System Control

A system can generally be defined as a physical, chemical, and/or biological process in which an input signal is transformed into an output signal. This implies that the output is merely the result of the interaction of the input with the internal components of the system, and that therefore, when the input is varied in a particular way, the output is also caused to vary in a particular way. The output depends on the input according to the properties of the system (Figure 13.5), which ideally can be expressed in the form of mathematical equations (a model).

A system input is called a control if it can be chosen in such a way as to induce the system to conform to some desired behaviour. In general, process control refers to an engineering operation by which a system 'makes decisions' about how to best manipulate available variables to obtain a desired output. Control theory has been instrumental in the successful design of most man-made complex systems.

Although control theory can also contribute to the understanding of biological systems, they are rarely analyzed mathematically. This is due partly to the complexity of the estimation of numerical parameters and partly to the numerous interlocking control loops that biological system normally include, which make it difficult to study one system in isolation.

Characteristics of biological systems include:

- extremely complex, efficient, robust, and high-performance,

- control systems overrule,

- networks dominated by feedback loops arranged in a hierarchy,

- relevant variables are often difficult to measure, control, or even identify,

- virtually impossible to isolate.

Various control schemes used in complex engineering systems are also found in biological systems. Among them, feedforward control and feedback control - which are two major control schemes - are nearly ubiquitous in biological systems.

Feedforward control is a control strategy by which the controller measures a process variable representing the disturbances that can affect the process. An example input

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