3.5: Bottom Range (Height) Sensor

Make sure that your drone is connected with Blockly before starting the lesson! If you are unsure of how to connect, go back to lesson 1.1 to review!

The goals for this lesson are:

1. Understand how the get_height() function simplifies calculations.
2. Learn how the drone uses barometer values to measure height.
3. Use get_height() at different points in a program.
4. Store the drone’s height as a variable and make decisions with it.
5. Identify differences between measured height and actual height.
6. Explore why program height might differ from reality.

The bottom range sensor relies on an infrared sensor chip. Underneath your drone, you will be able to find the bottom range sensor. See those two dots? One of those emits the infrared light, and the other receives the infrared that is reflected from the surface. The chip calculates the distance based on the time it took for the infrared light to come back to the receiver.

Note: The maximum range for the bottom range sensor is around 1.5 meters.

In the last lesson, you learned about the barometer as a way of estimating altitude. Now, you’re going to explore measuring height using the bottom range IR height sensor. There are two functions from the “Sensors” menu that you can use. The blocks get_range(bottom,cm) and get_height() will return the same values.

What is the average takeoff height for CoDrone EDU? Find out by running a test. Build a program that takes off, prints the height, then lands.

Now, run the experiment 5 times. Record the height for each time. What is the average? Does it change depending on the battery level?

You can use the get_height() function in a loop to measure height repeatedly. Start with a takeoff and set the throttle variable to 20. Fly up for one second and take a height reading. Repeat for a total of 3 times.

The height sensor can also be used to control throttle. In this program, CoDrone EDU will take off and rise up to a 1 meter (100cm). Here are the steps.

1. Create a variable called max_height and set it to 100 (this will be in centimeters).
2. Takeoff and set throttle to 20.
3. Use the height sensor to read the height in centimeters and store it in a variable called current_height.
4. In a while loop, move up while current_height returns a value less than max_height.
5. Update current_height by taking another reading of the height sensor.
6. Print the value of current_height.
7. Add a land outside the loop.

Run the code. Does the drone stop flying up at 100cm? You might notice some print statements return 999.9. Let’s improve the code in the next step.

Sometimes the height sensor will return a reading of 999.9 if the sensor was not able to retrieve a value or if there was an error. Fortunately, we can improve the code to account for this scenario. A sensor value of 999.9 is an error, so CoDrone EDU should continue to throttle up. To do this, we will add an or statement to the conditional. From the “Logic” menu, select the logical operators block.

Use the dropdown menu to change the operator to or. Then, temporarily remove your original conditional statement and replace it with this one.

The program should still keep flying if the current height has not increased past the max height. Return the original conditional to the left side of the or statement.

Finally, the program should also check if the height sensor returned an error. In other words, if the height sensor is below the maximum height OR the height was above 999 (an error), keep flying. Fill in the rest of the conditional and test your program.

You learned how to combine if statements to check and ignore any height sensor data that may not be useful for your program.

You have been using the height sensor to fly to various heights, but how accurate is it? For this challenge, you will create a relationship between the actual height of the drone and the data returned from the IR sensor. Grab a partner for this activity to take measurements more easily. Here are the steps:

• In a forever loop, write code that will print the height sensor data when the space bar is pressed
• In the loop, after each keyboard press, use a sleep()function for 0.5 seconds to delay the key press for the next measurement.
• Find a ruler or measuring tape to use for measuring the drone’s height. This is easier with a partner so that they can hold the measuring tape for you.
• Run your program so that it’s ready to receive key press commands.
• Move the drone with your hand to various heights. For example, measure 6 inches off the ground, 12 inches, 18 inches, etc. Press the space bar at each reading and record the result.

Finally, analyze the results. How sensitive is the height sensor? Does it change based on the flooring? Remember that the height sensor relies on infrared light to make its measurements. If light cannot reflect well off the surface, you may get an incorrect or inaccurate reading.