Automated Controls Glossary

Question 1

ACCURACY

Answer 1

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)

Question 2

ADC [A/D]

Answer 2

Analogue to digital converter

Question 3

ANALOGUE SIGNAL

Answer 3

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.

Question 4

ATTENUATION

Answer 4

A decrease in signal magnitude. Also, the reciprocal of gain.

Question 5

CLOSED LOOP

Answer 5

A signal path which includes a forward path, a feedback path,
and a summing point , and forms a closed circuit.

Question 6

CONTROLLED VARIABLE /PROCESS VARIABLE

Answer 6

The parameter that is
desired to be controlled. e.g. temperature, flow rate,
pressure, liquid level.

Question 7

CONTROLLER

Answer 7

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.

Question 8

DAC

Answer 8

Digital to analogue converter

Question 9

DCS

Answer 9

Distributed control system. Using DDC and Supervisory control for
process control sharing a common communications facility(Data
Highway).

Question 10

DDC

Answer 10

Distributed(Direct) Digital Control – A building automation system in which
a microprocessor based control system performs building automation
digitally.

Question 11

DEAD-BAND

Answer 11

The range through which an input can be varied without initiating
an observable response.

Question 12

DERIVATIVE (RATE) CONTROL ACTION

Answer 12

control action in which the output is
proportional to the rate of change of
the input.

Question 13

DIGITAL SIGNAL

Answer 13

Information represented by a set of binary numbers.

Question 14

DIRECTLY CONTROLLED VARIABLE

Answer 14

In a control loop, it is the variable which
is sensed to originate a feed-back
signal.

Question 15

DISCRETE SIGNAL

Answer 15

Information represented by a set of separate and distinct

numbers or values. E.g. on/off or integers.

Question 16

ELEMENT

Answer 16

A component of a device or system.

Question 17

ERROR SIGNAL

Answer 17

This is the signal derived in a closed loop control system from
the difference between the set-point signal and the feed-back
signal.

Question 18

FEED-BACK SIGNAL

Answer 18

The return signal which is generated from the

measurement of the directly controlled variable.

Question 19

FINAL CONTROL ELEMENT-

Answer 19

(final controlling element)- It is the forward
controlling element which directly changes the
value of the manipulated variable.

Question 20

GAIN, PROPORTIONAL

Answer 20

The ratio of the change in output due to proportional

control action to the change in input.

Question 21

INTEGRAL (RESET) CONTROL ACTION

Answer 21

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.

Question 22

MANIPULATED VARIABLE

Answer 22

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.

Question 23

NTC

Answer 23

Negative temperature coefficient. Resistance and temperature are
inversely proportional.

Question 24

OPEN LOOP

Answer 24

a signal path without feedback.

Question 25

PROCESS

Answer 25

Physical or chemical change of matter or conversion of energy. e.g.
change in temperature, pressure, electrical potential, speed etc.

Question 26

PROCESS VARIABLE

Answer 26

The parameter that is
desired to be controlled. e.g. temperature, flow rate,
pressure, liquid level.

Question 27

PROPORTIONAL CONTROL ACTION

Answer 27

control action in which there is a
continuous linear relation between the
output and the input.

Question 28

PTC

Answer 28

Positive temperature coefficient. Resistance and temperature are
directly proportional.

Question 29

RANGE

Answer 29

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.

Question 30

RESOLUTION

Answer 30

The smallest interval between sequential discrete values. e.g.
the number of degrees per address produced by an absolute
encoder.

Question 31

SENSING ELEMENT (SENSOR)

Answer 31

The element which directly responds to the

measured variable.

Question 32

SET-POINT

Answer 32

An input variable which determines the desired value of the
controlled variable. It may be manually set, automatically set or
programmed.

Question 33

SPAN

Answer 33

The algebraic difference between the upper and lower range values. If
the range were -20 to 200ºC, the Span would be 220ºC.

Question 34

TRANSDUCER

Answer 34

A device or element which converts energy from one type to

another.

Question 35

TRANSMITTER

Answer 35

A transducer which responds to a measured variable by
means of the sensor (sensing element) and converts it to a
standardized transmission signal.