E.6.1 Objective

The objective of this exercise is to demonstrate the direct relationship between process lag and stability, and to make students aware of the great difference between noise and stability in practice.

E.6.2 Operation

Call up the training application General single loop with interactive PID (real form).

In order to see the stability problem of closed loops in practice and without disturbances, confirm that K-DIST is 0. Call up auxiliary display F8 and change the gain for disturbance to 0 (K-DIST = 0) and the gain for noise to 0 as well (K-NOISE = 0). Then the auxiliary display should be as shown in Figure Ex. 6.1.

Ex. 6.1: Auxiliary Display Showing K-DIST = 0 and K-NOISE = 0

Call up detail display F3 as well, and make the changes K= 3.5, T _INT = 999 and T _DER = 0 as shown in Figure Ex. 6.2. The values shown in the detail display make it possible to create a continuous oscillation, purely based on the lag of the process. With these values, no integral and no derivative control action takes place and the controller will neither increase nor decrease the phase shift within the control loop. Therefore, the existing phase shift has been caused by the process only.

Ex. 6.2: Detail display

Change SPE from 1000 to 1100 to start continuous oscillation (see Figure Ex. 6.3).

Ex. 6.3: Continuous oscillation

E.6.3 Observation

In this situation, the controller performs P-control only. As shown in the...