It's working!

It's astonishing how much you can manage to do in just one day of coding. Or astonishing how long time coding takes, depending on how you look upon it. In one day we've started to familiarize ourselves with the NXT and we've created all kinds of smaller test programs, all of which you can find on our Google Code page.


The first basic program to show that the NXT and leJOS works properly. The same program that every programmer has done at least once during his early career. What it does is to simply write a String on our NXT's screen. The leJOS code for writing on the screen of the NXT is:


In this project with its very intuitive name we started playing with the Ultrasonic Sensor. This works like a sonar; it sends out ultrasonic sound and measures the time to the echo. To use the sensor you simply declare the sensor, which then has a number of predetermined methods. Note that you have to specify the port he sensor is located in as well.
UltrasonicSensor sensor = new UltrasonicSensor(SensorPort.S1);
int distance = sensor.getDistance();
LCD.drawInt (distance , 1, 1);

These lines will cause the sensor to measure the distance ahead of it and write out the distance in centimeters on the display.

We used this to create a program that measures the distance in a 180 degrees radius and then write it out on the LCD screen. Here we also used a motor to rotate the ultrasonic sensor. The motors also have a set of predetermined methods, but you don't have to declare them.

class HeadRotator extends Thread {
public void run() {
while (!this.isInterrupted()) {
Motor.A.rotateTo(-100, true);
while (Motor.A.getTachoCount() >= -90 && !this.isInterrupted()) {
try {
} catch (InterruptedException e) {
Motor.A.rotateTo(100, true);
while (Motor.A.getTachoCount() <= 90 && !this.isInterrupted()) { } try { Thread.sleep(200); } catch (InterruptedException e) { } Sound.beep(); } } }

This loop makes the sensor rotate back and forth in a 180 degree arc. In a separate thread we continuously gather data which we then translate into coordinates on the LCD and write it out.

We noted that the sensor isn't very accurate when facing at an angle to a flat surfaces, and when the sensor sweeped over a flat surface the surface would appear curved on the screen. This is something we'll have to take into consideration later on. Also, we noticed the limits of the NXT' computing capacity, as it didn't quite manage to keep up with the data income. Though the most difficult part for it was to manage to write all the data to the LCD fast enough, so the screen flickered.


To solve the problem with the NXT's limited computing capacity we wanted to be able to run most of the computation on a server, in this case a computer. Also we want to be able to show the results on a real screen instead of the very limited LCD of the NXT. Thus, we had to establish a communication link.

The first comm test was just a simple app sending the data from a touch sensor to the computer through USB. When the touchsensor was pressed the computer app should exit. leJOS included some really great examples for rthis so it wasn't that hard to establish the first connection.

We extended the data transfer so that it also transmitted the data from the soundsensor aswell as the ultrasonicsensor in real time. The PC recieved the data and painted it to an applet as a graph. The PC also always sends back a status code telling the NXT if the status is OK or not, if it's not OK everything should shut down.

Then it wasn't that hard to switch to Bluetooth mode instead. The use the same connector but with different protocols:

USB:NXTConnector conn = new NXTConnector();

Bluetooth:NXTConnector conn = new NXTConnector();

Though there are better ways to implement it which we will test later on. Anyhow we missed one important thing in the beginning, we forgot to add the reference to the BT package bluecove.jar. It took us at least one hour to figure out that this was the missing part.

Video of the CommunicationTest:

So what we've learned so far will be really important. Now we will use our know knowledge in BT communication and ultrasonic data to build a simple bot that rotates, collects US data and sends it to the computer to display it.

Stay tuned!!
/Peter And Josef

http://lejos.sourceforge.net/nxt/nxj/api/index.html leJOS NXT API
http://lejos.sourceforge.net/nxt/pc/api/index.html leJOS PC API
http://lejos.sourceforge.net/nxt/nxj/tutorial/Communications/Communications.htm leJOS Communication Tutorial

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