Product Data Document
Y1900SS-1a
July 11, 2007 - Page 1
GLOSSARY OF TERMS FOR PROCESS CONTROL
Accuracy Conformity of an indicated value to an accepted standard value, or true value.
Accuracy, Reference A number or quantity which
denes the limit that errors will not exceed when the
device is used under reference operating conditions.
Note: Reference accuracy includes the combined conformity, hysteresis and repeatability errors. The units
being used are to be stated explicitly. It is preferred
that a + and - sign precede the number or quantity. The
absence of a sign infers a + and – sign. Reference
accuracy can be expressed in a number of forms. The
following examples are typical:
1. Reference accuracy expressed in terms of the
measured variable. Typical expression: The refer-
ence accuracy is + 1oF.
2. Reference accuracy expressed in percent of span.
Typical expression: The reference accuracy is +
½% of span.
3. Reference accuracy expressed in percent of actual
output reading. Typical expression: The reference
accuracy is + 1% of actual output reading.
Auctioneering Device A device which automatically
selects either the highest or the lowest input signal
from among two or more input signals.
Bode Diagram A plot of log amplitude ratio and phase
angle values on a log frequency base for a transfer
function.
Control Action The nature of the change of the output
affected by the input of a controller or a controlling
system.
Control Action, Adaptive Control action whereby
automatic means are used to change the type or inuence (or both) of control parameters in such a way as
to improve the performance of the control system.
Control Action, Cascade Control action where the
output of one controller is the setpoint for another
controller.
Control Action, Derivative (Rate) Control action in
which the output is proportional to the rate of change
of the input.
Control Action, Direct Digital Control action in
which control is performed by a digital device, which
establishes the signal to the nal controlling element.
Control Action, Feedback Control action in which a
measured variable is compared to its desired value to
produce an actuating error signal which is acted upon in
such a way as to reduce the magnitude of the error.
Control Action, Feedforward Control action in which
information concerning one or more conditions that can
disturb the controlled variable is converted into corrective action to minimize deviations of the controlled
variable. Note: Feedforward control action can be
combined with other types of control to anticipate and
minimize deviations of the controlled variable.
Control Action, High Limiting Control action in which
the output never exceeds a predetermined high limit
value.
Control Action, Integral (Reset) Control action in
which the output is proportional to the time integral of
the input; i.e., the rate of change of output is proportional to the input.
Control Action, Low Limiting Control action which
the output is never less than a predetermined low limit
value.
Control Action, Optimizing Control action that automatically seeks a d maintains the most advantageous
value of a specied variable, rather than maintain it at
one set value.
Control Action, Proportional Control action in which
there is a continuous linear relation between the output
and the input.
Control Action, Proportional Plus Derivative (Rate)
Control action in which the output is proportional to a
linear combination of the input and the time rate-ofchange of input.
Control Action, Proportional Plus Integral (Reset)
Control action is which the output is proportional to a
linear combination of the input and the time integral
of the input.
Control Action, Proportional Plus Integral (Reset)
Plus Derivative (Rate) Control action in which the
output is proportional to a linear combination of the
input, the time integral of input and the time rate0of0change of input.
Control Action, Shared Time Control action in which
one controller divides its computation or control time
among several control loops rather than acting on all
loops simultaneously.
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Glossary
Control Action, Supervisory Control action in which
the control loops operate independently subject to
intermittent corrective action; e.g., setpoint changes
from a external source.
Control Mode A specic type of control action such
as proportional, integral or derivative.
Control System A system in which deliberate guidance
or manipulation is used to achieve a prescribed value
of a variable. Note: It is subdivided into a controlling
system and a controlled system.
Control System, Multi-Element (Multi-Variable) A
control system utilizing input signals derived from two
or more process variables of the purpose of jointly affecting the action of the control system.
Control System, Non-Interacting A multi-element
control system designed to avoid disturbances to other
controlled variables due to the process-input adjustments, which are made for the purpose of controlling
a particular process variable.
