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Using Lego Mindstorms NXT in the Classroom






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Using Lego Mindstorms NXT in the Classroom Gabriel J. Ferrer Hendrix College ferrer@hendrix.edu http://ozark.hendrix.edu/~ferrer/ Outline NXT capabilities Software development options Introductory programming projects Advanced programming projects Purchasing NXT Kits
Using Lego Mindstorms NXT in the Classroom

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Slide 1

Using Lego Mindstorms NXT in the Classroom

Gabriel J. Ferrer

Hendrix College

ferrer@hendrix.edu

http://ozark.hendrix.edu/~ferrer/

Slide 2

Outline

  • NXT capabilities

  • Software development options

  • Introductory programming projects

  • Advanced programming projects

Slide 3

Purchasing NXT Kits

  • Two options (same price; $250/kit)

    • Standard commercial kit

    • Lego Education kit

      • http://www.lego.com/eng/education/mindstorms/

  • Advantages of education kit

    • Includes rechargeable battery ($50 value)

    • Plastic box superior to cardboard

    • Extra touch sensor (2 total)

  • Standard commercial kit

    • Includes NXT-G visual language

Slide 4

NXT Brick Features

  • 64K RAM, 256K Flash

  • 32-bit ARM7 microcontroller

  • 100 x 64 pixel LCD graphical display

  • Sound channel with 8-bit resolution

  • Bluetooth radio

  • Stores multiple programs

    • Programs selectable using buttons

Slide 5

Sensors and Motors

  • Four sensor ports

    • Sonar

    • Sound

    • Light

    • Touch

  • Three motor ports

    • Each motor includes rotation counter

Slide 6

Touch Sensors

  • Education kit includes two sensors

  • Much more robust than old RCX touch sensors

Slide 7

Light Sensor

  • Reports light intensity as percentage

  • Two modes

    • Active

    • Passive

  • Practical uses

    • Identify intensity on paper

    • Identify lit objects in dark room

    • Detect shadows

Slide 8

Sound Sensor

  • Analogous to light sensor

    • Reports intensity

    • Reputed to identify tones

      • I haven’t experimented with this

  • Practical uses

    • “Clap” to signal robot

Slide 9

Ultrasonic (Sonar) Sensor

  • Reports distances

    • Range: about 5 cm to 250 cm

    • In practice:

      • Longer distances result in more missed “pings”

  • Mostly reliable

    • Occasionally gets “stuck”

    • Moving to a new location helps in receiving a sonar “ping”

Slide 10

Motors

  • Configured in terms of percentage of available power

  • Built-in rotation sensors

    • 360 counts/rotation

Slide 11

Software development options

  • Onboard programs

    • RobotC

    • leJOS

    • NXC/NBC

  • Remote control

    • iCommand

    • NXT_Python

Slide 12

RobotC

  • Commercially supported

    • http://www.robotc.net/

  • Not entirely free of bugs

  • Poor static type checking

  • Nice IDE

  • Custom firmware

  • Costly

    • $50 single license

    • $250/12 classroom computers

Slide 13

Example RobotC Program

void forward() {

motor[motorA] = 100;

motor[motorB] = 100;

}

void spin() {

motor[motorA] = 100;

motor[motorB] = -100;

}

Slide 14

Example RobotC Program

task main() {

SensorType[S4] = sensorSONAR;

forward();

while(true) {

if (SensorValue[S4] < 25) spin();

else forward();

}

}

Slide 15

leJOS

  • Implementation of JVM for NXT

  • Reasonably functional

    • Threads

    • Some data structures

    • Garbage collection added (January 2008)

    • Eclipse plug-in just released (March 2008)

  • Custom firmware

  • Freely available

    • http://lejos.sourceforge.net/

Slide 16

Example leJOS Program

sonar = newUltrasonicSensor(SensorPort.S4);

Motor.A.forward();

Motor.B.forward();

while (true) {

if (sonar.getDistance() < 25) {

Motor.A.forward();

Motor.B.backward();

} else {

Motor.A.forward();

Motor.B.forward();

}

}

Slide 17

Event-driven Control in leJOS

  • The Behavior interface

    • boolean takeControl()

    • void action()

    • void suppress()

  • Arbitrator class

    • Constructor gets an array of Behavior objects

      • takeControl() checked for highest index first

    • start() method begins event loop

Slide 18

Event-driven example

class Go implements Behavior {

private Ultrasonic sonar =

new Ultrasonic(SensorPort.S4);

public boolean takeControl() {

return sonar.getDistance() > 25;

}

Slide 19

Event-driven example

public void action() {

Motor.A.forward();

Motor.B.forward();

}

public void suppress() {

Motor.A.stop();

Motor.B.stop();

}

}

Slide 20

Event-driven example

class Spin implements Behavior {

private Ultrasonic sonar =

new Ultrasonic(SensorPort.S4);

public boolean takeControl() {

return sonar.getDistance() <= 25;

