# Lesson 4: Distance Sensor

# Objective

Teachers will teach students how to read and use the XRP’s built-in distance sensor to measure distances and control robot behavior based on sensor input.

# Prerequisites

Completion of Lesson 3: Subsystems and Button Bindings.
XRP’s distance sensor is connected and functional.
Students understand Java conditionals and the command-based structure.

# Teaching Notes

This lesson introduces basic sensor integration. Focus on reading real-time data from hardware, interpreting it, and reacting to environmental changes.

# Lesson Outline

# 1. Discuss Sensor Purpose

Explain what the distance sensor does:

Emits infrared or ultrasonic signals.
Measures time until signal returns.
Reports distance in centimeters.

Show how sensors allow the robot to make autonomous decisions.

# 2. Creating DistanceSubsystem.java

package frc.robot.subsystems;

import edu.wpi.first.wpilibj2.command.SubsystemBase;
import edu.wpi.first.wpilibj.xrp.XRPRangefinder;

public class DistanceSubsystem extends SubsystemBase {
    private final XRPRangefinder distanceSensor = new XRPRangefinder();

    public double getDistanceIn() {
        return distanceSensor.getDistanceInches();
    }
}

# 3. Creating StopAtWall Command

This command drives forward until the robot is within a threshold distance from an obstacle.

package frc.robot.commands;

import edu.wpi.first.wpilibj2.command.Command;
import frc.robot.subsystems.XRPDrivetrain;
import frc.robot.subsystems.DistanceSubsystem;

public class StopAtWall extends Command {
    private final XRPDrivetrain drive;
    private final DistanceSubsystem sensor;

    public StopAtWall(XRPDrivetrain drive, DistanceSubsystem sensor) {
        this.drive = drive;
        this.sensor = sensor;
        addRequirements(drive, sensor);
    }

    @Override
    public void execute() {
        drive.arcadeDrive(0.6, 0);
    }

    @Override
    public void end(boolean interrupted) {
        drive.arcadeDrive(0, 0);
    }

    @Override
    public boolean isFinished() {
        return sensor.getDistanceIn() < 8.0; // Stop within 8 in of obstacle
    }
}

# 4. Binding to Button

In RobotContainer.java:

private final DistanceSubsystem distanceSubsystem = new DistanceSubsystem();

private final StopAtWall stopAtWall= new StopAtWall(m_xrpDrivetrain, distanceSubsystem);

private void configureBindings() {
    controller.rightTrigger().onTrue(stopAtWall);
}

# Teaching Tips

Have students log distance readings using System.out.println().
Place obstacles at different distances and test stopping accuracy.
Discuss sensor noise and real-world variability.

# Extensions

Use the sensor for automatic parking or alignment.
Plot sensor readings in AdvantageScope (preview for Lesson 7).
Introduce averaging or filtering for more accurate readings.

# Estimated Duration

45 minutes (concept + testing)

# Learning Outcomes

Understand how to interface with a distance sensor.
Use sensor feedback to stop or adjust robot behavior.
Build foundational logic for autonomous navigation.