Page 35 - Instrumentation and Measurement

3

CONTROL SYSTEMS

3.1 Introduction

The term automation is used to describe the automatic operation or control of a process. In modern
manufacturing, there is an ever-increasing use of automation, e.g. automatically operating machinery,
perhaps in a production line with robots, which can be used to produce components with virtually no
human intervention. Also, in appliances around the home and in the office, there is an ever-increasing
use of automation. Automation involves carrying out operations in the required sequence and
controlling outputs to required values.

3.2 Control Systems

As an illustration of what control systems can do, consider the following:

Control a variable to obtain the required value

1. The required temperature for a room is obtained by setting to the required temperature the
room thermostat of a central heating system. This is an example of a control system with the
controlled variable being the room temperature.

Control the sequence of events
2. The dials on an automatic washing machine are set to indicate that ‘whites’ are being washed
and the machine then goes through the complete washing cycle appropriate to that type of
clothing. This is an example of a control system where a controlled sequence of events occurs.

Control whether an event occurs or not

3. The automatic clothes washing machine has a safety lock on the door so that the machine will
not operate if the door is open. The control is of the condition which allows the machine to
operate.

A control system can be thought of as a system which for some particular input or inputs is used to
control its output to some particular value (Figure 3.1A), give a particular sequence of events (Figure
3.1B) or give an event if certain conditions are met (Figure 3.1C).

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