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Figure 9-3. Nonlinear pulse control
Sequential Control Systems
A sequential control system is one wherein the controller has a discrete set of states
with inputs and outputs consisting of discrete control signals. The controller might
enable or disable devices that themselves utilize discrete states, or they could be linear
(or nonlinear) subsystems. The main point is that the devices under control will be
either active or inactive, on or off, in a predefined sequence.
Sequential control systems are often found in applications where a series of timed steps
are performed in a specific sequence. An example would be an automatic sprinkler
system. Internally the sprinkler controller is based on a model composed of sprinkler
zones, which typically define groups of commonly plumbed sprinkler heads in certain
areas of a yard (or golf course, park, etc.).
Figure 9-4 shows the timing chart for a five-zone sprinkler system. The system is programmed to activate each zone at a certain time on a specific day of the week, and to
stay active for some specific duration.
In this example the sequence starts at 2200 hours (10 p.m.) and ends at 0400 (4 a.m.).
A typical automatic sprinkler control system is a form of sequential control that relies
solely on the current time; it has no other inputs to regulate its behavior. It doesn’t
matter if the soil is already damp, or if it is pouring rain; the grass will get watered
anyway at the designated times.
308 | Chapter 9: Control System Concepts