ABB Customer World: Marshalling field instruments the digital way


HOUSTON -- The advent of input/output (I/O) hardware systems that are fully flexible on a single-channel basis has remarkably wide-ranging implications when it comes to project risk. Each analog I/O channel and module starts out essentially identical to the next; entire I/O cabinets—fully assembled with redundant power supplies and network connections—can be ordered with a single part number.

Only late in the project execution cycle, often just before commissioning, does each channel take on its true identity, often through a plug-in signal conditioning module (SCM) as in the case of ABB’s Select I/O offering. When it comes to project execution, this persistent flexibility means that even analog, point-to-point signal loops behave much more like their I/O-independent digital counterparts.

Fieldbus instrumentation, packaged skids and other digital devices already plug directly into plant control and instrument networks without the need for specialized conditioning. Now, instrument installation techs can land their analog signal pairs on any convenient I/O channel without regard for signal conditioning specifics—much as they might when connecting a digital device to a digital network.

Importantly, that specialized conditioning is added to the channel late in the project, and only then is the instrument digitally marshalled to the plant’s full automation and information architecture and bound to the appropriate controller and applications.

This effectively eliminates custom I/O “engineering” as well as the remaining serial dependencies between automation hardware and software engineering. Control system hardware and software development tasks can proceed along parallel paths, dramatically compressing project delivery while maintaining the flexibility to accommodate change, which in turn reduces budget and schedule risk.

The common starting point for parallel application development and hardware tasks remains the list of necessary input and output points that emerges from a preliminary process design. In ABB’s xStream Engineering methodology, for example, each of these points will have been given a unique “signal” identifier, or field device tag. Functioning as both instrument signal and application signal, the consistent and coordinated use of these field device tags throughout hardware and software project execution contributes to the smooth and speedy start-up of a fully functioning system.

Engineers responsible for the hardware side of the control system equation can then order standard cabinets with the adequate number of generic I/O channels (plus spare capacity) based on the list of signals. Because the project is based on pre-designed and tested I/O cabinets that require no custom engineering, there’s no need for a factory acceptance test (FAT) when the cabinets arrive. Instead, technicians can proceed to install the cabinets, wire the field devices to the I/O, configure each I/O channel with the appropriate plug-in signal conditioning module and field device tag, and check that each loop functions as expected.

Meanwhile, those responsible for the software will have developed the necessary application logic, allocated the applications to specific controllers, and—in a virtual FAT—tested the software against emulated hardware and a simulated process to ensure that the code will perform as anticipated.

Finally, the tested application and hardware meet up on site. I/O points are digitally marshalled and bound to their designated controllers and applications, independent of physical I/O channel location. This ability to preserve automation system flexibility while performing hardware and software engineering tasks in parallel is a critical step toward moving automation off the critical path of the overall project schedule.

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