Week 13 of 15 – Terms	Name: Answer Sheet

In your own words, write a definition for each of the following terms. Plagiarism will be checked and not tolerated.

Points: 5 pts/each = 14 * 5 = 70 pts

Linear-lag (expand on the textbook definition)

Linear-lag is basically a time delay between two events, states or mechanisms. The term is used to describe the signal-delaying and signal-size-changing effects of a part of the control loop. The name comes from the form of the equation which represents the relation between output and input. The effects are evaluated by a “Time Constant”. Linear-Lag is most oven used to explain the First-Order-Lag which is the delay of the process to respond to a step-change in input.

Lead lag (expand on the textbook definition)

“Lead lag is a broadly used term in the processing industry that describes how a process reacts to a disturbance (e.g., a process reactor temperature change leads to a corresponding pressure change) or other manipulating conditions (a measurement change lags a controller output change). Lead lag is also used to describe how a control mode action reacts to an error (rate = lead; reset = lag), or which measured variable will be used to control a loop. It is even used to describe equipment utilization strategies.” (Center for the Advancement of Process Technology, 2010, p. 265)

Center for the Advancement of Process Technology. (2010). Instrumentation (1st ed.). (V. Anthony, Ed.) Upper Saddle River, New Jersey, United States of America: Pearson - Prentice Hall.

Ratio control (revisited)

1. A controller that maintains a predetermined ratio between two or more variables or that maintains the magnitude of a controlled variable at a fixed ratio to another variable. See also controller, ratio. 2. A controller that maintains a predetermined ratio between two variables. [ANSI/ISA-51.1-1979 (R1993)]

Parallel positioning control

A method of ratio control that maintains a ratio to multiple final control element.

Full metering control

A method of ratio control that includes an additional control loop to assist with the ratio controller.

Cross-limiting control

A method of ratio control to ensure that during transients, the ratio in the control loop stays in control.

Override control (constraint control)

1. Generally, two control loops that are connected to a common final control element. One control loop is normally in control, with the second loop being switched in by some logic element when an abnormal condition occurs so that constant control is maintained. 2. A technique in which more than one controller manipulates a final control element. The technique is used when constraint control is important.

Downcomer flooding

Downcomer flooding occurs in distillation columns. It is when the force of the rising vapors is greater than or equal to the force of the liquid that is falling. The rising vapors will cause the liquid to be suspended and backup in the column. This causes the column to flood and waste energy.

Reset wind-up [Review Unit 6 – Rate & Reset]

The saturation of the integral mode of a controller. It develops during times when control cannot be achieved, which causes the controlled variable to overshoot its set point when the obstacle to control is removed.

Selective control (as related to the process industry)

A name given to the application of signal selectors in a control strategy. Signal selectors are devices that choose the lowest, highest, or median signal from among two or more signals. In most applications, selective control is a form of multivariable control where the selectors facilitate the on-line modification of control strategies as a function of changing operating conditions. The selectors allow the strategies to be changed smoothly without disturbing the process. Some uses are; guarding against exceeding equipment or operating constraints, automatic startup and shutdown, protection against instrument failures and selection of extreme values.

Duplex or Split-Range control

A control in which two independent control elements share a common input signal to operate separate final control elements, both of which influence the value of the controlled condition.

Cutback control (Auto-Selector)

A controller which ensures a monitored parameter doesn't exceed a limit by adjusting a controlled parameter. For example speed regulator of an electric motor senses a high level of current or temperature and reduces the motor speed (load) to cutback the current flow and reduce the temperature to a safe level.

Dead time control

The period of dead-state of a system. A deadband (sometimes called a neutral zone) is an interval of a signal domain or band where no action occurs (the system is dead).

Transportation lag

The time required to transport material from one place to another.