# AERATION

 The state of air being suspended in a liquid such as a lubricant or hydraulic fluid.

## Related Terms

### POLE-POSITIONING

A design technique used in linear control theory in which many or all of a system's closed-loop poles are positioned as required, by proper choice of a linear state feedback law; if the system is controllable, all of the closed-loop poles can be arbitrarily positioned by this technique.

### PLANE STRESS

A state of stress in which two of the principal stresses are always parallel to a given plane and are constant in the normal direction.

### OPTIMAL PROGRAMMING

A subfield of optimal control theory in which the control variables are determined as functions of time for a specified initial state of the system.

### OPTIMAL FEEDBACK CONTROL

A subfield of optimal control theory in which the control variables are determined as functions of the current state of the system.

### ONSAGER RECIPROCAL RELATIONS

A set of conditions which state that the matrix, whose elements express various fluxes of a system (such as diffusion and heat conduction) as linear functions of the various conjugate affinities (such as mass and temperature gradients) for systems close to equilibrium, is symmetric when certain definitions are chosen for these fluxes and affinities.

### ON

Designating the operating state of a device or one of two possible conditions (the other being off'') in a circuit.

### OFF-LINE

1. A condition existing when the drive rod of the drill swivel head is not centered and parallel with the borehole being drilled. 2. A borehole that has deviated from its intended course. 3. A condition existing wherein any linear excavation (shaft, drift, borehole) deviates from a previously determined or intended survey line or course. 4. State in which an equipment or subsystem is in standby, maintenance, or mode of operation other than online.

### OFF

Designating the inoperative state of a device, or one of two possible conditions (the other being on'') in a circuit.

### OBSERVER

A linear system B driven by the inputs and outputs of another linear system A which produces an output that converges to some linear function of the state of system A. Also known as state estimator; state observer.

### OBSERVABILITY

Property of a system for which observation of the output variables at all times is sufficient to determine the initial values of all the state variables.

### Related questions

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