Page 96 - Dynamics and Control of Nuclear Reactors

90 CHAPTER 8 Reactor control

Input Output
X(t) Controller Process Y(t)

FIG. 8.1
An open-loop control system.

Reference Input
Based on desired output

e(t) G
Comparison c G p
X(t) Controller Process Y(t)

Measurements
H
FIG. 8.2
A closed-loop (feedback) control system.

Following are some of the features of a closed-loop control system:

1. When feedback is introduced, certain controlled systems can go unstable.
2. The introduction of feedback control reduces the sensitivity of a system to
disturbances and changes due to component aging.
3. Feedback control reduces the effect of natural fluctuations in process variables
and measurements. These fluctuations are usually called noise.
4. Since a closed-loop controller performs comparison of a desired (set point) value
and an actual value of the output, and provides feedback through measurements,
closed-loop control systems are always more expensive than open-loop control
systems.
The components of a system that affect the controller performance are: (1) The
dynamics of the process being controlled; (2) actuators such as valves, thrusters,
motors, heaters, sprays, etc.; (3) various sensors perform process measurements such
as temperature, pressure, flow, level, position, velocity; yaw, pitch and roll of an air-
plane; metal sheet thickness, sheet tension, force on a stand in a rolling mill, etc.
Chapter 16 addresses sensors that are important in nuclear reactors.
The complexity of control systems varies widely and depends on the complexity
of the process and the required control actions. Automatic control generally means
that all control actions are taken by the system, with minimum or no human inter-
ference. This is especially true when the process under study is large and complex,
the desired accuracy of the output or the quality of the product is high, or when