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.
Here we use the broader definition of linearity to mean constant dynamics. A linear control loop has a constant total loop dead time, constant primary and secondary time constants, and a constant open loop gain. This perspective reviews the sources of dynamics and causes of nonlinearity.
The speedup of a plant’s response can cause loops to go from a smooth to an oscillatory response. In actual plants, the faster rate of change of a process variable important for product quality such a temperature or composition occurs for various changes in operating conditions.
The primary reason why there are so many and so different schools of thought about control algorithms and tuning can be traced back to one parameter in the process response. What PID tuning and what PID structure is pronounced as best and even whether PID control should be used is...
If there were no unmeasured disturbances, feedback control would not be necessary. Process engineers and operators could home in on the best PID output and just leave it at this value. In fact many process engineers are much more comfortable with setting a stream flow per a process flow diagram...
Much of the differences in approaches to controller algorithms and tuning can be traced back to assumptions made about the type and importance of disturbances. Each method has merits based on the disturbance frequency, location, and time lag.
A young small company is able to take university graduates and immediately make them productive in industrial applications. The open, positive, and enabling culture fostered by management has recently elevated and accelerated employee knowledge by providing a means to gain key fundamentals and essential concepts.
The effects of measurement dynamics are similar to the effects of controller dynamics except there are often many more sources of lags and delays and the consequences are generally more severe. Delays are the result of transportation delays and discontinuous updates from periodic sampling and cycle times in digital signals,...
The measurement provides the window into the process. Smart transmitters have made the view clearer. Installation and operating condition effects are compensated for allowing the transmitter to have an installed accuracy close to the sensor capability.
The use of the term "process dead time" can mislead us in terms of recognizing the many sources of dead time. Also we don't often take into effect the profound effect of the speed and the entry point of a disturbance into the process.
Michel Ruel, a frequent source on process control improvement in both my Control Talk Blogs and Columns, offers his concise list of the more memorable mistakes made with actuators, compressed air systems, transmitters, and variable-speed drives.