For closed-loop controllers
it is easiest to think in terms of the error, the difference
between the commanded and measured values, of some quantity
(like RPM) instead of the measured value directly.
So we write the above as,
This proportional controller is very easy to implement, and is frequently used with good results. The controller does have a flaw in it; if the system is sitting at the desired output level (the set point) and is gets perturbed, the controller has a tendency to respond by oscillating about the set point. These oscillations cause the system to be at the set point on average, but it never settles down to stay there.