Don't overlook the virtues of PID when optimizing processes

Model predictive control isn't always the best choice

By Greg McMillan and Stan Weiner

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Stan: PID is the workhorse of the process industry. It has more than enough flexibility and capability to maximize the performance of PID loops distributed throughout the process for different objectives and difficult situations. However, the disparity is increasing between what PID can do and how it is used. I suspect this is due to an increase in the capability of PID, growing disagreement and misunderstandings in tuning rules, the lack of tools for automatically setting dynamic compensation of feedforward and decoupling signals, a decrease in the expertise onsite, and the lack of succinct guidance.

Greg: The solution more than ever is to get a consultant on-site. The Control Talk columns we have done with James Beall, Mark Coughran, Sigifredo Nino, Michel Ruel, and Jacques Smuts show us what consultants can do with getting the most out of PID. My concern is the number of consultants is declining, and they are so busy providing solutions that they don't have time to document their knowledge. While ideally we would want general concepts with a step-by-step approach for the major types of applications, just to hear about industrial solutions is a great help.

What I am seeing is that engineers who want to do something better, particularly where there is a feedforward signal, are turning to model predictive control (MPC) because the dynamics of the feedback and feedforward loops are automatically identified, and tuning often is relegated to a simple adjustment of move suppression (penalty on move). A penalty on error is used to place a different relative emphasis on the importance of controlled variables and constraint variables. Often the 1.0 default value can be used or adjusted based on a basic understanding of importance, and refined as more knowledge is gained.

Also read: "Model-based tuning methods for PID controllers"

Stan: To help us understand the role of PID and MPC in our future, we have invited Sigifredo Nino into this discussion. Sigifredo shared his expertise and accomplishments in the 2014 Control Talk columns "Giving thanks for process control achievements," "Distilled analysis of interaction," and "How to Get the most out of your PID by better tuning." The data shows that many advanced control solutions fall into disuse about a year or two after the consultant leaves the site. What is your experience?

Sigifredo: My solutions stay in service long after I leave. In fact, this is a basic requirement for me upfront. I assure the customer from the start that the solution will remain effective, and there should be no future need of my services for the particular application for as long as there are no substantial changes in the process and all field devices are working properly.

Greg: Maybe one of the advantages of PID is that once properly implemented, the operator can better understand what the PID system is doing compared to an MPC system. MPC does offer future trajectories, but these often dramatically change from the progression of unmeasured load disturbances. Still, there is an advantage to realizing that changes being made now do not start to have an effect until after the loop dead time. The simple computation of a process variable (PV) one dead time into the future for PID would greatly help operator understanding of where PID is going as detailed in the 6/28/2012 Control Talk Blog, "Future PV values are the future."

Sigifredo: It is easier to discern what a PID controller is doing in response to the behavior of the process variable, whereas the multivariable actions of an MPC approach are difficult to fathom, especially if we take into account that MPC does not have a closed-form solution.

Stan: Even if advances have been made in showing the operator the contribution of the controlled variables and constraint variables to what is seen in the action of a particular manipulated variable, being able to decide whether MPC is doing the right thing is a challenge. In override control, the controller output selected can be easily flagged to the operator. The principal concerns are the proper connections of the back-calculated signal in the configuration and the tuning of a PID that is only active for abnormal conditions. The "back-calculate" capability in the PID positional algorithm enables significant performance advantages over the velocity algorithm.

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