How do supervisory control direct digital control and hierarchy control differ in industrial plants?

In industrial control systems, digital computers are applied in three distinct ways to manage plant operations. These methods differ primarily in their scope and their relationship to the primary control hardware.1

Supervisory or Optimizing Control

In this capacity, the computer acts in a secondary or external role. It does not replace primary controllers but instead "trims" plant operation by adjusting the set points of existing control systems—such as resetting a thermostat—to optimize production or reduce costs. Because it operates externally, a computer malfunction in this mode does not adversely affect the plant's basic operation.1

Direct Digital Control

This method involves a digital computer replacing a group of conventional, single-loop analogue controllers. By utilizing its superior computational power, the computer manages the process directly, allowing for the implementation of more complex, advanced control techniques than would be possible with standard analogue hardware.1

Hierarchy Control

Hierarchy control represents the most comprehensive approach, attempting to integrate all plant-control situations simultaneously. It utilizes advanced computers to coordinate operations across every level of the facility, ranging from top-level management decisions down to the physical movement of a valve.1