SAE Baja Reading RPM and Displaying

Hello everyone. I am a college student who is still quite a novice at electronics and would like some help on a project I have been working on. I would greatly appreciate it if people would guide me in a direction that may work, so that I may still learn; however, I would still consider myself a beginner. So, with that in mind, please clearly explain and really encourage posting sites that may help.

So, I am part of a SAE Mini Baja club and have been trying to get a data collecting system to confirm and adjust reasoning for how and why we made the car the way we did.

1 Problem: I am trying to read RPM from a Briggs and Stratton Model 19 with the circuit attached below. The readings are bouncing around. When the engine is idle, it produces about 1800 RPM but the readings could be anywhere from about 800 to 2000. I want to get readings to be more accurate such as to have the readings only be bouncing within -+ 300, preferably -+100 if possible. One way I was thinking of doing is to increase the number of windings of wire around the ring to hopefully get a stronger signal. There is only 10 windings of insulated 20 gauge wire around it.

2 Problem: I have notice that when the screen with Arduino MEGA is up at the front and the circuit is at the back, with wires for power ground and signal wire to detect the DC pulses from the circuit running part of the length of the car (roughly 3 feet), that the screen was kind of diming in and out some. So, does the distance effect how nice of a signal it can get from the RPM sensing circuit? The idea is to be able to display RPM and speed to the driver or whomever is tuning the cvt clutches and it also looks nice for judges.

Here is a previous post the explains some of how the RPM circuit works. https://forum.arduino.cc/index.php?topic=687223.0
The screen is a TFT LCD 3.4" screen display shield

The code is also attached, first one just reads rpm and the second one also reads rpm and displays it on the screen.

Thank you for your time reading and helping me out with this project!

Tachometer_Using_micros.ino (11.7 KB)

Screen_w_RPM.ino (12.9 KB)

There is a section at the top of the Forum - How to use the Forum - please review it and pay extra attention to the part about posting your code using code tags, most of the people who help here will not go the the hassle of downloading someones code and loading it in the IDE - you will get better answers if you make it easy for people to help

A few thoughts - engines are very noisy with crap spikes that make them hard to instrument - shielding will probably be required - - for bounce there are many ways to do some running averaging - adjust the sample rate to give a reliable number -

Don’t use magnets and inductance. Electricity and magnets are like trying to control fish.

Do it digital. Just use a light dark on the flywheel and an IR LED. ON off ON off ON off ON off ON off ON off How long does each take?

-jim lee

#include <Adafruit_GFX.h>    // Core graphics library
#include <MCUFRIEND_kbv.h>   // Hardware-specific library
MCUFRIEND_kbv tft;

#include <Fonts/FreeSans9pt7b.h>
#include <Fonts/FreeSans12pt7b.h>
#include <Fonts/FreeSerif12pt7b.h>

#include <FreeDefaultFonts.h>

#define BLACK   0x0000
#define RED     0xF800
#define GREEN   0x07E0
#define WHITE   0xFFFF
#define GREY    0x8410


const byte PulsesPerRevolution = 2;  // Set how many pulses there are on each revolution. Default: 2.


// If the period between pulses is too high, or even if the pulses stopped, then we would get stuck showing the
// last value instead of a 0. Because of this we are going to set a limit for the maximum period allowed.
// If the period is above this value, the RPM will show as 0.
// The higher the set value, the longer lag/delay will have to sense that pulses stopped, but it will allow readings
// at very low RPM.
// Setting a low value is going to allow the detection of stop situations faster, but it will prevent having low RPM readings.
// The unit is in microseconds.
const unsigned long ZeroTimeout = 100000;  // For high response time, a good value would be 100000.
// For reading very low RPM, a good value would be 300000.


// Calibration for smoothing RPM:
const byte numReadings = 2;  // Number of samples for smoothing. The higher, the more smoothing, but it's going to
// react slower to changes. 1 = no smoothing. Default: 2.

/////////////
// Variables:
/////////////

volatile unsigned long LastTimeWeMeasured;  // Stores the last time we measured a pulse so we can calculate the period.
volatile unsigned long PeriodBetweenPulses = ZeroTimeout + 1000; // Stores the period between pulses in microseconds.
// It has a big number so it doesn't start with 0 which would be interpreted as a high frequency.
volatile unsigned long PeriodAverage = ZeroTimeout + 1000; // Stores the period between pulses in microseconds in total, if we are taking multiple pulses.
// It has a big number so it doesn't start with 0 which would be interpreted as a high frequency.
unsigned long FrequencyRaw;  // Calculated frequency, based on the period. This has a lot of extra decimals without the decimal point.
unsigned long FrequencyReal;  // Frequency without decimals.
unsigned long RPM;  // Raw RPM without any processing.
unsigned int PulseCounter = 1;  // Counts the amount of pulse readings we took so we can average multiple pulses before calculating the period.

unsigned long PeriodSum; // Stores the summation of all the periods to do the average.

unsigned long LastTimeCycleMeasure = LastTimeWeMeasured;  // Stores the last time we measure a pulse in that cycle.
// We need a variable with a value that is not going to be affected by the interrupt
// because we are going to do math and functions that are going to mess up if the values
// changes in the middle of the cycle.
unsigned long CurrentMicros = micros();  // Stores the micros in that cycle.
// We need a variable with a value that is not going to be affected by the interrupt
// because we are going to do math and functions that are going to mess up if the values
// changes in the middle of the cycle.

