Servo - November 2009

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A Brain For All Seasons

With a “brain,” your robot becomes more than just a simple automated machine. To be a true robot, the brain processes outside influences whether they be light sensors, accelerometers, or lowly bumper switches. Then, from these senses, the robot determines a proper course of action.

In most robots, a computer of one type or another is the most common type of brain. A robot control computer can be — but is only occasionally — like the PC on your desk. Rather, the modern-day robot uses a specialized form of computer called the microcontroller or single board computer, both of which are small, lightweight, and relatively inexpensive. Of course, not all robot brains are even computerized. As we discussed in the September ‘09 installment of Robotics Resources, a perfectly good robot brain can be a simple assortment of electronic components.

In this month’s column, we’ll provide a quick review of the major types of “brains” available for your robot. In choosing the right kind of brain for your bot, you balance the computational power you need with size, weight, and power consumption.

Brains from Small Parts

We already covered creating robot brains out of discrete components (transistors, resistors, linear and digital ICs) last September,

so we won’t spend much time with the subject here. But in summary, you can create a perfectly agile and workable robot using the simplest of electronics. In fact, this is the concept of the BEAM robotics philosophy. Simple functions such as seeking or avoiding light, or reacting to contact sensed by a mechanical bumper switch need only a simple circuit.

You may need to augment the basic electronic designs with relays or power transistors in order to control larger motors. However, this is a requirement of most any control circuit, even one based on the latest microprocessors. Most robot brains operate at 3. 3 to 5 volts, and their components are rated to handle only minimal currents — most up to tens of milliamps. Anything more and you run the risk of burning things out. Relays, power transistors, and other power-handling components allow the low-current components to operate even the largest of motors.

Single-Chip Microcontrollers

Since the early 1990s, a preferred method of endowing a robot with smarts is the microcontroller. Microcontrollers are stand-alone programming computers that are designed to connect with external components. They are inexpensive, have simple power requirements (usually just five volts or less), and most can be programmed using software on your PC. Once programmed,

the microcontroller is disconnected from the PC and operates on its own.

Microcontrollers are, in effect, programmable integrated circuits. Using a standard programming language, you define how the various inputs and outputs of the chip behave. Following the cues of your program, the microcontroller accepts input, analyzes it in one way or another, and outputs some value. This is fundamentally the same as any computer, except that a microcontroller is primarily designed to operate things (motors, relays, lamps, etc.) rather than interact with people through the use of a keyboard and display monitor.

Assembly language is a common method to programming microcontrollers, but it is by no means the only method. At least for amateur robotics, a high-level language such as C, Basic, or Pascal is used to program the chip. This process involves using a compiler that converts the syntax of the high-level language to a machine readable form, which is then downloaded from the PC to the controller.

There is also another popular programming approach some microcontrollers use. With these, a high-level language interpreter is built into the chip. You still write your program on a PC but, in this case, your instructions are loaded into the chip, and a resident program interprets your program one step at a time. This is the method use by the venerable BASIC Stamp, for example.