Observer: Usually another name for a state-estimator. Other variables such as disturbances or faults could also be observed.

OPC or OLE for Process Control: A standard set by the OPC Foundation for fast and easy connections to controllers.

Open Loop: Controller in manual mode.

Open Systems: Those systems that can be supplied by hardware components from multiple vendors, and whose software can be operated from different platforms. They are opposite to closed or proprietary systems.

Operating condition: A nominal set of conditions (input, outputs and internal states) for a certain system operation. For example, in a steel mill, different sets of operating conditions will be required to produce different grades/thicknesses of steel strip.

Optimal Control: Design of a controller by optimizing a cost function. Needs a model of the process and the controller may require the states to be measurable otherwise a state-estimator is used.

Output: Output of the PID controller. In auto mode the controller calculates the output based its calculation using the error signal (difference between setpoint and PV). In manual mode, the user sets the output.

Over-Damped: A linear system that responds slower than the critical damped case is said to be over-damped. This applies to 2nd-order or higher-order systems.

Overshoot: If the time response of a system temporarily attains a value higher that which it finally settles at, it is said to exhibit overshoot. First-order systems cannot have overshoot. For second-order systems, overshoot is related to damping ratio.

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All of the information in this glossary has been carefully compiled and we believe it to be accurate. However, since any such definitions need to be placed in the context of specific applications, we assume no liability for the usage of information contained in this glossary.

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