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Building  Robots  with  NXT  and  LEJOS    

Introduc<on  

Michael  Wooldridge  (mjw @ liv.ac.uk)

What  is  the  NXT  Robot  Pla@orm?  

• A  simple,  cheap,  well  supported  pla6orm  for  experimental  robo;cs,  manufactured  by  LEGO  

• Comes  with  a  collec;on  of  LEGO-­‐manufactured  sensors  and  actuators,  and  has  a  number  of  third  party  sensors  &  actuators  available  

• The  “brain”  is  the  NXT  brick  

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The  NXT  Brick  

The  NXT  Brick  

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The  NXT  Brick  •  3  ports  for  actuators  (named  A  to  C)  

• Mainly  used  for  motors  

•  4  ports  for  sensors  (named  1  to  4)  connected  via  RJ12  (phone-­‐like)  connector  

•  Touch  sensors,  colour  sensors,  …  •  LCD  display  panel  (monochrome,  100x64  resolu;on)  

•  Programs  can  write  to  this  panel  at  pixel  level  

•  4  Input  buWons  (leX,  right,  enter,  escape)  • USB  port  • Mainly  used  for  downloading  programs  to  the  brick  

•  Bluetooth  connec;on  •  Allows  robots  to  communicate  with  each  other  

The  NXT  Brick  

• The  brick  is  a  baWery-­‐powered  computer,  specifically  intended  for  robot  control  

• 32-­‐bit  ARM7  processor,  running  at  48MHz  

• 256  Kbytes  non-­‐vola;le  flash  storage  • mainly  used  for  storing  programs    

• 64  Kbytes  RAM    

• run;me  memory  

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Ultrasonic  Sensors  

• Essen;ally  sonars  • Used  to  find  the  distance  to  obstacles  • Work  by  transmidng  ultrasonic  signal,  and  ;ming  how  long  it  takes  for  the  signal  to  get  back  

• Works  up  to  about  255cm,  precision  of  ±3  cen;meters  

• !!CAUTION!!  Ultrasonic  sensors  interfere  with  each  other,  if  others  are  using  ultrasonics  nearby,  your  robot  might  get  confused  

Colour  Sensors  

• Can  be  used  to  iden;fy  the  colour  of  objects  immediately  in  front  of  them  

• Simplest  use  is  to  detect  colour  patches  (eg  tracks)  immediately  underneath  the  robot  

• Mainly  good  for  red,  green,  blue,  white,  black  

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Touch  Sensors  

• Detect  sensor  being  pressed  • Usually  used  to  make  bumpers  for  forward  colision  detec;on,  and  on  other  sides  to  detect  other  vehicles  driving  into  us  

• Return  a  boolean  value,    • True  =  depressed  • False  =  not  depressed  • If  we  want  many  touch  sensors,  we  can  use  a  mul-plexer  

Servomotors  • The  basic  actuators  we  will  use  • Not  simply  “on/off”  motors,  but  servo  motors  

• Allow  precise  posi;oning  of  motors,  fine  degree  of  control  of  rota;on  

• Surprisingly  powerful,  with  sophis;cated  gear  mechanism  

• Simplest  use  is  to  connect  to  wheels  and  use  for  locomo;on  

• More  sophis;cated  uses  include:  

• Building  robot  arms  

• Controlling  ultrasonic  sensor  to  give  360°  coverage  

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Standard  Configura<on  Robot  

• For  most  of  the  course,  we  will  use  robots  in  a  standard  configura;on  

• Sensors  • Port  1  –  ultrasonic  (facing  forward)  • Port  2  –  colour  sensor  (at  rear)  • Port  3  -­‐  empty  

• Port  4  –  touch  sensor  (front  bumper)  

Standard  Robot  Configura<on  

• Robot  with  caterpillar  tracks  • Output  port  A  –  right  motor  (i.e.,  right  from  robot’s  perspec;ve)  