// We get the RPM by measuring the time between 2 or more pulses so the following will set how many pulses to
// take before calculating the RPM. 1 would be the minimum giving a result every pulse, which would feel very responsive
// even at very low speeds but also is going to be less accurate at higher speeds.
// With a value around 10 you will get a very accurate result at high speeds, but readings at lower speeds are going to be
// farther from eachother making it less "real time" at those speeds.
// There's a function that will set the value depending on the speed so this is done automatically.
unsigned int AmountOfReadings = 1;

unsigned int ZeroDebouncingExtra;  // Stores the extra value added to the ZeroTimeout to debounce it.
// The ZeroTimeout needs debouncing so when the value is close to the threshold it
// doesn't jump from 0 to the value. This extra value changes the threshold a little
// when we show a 0.

// Variables for smoothing tachometer:
unsigned long readings[numReadings];  // The input.
unsigned long readIndex;  // The index of the current reading.
unsigned long total;  // The running total.
unsigned long average;  // The RPM value after applying the smoothing.


int MPH = 0;
int tireSize = 22;
int pi = 3.1415;
int actualSeconds = 0;
int multiplier = 0;


void setup(void)
{
  Serial.begin(9600);


  attachInterrupt(digitalPinToInterrupt(21), Pulse_Event, RISING);  // Enable interruption pin 21 when going from LOW to HIGH.

  delay(1000);  // We sometimes take several readings of the period to average. Since we don't have any readings
  // stored we need a high enough value in micros() so if divided is not going to give negative values.
  // The delay allows the micros() to be high enough for the first few cycles.


  uint16_t ID = tft.readID();
  if (ID == 0xD3D3) ID = 0x9481; //force ID if write-only display
  tft.begin(ID);
  tft.setRotation(1);
  tft.fillScreen(BLACK);

  tft.setTextSize(5);
  tft.setCursor(30, 120);
  tft.print("Initalizing...");
  delay(1000);
  tft.fillScreen(BLACK);

  //  tft.fillRect(345, 55, 90, 90, WHITE);

  //   tft.fillRect(350, 60, 80, 80, RED);
}

void loop() {
  // put your main code here, to run repeatedly:
  int mill = 2;
  int seconds = millis() / 1000;
  int minutes = millis() / 60000;
  char O = O;
  char zero [1];
  zero[1] = O;

  actualSeconds = seconds - minutes * 60;



  tft.setTextColor(GREEN, BLACK);
  tft.setTextSize(5);

  /*    tft.setCursor(30, 120);
      tft.print(PeriodBetweenPulses);

      tft.setCursor(30, 220);
      tft.print(AmountOfReadings);

      tft.setCursor(30, 20);
      tft.print(FrequencyReal);
  */
  tft.setCursor(200, 120);
  tft.print(RPM);

  //    tft.setCursor(200, 220);
  //    tft.print(MPH);

  tft.setCursor(200, 20);
  tft.println(minutes);

  tft.setCursor(260, 20);
  tft.println(actualSeconds);


  tft.setCursor(200, 220);
  tft.print(seconds);



  // The following is going to store the two values that might change in the middle of the cycle.
  // We are going to do math and functions with those values and they can create glitches if they change in the
  // middle of the cycle.
  LastTimeCycleMeasure = LastTimeWeMeasured;  // Store the LastTimeWeMeasured in a variable.
  CurrentMicros = micros();  // Store the micros() in a variable.


  // CurrentMicros should always be higher than LastTimeWeMeasured, but in rare occasions that's not true.
  // I'm not sure why this happens, but my solution is to compare both and if CurrentMicros is lower than
  // LastTimeCycleMeasure I set it as the CurrentMicros.
  // The need of fixing this is that we later use this information to see if pulses stopped.
  if (CurrentMicros < LastTimeCycleMeasure)
  {
    LastTimeCycleMeasure = CurrentMicros;
  }
 // Calculate the frequency:
  FrequencyRaw = 10000000000 / PeriodAverage;  // Calculate the frequency using the period between pulses.


  // Detect if pulses stopped or frequency is too low, so we can show 0 Frequency:
  if (PeriodBetweenPulses > ZeroTimeout - ZeroDebouncingExtra || CurrentMicros - LastTimeCycleMeasure > ZeroTimeout - ZeroDebouncingExtra)
  { // If the pulses are too far apart that we reached the timeout for zero:
    FrequencyRaw = 0;  // Set frequency as 0.
    ZeroDebouncingExtra = 2000;  // Change the threshold a little so it doesn't bounce.
  }
  else
  {
    ZeroDebouncingExtra = 0;  // Reset the threshold to the normal value so it doesn't bounce.
  }

  FrequencyReal = FrequencyRaw / 10000;  // Get frequency without decimals.
  // This is not used to calculate RPM but we remove the decimals just in case
  // you want to print it.


