Breo's IT, Telecomms, Project Management & some reflections Blog

The past year (2010, last 5-year degree course) I studied real real-time systems design and I did an (optional and) very interesting classwork with an Lego MindStorms NXT, in which I had to see how it works and how to programming it (a very small research )

I had a floor with gray tiles and a roll of white adesive tape… And with this material we build a “racing circuit”, in which the Lego had to run over the white tape.

Introducing NXC (a version of C)

NXC is very similar to C language. You can learn a lot about it in the official site, in the NXC API section. It is on the Site of Bricx Command Center (BrixCC). There you can download the IDE (problem: only windows version, but actually you have Linux or Mac OS X alternatives, the NeXT Tools).

Code in C (NXC)

Tested and working (with the gray tiles and white adesive tape parameters).

Sorry, comments are in Galician language…

int CINTA_BLANCA_MIN = 47;				 // white tape: minimum value

int curve_time = 50;						   // 0,1 seconds by default
int move_time = 100;						   // 0,2 seconds by default

int speed_curve = 75;						   // % of maximum engine capacity
int opposite_speed_curve = 25;
int speed_straight = 50;					 // % of engine capacity to go straight

task main()
{

	SetSensorLight(IN_1);					// Indicates that this port (right side) is connected to an optical sensor
	SetSensorLight(IN_4);					// " (but left side)

	while (true)
	{
/*
		// RIGHT AND LEFT SENSOR CHECK THE AT SAME TIME (if it detects the tape): We assume that the NXT is perpendicular to the tape ...
		if ( (Sensor(IN_1) > CINTA_BLANCA_MIN) & (Sensor(IN_4) > CINTA_BLANCA_MIN) )
		{
			OnFwd(OUT_C, speed_curve);		       // turn left
			Wait(curve_time);				             // continue path during curve_time
		}
*/
		// RIGHT SENSOR CHECK (if it detects the tape)
		if (Sensor(IN_1) > CINTA_BLANCA_MIN)
		{
      // turn left
			OnFwd(OUT_A, speed_curve);            // accelerate engine (side we want to turn)
			OnFwd(OUT_C, opposite_speed_curve);  	// decelerate opposite engine
			Wait(curve_time);			                // continue path during curve_time
		}
		// LEFT SENSOR CHECK (if it detects the tape)
		else if (Sensor(IN_4) > CINTA_BLANCA_MIN)
  {
      // turn right
			OnFwd(OUT_C, speed_curve);
			OnFwd(OUT_A, opposite_speed_curve);
			Wait(curve_time);
		}
		else	// go straight!
		{
			OnFwd(OUT_AC, speed_straight);  			// speed up and go straight
			Wait(move_time);				              // continue path during move_time
		}
	}
	Off(OUT_AC); // really, this order must be never executed...
}

Explanation about leJOS: NXJ, a Java version for Lego NXT

LeJOS (according to the official website read pronounced as the Spanish word “lejos”, meaning far) is a small Java virtual machine. In 2006 it was ported to the LEGO NXT brick.
This virtual machine is a firmware for the NXT LEGO block: you have to install the new firmware NXT, overwriting the existing (it can be a risk).
The firmware installer and compiler development for the computer is available for 32-bit Windows, GNU/Linux or Apple Mac OS-X in http://lejos.sourceforge.net/nxj-downloads.php

By this way, your lego supports what is called NXJ, “Not Exactly Java”, very similar to Java but not exactly Java (remember NXC)

Code in Java… better said, in Not eXactly Java (NXJ)

Not tested (the Lego wasn’t mine and the owner, the Professor, decided it was too risk changing the firmware, and I agree with him, but if you have enough money and like the risk, you can try; when I buy a NXT, I will convert it in a NXJ)

import lejos.nxt.*;
import lejos.util.*;

public class circuito1 {
	static int  CINTA_BLANCA_MIN = 47;				 // mínimo valor en cinta blanca

	int curve_time = 50;						   // 0,05 segundos por defecto
	int move_time = 100;						   // 0,1 segundos por defecto

	/* Velocidades para usar con Motor.setSpeed.
		En Motor.setSpeed configúrase o motor en º (grados) por segundo.
		Según a información atopada nos Java Docs do API, o tope (sin conectar o LEGO a 8V) parece é 900
		NOTA: habería que revisar na práctica estes valores, aquí son calculados teoricamente.
	*/
	// NXC -> 75%. 75% of 900 = 675 degrees per second
	int jspeed_curve = 675;						   // 75% of Max. encine capacity
	// NXC era 25%. 25% de 900 = 225 degrees per second
	int jopposite_speed_curve = 225;		 		// 25% of Max. engine capacity
	// NXC era 50%. 50% de 900 = 450 degrees per second
	int jspeed_straight = 450;					 // 60% of Max. engine capacity
	TimerListener tL;
	Timer tmr = new Timer(move_time, tL); // Timer(int theDelay, TimerListener el)

	public void circuito1()
	{
	}; // circuito1

	public void corre(){

		LightSensor light1 = new LightSensor(SensorPort.S1);		// Indicates that this port (right side) is connected to an optical sensor
		LightSensor light4 = new LightSensor(SensorPort.S4);		// " (left side) Indicamos que a ese porto está conectado un sensor óptico. Será o do lado esquerdo

		while (true)
		{
			// CHEQUEAMOS SENSOR DEREITO (se detecta a cinta)
			if (light1.readNormalizedValue() > CINTA_BLANCA_MIN)
			{
				// xiramos á esquerda
				Motor.A.setSpeed(jspeed_curve);           // aumentamos velocidad en motor del lado que deseamos girar
				Motor.C.setSpeed(jopposite_speed_curve); // reducimos velocidad en motor contrario
				tmr.setDelay(curve_time);			     // continuamos a traxectoria durante curve_time
				tmr.start();

			} // if
			// CHEQUEAMOS SENSOR ESQUERDO (se detecta a cinta)
			else if (light4.readNormalizedValue() > CINTA_BLANCA_MIN)
			{
				// xiramos á dereita
				Motor.C.setSpeed(jspeed_curve);            // aumentamos velocidad en motor del lado que deseamos girar
				Motor.A.setSpeed(jopposite_speed_curve);	// reducimos velocidad en motor contrario
				tmr.setDelay(curve_time);			       // continuamos a traxectoria durante curve_time
				tmr.start();
			} // else if
			else	// go straight!
			{
				// aceleramos e seguimos recto
				Motor.A.setSpeed(jspeed_straight);
				Motor.C.setSpeed(jspeed_straight);
				tmr.setDelay(move_time);			     // continuamos a traxectoria durante curve_time
				tmr.start();
			} // else
		} // while
	} // corre()

	public static void main (String[] args) {
		circuito1 c1 = new circuito1();
		c1.corre();
	} // public static void main

}

I’m sorry, I didn’t recorded the Lego doing the “adesive tape circuit”

Paper download/visualization

You can see or download the PDF (in Spanish) here

More info:

• NXC Programmer’s Guide
• Interesting blog I discovered now: The NXT Step

More about Lego (and Lego & NASA )

• Interesting site I discovered recently: Lego Space experience… At section “videos” you can find “How to build a Satellite“…
• Lego Space at Wikipedia