Page 39 - Instrumentation and Measurement

3.3 Basic Elements

Figure 3.7 shows the basic elements of an open-loop control system. The system has three basic
elements: control, correction and the process of which a variable is being controlled.
1. Control element

This determines the action to be taken as a result of the input of the required value signal to the
system.
2. Correction element
This has an input from the controller and gives an output of some action designed to change the
variable being controlled.
3. Process
This is the process of which a variable is being controlled.

FIGURE 3.7 Basic elements of an open-loop control system.
With open loop, there is no changing of the control action to account for any disturbances which
change the output variable. Figure 3.8 shows an example of an open-loop system which a motor is
used to drive a gearbox and so rotate an aerial dish picking up signals from a satellite. With open
loop, changes in set point have to be used to accommodate disturbances or changes in loading. So, if
the aerial is required to track a satellite while a person could keep adjusting the input set value what
is really required is a closed-loop system in which feedback about the actual position of the satellite
is fed back to automatically adjust the input to the system.

FIGURE 3.8 An open-loop control system for an aerial dish.

3.3.1 Basic Elements of a Closed-Loop System

Consider the task of driving an automobile along a road. The driver observes the direction of the road
and sets the direction by use of the steering wheel. If there are disturbances, such as perhaps traffic,
then the driver observes the disturbances and corrects the direction of travel by means of the steering
wheel. This is an example of a closed-loop control system and is illustrated in Figure 3.9.

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