  // Calculate the RPM:
  RPM = FrequencyRaw / PulsesPerRevolution * 60;  // Frequency divided by amount of pulses per revolution multiply by
  // 60 seconds to get minutes.
  RPM = RPM / 10000;  // Remove the decimals.

  // Smoothing RPM:
  total = total - readings[readIndex];  // Advance to the next position in the array.
  readings[readIndex] = RPM;  // Takes the value that we are going to smooth.
  total = total + readings[readIndex];  // Add the reading to the total.
  readIndex = readIndex + 1;  // Advance to the next position in the array.

  if (readIndex >= numReadings)  // If we're at the end of the array:
  {
    readIndex = 0;  // Reset array index.
  }

  // Calculate the average:
  average = total / numReadings;  // The average value it's the smoothed result.



  // Print information on the serial monitor:
  // Comment this section if you have a display and you don't need to monitor the values on the serial monitor.
  // This is because disabling this section would make the loop run faster.
  Serial.print("Period: ");
  Serial.print(PeriodBetweenPulses);
  Serial.print("\tReadings: ");
  Serial.print(AmountOfReadings);
  Serial.print("\tFrequency: ");
  Serial.print(FrequencyReal);
  Serial.print("\tRPM: ");
  Serial.print(RPM);
  Serial.print("\tTachometer: ");
  Serial.println(average);


}

void showmsgXY(int x, int y, int sz, const GFXfont *f, const char *msg)
{
  int16_t x1, y1;
  uint16_t wid, ht;
  tft.drawFastHLine(0, y, tft.width(), WHITE);
  tft.setFont(f);
  tft.setCursor(x, y);
  tft.setTextColor(GREEN);
  tft.setTextSize(sz);
  tft.print(msg);

}


void Pulse_Event()  // The interrupt runs this to calculate the period between pulses:
{

  PeriodBetweenPulses = micros() - LastTimeWeMeasured;  // Current "micros" minus the old "micros" when the last pulse happens.
  // This will result with the period (microseconds) between both pulses.
  // The way is made, the overflow of the "micros" is not going to cause any issue.

  LastTimeWeMeasured = micros();  // Stores the current micros so the next time we have a pulse we would have something to compare with.

  if (PulseCounter >= AmountOfReadings) // If counter for amount of readings reach the set limit:
  {
    PeriodAverage = PeriodSum / AmountOfReadings;  // Calculate the final period dividing the sum of all readings by the
    // amount of readings to get the average.
    PulseCounter = 1;  // Reset the counter to start over. The reset value is 1 because its the minimum setting allowed (1 reading).
    PeriodSum = PeriodBetweenPulses;  // Reset PeriodSum to start a new averaging operation.


    // Change the amount of readings depending on the period between pulses.
    // To be very responsive, ideally we should read every pulse. The problem is that at higher speeds the period gets
    // too low decreasing the accuracy. To get more accurate readings at higher speeds we should get multiple pulses and
    // average the period, but if we do that at lower speeds then we would have readings too far apart (laggy or sluggish).
    // To have both advantages at different speeds, we will change the amount of readings depending on the period between pulses.
    // Remap period to the amount of readings:
    int RemapedAmountOfReadings = map(PeriodBetweenPulses, 40000, 5000, 1, 10);  // Remap the period range to the reading range.
    // 1st value is what are we going to remap. In this case is the PeriodBetweenPulses.
    // 2nd value is the period value when we are going to have only 1 reading. The higher it is, the lower RPM has to be to reach 1 reading.
    // 3rd value is the period value when we are going to have 10 readings. The higher it is, the lower RPM has to be to reach 10 readings.
    // 4th and 5th values are the amount of readings range.
    RemapedAmountOfReadings = constrain(RemapedAmountOfReadings, 1, 10);  // Constrain the value so it doesn't go below or above the limits.
    AmountOfReadings = RemapedAmountOfReadings;  // Set amount of readings as the remaped value.
  }
  else
  {
    PulseCounter++;  // Increase the counter for amount of readings by 1.
    PeriodSum = PeriodSum + PeriodBetweenPulses;  // Add the periods so later we can average.
  }

}  // End of Pulse_Event

jimLee:
Don’t use magnets and inductance. Electricity and magnets are like trying to control fish.

Do it digital. Just use a light dark on the flywheel and an IR LED. ON off ON off ON off ON off ON off ON off How long does each take?

-jim lee

Thanks for the idea but one of the rules is that the engine cant be modify in any way. That is why I am trying to get the signal off the spark plug. Though I could try to but something similar on the motor axle but there really is no where to put it and we also go through lots of mud, dust, and dirt. That stuff can easily get in and block it.

Put a sprocket on the output shaft, time the spokes as they pass by? There are myriad of ways to get a visual signal. But maybe the rules are trying to mitigate such things?

-jim lee

Try gluing a small magnet to the output shaft and use a hall effect pick up. Mud problem solved.