Hall sensors, interrupts etc.

Forum 2005-2010 (read only) Hardware Interfacing
1

I read the other thread but couldnt make much sense of it.

I tried modifying the Fan speed sensor code, and that didnt work either.

Im trying to make a speedometer that detects the speed of the output shaft of the diff, and then workout the speed of the vehicle. Using a hall sensor to detect axle/wheel speed.

Here is my code, it is not finished and I have some questions:

/* Speed sensor
 * Detects diff ouput speed and works out vehicle speed.
 */

volatile byte rpmcount;
unsigned int rotations;
unsigned long timeold
int tyresize;
int speed;


void setup()
{
  Serial.begin(9600);
  attachInterrupt(0, rpm_fun, RISING);
  rpmcount = 0;
  rotations = 0;
  timeold = 0;
  tyresize = 2345; //Enter the circumfrence of your tyre size here, meausred in mm
}

void loop()
{
  if (rpmcount >= 20) {
    //Update RPM every 20 counts, increase this for better RPM resolution, decrese for faster update
    rotations = 30*1000/(millis() - timeold)*rpmcount;
    timeold = millis()
    rpmcount = 0;
    speed = (rotations * tyresize) //Calculation to work out speed.
    Serial.PrintIn(speed,DEC);
  }
}

void rpm_fun()
{
  rpmcount++;
}

But i get a error when I try and compile it:

error: expected intializer before 'int'

error: expected initializer before 'int' In function 'void setup()':
In function 'void loop()':'

:cry:

I know that my calculation for speed is wrong.

I dont understand what this part of the code does

rotations = 30*1000/(millis() - timeold)*rpmcount; (remember its modifed code from the fan rpm code)

I need a way to work out rpm of the wheel so I can work out speed. since i am planning on using this calculation to work out vehicle speed:

meterspm = wheelrpm * wheelcircumfrence / 1000 //times wheelrpm and wheel circumfrence (in millimeters) to give millimeters a minute, devide by 1000 to give meters per minute
speed = (meterspm /1000) *60 //devide meterspm by 1000 to give kilometers per minute, times 60 to give kilometers per hour.

So basically, I need to read RPM of a spinning axle with a hall effect sensor (or maybe some other sensor?) So I can work out vehicle speed. I think once I can get RPM into a varible I will be able to work out vehicle speed easily.

Sorry if this is hard to understand, my head is spinning.

Also, I cant get the original Fan speed RPM code to compile, which is:

 volatile byte rpmcount;

 unsigned int rpm;

 unsigned long timeold;

 void setup()
 {
   Serial.begin(9600);
   attachInterrupt(0, rpm_fun, RISING);

   rpmcount = 0;
   rpm = 0;
   timeold = 0;
 }

 void loop()
 {
   if (rpmcount >= 20) { 
     //Update RPM every 20 counts, increase this for better RPM resolution,
     //decrease for faster update
     rpm = 30*1000/(millis() - timeold)*rpmcount;
     timeold = millis();
     rpmcount = 0;
     Serial.println(rpm,DEC);
   }
 }

 void rpm_fun()
 {
   rpmcount++;
   //Each rotation, this interrupt function is run twice
 }
2

I got the Fan rpm code to compile by moving the rpm_fun function above void_setup.

Is this the correct way to do it? So it now looks like this,

 volatile byte rpmcount;

 unsigned int rpm;

 unsigned long timeold;

 void rpm_fun()
 {
   rpmcount++;
   //Each rotation, this interrupt function is run twice
 }
 
 void setup()
 {
   Serial.begin(9600);
   attachInterrupt(0, rpm_fun, RISING);

   rpmcount = 0;
   rpm = 0;
   timeold = 0;
 }

 void loop()
 {
   if (rpmcount >= 20) { 
     //Update RPM every 20 counts, increase this for better RPM resolution,
     //decrease for faster update
     rpm = 30*1000/(millis() - timeold)*rpmcount;
     timeold = millis();
     rpmcount = 0;
     Serial.println(rpm,DEC);
   }
 }
3

Ok, I think I have maybe figured this out myself.

 volatile byte rpmcount;

 unsigned int rpm;

 unsigned long timeold;
 
 int tyresize = 4320; //Tyre circumfrence in millimeters
 
 int speed; 
 
 void rpm_fun()
 {
   rpmcount++;
   //Each rotation, this interrupt function is run twice
 }
 
 void setup()
 {
   Serial.begin(9600);
   attachInterrupt(0, rpm_fun, RISING);

   rpmcount = 0;
   rpm = 0;
   timeold = 0;
 }

 void loop()
 {
   if (rpmcount >= 20) { 
     //Update RPM every 20 counts, increase this for better RPM resolution,
     //decrease for faster update
     rpm = 30*1000/(millis() - timeold)*rpmcount;
     timeold = millis();
     rpmcount = 0;
     speed = (rpm*tyresize)/1000/1000*60;
     Serial.println(speed,DEC);
   }
 }

;D ;D

I dont have an Arduino to test it yet, but it compiled fine.

Although there are two things I am not sure about.

(rpmtyresize)/1000/100060;

Do I need spaces between /1000/100*60 etc? I am trying to do this:

rpm TIMES tyresize DEVIDE by 1000 DEVIDE by 1000 TIMES 60.

Will my code work for that?

Also,
Serial.println(speed,DEC);

What does the DEC do?

Cheers.

4

First, the Serial.Println command syntax can be found here:

As for the math; do you want to do:
((rpm*tyresize)/1000/1000)*60;

or

(rpmtyresize)/1000/(100060);

I would assume it would be the second otherwise you could change your code to:
((rpm*tyresize)/1000000)*60;

And you don't need spaces between operators (multiply and divide).