Figure 5.9 : Response of (the manipulated variable)for P (- - - -) and PI (------) control.
The flow out of the tank for the P control rises slowly to the value of the input step change(to force the flow out to become equal to the flow in), while the PI control forces the flow out to a periodic function, the maximum value of which is approximately 1.5 times larger than the initial disturbance(of 0.0001 /s). Thus the P control gives a smooth transfer between two steady-states, while the Pl control magnifies the inlet disturbance so that the disturbance to the outlet flow is up to 1.5 times as large. This may cause serious control problems downstream of the tank. Note that this discussion is only relevant to level control where the magnitude of the flow out of the tank is as important as the level(i.e. the value of the manipulated variable is as important as the value of the controlled variable). In most cases the controlled variable is much, more important than the manipulated variable, and PI control is more suitable, as the controlled variable will be forced back to the set-point.