Hallintapiiri, suljettu

w: Set-point value or reference variable (required filling level)

x: Actual value or controlled variable (measured filling level)

x_w: Control derivation (actual value - set-point value), also known as x_d

y: Controller out put signal (control variable)

z₁: Disturbance variable 1 (outflow from the tank)

z₂: Disturbance variable 2 (evaporation of liquid from the tank)