Herding Cats: Successfully Manage All Those Process Variables With a Systematic Approach to Plant-Wide Control
By Sigurd Skogestad
A chemical plant may have thousands of measurements and control loops. By the term “plant-wide control,” I do not mean the tuning and behavior of each of these loops, but rather the control philosophy of the overall plant with emphasis on the structural decisions:
- Selection of controlled variables (CVs, “outputs”)
- Selection of manipulated variables (MVs, “inputs”)
- Selection of (extra) measurements
- Selection of control configuration (structure of overall controller that interconnects the controlled, manipulated and measured variables)
- Selection of controller type (PID, decoupler, MPC, linear, quadratic, Gaussian (LQG), aka, optimal control, ratio, etc.).
In practice, the control system is usually divided into several layers, separated by the time scale (Figure 1).
The time factor
Figure 1. Typical control hierarchy in a chemical plant.
Control structure design (plant-wide control) thus involves all the decisions necessary to make a block diagram or process and instrumentation diagram that includes the control system for the entire plant, but does not involve the actual design of each individual controller block.