Figure 1.

A PID controller [46]. The prediction error $e\left(t\right)$ is given by the difference between a reference signal $r\left(t\right)$, $y_r$ in our formulation, and the output $y\left(t\right)$ of a process. The different terms, one proportional to the error (P term), one integrating the error over time (I term) and one differentiating it (D term), drive the control signal $u\left(t\right)$.