Controlling a DC motor by PWM at constant speed

Other Hardware Motors, Mechanics, Power and CNC
2

The best way to do this is with a PID controller - take the difference between desired and actual speeds as the error value, let the PID control the PWM output.

Failing that a simple approach is to increase the PWM by 1 each time you sense the speed is too low, and decrease by 1 each time it is too slow, clunky but might be OK.

BTW that float rpm computation can be done faster like this:

    // incercare
 
  if (trigger != 0) 
  {
       rpm = 60000000UL / (t1 - t0) ;
       trigger = 0; 
  }

Floats aren't needed at all.