We will give a simple example of process control.
Figure 1.1 Example of Manual Control
The diagram above (see Figure 1.1) shows what is called MANUAL
control. The process is temperature control. The indicator is
a thermometer. The correcting unit is the gas control valve.
The controller is the operator who uses his own judgement to
keep the water temperature constant.
Manual control has its uses as it is cheap to install and maintain,
and simple to operate. However, it is very seldom used in industry
• The operator must remain in position at all times.
• It cannot be used if the operator is placed in a dangerous area.
• The process changes faster than the operator can react.
• A mistake by the operator can have dangerous results.
These problems are avoided by using automatic control.
Modern household appliances now use automatic control to make
• Refrigerators and air conditioners use automatic temperature control.
• Washing machines use automatic heating and water control.
Figure 1.2 Simple Example of Automatic Control
The above diagram (see Figure 1-2) shows a simple automatic
controller. The boiler now has the loop closed and no operator
The following items are added.
• The temperature transmitter (T.T) which measures (senses) the temperature of the hot water and changes it to a standard signal.
• A signal line from the transmitter to the controller, the signal
may be either pneumatic or electrical.
• A controller which keeps the temperature of the hot water at
a position set by the operator (set point)
• The controller adjusts the correcting unit (automatic control valve) using an output signal line similar to the input line from the
• The controller may provide alarm signals to alert the operator if
the system fails. It may also shut off the gas if the water starts
• This is a very simple example of automatic control.
• Process Control Instruments can be very complex.
• If we take each part or device that is in a complex unit one by
one the system is easy to understand.