2
$\begingroup$

Rubik's Cube Robot

I am trying to the read the quadrature encoder on a ServoCity 624 RPM Premium Planetary Gear Motor w/Encoder with a SparkFun ESP32 Thing.

I am supposed to see 228 counts per revolution, but I see 230-232 instead.

Here is my Arduino code:

// Global variables modified by ISR
int state;
unsigned int count;

int error;

// ISR called on both edge of quadrature signals
void handleInterrupt()
{
  // Shift old state into higher bits
  state = ( state << 2 ) & 15;

  // Get current state
  if( digitalRead(34) ) state |= 2;
  if( digitalRead(35) ) state |= 1;

  // Check state change for forward or backward quadrature
  // Flag any state change errors
  switch( state )
  {
    case 1:
    case 7:
    case 14:
    case 8:
      count--;
      break;
    case 11:
    case 13:
    case 4:
    case 2:
      count++;
      break;
    default:
      error++;
  }
}

void setup()
{
  pinMode(33, OUTPUT); // PWM
  pinMode(32, OUTPUT); // DIR

  pinMode(34, INPUT_PULLUP); // QB
  pinMode(35, INPUT_PULLUP); // QA

  attachInterrupt( digitalPinToInterrupt(34), handleInterrupt, CHANGE );
  attachInterrupt( digitalPinToInterrupt(35), handleInterrupt, CHANGE );

  Serial.begin(74880);
}

void loop()
{
  int new_state;
  int old_state;

  // Start motor
  digitalWrite(32, HIGH); // clockwise
  digitalWrite(33, HIGH);

  // 57*4-17 found by trial and error to make a complete revolution
  for( int i = 0; i < (57*4-17); i++ )
  {
    // Busy wait for change
    do
    {
      // Get current state
      new_state = 0;
      if( digitalRead(34) ) new_state |= 2;
      if( digitalRead(35) ) new_state |= 1;
    }
    while( old_state == new_state );
    old_state = new_state;
  }

  // Stop motor
  digitalWrite(33, LOW);
  delay( 1000 );

  Serial.print( " state=" );
  Serial.print( state );
  Serial.print( " count=" );
  Serial.print( count % 228 );
  Serial.print( " error=" );
  Serial.println( error );
}
  1. On my scope, quadrature signals are very clean
  2. I instrumented the code to look at interrupts and they appear to be in the right places and not too close together.
  3. I don't see mechanical slipping, and it would be obvious because over 100 revs, the count slips an entire revolution.

But the bottom line is: How can this be failing? If the CPU is missing transitions, I would get illegal transitions and errors. Noise would also cause errors. But I am not getting any errors at all (except a single error at startup).

$\endgroup$
  • $\begingroup$ How are you measuring a revolution? Could it be that you are rotating by an extra 3 degrees? $\endgroup$ – combo Jun 10 '17 at 7:38
  • $\begingroup$ How are you determining that it has actually gone a full revolution? It looks like you have a magic number, and I totally suspect that's to blame. How did you arrive at that number? $\endgroup$ – Chuck Jun 10 '17 at 8:35
  • $\begingroup$ I am determining a full revolution visually observing the rubik's cube the motor is attached to. the loop() turns a full rev, stops for a second and then loops. After about 100 loops, the counter has returned to the same value, but i'm pretty sure i didn't miss seeing an entire revolution. $\endgroup$ – Alan Nishioka Jun 11 '17 at 11:07
1
$\begingroup$

ServoCity 624 RPM Premium Planetary Gear Motor w/Encoder has the encoder on the motor and a 19:1 gear reduction on the output shaft.

Mechanical tolerance in the gearbox accounts for the difference between 228 counts per revolution that I expected and 230.7 counts per revolution that I measured.

| improve this answer | |
$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.