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.
In many publications on process control, the common metric you see is integrated absolute error for a step disturbance on the process output. In many tests for tuning, setpoint changes are made and the most important criteria becomes overshoot of setpoint.
Having the blind wholesale goal of reducing variability can lead to doing the wrong thing that can reduce plant safety and performance. Here we look at some common mistakes made that users may not realize until they have better concept of what is really going on.
Inverse response, negative resistance, positive feedback and discontinuities can cause processes to jump, accelerate and oscillate confusing the control system and the operator. Not properly addressing these situations can result in equipment damage and plant shutdowns besides the loss of process efficiency.
The operator is by far the most important person in the control room having the most intimate knowledge and “hands on” experience with the process. Engineers who are most successful with process improvements realize they need to sit with and observe what operators are doing to deal with a variety...
The usual concern is whether an automation system is too slow. There are some applications where an automation system is disruptive by being too fast. Here we look at what determines whether a system should be faster or slower and what are the limiting factors and thus the solution to...
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...