I had wanted to build a robot for years and my friendly competition finally gave me a motivation: to complete my robot before Jacob’s and throw my success right in his face.

Our Problem:

It seems simple enough at first glance. We just need to create robots capable of autonomously mapping a small, indoor space such as a 1 bedroom apartment. The robots need to determine the outline of the room and map all of the obstacles (furniture, tool boxes, piles of clothes, etc) within the room. We even made a couple of concessions to speed things up: The robots do not have to operate on carpet and they are allowed to use fixed points (such as RF beacons) for navigation. Easy, right?

Of course, neither of us know very much about robotics or programming, but learning-on-the-fly is sort of the point of this exercise.

Physical Requirements:

An abandoned early design.

I decided to sort out my robot’s physical design before worrying about software, which would likely be far more difficult. So I need to ask, what does my robot need to do?

• Move around
• Control its own movements
• Power itself
• Transmit map data to a remote computer
• Sense its surroundings
• Sense its location

I realized from the outset that I would probably need to change the design in response to unforeseen problems, so I also wanted my robot to be easy to rewire and reconfigure. Also, I’m not a rich man, so I need a robot that will fulfill these requirements without being absurdly expensive. This means I need to use cheap, off-the-shelf parts as much as possible and keep custom fabrication to a minimum.

Finally, I my robot needed a name. Referring to it as “my robot” is just too cumbersome. I named it Colin, after the happy security robot from Mostly Harmless by Douglas Adams.

Design

For movement I elected to go with a differential drive system, like you would find on a Roomba. It has two parallel wheels, each with its own motor. Colin can move forward or backward by running his motors at the same speed and he can rotate by running them at different speeds. I also considered omni-wheels for added maneuverability but the extra complication in design and programming did not seem to be worth the benefit. I chose a pair of gearmotors and wheels from Pololu. I made sure to get motors with extended backshafts so they could be used with encoders. This will be important later. Also, most controllers can’t directly power a motor, so a motor driver circuit will be necessary as well.

For control I decided to use an Arduino Duemilanove. It is functionally equivalent to the newer Arduino UNO and I had one lying around from a previous project. The computation requirements of this project are probably greater than the Arduino’s ATmega328 can handle, but it will suffice for awhile. I can always change it out later.

For power I’m using four AA batteries. I badly wanted to use a lithium polymer battery like one of these from SparkFun but the extra expense did not seem worth the slight convenience.

To transmit map data I’m using a bluetooth radio. It probably would have been easy enough to do this via USB but bluetooth gives more versatility. Colin can’t be tethered by a USB cable while he’s mapping a room, but bluetooth presents no such problem. He can send back error messages and status updates to let me know how his job is progressing. Also, it seemed like a good idea to learn how to use bluetooth.

For sensing I decided to start with HC-SR04 ultrasonic sensors. They’re fairly accurate and Arduino already has a good code library to handle them thanks to Tim Eckel. The main advantage though, is that they’re dirt cheap. If you shop around on amazon you can get them for 2 or 3 dollars apiece including shipping.

For localization (telling Colin where he is) I’m using shaft encoders on my motors. These can be used to record how far the motor shaft has turned or how fast it’s turning, allowing Colin to calculate his present position relative to where he started.

To hold everything together I’m using a very simple chassis made from laser cut ABS plastic. I made my own designs using Inkscape and had the plastic custom cut at Metrix Create:Space in Seattle.

Lastly, to ensure my design is easy to reconfigure and rewire I’m doing all of my connections using a breadboard. If you’ve never used a breadboard I suggest you make learning to use one your first step before starting any electronics project. They allow you to build circuits without soldering, so rewiring is a snap.

I’ll include details on how to put these elements together into a functioning robot in later posts. Also stand by for tips on programming and example code!