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Digital Control

The latest generation of field devices and controllers, are part of the digital networks as represented by the various fieldbuses. These devices are yet again more accurate than their analogue predecessors because they do not have to convert the signal from analogue to digital to transmit it into the host system. In addition, these devices also transmit a status variable with each process reading update so that the control system knows if a signal should not be used for control.

Figure 3 shows how reducing the variability of the process provides the opportunity for a facility to operate closer to the control limits, whether that is a quality target or the physical limit of a piece of equipment. As can be seen, it is possible to move the mean, which becomes the new set point under each of these conditions, slightly closer to the limit. It is this ability to operate closer to the limits that forms the basis to justify most control projects/investments. The assumption, of course, is that the control limit remains unchanged for all four methods of control.

Figure 3: Impact of Closed Loop Control

Optimisation/Integration

As with all things electronic and microprocessorbased, the trend in the automation and control world is also to a greater and easier integration. Typical of this is the move by control systems to using Ethernet, Windows™ and Web-based graphics for several key components of their offerings.

The interfaces to the process such as the operator and engineering stations are predominantly Windows and Web-based devices, while the backbone to connect all these components as well as some of the computers used to gather data from the field use Ethernet as their communications carrier. Some of the computers that form part of these modern control systems to perform the multivariate and optimisation calculations also run on the Windows platform. As Figure 4 illustrates there are a large number of common components on each of the levels of an integrated enterprise, making it easier to transfer data between each layer and consequently improve the ease with which the information necessary to optimise an operation can be made available.

Figure 4: Today's Integrated Enterprise

This same ease of integration across the enterprise when combined with the use of commercial off the shelf (COTS) technology such as Windows, Ethernet and Web graphics also increases the risk of external contamination. As a result, process control system security is now another issue that needs to be addressed as part of the design and operation of a process facility. Figure 4 shows firewalls between each of the levels of the enterprise, as one step in the suite of activities necessary to ensure safe, reliable operations. Another critical step is the development and adherence to a strict control system security policy. Much research is being conducted in the area of process control security by government, industry and standards organisations in this important field.

Every business is in operation to provide benefits to its client owners. As shown above, today’s modern control systems, when properly installed, managed and maintained, can and do make a significant contribution to the ability of a facility to remain safe, reliable and profitable.

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