The Control Talk Blog provides guidance from a user's viewpoint on the design of automation systems, equipment, and piping for process control improvement. Details are offered on the selection and installation of PID controllers, control valves, variable speed drives, and measurements to maximize loop performance. The blogs are often more intensive and extensive and less vendor specific than a white paper. The goal is an advancement of the profession by sharing conceptual principle based knowledge.
Greg McMillan is a retired Senior Fellow from Solutia/Monsanto and an ISA Fellow. At present, McMillan is a part time modeling and control consultant in Technology for Process Simulation for Emerson Automation Solutions specializing in the use of the Virtual Plant for exploring new opportunities. He spends most of his time writing, teaching and leading the ISA Mentor Program he founded in 2011. He received the ISA Kermit Fischer Environmental Award for pH control in 1991, received the Control magazine Engineer of the Year Award for the Process Industry in 1994, was inducted into the Control magazine Process Automation Hall of Fame in 2001, was honored by InTech magazine in 2003 as one of the most influential innovators in automation, and received the ISA Life Achievement Award in 2010.
Here we looks at applications where feedforward can do more harm than good and what to do to prevent this situation. This problem is more common than one might think. In the literature we mostly hear how beneficial feedforward can be for measured load disturbances.
There are many mistakes but some are repeated over and over again even though the automation engineer is attentive and experienced and has the best intentions. Part of the problem is overload in terms of tasks and the time crunch.
We learned in control theory courses that too high a PID gain causes oscillations and can lead to instability. Operators do not like the large sudden changes in PID output from a high PID gain. Operators may see what they think is the wrong valve open in split range control...
The key to a much brighter future of our profession depends upon management providing the funding and support and millennials seeking to improve process performance by the use of the best automation and process control. There are some common approaches to these seemingly very different groups.
Here we look at a myriad of metrics on process and control loop performance and show how to see through the complexity and diversity to recognize the commonality and underlying principles. We will see how dozens of metrics simplify to two classes each for the process and the loop.
While there are some cases where deadband is helpful, in most applications the effect is extremely detrimental and confusing. Deadband can arise from any sources either intentionally or inadvertently. Deadband creates deadtime and for certain conditions excessive and persistent oscillations.
Deadtime is the easiest dynamic parameter to identify and the one that holds the key to better control. Deadtime found visually or by a simple method can tell you what is limiting the ability of the loop and what the remedy is.
In some applications, throttling of the manipulated flows is difficult or impossible. In the biochemical industry, where precise (good resolution and sensitivity) throttling valves without any crevices (to meet sanitary requirements) are rather limited (there are exceptions such as the Fisher Baumann 83000-89000 series).