All industrial processes are analog in nature. As such, one of the foundational requirements of process automation is the translation of analog process variables into the bits and bytes that today’s digital controllers can understand. A controller’s digital decisions, in turn, must at some point be translated back into analog action such as a change in valve position or an adjustment in motor speed.
Not so long ago, this input/output (or simply I/O) function was the exclusive province of fixed-functionality, multi-channel I/O modules that bridged the divide between the digital process automation system and each of the hundreds or thousands of analog field devices in a typical process plant. As discussed earlier in this volume (see “The Problem with Projects”), the wire pair from each field device had to be physically marshalled to a specific type of I/O channel, which in turn had to be connected to the controller responsible for that particular variable. These serial design interdependencies made for inflexible I/O subsystems that could not gracefully accommodate change, resulting in redesign, rework and ultimately late projects that ran over budget as well.
Eliminate I/O where possible
Over the past several years, ABB together and other automation system suppliers have begun to chip away at these serial interdependencies. First, through the development of fieldbus and wireless instrument networks. These advances in digital communications technology increase system flexibility by effectively eliminating the need for traditional I/O hardware altogether. Any new measurement points or outputs added to a project must still be digitally mapped to the broader control system strategies, but that can be done relatively late in the project via software—an approach that’s always less disruptive than reengineering, reordering and/or reinstalling multiple new hardware components.
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The second means by which process plant designers have kept inflexible I/O designs from derailing project delivery is by leveraging modularized process units and other higher level subsystems such as intelligent electrical devices (IEDs) where possible and practical. These subsystems arrive at the project site not with a bundle of analog wires to physically marshal into the main automation system, but with a single (or redundant) Ethernet cable connection. The subtleties of lower-level field device connectivity are left to the subsystem supplier and the measurement and control parameters digitally mapped to broader strategies in the main control system. By some estimates, more than 50% of today’s measurement and control points are wired not at the project site, but come already embedded within larger pieces of equipment and skids.
Configurability tames remaining I/O
Despite these innovative ways of eliminating traditional I/O engineering dependencies, old habits die hard. Indeed, a recent survey of Control readers indicates that analog instruments still account for some 55% of capital project I/O points, whereas 38% is connected via fieldbus and 7% by wireless (Figure 1). Clearly, the process industries still appreciate the simplicity and familiarity of analog instrumentation. So, a third innovation was needed to free project engineers from the rigid tyranny of fixed-functionality I/O modules: single-channel, flexible I/O such as ABB’s Select I/O offering.
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Available for process automation and safety applications, this extension to the System 800xA family of Flexible I/O Solutions allows for each I/O channel to be individually characterized using a plug-in hardware module. This approach streamlines project execution in a number of ways.
First, because the base hardware for every type of signal is the same, automation system designers need only know an approximate I/O count at the design-freeze milestone. Designers can then order standard—not custom—I/O module bases and enclosures, knowing that they have full flexibility to alter the mix of I/O types at any point in the project. Further, since control system hardware components are now standard issue, the factory acceptance test (FAT) of control system hardware is a thing of the past.
Second, because each channel can take on any signal type and be digitally marshalled to any controller, the need for physical marshalling—and all those cabinets and terminations, too—disappears completely (Figure 3). In addition to lowering costs and speeding execution, this has the added benefit of significantly reducing overall system footprint, which can be critically important in an application such as an off-shore oil rig where floor-space and even allowable weight are at a premium. Instrument installation techs save time, too, because they can simply land their wire pairs on the most convenient pair of I/O terminations and move on to the next.
In the end, single-channel, configurable I/O does much to sever the serial design dependencies that plague the execution of automation projects. Hardware and software aspects of automation system design can proceed in parallel, compressing schedules and reducing risk. But ABB goes a step further, delivering software tools and new execution workflows to ensure that these streamlined, parallel engineering processes meet up in a fully tested, fully functional automation solution.
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