IRtoRF Link for the Lego RCX

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IR-to-RF Linkfor the Lego RCX

• Eric P. Walklet
• Senior Project - Computer Engineering - 2005
• Advisor Prof. John Spinelli

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Presentation Agenda

• Introduction to the RCX
• Statement of the Problem
• Project Considerations
• Block Diagram
• Programming the RCX
• The IR Circuitry
• Crossbow Motes and TinyOS
• Communicating Between Unlike Devices
• RFRCX Controlling the RCX with MATLAB
• Challenges and Conclusions

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Introducing the Lego RCX

• Part of the Lego Mindstorms package for many
  years
• Three motors and three sensors
• Programmed using a proprietary language created
  by Lego
• Interfaces with a host computer through a 935-nm
  IR link with a USB tower

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Purpose of the Project

• To design an interface for the Lego RCX that does
  not require a line of sight to operate such as
  an RF link
• To design a new programming language in MATLAB
  that can allow novice programmers to control the
  RCX
• Inspired by CSC-030

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Project Requirements

• Language must control all three motors and read
  all three sensors, at least
• Language must be simple enough to understand for
  a student with little programming experience
• Information must travel between MATLAB and the
  RCX via some type of RF link
• RCX is limited by a one-byte message buffer and
  an archaic protocol

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Crossbow Motes

• The building blocks of wireless sensor networks
• Operate at RF frequency of 900 MHz
• Capable of communication through RF or UART
• Ethernet Gateway allows communication with other
  devices through TCP/IP
• Programmed with nesC

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Block Diagram
MIB600CA Ethernet Gateway
MATLAB Users PC
TCP/IP
Mote
900 MHz RF
Mote
IR RX/TX Circuitry
935 nm IR
Lego RCX
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Making the Most of One Byte
From MATLAB to the RCX Motor Control
Even Parity
Odd Parity
Motor 1
Motor 2
Motor 3
8 voltage settings, two directions now limited
to 3 settings in each direction idle
From RCX to MATLAB Sensor Status
Even Parity
Odd Parity
Sensor 1
Sensor 2
Sensor 3
100 scaled levels of sensitivity now limited to
4 scaled levels of sensitivity
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Programming the RCX with LeJOS

• LeJOS is a Java VM that replaces the standard RCX
  firmware
• Small enough to fit in the 32 kB (!) of memory
• Allows RCX programmers to use a language that
  they are already familiar with, though there are
  limitations
• Support for the entire RCX feature set and beyond

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What the RCX Accomplishes

• Accepts a one-byte message from the IR receiver
• Uses that message to control the three motors
  accordingly
• Checks the status of the three sensors
• Assembles the status of the three sensors into a
  one-byte message
• Sends the message back through the IR transmitter

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I Cant Believe Its Not Java!

• if(motorNumber1)
• if((chArray0 1) (chArray1 1))
• return 3 // motor is floating
• else if((chArray0 1) (chArray1 0))
• return 2 // motor is on, forward
• else if((chArray0 0) (chArray1 1))
• return 1 // motor is on, reverse
• else return 0 // motor is off
• else if(motorNumber2)
• if((chArray2 1) (chArray3 1))

• while(true) // loop indefinitely
• // reads in a message from the mote, and converts
• // it to binary
• inBin integerToBinary(dis.readInt())
• // if the parity checks properly, set the motors
• // accordingly
• if (evenParityCheck(inBin) oddParityCheck(inBin
  ))
• setMotors(inBin)
• // assembles a message from the status of the
• // sensors, and adds parity
• outBin convertBinary(setParity(getSensors()))
• // sends the sensor message
• dos.writeInt(outBin)

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Do You Speak Infrared?

• Need an IR receiver and transmitter to
  communicate directly with the RCX
• A Sharp GP1UW70QS remote control receiver
  collects sensor information from the RCX
• A PIC12C508A microcontroller modulates the motor
  messages at 38 kHz so they can be detected by the
  RCX, and outputs them through an IR photodiode

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Circuit Diagram
What the IR Detector Sees
What the IR Diode Sends
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Mote 2 The Great Interpreter

• The RCX uses a unique protocol eight bytes sent
  for a one-byte message
• Crossbow motes use a variation of HDLC for both
  radio transmission and UART communication
• The mote that connects to the IR circuit must
  change the message to a structure the RCX can
  understand
• Sends translated message to RCX via UART, and
  sends decoded message to gateway via RF

