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3.4.3 Fluid Flow Control
Figure 3.15 shows the elements of a control system used to control the rate of flow of liquid to some
required value, regardless of any fluctuations in supply pressure or back pressure. Figure 3.16 shows
a block diagram of the system.

FIGURE 3.15 Control system for controlling the rate of flow of a liquid.

FIGURE 3.16 Control system for controlling the rate of flow of a liquid.

3.5 Discrete-Time Control Systems

Discrete-time control systems are control systems in which one or more inputs can change only at
discrete instants of time, i.e. the inputs are effectively on-off signals and so in digital form rather than
the analogue form which has been discussed earlier in this chapter. This form of control is often
called sequential control. It describes control systems involving logic control functions, e.g. is there
or is there not a signal from sensor A or perhaps an AND logic system where the issue is whether
there is an input from sensor A and an input from sensor B, in order to determine whether to give an
output and so switch some device on or off.
As a simple illustration of sequential control, consider the automatic kettle. When the kettle is
switched on, the water heats up and continues heating until a sensor indicates that boiling is occurring.
The sensor is just giving an on-off signal. The kettle then automatically switches off. The heating
element of the kettle is not continuously controlled but only given start and stop signals.

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