Why do large multivariable systems need more than single-loop control techniques?

In large multivariable systems, single-loop control techniques are insufficient because the internal variables of the process are interdependent; the value of one variable is often dependent upon the values of other related variables within the system.1

When variables are coupled in this manner, classical single-loop control cannot effectively manage the interactions, necessitating the use of multivariable-noninteracting control and other sophisticated techniques to develop appropriate control systems. These complex systems, such as power plants, oil refineries, or chemical factories, rely on the coordinated operation of numerous individual control devices—typically directed by a computer—to maintain stability and performance, as human intervention would be impractical.12