Servo - January 2008

Tune in each month for a heads-up on where to get all of your “robotics resources” for the best prices!

Small Brains for Your Bot

As a child, I imagined robots being governed by tubes and relays — not an unusual image given the science fiction movies of the time, like Robbie the Robot in Forbidden Planet. Today, fictional robots are depicted with miniature microelectronic brains, with “ emotion chips” the size of a fingertip. And no wonder, because these things actually exist! Far from science fiction, with today’s technology you can build a robot with a brain no larger than a caterpillar.

What’s more, these brains are designed to interface with the outside world. These so-called microcontrollers are part computer, part input/output. In one chip-sized package is an affordable programmable computer with a multitude of I/O for connecting to a robot’s various motors and sensors. The best news is that these microcontrollers are inexpensive. Some are as cheap as a dollar each; versions that come with a complete development kit are $50 to $150. In all, a bargain.

This month, we’ll explore a variety of options for inexpensive microcontrollers suited for small robotics. Though the microcontroller is basically a generic, universal device, some of these products are expressly designed for use in amateur robots. And those that aren’t are still very capable of the job, as they have the core ingredients needed to control most any real-world device.

Under the Hood

Microcontrollers are single-chip computers, capable of running user-defined programs, accepting input from switches and other devices, and control-

ling the state of one or more outputs. Microcontrollers are expressly designed to be used in so-called embedded applications, where control of some external device is the main goal. Typical uses for embedded chips include the on-board computer in your car, the “smarts” in a modern-day television, even the control circuitry in a coffee maker.

For the typical robot application, the microcontroller uses previously prepared custom programming to read one or more sensors. Based on the condition of those sensors, the controller then activates or deactivates outputs connected to motor drivers.

For example, suppose you’ve built a robot that senses the brightest light in the room. The robot is “trained” to go to that light. You’ve built your bot with two light sensors, both of which point straight ahead like headlamps on a car. These light sensors are connected to two analog inputs of the microcontroller (not all microcontrollers have analog inputs; this is just for illustration purposes). The program — which you’ve written and which constantly runs in the microcontroller — reads the intensity of the sensors. The controller will activate either of the robot’s two motors in order to steer the vehicle into the direction of the light.

The brains on board your bot know the light is straight ahead when the reading of the two sensors is the same. At that point, the controller activates both motors at the same time, causing the vehicle to move toward the light.

This is just one of many possible applications for microcontrollers in robotics. As you work with microcontrollers and

various types of sensors — be they optical, ultrasonic, or whatever — new and creative uses for your robot emerge. For instance, if you can interface an electronic compass to your robot, you can write a routine for the microcontroller that tells the robot which direction it’s facing.

A tilt sensor could be used on a self-balancing robot (two wheels, like a Segway); a GPS could inform the robot where it is on the Earth down to the nearest few feet; an optical distance sensor could tell the bot how far away it is from the nearest object, and so on.

Variations in Design and Features

Not all microcontrollers are the same. Sure, they all contain some kind of computational unit, some place to hold your program (microcontrollers retain their programming when switched off or disconnected from power), and some working RAM to run everything. But they do vary in things like the number and type of inputs and outputs, internal timers, and the amount of space for programs.

The most basic microcontroller has a small handful (four or five) of connecting pins that can be used as either inputs or outputs. These I/O pins — or lines — are purely digital. That is, as inputs, the lines can be low or high (0 or five volts, respectively). Same if the pins are acting as outputs. This means you can read on/off type sensors such as switches, and control on/off devices such as LEDs and motors.