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.
Most of the control literature focuses on minimizing the integrated absolute error (IAE) for a step disturbance, often in a linear system. In the process industry, there are many other objectives and complications that require special attention.
The power of the PID largely remains untapped. I have recently documented the extensive capability of the PID but being a realist, I expect MPC is going to take over more and more of the role of the PID.
The amount of information that the generalist needs to know is staggering. The knowledge needed is buried in a hundred thousand pages of publications and presentations that are oriented toward the specialist. Here I provide the essentials for the best field instrumentation system.
A lot of time and money can be spent deciding which valves need positioners and which flows need measurement. We tend to look at short term costs such as hardware and not the cost of troubleshooting and the implications as to plant performance.
Each process test will typically give a different result in the process dynamics identified and consequential tuning settings calculated. Here we look at the sources of this lack of repeatability, the implications, and what can be done to improve tuning tests.
There are some simple diagnostic checks and rules of thumb on tuning adjustments that can be used to find out if there is a problem with the PID tuning and what is the solution. This guidance in conjunction with good tuning software can reduce process variability introduced or aggravated by...
When is a controller in automatic not able to do anything to reduce an oscillation? When will a controller amplify an oscillation? In both of these cases, the controller is doing more harm than good by wearing out valves and upsetting other loops.
Sensor lags, transmitter damping, and PID signal filters can make oscillations look better but is this really a good a thing? Here we look at how these dynamics affect what you see and how much of a problem it can be.
What comes at you too fast? How do people know you are a process control engineer? What is more disturbing than talk shows? Is a flea market indicative of a project behind schedule? These and other questions you have not asked will be answered.
Before the 1990s relatively few choices in PID structure were offered. There were also various supplier specific rules as to how to set the proportional mode and integral mode tuning settings to get proportional-only and integral-only control. A different model controller may have been needed for a different structure.