}

Slide 21

Event-driven example

public void action() {

Motor.A.forward();

Motor.B.backward();

}

public void suppress() {

Motor.A.stop();

Motor.B.stop();

}

}

Slide 22

Event-driven example

public class FindFreespace {

public static void main(String[] a) {

Behavior[] b = new Behavior[]

{new Go(), new Stop()};

Arbitrator arb =

new Arbitrator(b);

arb.start();

}

}

Slide 23

NXC/NBC

  • NBC (NXT Byte Codes)

    • Assembly-like language with libraries

    • http://bricxcc.sourceforge.net/nbc/

  • NXC (Not eXactly C)

    • Built upon NBC

    • Successor to NQC project for RCX

  • Compatible with standard firmware

    • http://mindstorms.lego.com/Support/Updates/

Slide 24

iCommand

  • Java program runs on host computer

  • Controls NXT via Bluetooth

  • Same API as leJOS

    • Originally developed as an interim project while leJOS NXT was under development

    • http://lejos.sourceforge.net/

  • Big problems with latency

    • Each Bluetooth transmission: 30 ms

    • Sonar alone requires three transmissions

    • Decent program: 1-2 Hz

Slide 25

NXT_Python

  • Remote control via Python

    • http://home.comcast.net/~dplau/nxt_python/

  • Similar pros/cons with iCommand

Slide 26

Developing a Remote Control API

  • Bluetooth library for Java

    • http://code.google.com/p/bluecove/

  • Opening a Bluetooth connection

    • Typical address: 00:16:53:02:e5:75

  • Bluetooth URL

    • btspp://00165302e575:1; authenticate=false;encrypt=false

Slide 27

Opening the Connection

import javax.microedition.io.*;

import java.io.*;

StreamConnection con = (StreamConnection) Connector.open(“btspp:…”);

InputStream is = con.openInputStream();

OutputStream os = con.openOutputStream();

Slide 28

NXT Protocol

  • Key files to read from iCommand:

    • NXTCommand.java

    • NXTProtocol.java

Slide 29

An Interesting Possibility

  • Programmable cell phones with cameras are available

  • Camera-equipped cell phone could provide computer vision for the NXT

Slide 30

Introductory programming projects

  • Developed for a zero-prerequisite course

  • Most students are not CS majors

  • 4 hours per week

    • 2 meeting times

    • 2 hours each

  • Not much work outside of class

    • Lab reports

    • Essays

Slide 31

First Project (1)

  • Introduce motors

    • Drive with both motors forward for a fixed time

    • Drive with one motor to turn

    • Drive with opposing motors to spin

  • Introduce subroutines

    • Low-level motor commands get tiresome

  • Simple tasks

    • Program a path (using time delays) to drive through the doorway

Slide 32

First Project (2)

  • Introduce the touch sensor

    • if statements

      • Must touch the sensor at exactly the right time

    • while loops

      • Sensor is constantly monitored

  • Interesting problem

    • Students try to put code in the loop body

      • e.g. set the motor power on each iteration

    • Causes confusion rather than harm

Slide 33

First Project (3)

  • Combine infinite loops with conditionals

  • Enables programming of alternating behaviors

    • Front touch sensor hit => go backward

    • Back touch sensor hit => go forward

Slide 34

Second Project (1)

  • Physics of rotational motion

  • Introduction of the rotation sensors

    • Built into the motors

  • Balance wheel power

    • If left counts < right counts

      • Increase left wheel power

  • Race through obstacle course

Slide 35

Second Project (2)

if (/* Write a condition to put here */) { nxtDisplayTextLine(2, "Drifting left");

} else if (/* Write a condition to put here */) { nxtDisplayTextLine(2, "Drifting right");

} else {

nxtDisplayTextLine(2, "Not drifting");

}

Slide 36

Third Project

  • Pen-drawer

    • First project with an effector

    • Builds upon lessons from previous projects

  • Limitations of rotation sensors

    • Slippage problematic

    • Most helpful with a limit switch

  • Shapes (Square, Circle)

  • Word (“LEGO”)

    • Arguably excessive

Slide 37

Pen-Drawer Robot

Slide 38

Pen-Drawer Robot

Slide 39

Fourth Project (1)

  • Finding objects

  • Light sensor

    • Find a line

  • Sonar sensor

    • Find an object

    • Find freespace

Slide 40

Fourth Project (2)

  • Begin with following a line edge

    • Robot follows a circular track

    • Always turns right when track lost

    • Traversal is one-way

  • Alternative strategy

    • Robot scans both directions when track lost

    • Each pair of scans increases in size

Slide 41

Fourth Project (3)

