Implementation in Arduino Servo Libary for Rotation with Less Than 1 Degree Granularity

Arduino is popular open-source software/hardware platform for DIY. Using Arduino platform, one can control all kinds of device such as sensor, servo, LCD etc. Servo is a motor with feedback loop for rotation measurement. The feedback loop helps the servo to achieve precise control. If you tell a servo to rotate 90 degrees, it will do that. This makes servo an essential part for projects like robots. However, the smallest rotation the existing Arduino servo library can achieve is 1 degree. The reason is that the existing library only allows one to program integer number of degrees for rotation. Therefore, if one want to program servo for rotation less than 1 degree to achieve more smooth transition, the library has to be modified but the modification is rather straightforward. This post gives an example of how to modify the library.

The existing servo rotation function looks like this: an integer value is used as the input. First, the function will make sure that this integer is between 0 and 180 since that is the scope of the servo rotation. Then this integer will be mapped into a value between SERVO_MIN() and SERVO_MAX(). Here is how the map() function work: assuming that y = map(x, 0, 180, SERVO_MIN, SERVO_MAX), then (x - 0)/(180 - x) = (y - SERVO_MIN)/(SERVO_MAX - y). This y will be used to drive the servo operation. Below is the existing function in the Arduino servo library:

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);
}


To make sub-degree rotation possible, we create a new function with floating-point number as input.

// To write angle in floating value /Nan Zhang
void Servo::writeFloat(float 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 < 0.0) value = 0.0;
    if(value > 180.0) value = 180.0;
    value = map((int) (value*10), 0, 1800, SERVO_MIN(),  SERVO_MAX());
  }
  this->writeMicroseconds(value);
}

In this new function, we will first check whether this input is between 0.0 and 180.0. After that, the rounded value of (input*10) will be injected into the map function as map((int) (value*10), 0, 1800, SERVO_MIN(),  SERVO_MAX()). Essentially, this makes the smallest achievable rotation to be 0.1 degree. You may want to further increase the granularity by doing things like map((int) (value*100), 0, 18000, SERVO_MIN(),  SERVO_MAX()). But you will face physical limitation of how precise the servo can operate. For example, the default value for MAX and MIN are:

#define MIN_PULSE_WIDTH       544     // the shortest pulse sent to a servo
#define MAX_PULSE_WIDTH      2400     // the longest pulse sent to a servo

That means the best the device can do is (180)/(2400-544)=0.097 degree. Therefore, increasing granularity beyond 0.1 degree does not make sense for this servo device.