Page 45 - Instrumentation and Measurement

As another example, the filling of a container with water might have a sensor at the bottom to register
when the container is empty and give an input to the controller to switch the water flow on and a
sensor at the top to register when the container is full and give an input to the controller to switch off
the flow of water (Figure 3.17). We have two sensors giving on-off signals in order to obtain the
required sequence of events.

FIGURE 3.17 Discrete-event control with the controller switching the valve open when empty
signal received and closed when the full signal.
As an illustration of the type of control that might be used with a machine, consider the system for a
drill which is required to automatically drill a hole in a workpiece when it is placed on the work table
(Figure 3.18). A switch sensor can be used to detect when the workpiece is on the work table, such a
sensor being an on-off sensor. This then gives an on-input signal to the controller and it then gives
an output signal to actuate a motor to lower the drill head and start drilling. When the drill reaches
the full extent of its movement in the workpiece, the drill head triggers another switch sensor. This
provides an on input to the controller and it then reverse the direction of rotation of the drill head
motor and the drill retracts.

FIGURE 3.18 An automatic drill.

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