Page 48 - Instrumentation and Measurement

The term fieldbus is used for such a bus system that conforms to the IEC 61158 specification, there
being a number of such bus systems. The standard defines the physical means by which the buses
can be realised and the means by which devices connected to a bus can communicate with a controller
and receive control instructions. Fieldbus are bi-directional, digital serial networks with the data
being put on bit by bit along a single connector. In process control, the main buses used are the
FOUNDATION Fieldbus and PROFIBUS PA. FOUNDATION Fieldbus is a digital, serial, two-way
communication system widely used as the network in process industries. PROFIBUS was designed
specifically for use with programmable logic controllers in control networks (see Chapter 7). The bus
most used in automobiles is the CAN bus (see Chapter 13 for more discussion of bus systems).

3.7 Hierarchical Control

The term hierarchical control is used for the form of control systems where we have one control
system being used to control other control systems to give a pyramid of control (Figure 3.22).

Thus, we might have one control system controlling one aspect of a manufacturing process and
another controlling a different aspect, the two control systems then being supervised. Indeed, there
may be a number of such supervised controls with the supervisory controls in turn being supervised
by a management controller. Such supervision might be used to change the set points of control
systems in order to cope with some change in circumstances.

FIGURE 3.22 Hierarchical control.

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