Unformatted text preview: is given in Figure 3. From this figure we can appreciate that the Nyquist plot is exactly crossing the crit ical po int (1;0) thus indicating that this is the critical gain ( k cu ). Critical point Figure 3: Nquist diagram for k cu = 2.4 . The corresponding closedloop response for the same value o f the controller gain is give n in Figure 4. From this Figure, the ult imate period can be estimated as Pu = 2.5 . From the ZN settings using Table 12.3, we find for a PID controller: k c = 1.44 t I = 1 25 t D = 0 31 ; . ; . Figure 4: Closedloop response for P controller with k c = 2.4 . Figure 5 illustrates the closedloop response for a setpoint change using the PIDZN controller settings. Figure 5: Closedloop setpoint response with a PID controller using k c = 1.44 t I = 1 25 t D = 0 31 ; . ; . .
2. For the blending process (see Continuing Problem), and using the transfer funct ions developed in Chapter 5, we would like to implement a feedback control scheme to control the level o f the tank using F1 as manipulated input. F2 , in this case, is a disturbance:
1. Write the characterist ic...
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 Fall '08
 Hjortso,M
 controller gain

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