Fieldbus systems represent the best way to use the knowledge available from intelligent process devices. One example is in reducing the cost of maintaining instrumentation.
Manufacturers are increasingly equipping their instruments with smart diagnostics. These devices know exactly when a measured value is no longer dependable and when they must be calibrated, cleaned or replaced. They can even tell when they are being operated outside specifications, or when a pump is running dry. This information is typically available weeks before the fault starts to have serious consequences for the process, and is valuable in reducing maintenance costs.
The traditional approach to servicing instruments, in contrast, depends on reliability statistics collected over decades of operation in different process environments. Maintenance at fixed intervals, with a frequency depending on the application, has been successful in keeping unscheduled downtime to a minimum, maintaining high accuracy in the measured values, and generally increasing productivity.
Such maintenance schedules are based on the average performance of each instrument type, yet in practice instruments exhibit a wide range of performance compared to the average.
Condition-based Maintenance Based on Smart Diagnostics Can Help
On one hand, devices may fail sooner than expected: many production problems can be traced to corrupt measurements due to corrosion, deposits or premature failure. On the other hand, many instruments are still in good working order when they are serviced or replaced on a fixed maintenance schedule. Condition-based maintenance based on smart diagnostics can help to avoid both these problems.
Smart diagnostics and other cost-saving features of modern instrumentation demands a digital fieldbus (see box). Making the most of such an installation in turn requires a suitable choice of equipment by the plant operator.
Most devices are available with Foundation Fieldbus H1 or Profibus PA interfaces. They can be parameterized and diagnosed using a DTM (Device Type Manager) that is conveniently integrated into the control system. The Foundation Fieldbus (FF) H1 and Profibus PA interfaces, which have been specifically developed for use in process-related applications, use the same two wires for both data and power.
These fieldbus systems are robust thanks to their use of Manchester encoding for exchanging data. They differ only in terms of the communication protocol used.
Foundation Fieldbus H1 uses the publisher-subscriber principle. This allows closed-loop control algorithms to be shifted from the process control system into the field level (Control in the Field, or CIF). Especially in larger plants, CIF reduces the load on the distributed control system (DCS) and increases the performance that can be obtained from the relatively long cycle time of FF-H1, which has a data rate of just 31.25 kB/s.
Profibus PA employs the master-slave principle, which is particularly suited for hybrid applications, such as those found in the pharmaceutical and food industries. As a result of the standard Profibus protocol, the fast Profibus DP operations in factory automation — such as filling and packaging processes — are connected with the somewhat slower, process-related plant sections such as mixing, conveying or storing, which, however, have a high availability.
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