Controller A device which operates automatically to
regulate a controlled variable.
Controller, Direct–Acting A controller in which the
absolute value of the output signal increases, as
the absolute value of the input (measured variable)
increases.
Controller, Integral (Reset) (I) A controller which
produces integral control action only. Note: may also be
referred to as controller, proportional speed oating.
Controller, Multi-position A controller having two or
more discrete values of output.
Controller, On-off A multi-position controller having
two discrete values of output, fully on, or fully off.
Controller, Proportional (P) A controller which produces proportional control action only.
Controller, Proportional Plus Derivative (Rate), (PD)
A controller which produces proportional plus derivative
(rate) control action.
Controller, Proportional Plus Integral (Reset), (PI)
A controller which produces proportional plus integral
(reset) control action.
Controller, Proportional Plus Integral (Reset), Plus
Derivative (Rate), (PID) A controller which produces
proportional plus integral (reset) plus derivative (rate)
control action.
Controller, Ration A controller that maintains a predetermined ration between two or more variables.
Controller, Reverse-Acting A controller in which
the absolute value of the output signal decreases, as
the absolute value of the input (measured variable)
increases.
Controller, Sampling A controller using intermittently
observed values of a signal such as the setpoint signal,
the actuating error signal, or the signal representing
the controlled variable, to effect control action.
Controller, Self-Operated (Regulator) A controller
in which all the energy to operate the nal controlling
element is derived from the controlled system, through
the sensing element.
Controller, Time Proportioning A controller whose
output consists of periodic pulses whose duration is
varied to relate, in some prescribed manner, the time
average of the output to the actuating error signal.
Controller, Time Schedule A controller in which the
setpoint (or reference input signal) automatically adheres to a predetermined time schedule.
Controlling System 1. Of a feedback control system,
that portion which compares functions of a directly
controlled variable and a setpoint; and adjusts a ma-
nipulated variable as a function of the difference. It
includes the reference input elements; summing point;
forward and nal controlling elements; and feedback
elements (including sensing element). 2. Of an automatic control system without feedback, that portion of
the control system which manipulates the controlled
system.
Damping The progressive reduction or suppression
of the oscillation of a system. Note: When the time
response to an abrupt stimulus is as fast as possible
without overshoot the response is said to be “critically
damped”; “underdamped” when overshoot occurs;
“overdamped” when response is slower than critical.
Damping, Relative For an underdamped system, a
number expressing the quotient of the actual damping of a second-order linear system or element by its
critical damping. Note: a second order linear system
is a system, which is the product of two-second order
lags.
Dead Band The range through which an input can be
varied without initiating response. Note: Dead Band is
usually expressed in percent of span.
Deviation Any departure from a desired or expected
value or pattern.
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Glossary
Deviation, System The instantaneous value of the
ultimately controlled variable minus the setpoint.
Drift A change in the output-input relationship over a
period of time.
Element A component of a device or system.
Element, Final Controlling That forward controlling
element which directly changes the value of the manipulated variable.
Element, Sensing The portion of a device directly
responsive to the value of the measure quantity.
Elements, Feedback Those elements in the controlling
system which change the feedback signal in response
to the directly controlled variable.
Elements, Forward Controlling Those elements
in the controlling system which change a variable in
response to the actuating error signal.
Error The algebraic difference between the indica-
tion and the true value of the measured signal. It is
the quantity, which algebraically subtracted from the
indication gives the true value.
Error, Span The difference between the actual span
and the ideal span.
Error, Zero The error of a device operating under the
specied conditions of use when the input is at the
lower range-value.
Frequency Response Characteristic The frequency
dependent relation, in both amplitude and phase, between steady-state sinusoidal inputs and the resulting
fundamental sinusoidal outputs.
Gain, Closed Loop The gain of a closed loop system,
expressed as the ration of the output change to the
input change at a specied frequency.
