write() pulse width limitations

Other Hardware Motors, Mechanics, Power and CNC
1

I was testing out the differences between write() and writeMicroseconds(). I'm using the Parallax Continuous Rotation Servos for simple robot motion. I have been using writeMicroseconds() for years with my students, but a recent switch to MATLAB highlighted the difference since they do not include writeMicroseconds in their Arduino library.

using: leftservo.attach(12, 1400, 1600);

comparing these commands:
A - leftservo.writeMicroseconds(YYYY); // where YYYY = 1400 - 1500 for full speed to stop.

B - leftservo.write(X); // where X = 0 - 90 for full speed to stop

I measured the pulse widths and Option A with the expected results and variable speed within the range. However, in Option B the X value did not vary the linearly between the defined min and max pulse widths. For some reason the min value was 375ms and the max was 2620ms in practice.

Could someone explain why the write() command is not applying the min and max pulse width values? This is true in both Arduino and using MATLAB with Support Package for Arduino Hardware.

2

You have the Servo library source code, as this is all open source, so you can go and read the code for write()
and writeMicroseconds() yourself you know...

My version of the library (probably ages old!) has this defn of write():

void Servo::write(int value)
{  
  if(value < MIN_PULSE_WIDTH)
  {  // treat values less than 544 as angles in degrees (valid values in microseconds are handled as microseconds)
    if(value < 0) value = 0;
    if(value > 180) value = 180;
    value = map(value, 0, 180, SERVO_MIN(),  SERVO_MAX());      
  }
  this->writeMicroseconds(value);
}

You might have to burrow around in the sources a bit to see if it gets it consistent w.r.t. writeMicroseconds()

3

What attach line did you use for "B"?