UltraSonic sensor doesn't work ??

[eddynamique]

Hello,

I've a problem with the UltraSonic sensor on the NXT brick. When I call the method getDistance(), it always return me "255". I tried with a test program but it doesn't work...
I give you the code :

Code: Select all


  public class testDeplace{

   public static void main(String [] args) throws Exception{
      
      UltrasonicSensor ultra = new UltrasonicSensor(SensorPort.S4);
      
      while(!Button.ESCAPE.isPressed()){
         LCD.drawString("Distance : "+ultra.getDistance(), 1, 1);
      }
   }
   
  }



Thanks to answer me.

eddynamique

[gloomyandy]

Hi,
There is a bug in 0.85 that means that i2c sensors do not work on port 4. Try another port...

Andy

[eddynamique]

Thanks Andy for your answer. I switched to the port 3 but I've got a new problem now... getDistance() returns an INT that start from 235 instead of 0 and I don't understand why...
I give you the code of my test program :

Code: Select all

public class testDeplace{

   public static void main(String [] args) throws Exception{
   
      while(!Button.ESCAPE.isPressed()){
         
         LCD.drawString("Distance : "+ultra.getDistance(), 0, 1);
         LCD.refresh();   
      }      
   }   
}


Thanks

eddynamique

[gloomyandy]

Hi,
You may want to clear the screen before displaying new results, you are probably seeing old values and new values being combined.

Andy

[eddynamique]

You're right Andy. And I notice that I must wait around two seconds before the distance be correct...

Thanks for yours answers

eddynamique

[gloomyandy]

I don't think that is correct you should not have to wait any time for the distance to become correct (certainly not 2 seconds), so what happens if you don't wait? What sort of readings do you get, and what sort of thing are you doing to test these readings? You may get spurious readings from the sensor (welcome to the world of sensor handling), but you should be able to cleanup the data and ignore them...

Andy

[eddynamique]

In fact, I noticed that I have to call getDistance() three times to get a correct value. The first and the second call return 255...
My code :

Code: Select all

public class testDeplace  {

   public static void main(String [] args) throws Exception{
   
      UltrasonicSensor ultra = new UltrasonicSensor(SensorPort.S3);
         
      LCD.drawString("Distance : "+ultra.getDistance(), 0, 1);
      Thread.sleep(1000);
      LCD.drawString("Distance : "+ultra.getDistance(), 0, 2);
           Thread.sleep(1000);
      LCD.drawString("Distance : "+ultra.getDistance(), 0, 3);
      Thread.sleep(5000);
   }
}


[gloomyandy]

Ah, ok so what I think you are seeing is the sensor powering on. What happens after the initial set of readings? You should not have to wait between readings to get stable data...

Andy

[eddynamique]

To test my code, there are 20cm between the wall and the sensor and I read the distance on the LCD display.

If I do that :

Code: Select all

public class testDeplace  {

   public static void main(String [] args) throws Exception{
   
      UltrasonicSensor ultra = new UltrasonicSensor(SensorPort.S3);
   
      LCD.drawString("Distance : "+ultra.getDistance(), 0, 1);
      LCD.drawString("Distance : "+ultra.getDistance(), 0, 2);
      LCD.drawString("Distance : "+ultra.getDistance(), 0, 3);
           Thread.sleep(5000);
   }
}


I got this on the screen :

Code: Select all

     Distance : 255
     Distance : 255
     Distance : 255


And if I do this :

Code: Select all

public class testDeplace  {

   public static void main(String [] args) throws Exception{
   
      UltrasonicSensor ultra = new UltrasonicSensor(SensorPort.S3);
   
      LCD.drawString("Distance : "+ultra.getDistance(), 0, 1);
                Thread.sleep(1000);
      LCD.drawString("Distance : "+ultra.getDistance(), 0, 2);
                Thread.sleep(1000);
      LCD.drawString("Distance : "+ultra.getDistance(), 0, 3);
           Thread.sleep(5000);
   }
}


I got this :

Code: Select all

     Distance : 255
     Distance : 255
     Distance : 22


That's why I think I must wait few seconds to get a stable value... What do you think about my test?

[gloomyandy]

Hi,
The problem with your test is that it is only testing what happens when you first use the sensor (just after your program starts). It is normal for devices to take a short while to start up. The more interesting question is what happens after the device has started. Do you still have to wait between readings? I'm pretty sure that you should not have to do so...

Andy