Page 455 - Mechanical Engineers' Handbook (Volume 2)

446 Closed-Loop Control System Analysis

Figure 2 Open-loop system configuration.

Such a multilevel approach becomes useful in controlling very complex processes, with large
numbers of control loops with many inputs and controlled variables. In the hierarchical
approach the system is subdivided into a hierarchy of simpler control design problems rather
than attempting direct synthesis of a single comprehensive controller for the entire process.
Thus the controller on a given ‘‘level of control’’ can be less complex due to the existence
of lower level controllers that remove frequently occurring disturbances. At each higher level
in the hierarchy, the complexity of the control algorithm increases, but the required frequency
of execution decreases. Such a system is shown in Fig. 4.

1.4 Direct Digital Control (DDC)

In direct digital control all the analog controllers at the lowest level or intermediate levels
are replaced by a central computer serving as a single time-shared controller for all the
individual feedback loops. Conventional control modes such as PI or PID (proportional
integral differential) are still used for each loop, but the digital versions of the control laws
for each loop reside in software in the central computer. In DDC the computer input and
output are multiplexed sequentially through the list of control loops, updating each loop’s
control action in turn and holding this value constant until the next cycle. A typical DDC
configuration is shown in Fig. 5.

1.5 Hybrid Control
Combinations of analog and digital methods based on individual loop controllers is known
as hybrid control. It is clear that any real control system would have both analog and digital
features. For example, in Fig. 5, the plant outputs and inputs are continuous (or analog)
quantities. The inputs and outputs of the digital computer are nevertheless digital. The com-

Figure 3 Supervisory control configuration.