Gain, Dynamic The magnitude ration of the steadystate amplitude of the output sign from an element or
system to the amplitude of the input signal to that ele-
ment or system, for a sinusoidal signal. Note: It may
be expressed as a ratio, or in decibels as 20 times the
log of that ratio for a specied frequency.
Gain, Loop The ratio of the change in the return
signal to the change in its corresponding error signal
at a specied frequency. Note: The gain of the loop
elements is frequently measured by opening the loop,
with appropriate terminations. The gain so measured
is often called the open loop gain.
Gain, Proportional The ratio of the change in output
due to proportional control action to the change in
input.
Gain, Static The value of the gain approached as a
limit as frequency approaches zero.
Hysteresis When used as a performance specication,
the maximum difference for the same input between
the upscale and downscale output values during a full
range traverse in each direction.
Impedance, Input The impedance presented by a
device to the source.
Impedance, Load The impedance presented to the
output of a device by the load.
Impedance, Output The impedance presented by a
device to the load.
Impedance, Source The impedance presented to the
input of a device by the source.
Indicator Travel The length of the path described by
the indicating means or the tip of the pointer in moving
from one end of the scale to the other.
Integral Action Rate (Reset Rate) Of proportional
plus integral or proportional plus integral plus derivative control action devices, for a step input, the ratio
of the initial rate of change of output due to integral
control action to the change in steady-state output due
to proportional control action. Note: Integral action
rate is often expressed as the number of repeats per
minute because it is equal to the number of times per
minute that the proportional response to a step input
is repeated by the initial integral response.
Interference, Common Mode A form of interference
which appears between measuring circuit terminals and
ground. Note: May also be referred to as interference,
longitudinal, however interference, common mode is
the preferred term.
Interference, Differential Mode See interference,
normal mode.
Interference, Electrical Any spurious voltage or cur-
rent arising from external sources and appearing in
the circuits of a device.
Interference, Electrostatic Field A form of interference induced in the circuits of a device due to the
presence of an electrostatic eld. Note: It may appear
as common mode or normal mode interference in the
measuring circuits.
Interference, Longitudinal See interference, common mode.
Interference, Magnetic Field A form of interference
induced in the circuits of a device due to the presence of
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Product Data Document
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Glossary
a magnetic eld. Note: I may appear as common mode
or normal mode interference in measuring circuits.
Interference, Normal Mode A form of interference
which appears between measuring circuit terminals.
Note: May also be referred to as interference, differential
mode or interference, transverse, however interference,
normal mode is the preferred term.
Interference, Transverse See interference, normal
mode.
Linearity The closeness to which a curve approximates a straight line. Note: It is usually measured as a
nonlinearity and expressed a linearity; e.g. a maximum
deviation between an average curve and a straight
line. The average curse is determined after making
two or more full range traverses in each direction.
The value of linearity is referred to the output unless
otherwise stated.
Load, Regulation The change in output (usually
speed or voltage) from no-load to full-load (or other
specied limits).
Longitudinal Interference See Interference, com-
mon mode.
Loop, Closed (Feedback Loop) A signal path which
includes a forward path, a feedback path and a summing point, and forms a closed circuit.
Loop, Open A signal path without feedback
Modulation The process, or result of the process,
whereby some characteristic of one wave is varied in accordance with some characteristic of another wave.
Mounting Position The position of a device relative
to physical surroundings.
Noise An unwanted component of a signal or vari-
able which obscures the information content. Note: It
may be expressed in units of the output or in percent
of output span.
Offset The steady-state deviation when the setpoint
is xed. See also deviation, steady-state.
Operating Conditions Conditions (such as ambient temperature, ambient pressure, vibration, etc.) to
which a device is subject, but not including the variable
measured by the device.
Operating Conditions, Normal The range of operating conditions within which a device is designed to
operate, and under which operating inuences are
usually stated.
Operative Limits The range of operating conditions
to which a device may be subjected without permanent
impairment of operating characteristics.
