What is Non-Steady State Modeling or NSS Modeling?

Why LOOP-PRO is the Industry-Leading PID Tuning Software

Non-Steady State (NSS) Modeling is the ability to calculate an accurate process model when the associated data is oscillatory, is noisy, or has extensive Dead-Time.  It’s a highly valuable capability when tuning difficult PID loops, and it was pioneered by Control Station.

Before software can provide good tuning parameters and improve a controller’s performance, it first needs to understand the dynamics of the associated process. In general tuning software establishes a cause-and-effect relationship between a disturbance and how the process responds to how far it shifts (Gain), how quickly it corrects (Time Constant), and with how much delay (Dead-Time). The software’s ability to generate a model that accurately describes oscillatory and/or noisy process data is of critical importance. Oscillatory and noisy data is difficult to model accurately, but it is typical of industrial production processes.

For decades the state-of-the-art in PID controller tuning software required users to establish a steady-state condition before introducing a controlled disturbance. Without a steady-state condition the software would either reject the data or generate a flawed model. That was true for integrated auto-tuners as well as for after-market software products, and neither outcome was good. The irony is that when a control loop can be maintained at a steady-state before introducing a controlled process step change, it is relatively easy to tune manually. Viewed differently: Users look to software to tune their difficult loops, but traditional software only works when loops are controlled well enough that they’re easy to tune by hand. Go ahead and scratch your head over that one!

The term NSS Modeling is applied broadly and oftentimes in a misleading manner. Some vendors reference NSS Modeling capabilities, but their products only model the non-steady dynamics of integrating processes. That’s great if all you have are Level loops.  Besides, techniques for modeling open loop integrating loops are widely published. Although the ability to model integrating processes is essential, those loops represent maybe 15% of industrial control loops.

Control Station first introduced the term NSS Modeling and changed the state-of-the-art in 2008. We filed a patent for a new method by which dynamic process data is modeled. By NSS Modeling we mean the accurate modeling of oscillatory and noisy data regardless of the type of process.  Think Flow, Pressure, and Temperature. We even mean processes with long Dead-Times like pH. Those are the plurality of industrial processes that practitioners work to control and for which software is truly needed. 

NSS Modeling is incorporated it into all of Control Station’s tuning solutions both LOOP-PRO and other private-labeled products.  If your processes are typical and involve oscillatory and/or noisy dynamics, we’ll be happy to showcase the power of NSS Modeling and to discuss how it can improve manufacturing efficiency at your production facility.


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