Digital systems
System modification

Disadvantage of presented example is, that there is no way to lower reached level (if evaporization and leakage is neglected) of water in the tank – we cannot set negative flux as a control variable. It disables a control of the level in downward direction.

Fig. 8.4: Hydraulic system of integral type

Let us consider an extension of present example by a pump, able to drain water away from the tank. Its suction pipe is connected at the bottom plate. Let us assume constant flux incoming by pipe, as well as a flux drawn by the pump, both switched between zero and constant value. Dynamics of the pump is neglected.

During drainage, a similar behavior to filling is exhibited, only the level is falling down continuously by a constant velocity, until it reaches level of suction pipe, where it stops. It is an integral phenomenon as well, only the input flux (control variable) is a negative value. If the flux of filling and drainage is equal, the system is linear in the operation range, given by limits (saturation) on upper and bottom ends. If there are different values of flux during these two operations, the system becomes nonlinear – it is an asymmetric system with discontinuous variable parameter – the integration constant (different for drainage than for the filling).