Encoder coupled with motor shaft.Encoder is giving larger redings than expected

Question

I am a rookie and i dont have much experience on arduino. please help me. I intend to build a robot that will drive on a straight path even on nonuniform surfaces like the ones we see in agricultural fields(It is just a prototype, so i want to run it on normal offroad surfaces).The robot has 4 motors, one for each wheel. I plan to use encoder feedback method to drive straight. Right now, I have coupled a slotted disc to the shaft of a dc motor and i am trying to get the counts from the encoder and display the count after a certain interval(5 seconds) on serial monitor. The count that i get is way off the mark. The disc is having 30 slots. With just 2 to 3 rotations I am getting a count of around 180.

I am using arduino R3. Encoder output:- Pin2 (external interrupt) motor Pins :- 8,9 I am attaching the code below.

const int motorPin1  = 8;  // Pin 14 of L293
const int motorPin2  = 9;  // Pin 10 of L293
const int encoder_pin = 2;
volatile unsigned int count = 0;
 int time1 = 0;
 int time2 = 0;
void setup() {
  // put your setup code here, to run once:
  pinMode(motorPin1, OUTPUT);
    pinMode(motorPin2, OUTPUT);
    pinMode(encoder_pin,INPUT);
    digitalWrite(encoder_pin,HIGH);
    attachInterrupt(0,encoder,RISING);
    Serial.begin(9600);
     digitalWrite(motorPin1,HIGH);
    digitalWrite(motorPin2,LOW);
    time1 = millis();
}

void loop() {
  // put your main code here, to run repeatedly:
    time2 = millis();
    if(abs((time2 - time1) > 5000))
    {
    digitalWrite(motorPin1,LOW);
    digitalWrite(motorPin2,LOW);
    Serial.println(count);
    }
}

 void encoder()
 {
  count++;

 }

the end of code

The disc that i have mounted on the shaft doesn't rotate in vertical plane exactly. But i do get the blinks on the encoder when i rotate the shaft with my hand.Can that misalignment be the reason.

Answer 1

I guess this is a kind of debouncing problem, caused by noise on the interrupt pin. The behaviour might also get worsened by the disalignment you mentioned. You could verify this by manually rotating the shaft, even though you "get the blinks" I doubt they are accurate in count.

If that is the case there are two things you could do:

• add a capacity or a RC filter to the interrupt pin to smoothen out voltage peaks if there are some. This will decrease the chance that the interrupt gets triggered accidentially (and will not solve the problem), but as a low pass it also limits the maximum count rate you are able to detect.
• implement debouncing in software. This is usually done by ignoring the input signal for a specific amount of time ("deadtime") after an interrupt has occured. As you are expecting a periodic signal and debouncing behaviour might be different at different speed it could be a good idea to capture the time span between two "blinks" and make the deadtime dependend on this calculated speed.

Something else: the way you check for the timestamp is little incorrect. You should instead do something like this:

void setup() {
  unsigned long starttime=millis();
}

void loop() {
  if (millis()-starttime > 5000) {
    starttime=millis();
    // put code here to run every 5000 ms
  }
}

Answer 2

Usually a rotary encoder has at least 2 sensors.

Incremental rotary encoders usually only have 2 sensors. Absolute or positional encoders have more, and let you figure out the absolute orientation of your sensor by reading a bit pattern.

To read an incremental encoder you write your code to only pay attention to alternating changes in the sensors. That gives you automatic debouncing of the contacts. I suggest you search for Arduino code for decoding rotary encoders.