Aasimon.org Wiki

Date: November 7th 2008
Duration of activity: 8-13
Participants: Kasper, Bent and Johnny

Implement a working Rodney Brooks layered control system on the NXT ¹⁾

• Modify code by Ole Caprani²⁾ to work with the our own physical implementation of Marvin.
  • Test individual behaviors seperately and re-calibrate constants.
• Make modular priority system.
• Investegate the functionality of java threads in order to implement a “Drive towards light” behavior as seen in lab session 7 ³⁾

We modified the code to be more modular. This was not part of the exercise, but it made it a lot easier to test individual modules. Below follows the modifications made.

• The constant tooCloseThreshold was initially set to be 20 cm, but this was not enough to take into account our modified Marvin which has a plate seperating the two lightsensors. There this constant was changed to 35 instead.
• The constants in the thread RandonDrive.java has been change to accomdate more action, less waiting. We wanted Marvin to move longere before there were the tread ended. This caused Marvin to be more “exploring.”

Video of Marvin running with default code

Prints an f on the screen to show the direction of movement. The thread drives forward with different random numbers between 50 and 100 on each motor. This means that Marvin not nessesarily (infact not likkely) will drive straight ahead. It will do this between 1 and 3 seconds. Then print s on the screen to indicate a stop and wait between 0.5 and 1 second. All these intervals are computed using a random function from Math library.

Implemented using a sonic sensor. Prints a b on the screen to indecate a backwards motion, and set full power to both motors and do this for 1 second to get a away from the obstacle. Afterwards Marvin stops both motors, and sets right motor to full power to make a left turn. Marvin does this for 0.8 seconds, and it seems this corresponds to a 90 degree turn to the left. This thread shows a f while this happens. Lastly i shows a s en stops for 0.5 seconds.

This thread waits for 10 seconds, then stops and plays a sound while printing an s on the screen.

The priority handling was in the initial code created by handing pointers to all lower priority Behavior to each Behavior, as shown in the code below:

rd = new RandomDrive("Drive",1);
af = new AvoidFront ("Avoid",2,rd);
ps = new PlaySounds ("Play ",3,af,rd);

This however makes shuffling of the priorities, as well as making the introduction of new Behaviors, a very tedious task. We therefore rewrote the priority system to make it more modular and flexible.
Below is the new stucture:

ArrayList behaviors = new ArrayList();
 
RandomDrive rd;
AvoidFront af;
PlaySounds ps;
DriveTowardsLight dl;
 
rd = new RandomDrive("Drive", behaviors); behaviors.add(rd);
dl = new DriveTowardsLight("Light ", behaviors); behaviors.add(dl);
af = new AvoidFront("Avoid", behaviors); behaviors.add(af);
ps = new PlaySounds("Play ", behaviors); behaviors.add(ps);

The idea is to use an array of Behaviors containing all lower priority Behaviors in each Behavior.
this array is copied and stored in the constructor of each Behavior, and can then be iterated whenever suppression of the lower levels are needed.

private ArrayList behaviors;;
 
public Behavior(String name, ArrayList behaviors)
{
  .
  .
  .
 
  // Clone the Behavior list.
  this.behaviors = new ArrayList();
  for(int i = 0; i < behaviors.size(); i++) {
    Behavior b = (Behavior)behaviors.get(i);
    this.behaviors.add(b);
  }
}
 
public void suppressLower()
{
  for(int i = 0; i < behaviors.size(); i++) {
    Behavior b = (Behavior)behaviors.get(i);
    b.setSuppress(true);
  }
}

The Behavior class extends Thread which makes it run as a thread in the JVM. The thread is configured to run as daemon which makes it automatically terminate together with the main thread⁴⁾.

Bent writes this part

Video for “Drive towards light test”

The layered behavior model seems to work very well for the implementation of an autonomous roboot. We believe that with an added push sensor Marvin would be able to navigate in any room avoiding any obstacles and find the exit with lights.