• Output  port  B  –  leX  motor    

• Output  ports  C,D  -­‐-­‐  empty  

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Standard  Configua<on  Robot  

Standard  Configura<on  Robot  

Front bumper Touch sensor

Front-facing Ultrasonic

sensor

motors on left and right

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Standard  Configura<on  Robot  

ground facing colour sensor

at rear

Programming  NXT  Robots:  The  Default  Programming  Environments  

• Our  of  the  box,  NXT  bricks  come  with  a  LEGO-­‐provided  opera;ng  system  

• Programs  are  developed  on  a  PC  with  a  graphical  development  environment,  and  downloaded  to  the  robot  via  USB  

• This  development  approach  is  good  for  small  children,  wimps,  and  wusses  

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LEJOS  • LEJOS  provides  an  alterna;ve  opera;ng  system  for  NXT  robots  

• An  implementa;on  of  the  JAVA  virtual  machine  for  NXT  

• Not  all  Java  language  features  supported!  • Not  all  Java  classes  supported!    • The  LEJOS  environment  provides  a  much  richer,  object-­‐oriented  

development  environment  

• Special  classes  provide  interface  for  sensors  and  actuators  • Main  drawbacks:  limited  onboard  RAM,  limited  onboard  flash  

memory  for  storing  programs  (JAVA  not  renowned  for  being  compact)  

LEJOS  Development  Cycle  

1.  Develop  code  on  your  PC  (using  eg  Eclipse)  2.  Compile  &  link  

3.  Download  linked  binary  (.nxj)  to  robot  via  USB  4.  Run  Main  problem:  debugging  problem  code  on  robots  is  hard  –  very  limited  UI,  oXen  no  feedback  on  what  problem  is.  

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HelloWorld.java  

import lejos.nxt.*; public class HelloWorld { public static void main (String[] args) { System.out.println("Hello World"); Button.waitForPress(); } }

import lejos.nxt.*;  

• Makes  all  the  LEJOS  classes  available  by  default  

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public class HelloWorld  

• Just  as  in  regular  Java,  LEJOS  development  is  based  on  OO  principles  

• So,  the  program  is  built  from  a  set  of  classes  

public static void main (String[] args)  

• Just  like  regular  Java,  LEJOS  needs  a  class  that  contains  a  method  called  main• This  method  must  be  declared  •  public•  static •  void• With  arguments  as  indicated  

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System.out.println("Hello World"); Button.waitForPress();  

• If  our  main(…) method  just  contained  the  println(…) statement,  then  the  text  would  print,  but  then  the  program  would  terminate  and  the  LCD  panel  would  revert  to  the  default  –  text  would  disappear  

•  Button.waitForPress() causes  the  program  to  suspend  un;l  a  buWon  (any  buWon)  is  pressed.  

Using  Motors:  The  NXTMotor  Class  import lejos.nxt.*;import java.lang.*; // for Threadpublic class ProgOne { public static void main (String[] args) throws Exception { NXTMotor MA = new NXTMotor(MotorPort.A); NXTMotor MB = new NXTMotor(MotorPort.B); MA.setPower(100); //power to 100% MB.setPower(100); Thread.sleep(2000); // suspend for 2 seconds MA.stop(); // stop MB.stop(); }}

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Variables  

• No;ce  that  we  have  completely  standard  Java  variable  declara;ons:  

NXTMotor MA = new NXTMotor(MotorPort.A);NXTMotor MB = new NXTMotor(MotorPort.B);

• Here,  MA  and  MB  are  just  standard  (object)  variables  

• Can  use  int,  float,  etc  

The  NXTMotor  Class  • Class  constructor  takes  a  single  argument,  which  must  be  one  of  the  constants  A,  B,  or  C  defined  in  the  MotorPort  class  

• Other  methods  (actually  from  BaseMotor  interface,  which  NXTMotor  implements):  forward()  motor  forward  

backward()  motor  backwards  

flt()  cuts  power  to  motor,  which  “floats”  to  a  halt  

stop() stops  the  motor  (not  the  same  as  flt!)  

