGUI shows field but no robot

Quick fixes:

1. Check if pose is being published:

// In Drivetrain subsystem
@Override
public void periodic() {
    // Send pose to dashboard
    SmartDashboard.putData("Robot", field.getObject("robot"));
    field.getObject("robot").setPose(pose);
}

2. Check field object name:

// In SimGUI
// Make sure field object name matches code!
field.getObject("robot")  // Must match this name!

3. Send pose to NetworkTables:

// In periodic()
SmartDashboard.putNumber("PoseX", pose.getX());
SmartDashboard.putNumber("PoseY", pose.getY());
SmartDashboard.putNumber("PoseTheta", pose.getRotation().getDegrees());

Problem: Joysticks don’t work in simulation

Symptom: Joystick input does nothing

Quick fixes:

1. Check if joystick is connected:

@Override
public void periodic() {
    double x = joystick.getX();
    double y = joystick.getY();

    SmartDashboard.putNumber("JoystickX", x);  // Check dashboard
    SmartDashboard.putNumber("JoystickY", y);
}

If values stay at 0, joystick not detected.

2. Check joystick port:

// Make sure port matches joystick selection
private final XboxController driver = new XboxController(0);  // Port 0

// Or check all ports:
for (int i = 0; i < 6; i++) {
    XboxController test = new XboxController(i);
    if (test.getAButton()) {
        System.out.println("Joystick on port " + i);
    }
}

3. Use keyboard in simulation:

// Add keyboard controls for simulation
@Override
public void periodic() {
    double x = 0;
    double y = 0;

    if (DriverStation.isSimulation()) {
        // Keyboard controls
        if (Keyboard.isKeyDown(Key.W)) y = -1;
        if (Keyboard.isKeyDown(Key.S)) y = 1;
        if (Keyboard.isKeyDown(Key.A)) x = -1;
        if (Keyboard.isKeyDown(Key.D)) x = 1;
    } else {
        // Real joystick
        x = joystick.getX();
        y = joystick.getY();
    }

    drivetrain.drive(x, y, 0);
}

Problem: Simulation is too slow

Symptom: Simulation lags or stutters

Quick fixes:

1. Reduce physics complexity:

// Don't update physics too fast
@Override
public void periodic() {
    if (Timer.getFPGATimestamp() - lastPhysicsUpdate > 0.02) {
        updatePhysics();
        lastPhysicsUpdate = Timer.getFPGATimestamp();
    }
}

2. Reduce logging:

// Don't log every iteration
private double lastLogTime = 0;

@Override
public void periodic() {
    double now = Timer.getFPGATimestamp();

    // Only log every 1 second
    if (now - lastLogTime > 1.0) {
        System.out.println("Pose: " + pose);
        lastLogTime = now;
    }
}

3. Close unnecessary programs:

Close web browsers.
Close other IDE windows.
Check CPU usage in Task Manager.

Problem: Units don’t match

Symptom: “Robot moves wrong distance in sim”

The problem: Units confusion!

Common unit mistakes:

// WRONG: Mixing units
double velocityMetersPerSecond = motor.getVelocity();  // Returns RPM!

// RIGHT: Convert units
double velocityRPM = motor.getVelocity();
double velocityMetersPerSecond = Units.RotationsPerMinute.toRadiansPerSecond(velocityRPM) * wheelRadius;

Use WPILib Units class:

import edu.wpi.first.units.Units;

// Convert anything!
double meters = Units.Inches.of(24).in(Units.Meters);  // 24 inches to meters
double rpm = Units.MetersPerSecond.of(5).in(Units.RotationsPerMinute);  // m/s to RPM

Advanced: Writing Simulation Tests

Unit Tests for Simulation

Test your simulation IO:

@Test
public void testDrivetrainSimulation() {
    // Create simulation
    Drivetrain drivetrain = new Drivetrain();

    // Simulate for 5 seconds
    for (int i = 0; i < 250; i++) {  // 250 * 20ms = 5 seconds
        drivetrain.drive(1.0, 0, 0);  // Drive forward
        drivetrain.periodic();
        Timer.delay(0.02);
    }

    // Check robot moved forward
    assertTrue(drivetrain.getPose().getX() > 1.0, "Robot should move forward");
}

Test vision in simulation:

@Test
public void testVisionInSimulation() {
    VisionSubsystem vision = new VisionSubsystem();

    // Simulate AprilTag detection
    vision.simulateDetectedTag(new AprilTag(1, new Pose3d(5, 0, 0, new Rotation3d())));

    vision.periodic();

    // Check target detected
    assertTrue(vision.hasTarget(), "Should detect AprilTag");
    assertEquals(5.0, vision.getTargetDistance(), 0.1, "Target at 5 meters");
}

Simulation Best Practices

Do’s and Don’ts

DO:

✅ Test everything in simulation first.
✅ Use simulation IO classes.
✅ Add debug prints.
✅ Test edge cases.
✅ Write simulation tests.

DON’T:

❌ Assume simulation = real world.
❌ Ignore simulation warnings.
❌ Skip simulation testing.
❌ Test only on real robot.
❌ Assume perfect physics.

Quick Simulation Checklist

Before testing in simulation:

When simulation runs:

Before testing on real robot:

Everything works in simulation.
Simulation tests pass.
Code reviewed by teammate.
Safety check: Can’t move unexpectedly.

Common Simulation Commands

# Run simulation
./gradlew simulateJava

# Run specific simulation test
./gradlew simulateJavaTest --tests DrivetrainSimulationTest

# Clean and rebuild
./gradlew clean build simulateJava

# Run with more memory
./gradlew simulateJava -Dorg.gradle.jvmargs="-Xmx4096m"

# Run simulation in debug mode
./gradlew simulateJava --debug-jvm

Getting Help with Simulation

When simulation problems persist:

Gather this information:

What command did you run?
What error message appeared?
What did you expect to happen?
What actually happened?
Your OS and Java version.

Where to get help:

Include:

Error messages (full text)
Code snippets (relevant parts)
Screenshots (if helpful)

Next Steps

Once simulation works:

Remember: Good simulation testing saves hours of debugging on real robot!

Still stuck? Share your simulation problem - we’ll help you figure it out!