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.
A unified approach to tuning has been found that enables a common and simplified method for setting PID tuning parameters. Key features can be used to eliminate the need for retuning to deal with different dynamics and objectives.
A unified approach to PID Control has been found that enables a common and simplified method for setting PID tuning parameters. Key features can be used to eliminate the need for retuning to deal with different dynamics and objectives.
A PID tuned for maximum disturbance rejection in a composition, temperature, and gas pressure loop will exhibit excessive overshoot in the setpoint response unless one of several PID features is used. The options are relatively easy to configure. Here we discuss performance metrics and test results.
The process variable has slow decaying oscillations. Control theory text books indicate decreasing the PID gain should make the loop more stable. You decrease the PID gain. The oscillation gets worse. You decrease the gain again. The amplitude and the period get bigger. You repeatedly decrease the PID gain.
We continue this series with the steps for designing control strategies. Simple rules of thumb will be offered for setting up cascade, composition, flow, level, pH, pressure, and temperature control systems. Examples of common unit operations will help provide understanding of the steps involved. The objective is to set the...
We start this series with the steps for the selection and installation of the field measurements and control valves or variable speed drives to enable the control system to meet plant objectives. The measurement is the essential window into the process and manipulating a flow is the essential way to...
We conclude this series with a look at how to tune a controller when the objective is to maximize the absorption of variability rather than tight control of the process variable. The details for the most common case of surge tank level control are provided.
We are aware that too high of a PID gain can cause excessive oscillations and even instability. The ultimate gain for processes with no steady state on PID horizon is usually much higher than our comfort level.
Many of the most important process variables, such as vessel and column composition, pressure and temperature, do not reach a steady state in the time frame of PID action. Batch composition, pH and temperature and, of course, level have no steady state.
The PID is by far the most prevalent controller in the process industry. Here we step back for a view of the basics of the proportional, integral, and derivative modes. These PID controller modes have distinct advantages and disadvantages and consequences if one mode dominates.