Overrange Of a system or element, any excess value
of the input signal above its upper range-value or below
its lower range-value.
Parameter A controllable or variable characteristic of a
system or device, temporarily regarded as a constant,
the respective values of which serve to distinguish the
various specic states of a (the) system or device.
Power Consumption The maximum wattage used by
a device within its operating range during steady-state
signal condition.
Pressure, Ambient The pressure of the medium surrounding a device.
Pressure, Process The pressure of the process
medium at the sensing element.
Pressure Supply The pressure at the supply port of
the device.
Primary Element The system element that quantitatively converts measured variable energy into a form
suitable for measurement.
Process The collective functions performed in and
by the equipment in which the variable(s) is (are) to
be controlled.
Proportional Band The change in input required to
produce a full range change in output, due to proportional control action.
Range The region between the limits within which
a quantity is measured, received, or transmitted,
expressed by stating the lower and upper range values.
Range, Elevated-Zero A range where the zero value
of the measured variable, measured signal, etc., is
greater than the lower range-value. Note: The zero
may be between the lower and upper range values,
at the upper range value, or above the upper range
value. Note 2: Terms suppression, suppressed range
or suppressed span is frequently used to express the
condition in which the zero of the measured variable is
greater than the lower range value. The term range,
elevated zero is preferred.
Range, Suppressed-Zero A range where the zero
value of the measured variable is less than the lower
range value. Zero does not appear on the scale.
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Glossary
Range-Limit, Lower The lowest quantity that a device
can be adjusted to measure.
Range-Limit, Upper The highest quantity that a device
can be adjusted to measure.
Reference Performance Performance attained under
reference operating conditions. Note: Performance
includes such things as accuracy, dead band, repeatability, hysteresis, linearity, etc.
Repeatability The closeness of agreement among a
number of consecutive measurements of the output for
the same value of the input under the same operating
conditions, approaching from the same direction, for
full range traverses. Note: It is usually measured as
a non-repeatability and expressed as repeatability in
percent of span. It does not include hysteresis.
Reproducibility The closeness of agreement among
repeated measurements of the output for the same value
of input, made under the same operating conditions
over a period of time, approaching from both directions.
Note 1: It is usually measured as a non-reproducibility
and expressed as reproducibility in percent of span for
a specied time period. Normally, this implies a long
period of time, but under certain conditions, the period
may be a short time during which drift may not be
included. Note 2: Reproducibility includes hysteresis,
drift and repeatability.
Resolution The degree to which equal values of a
quantity can be discriminated by the device.
Response, Dynamic The behavior of the output of
a device as a function of the input, both with respect
to time.
Response, Ramp The total (transient plus steadystate) time response resulting from a sudden increase
in the rate of change in the input from zero to some
nite value.
Response, Step The time response of an instrument
when subjected to an instantaneous change in input
from one steady-state value to another.
Response, Time The time response of an instrument
when subjected to an instantaneous change in input
from one steady-state value to another.
Sampling Period The time interval between observations in a periodic sampling control system.
Sensitivity The ratio of a change in output magnitude
to the change of input which causes it after the steady-
state has been reached. Note 1: It is expressed as a
ration with the units of measurement of the two quantities
stated. (The ratio is constant over the range of a linear
device. For a nonlinear device the applicable input level
must be stated). Note 2: Sensitivity has frequently been
used to denote the deadband. However its usage in
this sense is deprecated since it is not in accord with
accepted standard denitions of the term.
Setpoint (Command) An input variable which sets
the desired value of the controlled variable.
Signal Information about a variable that can be
transmitted.
Signal, Error In a closed loop, the signal resulting
from subtracting a particular return signal from its corresponding input signal.
Signal, Feedback That return signal which results from
a measurement of the directly controlled variable.
Signal, Input A signal applied to a device, element
or system.
Signal, Measured The electrical, mechanical, pneumatic, or other variable applied to the input of a device.
It is the analog of the measured variable produced by
a transducer (when such is used).
