Interfacing the LEGO RCX to the outside world

1
Interfacing the LEGO RCX to the outside world

• John M. Larkin
• Whitworth College
• Spokane, WA

2
Outline

• Electronics at Whitworth
• Motivation for the Robotics Project
• Description of the Robotics Project
• Conclusions

3
Who takes electronics at Whitworth?

• Electronics is a required course for physics
  majors (both B.A. and B.S.) and computer science
  majors (only B.S.)
• It is offered each spring
• typically taken by physics majors as sophomores
• typically taken by CS majors as juniors or
  seniors
• Average enrollment is 16-20 students

4
Structure of the electronics course

• Content
• emphasis on analog electronics for scientific
  instrumentation applications
• brief introduction to digital electronics
• Format
• two 55-minute lectures per week
• two 3-hour labs per week

5
Structure of the electronics labs

• 16 sessions working on guided labs
• DC and AC circuits
• RC and RLC circuits
• diodes and DC power supplies
• transistors (simple amplifier and follower)
• op-amps
• light detection (photodiodes phototransistors)
• sound filtering with CD player source
• 10 sessions working on robotics project
• Presentation of project to campus-wide audience

6
Motivation for the robotics project

• We want students to
• collaborate with a student from a different
  department
• leave the course enthusiastic about electronics
• integrate and apply knowledge from throughout the
  course
• leave the course confident that they can design
  analog circuits

7
Project Description

• General project requirements
• construct 2 custom transducers that can interface
  with the LEGO RCX
• use those transducers, the RCX, plus other
  materials to do something interesting
• Students are required to submit a project
  proposal before project work days begin
• feedback on appropriate level of difficulty
• lead time to order specialized components

8
What is the RCX?

• Developed in a collaboration between MITs Media
  Lab and LEGO
• RCX details
• 16 MHz CPU
• 32 kB RAM
• 3 motor outputs
• 3 sensor inputs
• IR port

9
Overview of sensor inputs

• LEGO packages RCX with basic light and touch
  sensors as part of kit for schools
• Sensor inputs have a 10 bit A/D converter
• Sensor modes (software selected)
• passive
• active

10
Passive sensor mode

• Measures resistance with internal 10 kW resistor
  as part of voltage divider
• A/D conversion passes a value to software given
  by

11
Active sensor mode

• RCX alternates input between two states when
  put into active mode
• For 3 ms, sensor input is actually an output
  providing approximately 8 V to power the sensor
  circuit
• For 0.1 ms, an A/D converter captures the voltage
  generated by the sensor
• 0 V ? 0
• 5 V ? 1023

12
Useful active sensor interface circuit
13
Overview of outputs

• RCX outputs designed to power DC motors
• Output provides 8 V
• Pulse width modulation varies motor speed
• Eight power levels can be software selected
• power level 7 continuous output
• power level 6 on for 7 ms, off for 1 ms
• power level 0 on for 1 ms, off for 7 ms

14
Programming the RCX with leJOS

• leJOS is a version of Java for the RCX
• Why leJOS?
• many of our CS majors have previously used the
  RCX and leJOS in an artificial intelligence class
• freeware so students can install on their own
  computers for development outside of the lab
• more traditional programming language than the
  LEGO graphical programming system

15
Sample motor control code

• import josx.platform.rcx.
• public class GoAndTurn
• public static void main (String aArg)
• throws Exception
• Motor.A.setPower(2)
• Motor.C.setPower(2)
• Motor.A.forward()
• Motor.C.forward()
• Thread.sleep(3000)
• Motor.C.backward()
• Thread.sleep(1000)
• Motor.C.stop()
• Motor.A.stop()

16
Sample sensor interface code

• import josx.platform.rcx.
• public class ReadAndRespond
• public static void main (String aArg)
• throws Exception
• Sensor.S1.setTypeAndMode(SENSOR_TYPE_RAW,
  SENSOR_MODE_RAW)
• Sensor.S1.activate()
• int x Sensor.S1.readRawValue()
• if (x gt 512) Sound.twoBeeps()

17
Example a loudest-sound-seeking robot
18
What are the challenges?

• First portion of course must be fast paced so
  students have background for project
• Must help 8-10 teams working on different
  projects
• Transition from highly structured labs to
  open-ended project is too abrupt for some
  students

19
What are the rewards?

• Students enjoy the project and are proud of their
  work
• Students discover that they really learned
  something
• Faculty discover that the students really learned
  something
• Presentation day is good PR for physics on campus

20
Resources

• Web sites
• leJOS homepage
• http//lejos.sourceforge.net
• Enthusiastic hobbyists
• http//www.plazaearth.com/usr/gasperi/lego.htm
• http//www.philohome.com/

21
Resources

• Books
• Programming LEGO Mindstorms with Java by Giulio
  Ferrari, et al
• Core LEGO Mindstorms Programming by Brian Bagnall
• LEGO Mindstorms Interfacing by Don Wilcher