Page 86 - Instrumentation and Measurement

5.6 Case Studies

5.6.1 A Liquid Level Process Control System
Figure 5.30 shows one method of how a flow control valve can be used to control the level of a liquid
in a container. Because there may be surface turbulence as a result of liquid entering the container or
stirring of the liquid or perhaps boiling, such high frequency ‘noise’ in the system is often filtered
out by the use of a stilling well, as shown in Figure 5.30. However, it must be recognised that the
stilling well constitutes a U-tube in which low frequency oscillations of the liquid level can occur.
Vertical movement of the displacer results in a signal being passed to the controller. This may be
achieved by the movement causing the slider of a potentiometer to move across its track.

FIGURE 5.30 Liquid level control.
5.6.3 A Robot Control System
Figure 5.31 shows how directional control valves can be used for a control system of a robot. When
there is an input to solenoid A of valve 1, the piston moves to the right and causes the gripper to
close. If solenoid B is energised, with A de-energised, the piston moves to the left and the gripper
opens. When both solenoids are de-energised, no air passes to either side of the piston in the cylinder
and the piston keeps its position without change. Likewise, inputs to the solenoids of valve 2 are used
to extend or retract the arm. Inputs to the solenoids of valve 3 are used to move the arm up or down.
Inputs to the solenoids of valve 4 are used to rotate the base in either a clockwise or anticlockwise
direction.

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