Process control, PID and adaptive control - Intelligent Buildings and Building Automation - page 114

passes through the sequence: room temperature, temperature sensor, indica-
tor, eye, brain, muscle, control valve, heating flow rate and room temperature
again. The physical nature of the signal is different in various parts of the
loop, but each part affects the next in turn and a change at any one point will
propagate round the loop, as shown in Figure 7.3, which shows the block
diagram equivalent of the operation process. It is important to notice that a
feedback control loop can achieve its objective (i.e. to maintain the controlled
process output at its set-point) only when the feedback of the closed loop is
negative; that is, a negative feedback control loop.
A control system is defined as an interconnection of components forming
a system configuration that will provide a desired system response. Because
a desired system response is known, a signal, somehow, proportional to the
error between the desired and the actual response, is generated. The utiliza-
tion of this signal to control the process results in a closed-loop sequence of
operations that is called a feedback system.
In this example, we find that the introduction of the feedback control
dramatically reduced the complexity of open-loop control systems and the
process output can be controlled accurately at the desired value. We may
summarize the benefits of using a feedback control system as follows:
￿ ease of control and adjustment of the transient response of the system;
￿ great reduction in external disturbances (except those associated with
sensors) on the controlled variables;
￿ greater reduction of steady-state errors of the system;
￿ a decrease in the sensitivity of the system to variations in the parameters
of the process or tolerant variations (due to wear, age and environmental
effects).
The addition of feedback to a control system results in the advantages out-
lined above. However, it is natural that these advantages have an attendant
cost. The cost of feedback is the introduction of the possibility of instability,
which is caused by the overcorrection of the process input as the results of
delay and component dynamics. While the open-loop system is stable, the
closed-loop system may not always be stable. The instability problem of
the closed-loop control system is often an important and difficult matter in
control system design.
The addition of feedback to dynamic systems results in several additional
problems for the designer. However, in most cases, the advantages far out-
weigh the disadvantages, and a feedback system is utilized. Therefore it is
necessary to consider the additional complexity and the problem of stability
when designing a control. Finally, we should keep in mind that if an open-
loop control, of simple design, for a process can achieve satisfactory accuracy,
an open-loop system should be considered first.