We are going to start out with a simple program and will gradually add complexity to it. If you are using the Ultrasonic Sensor, it should be plugged into port 4, and this program is on the top line. If you are using the IR sensor, it should be plugged into port 1 and this program is at the bottom line. In this program, the robot moves forward until the Wait block tells it to stop 25 units from a wall or other barrier. You will find that the Ultrasonic Sensor can be set to stop in units of inches or centimeters. The Ultrasonic Sensor emits high-frequency sound waves (above the range of human hearing) and measures the time delay between the emission of the sound waves and when the reflection off an object is measured by the sensor.
In everyday conditions, we can assume that the speed of sound is constant, and thus the Ultrasonic Sensor can give precise distance measurements to the nearest centimeter. As mentioned in Chapter 4, Sensors and Control, using presence we could use the Ultrasonic Sensor to detect other robots or a loud sound. In other programming languages, you could even use the Ultrasonic Sensor to transmit data between two robots. The IR sensor emits infrared light and has an IR-sensitive camera that measures the reflected light. The sensor reading does not give exact distance units because the strength of the signal depends on environmental factors such as the reflectivity of the surface. What the IR sensor loses in precision in proximity measurements, it makes up for in the fact that you can use it to track on the IR beacon, which is a source of infrared light. In other programming languages, you could actually use the IR sensor to track on sources of infrared light other than the beacon (such as humans or animals).
In the following screenshot, we have a simple program that will tell the robot to stop a given distance from a barrier using a Wait for the sensor block. The program on the top of the screenshot uses the Ultrasonic Sensor, and the program on the bottom of the screenshot uses the IR sensor. You should only use the program for the sensor you are using. If you are downloading and executing the program from the Packt Publishing website, you should delete the program that you do not need.
When you execute the program in the preceding screenshot, you will find that the robot only begins to stop at 25 units from the wall, but cannot stop immediately. To do this, the robot will need to slow down before it gets to the stopping point.