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, he contracts with Emerson DeltaV R&D via CDI Process & Industrial in Austin and consults for MYNAH Simulation Technologies in Saint Louis. Greg 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.
Batch process control can seem like another world compared to continuous process control. In batch operations, process conditions are constantly changing and control loops are going in and out of service. PID control may take a back seat to sequential scheduling of manipulated flows.
What if you could use at-line analyzers and even off-line analyzers for control of batch and continuous processes without the need to retune the PID despite long and variable cycle times? What if the lab analyzers could be used for closed loop control without having to be concerned about the...
All feedforward control systems can be reduced to a common form that enables a better understanding and recognition that leads to the best performance and the best interface for the operator. For “smart controls” to be fully appreciated and utilized, the operator needs to know what is going on and...
Often arguments as to whether a linear or equal percentage trim is best are based on the theoretical inherent flow characteristics. Valve rangeability is often stated as simply a deviation of the catalog flow characteristic from the theoretical characteristic.
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...