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### Topic: help in making engine digital RPM meter (Read 19055 times)previous topic - next topic

#### Joy

##### Sep 08, 2011, 03:26 pmLast Edit: Sep 09, 2011, 07:32 am by Joy Reason: 1
I am trying to build an RPM meter for an engine.

The process I have selected is by Capacitive RPM sensing circuit inspire by

Now on each rotation of the engine there is one spark. That is I get one pulse in each rotation..

But how do I now calculate it in rotations per minutes..??

Is it I will have to find the time difference between pulses and multiply it with 60...??

Or there is some other way too..??

#### Magician

#1
##### Sep 08, 2011, 03:33 pm

http://tronixstuff.wordpress.com/2011/06/22/tutorial-arduino-timing-methods-with-millis/

#2
##### Sep 08, 2011, 03:43 pm
There are a couple common methods of calculating RPMs, and which is best to use depends on a variety of factors.

1. Measure the time between pulses.  Use that time to calculate RPMs.
If the time between pulses is measured in milliseconds, then RPM can be calculated using:
60000/time_between_pulses

2. Count the number of pulses that occur in an arbitrary period of time.  Use that value to calculate RPMs.
If you count the number of pulses that occur in 1 second, then RPM can be calculated using:
pulse_count*60

Both these calculations assume you have 1 pulse per revolution (as you mentioned).  Below are versions of the formula that add a pulses_per_rev parameter (which, when equal to one, will simplify to the above formulas)
60000/(time_between_pulses*pulses_per_rev)
(pulse_count/pulses_per_rev)*60

If, in the second formula you are counting the number of pulses in a period of time that is not 1 second, then you need to normalize the formula to a 1 second count period.

#### Joy

#3
##### Sep 08, 2011, 03:49 pm

There are a couple common methods of calculating RPMs, and which is best to use depends on a variety of factors.

1. Measure the time between pulses.  Use that time to calculate RPMs.
If the time between pulses is measured in milliseconds, then RPM can be calculated using:
60000/time_between_pulses

2. Count the number of pulses that occur in an arbitrary period of time.  Use that value to calculate RPMs.
If you count the number of pulses that occur in 1 second, then RPM can be calculated using:
pulse_count*60

Both these calculations assume you have 1 pulse per revolution (as you mentioned).  Below are versions of the formula that add a pulses_per_rev parameter (which, when equal to one, will simplify to the above formulas)
60000/(time_between_pulses*pulses_per_rev)
(pulse_count/pulses_per_rev)*60

If, in the second formula you are counting the number of pulses in a period of time that is not 1 second, then you need to normalize the formula to a 1 second count period.

I feel its better to go with the 1st option...

because if I am going with the pulse per second and i have each pulse every 1.5 seconds then the calculation will be lil hard...

And more over my engine's idle RPM is 600...

#### Joy

#4
##### Sep 08, 2011, 03:51 pm
How do I calculate the      time_between_pulse..??

#5
##### Sep 08, 2011, 04:04 pm

How do I calculate the      time_between_pulse..??

Magician's link provides some good information on how to do that.

#### PaulS

#6
##### Sep 08, 2011, 04:20 pm
Quote
because if I am going with the pulse per second and i have each pulse every 1.5 seconds then the calculation will be lil hard...

Quote
How do I calculate the      time_between_pulse..??

Seems to me you already told us the time between pulses...

#### Joy

#7
##### Sep 08, 2011, 04:41 pm

Quote
because if I am going with the pulse per second and i have each pulse every 1.5 seconds then the calculation will be lil hard...

Quote
How do I calculate the      time_between_pulse..??

Seems to me you already told us the time between pulses...

yes i told but it is not same all the time it increases and decreases

#### Techone

#8
##### Sep 09, 2011, 05:18 am
Quote
yes i told but it is not same all the time it increases and decreases

@Joy

+ or -  ? you talking about the engine ? car engine usally do not have acurate RPM, also Lawn Mower engine...

So what the range ?  example : 450 to 650

If you have +/- 100, well that good right ?

#### Joy

#9
##### Sep 09, 2011, 07:31 am

Quote
yes i told but it is not same all the time it increases and decreases

@Joy

+ or -  ? you talking about the engine ? car engine usally do not have acurate RPM, also Lawn Mower engine...

So what the range ?  example : 450 to 650

If you have +/- 100, well that good right ?

I will be reading the rpm of a single cylinder engine. That is of my bike..

the idle rpm of the engine is around 600

#### Joy

#10
##### Sep 09, 2011, 09:22 am
I tried this code idea from here http://www.arduino.cc/playground/Learning/Tachometer