  • Once scanning works, replace light sensor reading with sonar reading

  • Scan when distance is short

    • Finds freespace

  • Scan when distance is long

    • Follow a moving object

Slide 42

Light Sensor/Sonar Robot

Slide 43

Other Projects

  • “Theseus”

    • Store path (from line following) in an array

    • Backtrack when array fills

  • Robotic forklift

    • Finds, retrieves, delivers an object

  • Perimeter security robot

    • Implemented using RCX

    • 2 light sensors, 2 touch sensors

  • Wall-following robot

    • Build a rotating mount for the sonar

Slide 44

Robot Forklift

Slide 45

Gearing the motors

Slide 46

Advanced programming projects

  • From a 300-level AI course

  • Fuzzy logic

  • Reinforcement learning

Slide 47

Fuzzy Logic

  • Implement a fuzzy expert system for the robot to perform a task

  • Students given code for using fuzzy logic to balance wheel encoder counts

  • Students write fuzzy experts that:

    • Avoid an obstacle while wandering

    • Maintain a fixed distance from an object

Slide 48

Fuzzy Rules for Balancing Rotation Counts

  • Inference rules:

    • biasRight => leftSlow

    • biasLeft => rightSlow

    • biasNone => leftFast

    • biasNone => rightFast

  • Inference is trivial for this case

    • Fuzzy membership/defuzzification is more interesting

Slide 49

Fuzzy Membership Functions

  • Disparity = leftCount - rightCount

  • biasLeft is

    • 1.0 up to -100

    • Decreases linearly down to 0.0 at 0

  • biasRight is the reverse

  • biasNone is

    • 0.0 up to -50

    • 1.0 at 0

    • falls to 0.0 at 50

Slide 50

Defuzzification

  • Use representative values:

    • Slow = 0

    • Fast = 100

  • Left wheel:

    • (leftSlow * repSlow + leftFast * repFast) / (leftSlow + leftFast)

  • Right wheel is symmetric

  • Defuzzified values are motor power levels

Slide 51

Q-Learning

  • Discrete sets of states and actions

    • States form an N-dimensional array

      • Unfolded into one dimension in practice

    • Individual actions selected on each time step

  • Q-values

    • 2D array (indexed by state and action)

    • Expected rewards for performing actions

Slide 52

Q-Learning Main Loop

  • Select action

  • Change motor speeds

  • Inspect sensor values

    • Calculate updated state

    • Calculate reward

  • Update Q values

  • Set “old state” to be the updated state

Slide 53

Calculating the State (Motors)

  • For each motor:

    • 100% power

    • 93.75% power

    • 87.5% power

  • Six motor states

Slide 54

Calculating the State (Sensors)

  • No disparity: STRAIGHT

  • Left/Right disparity

    • 1-5: LEFT_1, RIGHT_1

    • 6-12: LEFT_2, RIGHT_2

    • 13+: LEFT_3, RIGHT_3

  • Seven total sensor states

  • 63 states overall

Slide 55

Action Set for Balancing Rotation Counts

  • MAINTAIN

    • Both motors unchanged

  • UP_LEFT, UP_RIGHT

    • Accelerate motor by one motor state

  • DOWN_LEFT, DOWN_RIGHT

    • Decelerate motor by one motor state

  • Five total actions

Slide 56

Action Selection

  • Determine whether action is random

    • Determined with probability epsilon

  • If random:

    • Select uniformly from action set

  • If not:

    • Visit each array entry for the current state

    • Select action with maximum Q-value from current state

Slide 57

Q-Learning Main Loop

  • Select action

  • Change motor speeds

  • Inspect sensor values

    • Calculate updated state

    • Calculate reward

  • Update Q values

  • Set “old state” to be the updated state

Slide 58

Calculating Reward

  • No disparity => highest value

  • Reward decreases with increasing disparity

Slide 59

Updating Q-values

Q[oldState][action] =

Q[oldState][action] +

learningRate *

(reward + discount * maxQ(currentState) - Q[oldState][action])

Slide 60

Student Exercises

  • Assess performance of wheel-balancer

  • Experiment with different constants

    • Learning rate

    • Discount

    • Epsilon

  • Alternative reward function

    • Based on change in disparity

Slide 61

Learning to Avoid Obstacles

  • Robot equipped with sonar and touch sensor

  • Hitting the touch sensor is penalized

  • Most successful formulation:

    • Reward increases with speed

    • Big penalty for touch sensor

Slide 62

Other classroom possibilities

  • Operating systems

    • Inspect, document, and modify firmware

  • Programming languages

    • Develop interpreters/compilers

    • NBC an excellent target language

  • Supplementary labs for CS1/CS2

Slide 63

Thanks for attending!

  • Slides available on-line:

    • http://ozark.hendrix.edu/~ferrer/presentations/

  • Currently writing lab textbook

    • Introductory and advanced exercises

  • ferrer@hendrix.edu


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