Signal Output A signal delivered by a device, element or system.
Signal, Reference-Input One external to a control
loop which serves as the standard of comparison for
the directly controlled variable.
Signal, Return In a closed loop, the signal resulting
from a particular input signal, and transmitted by the
loop and to be subtracted from the input signal.
Signal to Noise Ratio Ratio of signal amplitude to
noise amplitude.
Signal Transducer (Signal Converter) A transducer,
which converts one standardized transmission signal
to another.
Span The algebraic difference between the upper and
lower range-values.
Summing Point Any point at which signals is added
algebraically.
System, Directly-Controlled The body, process,
or machine directly guided or restrained by the nal
controlling element to achieve a prescribed value of
the directly-controlled variable.
System, Indirectly-Controlled That portion of the
controlled system in which the indirectly controlled
variable is changed in response to changes in the
controlled variable.
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Glossary
Temperature, Ambient The temperature of the medium surrounding a device. Note: allowable ambient
temperature limits are based on the assumption that
the device in question is not exposed to signicant
radiant energy sources.
Temperature, Process The temperature of the process
medium at the sensing element.
Thermal Shock An abrupt temperature change applied to a device.
Time, Dead The interval of time between initiation of
an input change or stimulus and the start of the resulting response.
Time, Ramp Response The time interval by which
an output lags an input, when both are varying at a
constant rate.
Time, Rise The time required for the output of a system
(other than rst order) to make the change from a small
specied percentage (often 5 or 10) of the steady-state
increment to a large specied percentage (often 90 to
95), either before or in the absence of overshoot.
Time, Settling The time required, following the initia-
tion of a specied stimulus to a system, for the output
to enter and remain within a specied narrow band
centered on its steady-state value.
Time, Step Response Of a system or an element, the
time required for an output to make the change from an
initial value to a large specied percentage of the nal
steady-state value either before or in the absence of
overshoot, as a result of a step change to the input.
Time Constant The value T in an exponential response
term A exp (—t/T) or in one of the transform factors
1 + sT, 1 + jwT, 1/(1 + sT), 1 (1 + jwT).
Where:
s = complex variable
t = time, seconds
T = time constant
J = √ - 1
w = frequency, rad/sec
Note: For the output of a rst-order (lag or lead) system
forced by a step or an impulse, T is the time required
to complete 63.2 percent of the total rise or decay; at
any instant during the proceed, T is the quotient of the
instantaneous rate of change divided into the change
to be completed. In higher order systems, there is a
time constant for each of the rst-order components
of the process. In a Bode diagram, breakpoints occur
at w = 1/T.
Transducer An element or device which receives
information in the form of one physical quantity and
converts it to information in the form of the same or
other physical quantity.
Transmitter A transducer which responds to a measured variable by means of a sensing element, and
converts it to a standardized transmission signal which
is a function only of the measurement.
Variable, Directly Controlled In a control loop that
variable whose value is sensed to originate a feedback
signal.
Variable, Indirectly-Controlled A variable which does
not originate a feedback signal, but which is related to
and inuenced by the directly controlled variable.
Variable, Manipulated A quantity or condition which is
varied as a function of the actuating error signal so as to
change the value of the directly controlled variable.
Warm-Up Period The time required after energizing
a device before its rated performance characteristics
apply.
Zero Elevation For an elevated-zero range, the
amount the measured variable zero is above the lower
range-value. It may be expressed either in units of the
measured variable or in percent of span.
Zero Shift Any parallel shift of the input-output
curve.
Zero Suppression For a suppressed-zero range, the
amount the measured variable zero is below the lower
range-value. It may be expressed either in units of the
measured variable or in percent of span.
Zone, Dead A zone in which no value of the output
exists.
CREDIT: Extracted from SAMA Standard PMC20-
2-1970. Process Measurement and Control Terminology, with the permission of the publisher, Process Measurement & Control Section, SAMA, 370
Lexington Ave., New York, NY 10017
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Glossary
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