Page 38 - Instrumentation and Measurement

Note that the comparison element in the closed-loop control system is represented by a circular
symbol with a (+) opposite the set value input and a (-) opposite the feedback signal. The circle
represents a summing unit and what we have is the sum.

+ set value - feedback value = error

This difference between the set value and feedback value, the so-called error, is the signal used to
control the process. If there is a difference between the signals then the actual output is not the same
as the desired output. When the actual output is the same as the required output then there is zero
error. Because the feedback signal is subtracted from the set value signal, the system is said to have
negative feedback.

Consider an example of a ball valve in a cistern used to control the height of the water (Figure 3.6A).
The set value for the height of the water in the cistern is determined by the initial setting of the pivot
point of the lever and ball float to cut the water off in the valve. When the water level is below that
required, the ball moves to a lower level and so the lever opens the valve to allow water into the tank.
When the level is at the required level the ball moves the lever to a position which operates the valve
to cut off the flow of water into the cistern. Figure 3.6B shows the system when represented as a
block diagram.

FIGURE 3.6 Ball valve used to control water level in a cistern.

In an open-loop control system, the output from the system has no effect on the input signal to the
plant or process. The output is determined solely by the initial setting. In a closed-loop control
system, the output does have an effect on the input signal, modifying it to maintain an output signal
at the required value.

Open-loop systems have the advantage of being relatively simple and consequently cheap with
generally good reliability. However, they are often inaccurate since there is no correction for errors
in the output which might result from extraneous disturbances.

Closed-loop systems have the advantage of being relatively accurate in matching the actual to the
required values. They are, however, more complex and so more costly with a greater chance of
breakdown as a consequence of the greater number of components.

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