Four advantages of state-based control
While state-based control (sequential function logic) generally presents significantly more work, it also provides significantly more benefits. To that end, we’ll pursue a discussion on the advantages of state-based control. It is up to the user to determine if the benefits are enough to justify the cost. For our discussion we will start with a step diagram of a distillation column as an illustration (see Figure 1).
Advantage 1: Mirrors operating discipline
One advantage of state-based control is that it mirrors operating discipline. Typically, to generate a “step diagram” for a process that is converting to state-based control, I start with the “operating procedure.” By comparing that against the instrumentation, you can draw conclusions to what can be easily converted to state-based control, and what additional instrumentation you might need. For example, in Figure 1 we see a “Fill” step. We can either enter this step when flows, pressures, and pump instrumentation tell us the column is being filled and/or upon entering this step, we can enable or turn on various controls and pumps to fill the column.
The point here is the steps mirror—and in essence—become the operating discipline building in repeatability and reliability by reducing human interaction. Clearly a balance must be struck between automation and human interaction. This typically makes itself clear based on the instrumentation and the operating procedure.
Advantage 2: Alarm reduction—making “smart alarms”
One of the greatest advantages of state-based control is its ability to reduce alarms. For example, there is no reason to have a low-level alarm in the fill step. In addition, “smart alarms” can be created utilizing steps to enable and disable all or parts of alarm calculations. For example, a “loss of flame” alarm only needs to be active when flame is expected, perhaps in a “Pilot Run” step. Consequently, the operator only gets alarms that are meaningful.
Advantage 3: Most useful in batch and complex processes
The use of state-based control is most obvious for batch process. However, it can also be readily applied to a process, such as burner management, where multiple sequences can be utilized to control equipment. Multiple sets of steps can be setup to cross-check between a burner management sequence, a process sequence, and even individual burner sequences. While this can add significant complexity to the automation, it can conversely create simplicity and safety for the operation.
Advantage 4: Useful in steady state processes (startup/shutdown time and safety)
Now you might think that for a steady state process that state-based controls would be a non-starter. That would not be true. In fact, state-based control is extremely valuable for steady state processes for the simple reason that no process stays in steady state forever.
Example: Two plants tripped off line because of the same feedstock issue. Both began their restart at the same time. One plant was entirely state based controlled and was back at capacity in 36 hours. The other was on loop controls and took eight days!
What is the reason for the differences? The operating discipline is built in so the complexity of startup is handled by the automation.
State-based control is clearly more expensive to implement and requires a sophisticated level of understanding the operating discipline and instrumentation, as well as automation techniques that take advantage of state-based control. The advantages can’t always be quantified but they can be categorized as improvements in:
– Quality of products.
All positively affected! Even in steady state processes a cost savings can be expressed in terms of startup time reduction and shutdown protection.
The cost is significantly higher but the benefits are also significantly higher.
This post was written by Rocky Chambers. Rocky is a control systems specialist at MAVERICK Technologies, a leading automation solutions provider offering industrial automation, strategic manufacturing, and enterprise integration services for the process industries. MAVERICK delivers expertise and consulting in a wide variety of areas including industrial automation controls, distributed control systems, manufacturing execution systems, operational strategy, business process optimization and more.