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10.8
To achieve stability, the feedback controller must have an integral mode.

10.9
The control system is functioning well. Which of the following would result in the feedback system becoming unstable?

	Increase the magnitude of the controller gain (K_c) by a large factor.
	Switch the sign of the controller gain (K_c).
	Increase the integral time (T_I) by a large factor.
	Decrease the initialization constant (bias, I) to zero.

10.10
What is the basis for the Bode stability test?

	The poles of the characteristic equation must have negative real parts.
	A sine introduced into the feedback system must not have its amplitude changed.
	A sine introduced into the feedback system must have its amplitude decrease.
	A sine introduced into the feedback system must have its amplitude decrease at the critical frequency.

10.11
The control system analyzed by the Bode plots given below is stable.

False

10.12
What is the basis for the Ziegler-Nichols tuning method?

	There is no theoretical basis, its purely based on experience.
	The controller is tuned so that the closed-loop system is on the boundary between stability and instability.
	The controller is tuned so that the closed-loop system has a stability margin of about 2.
	The controller is tuned so that the IAE for a set point change is minimized.

10.13
In a process plant, we would use the Ziegler-Nichols closed-loop (cycling) tuning method.

False

10.14
The feedback system was working properly; then, we doubled the tank volumes. How does the tuning have to be changed in response?

	There is no reason to retune the controller; it's still a PID.
	The ultimate gain and critical frequency have doubled.
	Kc is halved ; TI and Td are doubled.
	Only the critical frequency has doubled.
	Kc unchanged ; TI and Td are doubled.
	There is no way to answer this question without performing a process reaction curve experiment at the new conditions.