Week 02 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 = 20 * 5 = 100 pts

Closed loop

1. A combination of control units in which the process variable is measured and compared with the desired value (or set point). Any deviation from the set point is fed back into the control system in such a way as to reduce deviation. Usually called feedback control. 2. Pertaining to a system with feedback type of control, such that the output is used to modify the input. 3. An operation by which a computer applies control action directly to the process without manual intervention.

Summing unit a.k.a. Summing junction

Any point in a loop where signals are algebraically added. In closed loop systems, the summing junction will usually calculate the error in the system by algebraically adding the process variable to the set point.

Error = SP – PV or error = PV - SP

Control error (or simply, error)

1. In process instrumentation, the algebraic difference between the indication and the ideal value of the measured signal. It is the quantity that when algebraically subtracted from the indication gives the ideal value. Note: A positive error denotes that the indication of the instrument is greater than the ideal value: 2. The algebraic difference between the indicated value and the true value of the measurand.

Feedback

1. A process signal that is used in control as a measure of the response to control action. 2. The part of a closed loop system that automatically brings back information about the condition under control, for the sake of comparing that information with the desired condition. 3. A signal produced by a measuring device that is proportional to the magnitude of a controlled variable or the position of a control element.

Feedback elements

Those elements in the controlling system that act to change the feedback signal in response to the directly controlled variable.

Open loop

Pertaining to a control system in which there is no self-correcting action for misses of the desired operation condition, as there is in a close loop system.

Manual control

The operation of a process by a person adjusting final control elements manually.

Binary control

The simplest type of control where the control is either on or off. Binary control is also referred to as two-position control.

Direct acting

A controller in which the value of the output signal increases as the value of the input (measured variable) increases.

Reverse acting

A controller in which the value of the output signal decreases as the value of the input (measured variable) increases.

Differential gap control

Control in which the output of a controller remains at a maximum or minimum value until the controlled variable crosses a band or gap, causing the output to reverse. The controlled variable must then cross the gap in the opposite direction before the output is restored to its original condition.

Process load

The amount of material or energy that must be manipulated in order to control the controlled variable (CV).

Proportional control

1. A control mode in which there is a continual linear relationship between the deviation computer in the controller, the signal of the controller, and the position of the final control element. 2. Also known as single-mode control. The output can have any value from the lower to the upper output range.

Gain

1. The ratio of output signal magnitude to input signal magnitude. When the gain is less than one it is usually called attenuation. 2. The relative degree of amplification in an electronic circuit. 3. The ratio of the change in output to the change in the input that caused the change. 4. In a controller, the reciprocal of proportional band. For example, if the proportional band is set to 25 percent, the controller gain is 4. Proportional band can be expressed as a dimensionless number (gain) or as a percentage.

Offset

1. A sustained deviation of the controlled variable from set point. This characteristic is inherent in proportional controllers that do not incorporate reset action. 2. The steady-state deviation when the set point is fixed.

Proportional band

1. The change in input that is required to produce a full-range change in output as a result of proportional control action. Note: Proportional band is reciprocally related to proportional gain. It may also be stated in input units or as a percentage of the input span (usually the indicated or recorded input span). The preferred term is proportional gain. 2. The amount of deviation of the controlled variable from the set point that is required to move the final control element through the full range (expressed in percentage of span). 3. An expression of the gain of an instrument (the wider the band, the lower the gain.).

Integral control (Reset)

1. A method of changing the output of a controller by an amount proportional to an error and the duration of that error. 2. A form of control action that returns the value of the controlled variable to the set point when, without this action, sustained offset occurs. Also called Reset Control.

Derivative control (Rate)

1. A method of changing the output of a controller in proportion to the rate of change of the process variable (PV). 2. Change in the output that is proportional to the rate of change of the input. Also called Rate Control.

Two mode control

A modulating process control action that uses proportional and integral actions.

Three mode control

Another name for Proportional – Integral – Derivative (PID) control.