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Calibration problems with a Maxibotics XL-EZ4

Ok, here's the deal. Today, I got my fantastically awesome order from sparkfun. It included:

The Maxibotics XL-EZ4 Rangefinder

The JPEG Compression Camera

I managed to soder some wires to three pins of the EZ4 without burning the house down. I got GND, +V, and pin 2 (Digital signal)

I even managed to get some programming on my Arduino to work with it (Modded the PING))) library). However, the debug terminal does not go to 0 cm, although the sensor supposedly detects 0-range obstacles. I have tried using the number it came with, 29, and the number reccomended for conversion of time to distance by Maxibotics, 58. 58 let it get closer to 0, but I fear it messed up the actual length of a centimeter, as interpreted by the Arduino. Here is the code, with the number I need help with in bold and italic.

   This sketch reads a EZ4 ultrasonic rangefinder and returns the distance to the closest object in range. To do this, it sends a pulse to the sensor to initiate a reading, then listens for a pulse to return.  The length of the returning pulse is proportional to

   the distance of the object from the sensor.

  The circuit:

            * +V connection of the EZ4 attached to +5V

            * GND connection of the Ez4 attached to ground

            * SIG connection of the Ez4 attached to digital pin 10

   http://www.arduino.cc/en/Tutorial/Ping

 */

const int sonarPin = 10;

void setup() {

  // initialize serial communication:

  Serial.begin(4800);

}

void loop()

{

  // establish variables for duration of the ping,

  // and the distance result in centimeters:

  long duration,cm;

  // Give a short LOW pulse beforehand to ensure a clean HIGH pulse:

  pinMode(sonarPin, OUTPUT);

  digitalWrite(sonarPin, LOW);

  delayMicroseconds(2);

  digitalWrite(sonarPin, HIGH);

  delayMicroseconds(5);

  digitalWrite(sonarPin, LOW);

  // The same pin is used to read the signal from the EZ4: a HIGH

  // pulse whose duration is the time (in microseconds) from the sending

  // of the ping to the reception of its echo off of an object.

  pinMode(sonarPin, INPUT);

  duration = pulseIn(sonarPin, HIGH);

  // convert the time into a distance

  cm = microsecondsToCentimeters(duration);

  Serial.print(cm);

  Serial.print("cm");

  Serial.println();

  delay(100);

}

long microsecondsToCentimeters(long microseconds)

{

  // The speed of sound is 340 m/s or 29 microseconds per centimeter.

  // The ping travels out and back, so to find the distance of the

  // object we take half of the distance travelled.

  return microseconds / 29 / 2;

}

(29 gives me a minimum centimeter reading of 19, while 58 drops it to 9. which number do I use?)

Also, I can't seem to get the hang of serial stuff that I need for control of the camera. Any Arduino fans out there that can give me some pointers?

AttachmentSize
JComCamUserManual.pdf208.6 KB
XLSonarDatasheet.pdf406.42 KB

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From the datasheet "Objects from 0-cm to 20-cm range as 20-cm.'

Yes it can see things at < 20cm, but it cannot range them. At 20cm or less it's more of an object detector than a ranger.

Some other sensors report higher (if not highest) readings when the object is less than a certain distance away. So it would seem Maxbotix considers it a feature that their's reports low instead.

Speedreading sucks. I really need to curl up by the fireplace with a good users manual and get real into it...