Control Engineering - A guide for beginners - Chapter 2 ppsx

JUMO, FAS 525, Edition 02.04 29

2 The process

2.1 Dynamic action of technical systems

The process is the element of a system which has to be controlled in accordance with the applica￾tion duty. In practice, the process represents either an installation or a manufacturing process

which requires controlling. Normally, the process covers a number of elements within a system.

The input is the manipulating variable y received from the control device. The output is represented

by the process variable x. As well as these two variables there are the disturbances z which affect

the process to some extent, through external influences or process-dependent variations.

An example of a process is a gas-fired furnace (see Fig. 15). At the start of the process is the valve,

which has as its input the manipulating variable of the controller. The valve controls the gas flow to

the burner. The burner produces heat energy by burning the gas, which brings the charge up to a

higher temperature. If the temperature in the charge is measured (process value), this also forms

part of the process. The final component of the process here is the sensor, which has the job of

converting the temperature into an electrical signal. Disturbances here are all the variables in the

process which, when they change, result in a different temperature for the same valve setting.

Example: If the manipulating variable is just large enough to give the required temperature in the

charge, and a disturbance occurs due to a fall in outside ambient temperature, then, if the manipu￾lating variable is not changed, the temperature in the charge will also be lower.

Fig. 15: Input and output variables of a process

When designing a control loop, it is important to know how the process responds when there is a

change in one of the influencing variables mentioned above. On the one hand, it is of interest to

know the new process value reached when stable conditions have been attained, following such

changes. On the other hand, it is also important to find out how the process value varied with time

during the transition to the new steady-state value. A knowledge of the characteristics determined

by the process is essential and can help to avoid difficulties later on, when designing the process.