People have come to expect pixel-oriented graphic displays at the man-machine interface. While a great deal of effort is spent in providing drawing tools for the manager and engineer, the operator is sometimes neglected. The system should provide clear and fast graphic displays. Documentation should be clearly written. Online help screens are ideal documentation, because they are available while working on the system. Networking is another expectation for these small systems. With a PC-based distributed control system, one should expect the same features as in a large system.
PCs in the Process
The PC can bring several benefits to even very small plant processes in a cost effective manner. These benefits capitalize on the PC's abilities to gather, store, present, and manipulate data in much the same wav PCs are used in the office environment. In the office environment where PCs are a way of life, they are regarded as powerful information processing machines. Facts, figures, and the printed word are manipulated by professionals striving to maximize the useful work of this indispensable tool.
Industrial pc may someday become as widespread in the process industries as they are in the office. Process control specialists are beginning to look at the PC in much the same way as their business counterparts. Efficient information processing becomes the objective with a great deal of thought given to the amount of data needed and manner in which it may be presented, archived, or manipulated.
Off the shelf hardware and software available from a variety of sources has brought PCs within the reach of the typical plant for a variety of uses. Their greatest value lies in their ability to improve the productivity of the people using them. Information can be gathered, stored, and presented in a more useful manner that has direct benefits in terms of spotting problems quicker, facilitating control strategy improvement, and making better data analysis possible.
PCs can interact with the process in the following way:
• Monitor only device-all operation through other devices.
• Monitor and auxiliary operations-operate through other devices or PCs.
• Monitor and primary operations center-operate mainly through PC, with other devices used backup.
In each case the PCs function is to gather data, manipulate the data in some form, and present it in a light that enables plant personnel to take action upon the information in a beneficial manner. Differences only exist in the devices used to actually effect changes to the process. Implementation of the philosophy desired occurs when the process devices the PC is to interface to are selected along with matching software. The process devices include I/O or microprocessor-based controllers. The software provides communications drivers, menu driven configuration, and application programs such as standard displays.
As an operations tool, the PC provides many of the same benefits found in large computerized systems. It becomes a centralized point facilitating quick recognition, interpretation, and manipulation of the process. This is done through data grouping, data trending, and chronological alarm presentation. Data grouping is the presentation of related information in a logical format. By using the PC's programmable nature to easily place related data on the same display, operations personnel can see how a change in one variable affects another.
Industrial monitor in the form of semigraphic panels have been used for many years to serve as a starting point for discussion on the corporate tour, train new operations personnel, and quickly overview plant/unit status for experienced personnel. Touch screen display on PCs serve the same functions at a greatly reduced cost. The economic advantage is apparent also when a change in equipment usage occurs. Updating the semigraphic panel is much more expensive than rearranging a programmable graphic display. Status displays allow a much higher concentration of data within the view of an operator. This permits operators to gain a greater feel for the process. This type of display helps overcome problems where an important variable is far away in another control room or on another physical section of the panel not within a reasonable viewing distance. It is also useful when changes in equipment usage necessitate control strategy change, where the typical results due to panel space limitations or temporary fluxes that have a way of becoming permanent, force placement of control devices in undesirable locations. Again the PC allows easy rearrangement.
Data trending in real-time allows operations personnel to gain an understanding of the recent history of the process. This helps smooth the transition of shift changes especially when a recent upset in operations may have occurred. Training and learning of the time relationships that exist between certain variables is facilitated as one variable can be superimposed on another. The economics of data storage on PCs also favors recording many variables that may have only marginal utility, without justifying chart recorders and associated paper costs. A periodic review of these variables by operations or maintenance personnel may turn up impending equipment problems before they become serious. The advantage the PC brings to the plant chronological alarm presentation is the ability to time stamp alarm occurrence. During upset situations the first few alarms are often more important than later alarms in zeroing in on the problem source. The alarm summary page rapidly identifies these first few important alarms from the sometimes multiple later alarms that cloud the situation preventing immediate recognition and corrective action.
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