Hello .
In fact I do not measure the RPM. I don't need that.
I simply vary the PWM for the effects and the ball long.
I deduce approximate RPM .example:
255 = 5200 rpm
127 = 2600 etc.
Just allow time for the engine to regain its speed between two balls.
The goal is to set X modes of training ( forehand, backhand , volley ..... ) .
I use a bluetooth Module HC06 and android application made on the MIT app inventor 2 .