Automated Controls Glossary Flashcards
ACCURACY
How an indicated value conforms to an accepted standard value
or true value. Normally “reference accuracy” is used which defines
the limit that errors will not exceed when the device is used under
reference operating conditions. Reference accuracy is normally
expressed in a number of forms typically preceded by a + and a –
sign. e.g. ±0.1ºC, ± 2% of scale length. (Do not confuse with
precision)
ADC [A/D]
Analogue to digital converter
ANALOGUE SIGNAL
A signal representing a variable which may be
continuously observed and continuously represented.
i.e. The signal has an infinite number of points between
any two values.
ATTENUATION
A decrease in signal magnitude. Also, the reciprocal of gain.
CLOSED LOOP
A signal path which includes a forward path, a feedback path,
and a summing point , and forms a closed circuit.
CONTROLLED VARIABLE /PROCESS VARIABLE
The parameter that is
desired to be controlled. e.g. temperature, flow rate,
pressure, liquid level.
CONTROLLER
A device which operates automatically to regulate a controlled
variable. In process industries controller always means
“automatic “ but other industries the automatic is not implied so
the term “automatic controller” is used.
DAC
Digital to analogue converter
DCS
Distributed control system. Using DDC and Supervisory control for
process control sharing a common communications facility(Data
Highway).
DDC
Distributed(Direct) Digital Control – A building automation system in which
a microprocessor based control system performs building automation
digitally.
DEAD-BAND
The range through which an input can be varied without initiating
an observable response.
DERIVATIVE (RATE) CONTROL ACTION
control action in which the output is
proportional to the rate of change of
the input.
DIGITAL SIGNAL
Information represented by a set of binary numbers.
DIRECTLY CONTROLLED VARIABLE
In a control loop, it is the variable which
is sensed to originate a feed-back
signal.
DISCRETE SIGNAL
Information represented by a set of separate and distinct
numbers or values. E.g. on/off or integers.
ELEMENT
A component of a device or system.
ERROR SIGNAL
This is the signal derived in a closed loop control system from
the difference between the set-point signal and the feed-back
signal.
FEED-BACK SIGNAL
The return signal which is generated from the
measurement of the directly controlled variable.
FINAL CONTROL ELEMENT-
(final controlling element)- It is the forward
controlling element which directly changes the
value of the manipulated variable.
GAIN, PROPORTIONAL
The ratio of the change in output due to proportional
control action to the change in input.
INTEGRAL (RESET) CONTROL ACTION
control action in which the output is proportional to the time integral of the input. In other words, the rate of change of the output is proportional to the input.
MANIPULATED VARIABLE
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.
NTC
Negative temperature coefficient. Resistance and temperature are
inversely proportional.
OPEN LOOP
a signal path without feedback.
PROCESS
Physical or chemical change of matter or conversion of energy. e.g.
change in temperature, pressure, electrical potential, speed etc.
PROCESS VARIABLE
The parameter that is
desired to be controlled. e.g. temperature, flow rate,
pressure, liquid level.
PROPORTIONAL CONTROL ACTION
control action in which there is a
continuous linear relation between the
output and the input.
PTC
Positive temperature coefficient. Resistance and temperature are
directly proportional.
RANGE
The region between two limits within a quantity is transmitted,
measured, or received. It is expressed by stating the upper and lower
range values. e.g. -20 to 150 ºC. See span also.
RESOLUTION
The smallest interval between sequential discrete values. e.g.
the number of degrees per address produced by an absolute
encoder.
SENSING ELEMENT (SENSOR)
The element which directly responds to the
measured variable.
SET-POINT
An input variable which determines the desired value of the
controlled variable. It may be manually set, automatically set or
programmed.
SPAN
The algebraic difference between the upper and lower range values. If
the range were -20 to 200ºC, the Span would be 220ºC.
TRANSDUCER
A device or element which converts energy from one type to
another.
TRANSMITTER
A transducer which responds to a measured variable by
means of the sensor (sensing element) and converts it to a
standardized transmission signal.
