Event Driven Programming Minus the GUI Plus a Robot

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Event Driven ProgrammingMinus the GUIPlus a
Robot

• By Charlie Vick

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Why Event-Driven Robotics?

• This robot consists of sensors and motors.
• Event driven programming decouples sensor
  monitoring and motor controlling. Different
  classes specialize, compartmentalizing the
  hardware.
• Can more easily switch out sensors or motors, as
  long as everyone can use the same event system
  and events.

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Robot Gear

• Hardware Lego Mindstorm NXT - servos, sensors
  and an ARM processor
• Software open-source 3rd party firmware called
  leJOS, which runs compiled Java bytecode

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The old bot configuration
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Current bot configuration
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Architecture (part 1)

• Lejos library has a class, SensorPort
• SensorPort.addSensorPortListener(
  SensorPortListener aListener) is a public method
• SensorPortListener Interface, has one method
• void stateChanged(SensorPort source, int
  oldValue, int newValue)

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This seemed a real coup

• leJOS looked to be designed for event-driven
  programming
• Wire up SensorPortListeners to a few sensors,
  have each steer in different ways when triggered

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Problem

• API Documentation out of date. Forum search
  revealed SensorPortListener was deprecated for
  most sensors.
• Only works on one sensor I have, touch.
• Doesnt actually work.

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Program Architecture (part 2)

• Slightly more complex.
• One class, SensorReader, iterates through a
  couple of sensors. If certain values are hit,
  fire a certain event.
• Another class, EventHandler, receives these
  events and changes the motors accordingly.

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How do these classes communicate?

• Each runs on its own thread.
• They pass messages over a shared, synchronized
  queue.
• This means events are handled approximately in
  the order created, barring synchronization issues
• (Bonus What is a better / more granular / more
  responsive data structure than a regular queue,
  given events with different priorities?)

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DIAGRAM
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Code sample from Architecture 1

• public static void main (String aArg)
  
• Robot robot new Robot()
• SensorPort.S1.
• addSensorPortListener(robot)
  
• robot.moveForward()
• In Robot
• public void moveForward()
• Motor.A.setSpeed(20)
• Motor.B.setSpeed(20)
• while (!ts.isPressed()) //ts
  instanceof TouchSensor
• Motor.A.forward()
• Motor.B.forward()
• //bonus - what part is NOT very event-driven
  here?

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Code sample from Architecture 1

• public void stateChanged(SensorPort port, int
  oldValue, int newValue) //replace with
  diff values checking in oldValue, newValue
  //otherwise not very event-driven if
  (oldValue lt 500 newValue lt 500)
  Motor.A.stop()
• Motor.B.stop()
• Motor.A.setSpeed(20)
• Motor.B.setSpeed(20)
• Motor.A.backward()
• Motor.B.backward()
• Thread.sleep(100)
• Motor.A.stop()
• Motor.B.stop()
• Motor.A.forward()
• Motor.B.backward()
• Thread.sleep(250)
• Motor.A.stop()
• Motor.B.stop()
• moveForward()

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Architecture 2 Code

• public class SynchronizedQueueltEgt
• private final ArrayListltEgt queue
• public void push(E o)
• synchronized(queue)
• queue.add(o)
• queue.notify()
• public E pop()
• synchronized(queue)
• while (queue.isEmpty())
• queue.wait()
• return queue.remove(0)

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Architecture 2 code SensorReader(note ts is
a TouchSensor instance)

• public void run()
• while (true)
• if(ts.isPressed())
• queue.push(new RobotEvent(
• RobotEventInt.BACKUP))
• else
• queue.push(new RobotEvent(
• RobotEventInt.FORWARD))
• try
• Thread.sleep(200)
• catch (InterruptedException e)
• System.out.println("Sensor interrupted")

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Architecture 2 code EventResponders run method.
Both implement Runnable, an interface allowing
them to run in separate Threads.Bonus What is
missing?

• public void run()
• while(true)
• RobotEventInt curr queue.pop()
• if (curr.getName().equals(
• RobotEventInt.FORWARD))
• leftMotor.forward()
• rightMotor.forward()
• else if (curr.getName().equals(
• RobotEventInt.BACKUP))
• if (!leftMotor.isStopped()
• !rightMotor.isStopped())
• leftMotor.stop()
• rightMotor.stop()
• leftMotor.backward()
• rightMotor.backward()

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Interface Runnable

• One method
• public void run()
• Add a Runnable implementor to a Thread, run
  Thread.start(), and the code in run() executes
  asynchronously.

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RobotEvent

• Implements RobotEventInt, a collection of static
  String values (RobotEventInt.BACKUP) and one
  method, getName(), which should always be one of
  RobotEventInts static String values.

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Architecture 2 Working Yet?

• Still working at it. Robot throws an error that
  indicates not all classes loaded, which print
  statements dont seem to verify. Runs fine until
  queue.pop() in EventResponder.

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Lessons Learned

• If you learn about deprecated classes in a
  library from an online forum and NOT from the
  librarys API, try to switch libraries.
• (Also, if you shutdown and accidentally suspend
  an Ubuntu laptop and then power-cycle it, some
  perfect storm wipes out /sys and some of /sbin)
• (On a related note, Dropbox is great for backing
  up your work)