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280 Electric Drives and Electromechanical Systems

communicates to a central controller over the bus. The closure of the loop over the
bus requires the maintenance of synchronisation, this is achieved by clock cycle
synchronism, where the exchange of data is controlled by a master clock.

11.2 Supervisory control and data acquisition
Supervisory control and data acquisition (SCADA) is a control system architecture that
uses computers, networked data communications, and graphical human machine
interfaces for high-level process supervisory management and are found in many
industrial sectors, including energy generation and distribution, oil and gas,
transportation, and utilities.
A SCADA system also uses other peripheral devices such as programmable logic
controllers and other distributed controllers to interface to the process plant or
machinery. The operator interfaces, which enable monitoring and the issuing of process
commands, such as controller set point changes, are handled through the SCADA
supervisory computer system. It is important to note that, the real-time control logic or
controller calculations are performed by networked modules that connect to the field
sensors and actuators. The SCADA system is primarily concerned with the visualisation
of the process and the feedback of operations to the operators. The hierarchy of a SCADA
system is shown in Fig. 11.5.

FIG. 11.5 The hierarchy of a typical SCADA system. The key feature of which are the overall controller interfacing
both to the operator and the company’s network. The provision of a diagnostic feature provides additional
resilience to the process. It should be noted that a connection of the internet (and the intranet if required)
should be protected by a firewall.