How does direct digital control differ from supervisory control in an industrial plant?

In industrial plant operations, direct digital control and supervisory control serve distinct roles within a control system.

Supervisory Control

In supervisory or optimizing control, the computer functions in an external or secondary capacity. Its primary task is to "trim" plant operations to improve efficiency, such as lowering costs or increasing production, by adjusting the set points of the primary control system. Because it operates externally, a malfunction in the supervisory computer does not directly compromise the plant's basic operational safety.1

Direct Digital Control

In contrast, direct digital control involves a digital computer replacing a group of single-loop analogue controllers. By assuming the primary role of managing the process, the computer utilizes its superior computational power to execute more complex, advanced control techniques that would be difficult for conventional instrumentation to handle.1

While supervisory control acts as an overseer to optimize existing settings, direct digital control serves as the fundamental mechanism for executing the control actions themselves.1