Consider State-Based Control

This under-utilized approach can enhance operations and the bottom line.

By David Huffman, ABB Inc.

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• performance variability linked to operator experience and training; and
• need for frequent attention to automation configuration to maintain process performance at high levels.

If your facility isn't using state-based control (SBC) then you probably don't have acceptable answers for most of these questions. Even worse, your business is missing an opportunity to achieve potentially large improvements in OEE and return on net assets (RONA).

SBC is an automation design philosophy that recognizes processes have identifiable states and the control system nearly always can be the best operator to keep the plant running within those states and during state transitions. It isn't a new concept — it's long been implemented for batch production. Indeed, the ability to use logic to sequentially control a complete series of plant operations has been available in many process controllers for several decades. At the core, that's what underlies SBC. Dow Chemical has applied SBC techniques for many years and has documented a two times better RONA for processes that use SBC versus those that rely on conventional automation designs (Figure 2).

However, in most automation systems designing, coding and maintaining this type of logic has been costly as each piece of logic usually was a unique code block with nearly no reuse of code between applications.

Now, adoption of concepts for developing reusable code modules as presented by the ISA88 standard is increasing. While basically developed to deal with batch processes, the standard never was limited to them. ISA88 is being expanded and this should make even clearer that the principles apply across nearly all industries and not just to batch producers.

In addition, the latest generations of automation systems and their modern tools lower the hurdles to using SBC designs. Some automation systems now utilize object technologies that can significantly reduce initial engineering and lifecycle maintenance costs.

Visibility of information in the control hierarchy is a key feature operators need in a SBC design. Additionally, they must be able to easily view the active state logic to do troubleshooting. This is far different from today's conventional applications where operators only interact with control device faceplates and alarm lists.

Major Benefits
SBC's advantages stem from two main features:

1. Fewer interaction sites for an operator to deal with; and
2. Situational optimization that can be driven from the state control engine.

In SBC, an operator controls just units (larger equipment groupings like a distillation tower and its associated vessels, pumps and instrumentation or perhaps a complete boiler) or equipment modules (smaller groupings of equipment like the distillation tower's overhead section or the boiler superheater section). The operator can change a unit state, e.g., from process wait to starting, merely by selecting the new state. The state control engine configured in the automation system performs all actions needed to make the unit start up. This is more efficient than having the operator execute the necessary sequence of dozens of faceplate access actions and data entries — and avoids the risk of mistakes. A unit faceplate should contain access and interaction points to every equipment and control module grouped within the unit, should individual access be required (the visibility relationships). The unit faceplate should be able to call up an interactive view of the state diagram without using engineering tools or licensing. That diagram should allow easy display of the logic behind the steps and transitions — offering the operator an initial level of troubleshooting when things go wrong.

Even more important than significantly reducing interaction sites and potential mistakes that can be made is the opportunity for situational optimization. This isn't the model-based PhD-in-mathematics optimization of years past but rather a conditional-based optimization that's much simpler, perhaps more extensive, and thus more valuable.
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