isMoving() returns  true  if  motor  is  in  mo;on  

setPower(int p)  set  power  of  motor  (0  <=  p  <=  100)  

int getPower()    get  power  of  motor  (returns  value  in  range  0..100)  

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Using  Sensors:  The  TouchSensor  Class  import lejos.nxt.*;Import java.lang.*;public class ProgTwo { public static void main (String[] args) throws Exception { NXTMotor MA = new NXTMotor(MotorPort.A); NXTMotor MB = new NXTMotor(MotorPort.B); TouchSensor T = new TouchSensor(SensorPort.S4); MA.setPower(100); // go! MB.setPower(100); while(! T.isPressed()) { Thread.sleep(200); // motors will keep running! } MA.stop(); MB.stop(); }}

Using  Sensors:  The  TouchSensor  Class  

• The  TouchSensor  class  has  a  very  simple  interface!  

• Provides  one  method:  boolean isPressed()

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Using  Sensors:  The  UltrasonicSensor  Class  import lejos.nxt.*;Import java.lang.*; public class ProgThree { public static void main (String[] args) throws Exception { NXTMotor MA = new NXTMotor(MotorPort.A); NXTMotor MB = new NXTMotor(MotorPort.B); UltrasonicSensor us = new UltrasonicSensor(SensorPort.S1); MA.setPower(100); MB.setPower(100); while(!(us.getRange() < 20) ) { // 20cm threshold Thread.sleep(200); } MA.stop(); MB.stop(); }}

Using  Sensors:  The  ColorSensorHT  Class    

import lejos.nxt.*;import lejos.nxt.addon.ColorSensorHT;public class ProgFour { public static void main(String[] args) throws Exception{

ColorSensorHT cs = new ColorSensorHT(SensorPort.S2);

while( !Button.ESCAPE.isPressed() ) {

LCD.clear(); LCD.drawString("ID = "+cs.getColorID(),0,2); Thread.sleep(500);

}

}}

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getColorId()  

• 0  =  black  • 1  =  violet  • 2  =  purple  • 3  =  blue  • 4  =  green  • 5  =  lime  

• 6  =  yellow  • 7  =  orange  • 8  =  red  • 9  =  crimson  

• 10  =  magenta  

• 11  to  16  =  pastels  • 17  =  white  

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LEJOS  at  the  Command  Line  

• Three  main  tools:  

• nxjc  –  the  LEJOS  Java  compiler  

• nxjlink  –  the  LEJOS  linker  • nxjupload  –  uploads  files  to  NXT  brick    

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nxjc  –  The  LEJOS  Java  Compiler  

• The  LEJOS  replacement  for  javac  

• Takes  as  input  a  “.java”  file  • Produces  as  output  a  “.class”  file  

[mjw@mjw-imac]$ nxjc ProgFour.java [mjw@mjw-imac]$

nxjlink  –  The  LEJOS  Linker  

• Normally  in  JAVA,  linking  is  done  dynamically,  when  you  run  the  JAVA  interpreter/virtual  machine  over  your  “.class”  file  

• (Standard  JAVA  is  dynamically  linked:  referenced  classes  are  loaded,  as  needed  at  run  -me.  This  doesn’t  work  on  NXT  because  of  lack  of  memory  on  the  brick.  Classes  are  sta-cally  linked,  i.e.,  linked  at  compile  ;me.)  

[mjw@mjw-imac]$ nxjlink ProgFour -o ProgFour.nxj[mjw@mjw-imac]$

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nxjupload  –  Uploading  Files  to  the  NXT  Brick  

• Will  try  USB  and  Bluetooth  

• Make  sure  brick  is  switched  on  and  showing  main  menu  

[mjw@mjw-imac]$ nxjupload ProgFour.nxjFound NXT: NXT 001653119113leJOS NXJ> Connected to NXTleJOS NXJ> Upload successful in 997 milliseconds[mjw@mjw-imac]$