One of the most important things an autonomous robot must be able to do is see and understand its surroundings. There are many ways to accomplish this. If you have a lot of time or money to blow, you could use LIDAR or vision processing but these systems are often unnecessary.

In the past, I have used the ever-popular HC-SR04 ultrasonic distance sensor modules, however for an upcoming project I need something less bulky and more precise. I want to use IR distance sensors. IR distance sensors have two main components, an IR LED and an IR phototransistor. The LED shines on a nearby surface while the phototransistor measures how much of the IR light bounces back. A while back I bought some digital IR distance sensors off amazon but I needed an analog sensor. I cannibalized an LED and phototransistor from one of the Amazon modules and built a prototype. Here is my schematic:

I didn’t have any 100-ohm resistors so I used two 200-ohm resistors in parallel. Here is the sensor:

3.3V is blue, GND is green, and analog out is yellow. I know my colors are all wrong, but I was working with what I had.

The output from an IR distance sensor is not useful because the output is inversely proportional to the distance it measures. To get useful output data I roughly followed the steps on this webpage. Basically, you can convert the analog data into distance using this equation:

distance = (1 / (a * analogIn + b)) - k

I wasn’t super concerned about precision in this proof of concept so to find a, b, and k, I measured some known distances and recorded the corresponding analog output:

Analog value (0-1023)	Distance (mm)
971	20
810	25
440	60
270	120
210	240
192	300

I set up the data, variables, and distance function in a desmos plot and adjusted my a, b and k until the plot fit the points:

I ended up with a = 0.000043, b = -0.005, and k = 15. In order to make the calculation a little easier on your microcontroller the roboticlab page recommened rearanging some of the terms:

distance = (1 / (a * analogIn + b)) - k

distance = (1 / a) / (analogIn + B / a) - k

solving for 1/a and B/a, I got this final formula:

distance = (16667 / (analogIn - 133)) + 15

Now that I was reading distance (which was pretty cool by itself) I wanted to implement it on a robot. This chassis is from a previous project, a fairyweight combat robot with a custom controls system. I plan to one day document that project on this blog, but for now just know that it uses an Arduino Pro Mini as the “brain” and an L293D dual H-bridge for driving the motors (these components are underneath the robot).

In the first test, I wanted to have the robot maintain a constant distance from whatever was in front of it. It drove forward until it was 20mm from a wall if it became closer than 20mm it would back up. This is called bang-bang control and it was essentially useless as the robot oscillated dramatically. To improve performance I switched it to a PD loop and added a rolling average for the analog inputs. This produced much better results.

There is a lot that can be improved but for now, I am going to call this a success, in my next test I am going to rotate the sensor 90 degrees and see if I can get it to follow a wall while remaining a constant distance from it.