RPM Library for Arduino

Written by: Tyler Barnes.

What is the RPM Library?

The RPM.h library was created to make it incredibly easy to read RPM sensors. Simply call RPM.start() to initialize, and start reading data with RPM.get().

The library uses the input capture feature within the hardware timer of the microcontroller. This is an extremely accurate technique for measuring input signals, pulse width, and timing in general.

Why Use This Over Other Options?

The main focus for making this was creating something that returned an accurate and fresh value instantly without any waiting. It only takes a single revolution to calculate the RPM, which has already happened before calling RPM.get().

No setting up interrupts! Other libraries that I tried always required me to set up my own ISR(), and to attachInterrupt() manually. I might as well have written the entire thing myself at that point. So, that's what I did, so you don't have to.

Also, accuracy was a big focus to get right. While there might be some variation from the CPU being slightly off from the rated clockspeed, it is often negligable. You can even fine tune the accuracy if you want to.

The size of the library is also very compact. Using only 200 bytes of flash, and 6 bytes of RAM on an Arduino Nano.

Lastly, the library does not take away the External Interrupts from the user. So you are free to use attachInterrupt() as you please.

What are the Drawbacks?

The library has to use a timer to function, so it may take away a PWM channel that happens to use the same timer.

Here is a table containing which digital pins will lose the analogWrite() function.

Platform	Pins
UNO, Nano, Leonardo, Pro Mini, Micro, Fio, BT, Yun, Sparkfun Pro Micro	9, 10
Mega	6, 7, 8
Nano Every	6

AVR Chip	Pins
ATmega328P	PB1, PB2
ATmega168P	PB1, PB2
ATmega32U4	PB5, PB6
ATmega2560	PH3, PH4, PH5
ATmega4809	PF4

Is My Arduino Compatible?

Here is a list of boards I know to be compatible:

Platform
UNO
Nano
Nano Every
Leonardo
Pro Mini
Micro
Fio
BT
Yun
Sparkfun Pro Micro

Here is a list of boards that are not compatible:

Platform
LilyPad
Gemma
DFRobot Beetle
DFRobot Beetle BLE
ATtiny13A

Untested:

Platform
ATtiny10
ATtiny402
ATmega8, ATmega48, ATmega88, ATmega168

How to Install It

Method 1 (Recommended)

To install the library, click on the Sketch tab within the Arduino IDE, click on Include Library, then click on Manage Libraries....

Once the library browser launches, type RPM into the search field, then click on the RPM library's Install button.

Method 2

Alternatively, the RPM.h library can be downloaded from the releases section of the github repository.

Once the .zip file has been downloaded, click on the Sketch tab within the Arduino IDE, click on Include Library, then click on Add .ZIP Library....

The resulting dialog will allow you to locate the library within your computer's directories, and open it.

Hookup Instructions

The input capture method uses very specific pins on the Arduino. They usually do not overlap other powerful functions though.

Here is a table outlining which pin to connect to your RPM sensor's signal line.

Platform	Capture Pin
UNO, Nano, Pro Mini, Yun, Fio, BT	8
Nano Every	6
Micro, Leonardo, Sparkfun Pro Micro	4
Mega	49

If you are not using a premade developement board and instead using a compatible AVR microcontroller in your project, you can see which pin to use in this table:

AVR Chip	Capture Pin
ATmega328P	PB0
ATmega32U4	PD4
ATmega2560	PL0
ATmega4809	PF4
ATtiny10	PB1
ATtiny402	PA6
ATmega168P	PB0

How To Use It

To add its functionality to your sketch you'll need to reference the library header file. You do this by adding an include directive to the top of your sketch.

#include <RPM.h>

void setup(){

}

void loop(){

}

The library provides a global variable named RPM, you use this variable to access the library's methods. The methods provided in the RPM class are listed below.

Library Functions

RPM.start()

To initialize the library, call the RPM.start() method. This will set up the input capture registers and interrupts for you.

#include <RPM.h>

void setup(){
    RPM.start(); 
}

void loop(){

}

By default the libray assumes you have an external pullup resistor on your sensor. However, if you would like to use the internal pullup that the Arduino provides, then you can call INTERNAL_PULLUP inside the start() method.

#include <RPM.h>

void setup(){
    RPM.start(INTERNAL_PULLUP); 
}

void loop(){

}

You can also call EXTERNAL_PULLUP if you want to be explicit.

RPM.get()

The RPM.get() method returns a uint16_t containing the RPM value measured on the input capture pin.

#include <RPM.h>

void setup(){
    Serial.begin(9600); 
    RPM.start() 
}

void loop(){
    Serial.println(RPM.get());
}

An Important Note on Performance and Accuracy

Accuracy overall is about ±0.1% for general RPM ranges.

Lower RPMs are more accurate than higher ones.

The lowest RPM it will measure is 240RPM.

The highest RPM is around 60,000RPM.

The higher the RPM becomes, the less timer counts there are to measure. So, accuracy can drop when you are nearing 60,000.

Advanced Features

RPM.error(float value)

If the clock speed on your board is not running to specification, then the RPM calculation can be off. The input voltage will also have a direct effect on clockspeed, so it can vary from application.

My Arduino Mega for example was pretty far off, so I've included a way to correct it.

You can fine tune the RPM value by using the RPM.error() method. It takes in a float value.

I will demonstrate now how to calculate the value you'd want to enter.

First hook up a calibrated signal source to the input capture pin. I used a function generator set to a pulse wave.

Set the frequency of the signal source somewere in the 50Hz to 200Hz range. I chose 100Hz in my testing.

Convert the frequency into an RPM value by multiplying it by 60.

100Hz * 60 = 6000RPM

Next, run a sketch that spits out the RPM value into the Serial Monitor.

#include <RPM.h>

void setup() {
  Serial.begin(9600);
  RPM.start(); 
}

void loop() {
  Serial.println(RPM.get());
}

I got an output that looks like this:

6048
6048
6048
6048
6050
6050
6050
6050
6050
6050
...

Subract the measured value from the calculated one.

6000 - 6048 = -48

Then divide the result by the calculated value.

-48 / 6000 = -0.008

This is now the value you should pass into RPM.error().

#include <RPM.h>

void setup() {
  Serial.begin(9600);
  RPM.start(); 
  RPM.error(-0.008); 
}

void loop() {
  Serial.println(RPM.get());
}

The result should now be calibrated:

5999
5999
6001
6001
6001
6001
...