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,

where represents the correction value to apply to the controller.

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.