Code: [Select]
`//// This example shows one way of creating an optoswitch// using an IR LED as emiter and an IR LED receiver as// light sensor.// On this case it acts as a tachometer to count the// revolutions per second of an aeromodelism plane's// propeller.////           + GROUND                                 +GROUND          //           |                                        |  //           <                                        < //           > 220 ohm resistor                       > 220 omh resistor//           <                                        <      //           |                                        |  //           |                                        |//         -----                                    -----//          / \    >>IR LED emiter >>>>>>>>>>>>>>>>  / \   IR LED receiver//         -----                                    -----//           |                                        |//           |                                        |//           + +5VCD                                  +  ANALOG INPUT 0//int val;long last=0;int stat=LOW;int stat2;int contar=0;int sens=75;  // this value indicates the limit reading between dark and light,              // it has to be tested as it may change acording on the               // distance the leds are placed.int nPalas=2; // the number of blades of the propellerint milisegundos=500; // the time it takes each readingvoid setup(){  Serial.begin(9600);  pinMode(13,OUTPUT);}void loop(){  val=analogRead(0);  if(val<sens)    stat=LOW;   else    stat=HIGH;   digitalWrite(13,stat); //as iR light is invisible for us, the led on pin 13                           //indicate the state of the circuit.   if(stat2!=stat){  //counts when the state change, thats from (dark to light) or                      //from (light to dark), remmember that IR light is invisible for us.     contar++;     stat2=stat;   }   if(millis()-last>=milisegundos){     double rps=((double)contar/nPalas)/2.0*1000.0/milisegundos;     double rpm=((double)contar/nPalas)/2.0*60000.0/(milisegundos);     Serial.print((contar/2.0));Serial.print("  RPS ");Serial.print(rps);     Serial.print(" RPM");Serial.print(rpm);Serial.print("  VAL ");Serial.println(val);     contar=0;     last=millis();   }}`

in the code if I change    int nPalas=2; // the number of blades of the propeller     to    int nPalas=1;

as in my engine one pulse output is one rotation..

and the code checks for number of pulses in 500 miliseconds, the reading is not that accurate all the time..
it fluctuates a lot

#11
##### Sep 09, 2011, 02:43 pm
Quote
and the code checks for number of pulses in 500 miliseconds, the reading is not that accurate all the time..
it fluctuates a lot

This goes along the lines of what I was saying in my first post.  With only 1 pulse per rotation, and relatively low speed, you won't get many counts in a 500 millisecond window, which means any fluctuation  in counts is going to create a magnified fluctuation in RPMs.

The easiest way to remedy that is to increase the duration in which you're counting pulses, possibly increasing it to several seconds.  Unless you are using the RPM as an input to some other high speed process, like a PID, that shouldn't be a problem.

#### Techone

#12
##### Sep 10, 2011, 05:24 am
@Joy

First. Check your sensor and make sure it is "clean" <--- no bouncing

So you are using one cylinder engine  <-- Just like a lawn mower

My Theory :
Let assume the RPM is 600.  600 pulse / min  / 60 second = 10 pulse / second  +/- 100 rpm or 1 pulse / second ( the variation )

If using 500 ms, you should get 5 pulse / 500 ms = 10 pulse / second

In you code you have "contar" meaning count, can you modify so you can see the variale "contar" to check your "pulse", if you have 5 pulse +/- 1, well it work fine, if not, check the calculations, maybe check sensor, and maybe re-modify the code.

Code: [Select]
`int val;long last=0;int stat=LOW;int stat2;int contar=0;int sens=75;  // this value indicates the limit reading between dark and light,              // it has to be tested as it may change acording on the               // distance the leds are placed.int nPalas=2; // the number of blades of the propellerint milisegundos=500; // the time it takes each readingvoid setup(){  Serial.begin(9600);  pinMode(13,OUTPUT);}void loop(){  val=analogRead(0);  if(val<sens) // Software comparator  {    stat=LOW;  }   else  {    stat=HIGH;  }   digitalWrite(13,stat); //as iR light is invisible for us, the led on pin 13                           //indicate the state of the circuit.   if(stat2!=stat) {  //counts when the state change, thats from (dark to light) or                      //from (light to dark), remmember that IR light is invisible for us.     contar++;     stat2=stat;   }if(millis()-last>=milisegundos){ // isolated the calculations to check the pulse count and check the pulse counting routine/*     double rps=((double)contar/nPalas)/2.0*1000.0/milisegundos;     double rpm=((double)contar/nPalas)/2.0*60000.0/(milisegundos);     Serial.print((contar/2.0));Serial.print("  RPS ");Serial.print(rps);     Serial.print(" RPM");Serial.print(rpm);Serial.print("  VAL ");Serial.println(val);*/     Serial.println(contar); // to check the contar variable     contar=0; // reset contar variable     last=millis();  }}`

#### jmknapp

#13
##### Sep 10, 2011, 12:17 pm
You could feed the pulses to a frequency-to-voltage converter chip:

http://www.national.com/ds/LM/LM2907.pdf
http://parts.digikey.com/1/parts/483487-ic-converter-freq-volt-14-dip-lm2907n-nopb.html

...and then the rpm reading becomes just an analog read and scale factor & no need to worry about timing pulses. Note the "minimum component tachometer" on p. 8.

Joe

#### Joy

#14
##### Sep 11, 2011, 01:43 pm

You could feed the pulses to a frequency-to-voltage converter chip:

http://www.national.com/ds/LM/LM2907.pdf
http://parts.digikey.com/1/parts/483487-ic-converter-freq-volt-14-dip-lm2907n-nopb.html

...and then the rpm reading becomes just an analog read and scale factor & no need to worry about timing pulses. Note the "minimum component tachometer" on p. 8.

Joe

Thats a great thing I have come through,..

It will be great..
But can u help me out like how to do with it...??

how do I know what frequency input is what output voltage..

The datasheet is not clear to me...

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