TABLE OF CONTENTS Override control, also called selector control, exists when one process variable is the controlling variable in normal operation. During abnormal operation, however, another process variable assumes control to prevent some safety, process, or equipment limit from being exceeded.
A key element of an override control strategy is a selector switch, implemented either as a hardware device or a software function block. Depending on how it's configured, this selector switch passes the higher or lower of several input signals to its output. There are several ways of using selector switches in a control strategy. One is to select the higher or lower of several measurement signals to be passed on as the process variable to a feedback controller. For example, the highest of several process temperatures may be selected automatically to become the controlling temperature. As process conditions change, the location of the highest temperature may change also. The selector switch assures that, regardless of process conditions, the controlling point is the highest of the measured temperatures.
Placing a selector switch in the measurement side of a controller, though perhaps important from the vantage point of a particular process application, poses very little technical challenge for the control engineer. If each of the process sensors responds in a similar way to changes in the controller output, then the transition from one sensor to another will be virtually imperceptible.
There are also selectors which select the middle of three inputs. These are used primarily in high-criticality applications, where the failure...
