Clarifying the Lambda Tuning Method

The Automation List on Control.com recently had a question about IMC Tuning for Integrated Processes. I googled around for IMC or internal model control for a good definition and found these 2002 Introduction to Robust Control lecture notes:

The Internal Model Control (IMC) philosophy relies on the Internal Model Principle, which states that control can be achieved only if the control system encapsulates, either implicitly or explicitly, some representation of the process to be controlled.

The Automation List question asked how IMC can be implemented if the process time constant, process gain, control integral and controller gain are unknown. This person did a manual step test on the drum level feedwater control valve and the drum level starts to integrate (rise.) Measuring this occurrence provides dead time, level rate of change and change in control valve position.

The questioner writes:

I know you can implement Lambda Tuning, but from what I’ve seen with this, you end up with a very sluggish system that responds quite poorly due to the low value of Kc (please don’t comment here on 3 element control, as this is not apart of the discussion).

Am I missing something here, or have other people used different methods?

We’ve had several posts in the past on Lambda tuning, so I forwarded this question to Mark Coughran, a senior control engineering consultant on the Advanced Applied Technology team.

Mark notes:

Whatever method you use, it is important to understand each of the terms in the equations and the appropriate units of measure. Training is available to make clear how to measure the process dynamics, choose Lambda, and calculate the controller gain and reset. Emerson Educational Services offers the courses Process Dynamics, Control and Tuning Fundamentals (9030) and Modern Loop Tuning (9032). Tools and on-site services are also available.

Lambda tuning simply means the loop will not oscillate and you choose the speed of closed-loop response (Lambda), within some reasonable constraints. There is no reason to believe that Lambda tuning is arbitrarily “slow” or “fast”, since you choose the Lambda.

ZN or Ziegler-Nichols is a method to deliberately make the loop oscillate. This is not a good idea in any process plant.

Posted Friday, February 8th, 2008 under Asset Optimization, Education, Variability Management.

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