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Wednesday, April 22, 2009

A neural network for Java Lego robots - Learn to program intelligent Lego Mindstorms robots with Java

By Julio César Sandria Reynoso,, 05/16/05

This article shows how to develop a robot that can learn by using the backpropagation algorithm, a basic neural network, and implementing it on a Lego Roverbot. Using both the algorithm and Java, the Roverbot—a Lego robot vehicle—can learn some basic rules for moving forward, backward, left, and right.

In this article, we use the Lego Mindstorms Robotics Invention System 2.0 for building the Lego robot; leJOS 2.1.0, a little Java operating system for downloading and running Java programs inside the Roverbot; and J2SE for compiling the Java programs under leJOS.
Lego robots

The Lego Mindstorms Robotics Invention System (RIS) is a kit for building and programming Lego robots. It has 718 Lego bricks including two motors, two touch sensors, one light sensor, an infrared tower, and a robot brain called the RCX.

The RCX is a large brick that contains a microcontroller and an infrared port. You can attach the kit's two motors (as well as a third motor) and three sensors by snapping wire bricks on the RCX. The infrared port allows the RCX to communicate with your desktop computer through the infrared tower.

In this article, we use a Roverbot as it is constructed in the Lego Mindstorms Constructopedia, the guide for constructing robots. This Roverbot, as shown in Figure 1, has been configured to use all three sensors and two motors included in Lego Mindstorms RIS 2.0.

Figure 1. A Lego Roverbot with two touch sensors, one light sensor, and two motors

leJOS is a small Java-based operating system for the Lego Mindstorms RCX. Because the RCX contains just 32 KB of RAM, only a small subset of the JVM and APIs can be implemented on the RCX. leJOS includes just a few commonly used Java classes from java.lang,, and java.util, and thus fits well on the RCX.

You must load the RAM with the Lego firmware, or, in our case, with the leJOS firmware, and your programs. The firmware contains a bytecode interpreter, which can run programs downloaded from RCX code.

For setting up your leJOS installation, please take a look at Jonathan Knudsen's article "Imaginations Run Wild with Java Lego Robots," (JavaWorld, February 2001), Programming Lego Mindstorms with Java (Syngress Publishing, 2002), or the leJOS readme file contained in the leJOS zip file, which you can download from the leJOS homepage.
Neural networks

If we want to build intelligent machines, we should model the human brain. Early in the 1940s, the neurophysiologist Warren McCulloch and the mathematician Walter Pitts began working on the idea of building an intelligent machine out of artificial neurons. One of the earliest neural network models was the perceptron, an invention of F. Rosenblat in 1962. A perceptron can learn; it models a neuron by taking a weighted sum of its inputs and sending an output of 1 if the sum is greater than some adjustable threshold value, otherwise it sends 0. If a perceptron can compute, it can learn to compute. Figure 2 shows a neuron and Figure 3 shows a perceptron.

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