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TinyOS and nesC

• TinyOS is a native environment for small devices
  like the motes
• nesC is a modular language based on C
• Programs are formed by wiring interfaces and
  components to each other
• VERY DIFFICULT AT FIRST
• Each program requires a configuration and a
  module to operate

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A Look at the Configuration

• configuration UARTRCXPacket
• provides
• interface StdControl as Control
• interface BareSendMsg as Send
• interface ReceiveMsg as Receive
• implementation
• components RCXPacket as Packet, UART
• Control Packet.Control
• Send Packet.Send
• Receive Packet.Receive
• Packet.ByteControl -gt UART
• Packet.ByteComm -gt UART

• / Command to transfer an RCX-compatible packet
  /
• command result_t sendRCXPacket(TOS_MsgPtr msg)
  
• uint8_t rcx_packet
• result_t rval
• // form RCX-compatible frame
• rcx_packet0 (uint8_t)(PACKET_HEADER)
• rcx_packet1 (uint8_t)(0xFF PACKET_HEADER)
• rcx_packet2 (uint8_t)(OPCODE_MESSAGE)
• rcx_packet3 (uint8_t)(0xFF -
  OPCODE_MESSAGE)
• rcx_packet4 (uint8_t)(msg)
• rcx_packet5 (uint8_t)(0xFF -
  rcx_packet4)
• rcx_packet6 (uint8_t)(OPCODE_RETTOSUB)
• rcx_packet7 (uint8_t)(0xFF -
  OPCODE_RETTOSUB)
• rval call SendVarLenPacket.send(rcx_packet,
  PACKET_SIZE)
• return rval

To send one byte to the RCX 0xFF 0xFF
0xF7 0xF7 message message 0x08 0x08
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Mote 1 Not Quite as Glorious

• Messages received from the RF network (i.e. Mote
  2) are passed on to the Ethernet gateway, and
  further to the PC
• Messages received from the Ethernet gateway are
  sent to the wireless network to be
• Programming is the same as Mote 2, though lacks
  the RCX encoding

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MATLAB and the Motes

• Crossbow includes Java tools for sending and
  receiving messages to and from the wireless
  network
• Message Interface Generator (MIG) creates a
  message class unique to the programs installed on
  the motes
• Connection is made to the Ethernet gateway
  through a TCP/IP socket

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RFRCX

• Status of sensors and motors are stored to two
  global variables
• A variety of functions are assembled into a
  program to describe the RCXs behavior
• Each function updates the global variables
  depending on user input or received messages
• Always something to send! Always something to
  receive!

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RFRCX in Action

• function setMotorBackward(number)
• if number 1
• motorSettings(1) 0
• motorSettings(2) 1
• elseif number 2
• motorSettings(3) 0
• motorSettings(4) 1
• elseif number 3
• motorSettings(5) 0
• motorSettings(6) 1
• end
• setMotorParity
• asInt buildMessage
• oMsg net.tinyos.rfrcx.RCXMotorMsg(asInt)
• send(2,oMsg)

• drive forward
• when a wall is hit
• back up, and move forward
• again
• motorSettings resetMotors
• sensorSettings resetSensors
• setMotorForward(1)
• while getSensorValue(1) 0
• setMotorOff(2)
• end
• setAllMotors(-1,-1,0)
• continue(100)
• setMotorForward(2)
• continue(100)
• setMotorForward(1)

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Problems and Challenges

• UART on the motes is prone to errors and failures
• Poor battery life on the motes without
  power-saving features
• nesC is not well documented
• RCX hardware is proprietary
• RFRCX is not as user friendly as it could be
• Price

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The Future

• Use multiple messages to have more control over
  the motors and other features
• Control multiple motes through the same gateway
  to address price issue
• Replace MATLAB with a true Java interface
• Packaging
• A direct-connect solution

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Acknowledgments

• http//www.xbow.com
• http//www.tinyos.net
• Kekoa Proudfoot http//graphics.stanford.edu/keko
  a/rcx/
• http//www.csdm.qc.ca/SJdelaLande/lesclasses/4web/
  wwwrobotique/robotique_lego.htm
• Prof. Spinelli
• Crossbow (for their informative training seminar)
• The rest of the CN/CS/EE department

And a silent wave to all those in the room who
were inspired to become engineers by the Legos
they played with as a kid including me.
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Questions?