== Ultrasonic Rangefinder ==
Robots need to sense the environment around them, particularly when driving autonomously.  Ultrasonic sensors are like the sonar used by bats.  [[Image(UltrasonicSensor.jpg,right,250px,margin=10)]] They emit a chirp of sound and measure the time it takes to hear an echo of that chirp.  The longer the time, the further the sound traveled.  Since sound travels at a particular speed (about 1125 ft/s) through air, we can use the echo delay to calculate distance to the object was that reflected the sound.  You can read more about ultrasonic sensors on [https://wpilib.screenstepslive.com/s/currentCS/m/java/l/599715-ultrasonic-sensors-measuring-robot-distance-to-a-surface ScreenStepsLive]

Ultrasonic sensors are connected to the RoboRIO using DIO ports.  In the example below, a sensor's !Ping/Trig and Echo wires are connected to DIO8 and DIO9 respectively.

Create another program using the !TimedRobot java template and name it !UltrasonicTest. Modify the auto-generated template code in Robot.java as follows:

 * import the [https://first.wpi.edu/FRC/roborio/beta/docs/java/edu/wpi/first/wpilibj/Ultrasonic.html Ultrasonic] class
{{{
   import edu.wpi.first.wpilibj.Ultrasonic; 
}}}
 * declare a ultrasonic variable in the Robot class
{{{
   private Ultrasonic f_ultrasonic;
}}}
 * in robotInit() instantiate an ultrasonic object and set it to start automatically ranging
{{{
    // Sensor is connected to DIO8 (trig), DIO9 (echo)
    f_ultrasonic = new Ultrasonic(8,9);
    // Start sensor continuously pinging
    Ultrasonic.setAutomaticMode(true);
}}}
 * in robotPeriodic() read and display the range
{{{
    if (f_ultrasonic.isRangeValid()) {
       SmartDashboard.putNumber("Front range", f_ultrasonic.getRangeInches());
    }
}}}

Build and Deploy the program to your robot as you did in the [wiki:ControlSystems/SoftwareTeam/Training/GettingStarted/XboxController previous example] and observe the "Front range" value in the [https://wpilib.screenstepslive.com/s/currentCS/m/java/l/599724-displaying-data-on-the-ds-dashboard-overview Smart Dashboard] as you move the robot towards and away from the wall.

Notice that the measurements aren't always perfect; the sensor may receive echos from multiple surfaces.

=== Extra Credit ===
Explore more advanced ultrasonic sample program [wiki:ControlSystems/SampleCode/Ultrasonic here].  This program shows how to use two ultrasonic sensors (front and rear).  The page also contains a corrected Ultrasonic class since the default class provided in WPILib only supports one sensor.