Build a Model Planetary Lander with Arduino

This project follows the Engineering Design Process. Confirm with your teacher if this is acceptable for your project, and review the steps before you begin.

In this procedure, you will first assemble your circuit and test your code. This makes it easier to identify and fix any problems with your circuit or code. After you have everything working, you will build your model lander, attach your Arduino and circuit, and conduct drop tests. 

Assemble Your Circuit

Assemble your circuit as shown in Figures 2 and 3.

1. Connect the Arduino's 5V and GND pins to the breadboard's positive (+) and negative (-) buses respectively.
2. Connect the ultrasonic sensor:
   1. VCC pin to 5V
   2. Trig pin to Arduino pin 7
   3. Echo pin to Arduino pin 8
   4. GND pin to ground
3. Connect the four servo motors:
   1. Ground (brown) wires to ground
   2. Power (red) wires to 5V
   3. Signal (orange) wires to Arduino pins 2, 3, 4, and 5
4. Press the servo horns onto the servo motors, but do not screw them in place yet. 

Image Credit: Ben Finio / Science Buddies

Figure 2. Breadboard diagram for circuit.

Image Credit: Ben Finio / Science Buddies

Figure 3. Circuit schematic.

Test Your Code

1. Download our ultrasonic lander example code. You can also access a Tinkercad Circuits simulation here.
2. Read through the commented code so you understand how it works.
3. Pay close attention to the thresh, angle1, and angle2 variables. You may need to adjust these variables later depending on your lander design.
4. Open the serial monitor in the Arduino IDE (Tools→Serial monitor). 
5. Move your hand back and forth in front of the ultrasonic sensor.
   1. Watch the output in the serial monitor. If the output does not change when you move your hand, double-check that your ultrasonic sensor is wired correctly. 
   2. If the sensor has trouble detecting your hand, try using a large, flat object like a piece of cardboard instead.
   3. Do the servo motors move when you cross the threshold distance (100 cm in the example program)? If the servo motors do not move, double-check that none of your wires are loose. 

Build Your Lander

Now that you have your circuit and code working, it is time to build and test a lander. You need to protect your valuable payload (your Arduino and circuit) from damage. Here are some things to consider for your design:

1. Your lander needs to hold and protect your Arduino, breadboard, and 9V battery.
2. The ultrasonic sensor needs an unobstructed view of the ground.
3. The servo motors should be positioned to deploy landing gear (such as popsicle sticks glued to the servo horns) without getting snagged on any of the wires or parachute strings.
4. You should try to slow your lander down as much as possible while it is still in the air. Parachutes and streamers, which add air resistance, are a good way to accomplish this.
5. Your lander should have features that absorb some of the force upon impact. If you try to make a very stiff, rigid lander, it may just break when it hits the ground (and in turn, damage your Arduino). It is better to build some flexibility or springiness into the design of your lander to help absorb the impact.
6. Consider the aerodynamic stability of your lander. You do not want your lander to tip over while falling and land on its side or upside-down. 
7. Remember that you can use an in-line barrel plug switch to turn your Arduino on and off instead of unplugging the battery, but this will add more weight.
8. For many Arduino projects, you should not use a single 9V battery to power multiple motors since it will drain too quickly. However, since these motors only need to deploy once during landing, and you want to keep your lander as light as possible, it is OK to do so for this project. 
9. Figures 4 and 5 show top and bottom views of a very basic lander design, but your design can be different. 
   1. The electronics are all mounted on a flat, square piece of cardboard.
   2. The cardboard has two holes cut in it for the downward-facing ultrasonic sensor.
   3. The servo motors are attached to the four corners of the piece of cardboard, with popsicle sticks glued to the servo horns to act as landing gear.
   4. The servo horns are attached to the motors such that the popsicle sticks point straight up when the servo angle is set to 0, and straight down when the angle is set to 180. Depending on how you mount your servos, you may need to change the angle1 and angle2 variables in the code. 
   5. A parachute made from a plastic bag is attached to the four corners of the lander with strings. 

Image Credit: Ben Finio / Science Buddies

Figure 4. Top view of the model planetary lander. 

Image Credit: Ben Finio / Science Buddies

Figure 5. Bottom view of the model planetary lander. 

Test Your Lander

1. Comment out the Serial.begin and Serial.print lines in your code. These slow your program down, and you do not need them when your Arduino is no longer connected to your computer. You want your Arduino to react to changes in distance as quickly as possible. Re-upload the program to your Arduino. 
2. Do not drop your lander yet! Test it by holding it (either by the frame or letting it dangle from the parachute) and moving it up and down with your hand. Make sure that the servo motors switch positions when you cross the threshold distance, and that nothing gets tangled in the wires or parachute lines. 
3. Figure out a safe place to do some initial drop tests of your lander. You may want to drop it onto a pillow or other very soft surface to start. Start with a low drop height (although note that if you start out too low, your parachute might not have time to fully deploy).
   1. Note: the ultrasonic sensor may not work as well on soft, porous surfaces since they do not reflect sound as well as hard surfaces. If needed, try placing something flat and smooth (like a piece of paper or posterboard) on top of the soft surface (like a pillow or grass) to help it reflect sound better.
4. If possible, have someone use the slow-motion video mode on a phone to record your lander as it falls. 
5. Watch your lander fall, and if possible, review the slow-motion video. Based on your observations, think of possible ways to improve your design. Here are some suggestions:
   1. Do the legs fully deploy before the lander hits the ground? If not, how can you change the thresh variable to make the legs deploy earlier?
   2. Do the legs support the weight of the lander when it hits the ground? Do they slip out from under the lander, causing the servo motors to rotate backward? How could you potentially change the angle1 or angle2 variables to affect this result?
   3. How does the landing gear behave when it hits the ground? Can you modify the shape or materials used for the landing gear?
   4. What about the main body of your lander? Is it too flexible? Too rigid? How could you improve it?
   5. Is your lander going too fast when it hits the ground? How could you slow it down even more? What about a bigger parachute or multiple parachutes?
6. Make improvements to your lander design and test it again. Continue to iterate and improve your design. Can you build a lander that reliably lands upright without breaking, tipping over, or crash-landing too hard?