99 PLC SCADA Interview Question and Answers

99 PLC SCADA Interview Questions and Answers 

is Automation?
Automation is delegation of human
control functions to technical equipment for increasing productivity, better
quality, reduce cost & increased in safety working conditions.
are the different components used in automation?
The components of automation system include
Ø  Sensors for sensing the input
Ø  Transmitters for transmitting the
raw signal in electrical form
Ø  Control system which includes
PLC, DCS & PID controllers
Ø  Output devices/ actuators like
drives, control valves.
are the different control systems used in Automation?
Ø  PID Controller based control
Ø  PLC based control system
Ø  DCS based Control system
Ø  PC Based automation system
PID based control system.
PID (Proportional Integral Derivative) is the algorithm widely
used in closed loop control. The PID controller takes care of closed loop
control in plant. A number of PID controller with single or multiple loop can
be taken on network. PID Controllers are widely for independent loops. Although
some logic can be implemented but not much of sequential logic can be
implemented in PIDs.
between PLC & Relay?
Ø  PLC can be programmed whereas a
relay cannot.
Ø  PLC works for analog I/Os such a
PID loops etc. whereas a relay cannot.
Ø  PLC is much more advanced as
compared to relay.
6.         Difference between
DCS: The system
uses multiple processors, has a central database and the functionality is
distributed. That is the controller sub system performs the control functions,
the history node connects the data, the IMS node gives reports, the operator
station gives a good HML, the engineering station allows engineering changes to
be made.
PLC: The system has processor & I/O’s and some
functional units like basic modules and so on. Uses a SCADA for visualization.
Generally the SCADA does not use a central database.
7.         What is Encoder?
A feedback device which converts mechanical motion into electronic
signals. Usually an encoder is a rotary device that outputs digital pulses
which correspond to incremental angular motion. The encoder consists of a glass
or metal wheel with alternating clear and opaque stripes that are detected by
optical sensors to produce the digital outputs. 

Instrumentation System

8.         What types of
sensors are used for measuring different parameters?
Ø  Temperature sensors – RTD,
Thermocopule, Thermister
Ø  Pressure Sensor – Borden Tube,
Bellows, Strain gauge
Ø  Flow – sensor – Pitot tube
Ø  Level, Conductivity, Density, Ph
9.         What is
A transmitter is an electronic device that is generally mounted in
the field in close proximity to a sensor. The sensor (also known as a
transducer) measures a physical variable such as temperature or pressure and
outputs a very low level electronic signal. The basic function of the
transmitter is to provide the correct electrical power to turn on (or excite)
the sensor then to read the low level sensor signal, amplify it to a higher
electrical signal and send that signal a long distance to a control
or read-out device.
Since low-level electrical signals do not transmit long distances
with great accuracy, installing a transmitter generally gives a tremendous
improvement in the accuracy of the information delivered to a larger control
system. Typically the output form the transmitter is 4-20 mA or 0-10 V
10.      Why 4-20 mA
preferred over 0-10 V signal?
The 0-10 V signal has tendency to drop because of line resistance.
If the distance between sensor and input card is more the signal will not
properly represent the field value. The 4-20 mA will travel a long distance
without dropping signal value.

11.      Why 4-20 mA
preferred over 0-20 mA signal?
With 0-20 mA you can not distinguish between minimum field value
and connection break.  With 2-20 mA,
internal circuit can distinguish between connection break of minimum value.
Normally when the value is minimum the transmitter will give you 4 mA while in
case of connection breakage it will give 0 mA.
12.      Deference between 2
wire, 3 wire and 4 wire transmitter.  
In 2 wire transmitter the power and signal are transmitted through
same cable.
In 3 wire transmitter the data signal and power are with respect
to common ground.
In 4 wire transmitter two wires for power supply and two for
Only current transmitters can be used as 2 wire transmitters.
13.      What is a “Smart”
A “Smart” transmitter is a transmitter that uses a microprocessor
as the heart of the electronics. in addition, a “Smart” transmitter will output
some type of remote digital communications allowing you to read and set-up the
device from a remote position.
is Field bus?
Field bus is a general term for a digital only, high speed communications
protocol. The Key attribute to Field bus communications is higher speed
communications with the possibility of addressing multiple transmitters all on
the same field wiring. The foundation Field bus is a specific digital protocol
that is often shortened to just be called field bus. Other digital only
communications such as Prefabs are also Field bus protocols.
is Actuator?
In a closed- loop control system, the part of the final control
element that translates the control signal into action by the control device.
Working of RTDs
Resistance Temperature Device works on the principles that the
resistance of the material charges as its temperature charges Temperature is
determined by measuring resistance and then using the RTD Resistance vs Temp
characteristic to detect temperature. Typical elements used for RTD are Nickel,
copper and Platinum, Platinum is widely used in RTDs because of accuracy. PT
100 means at 0 deg temp 100 ohms resistance, A typical RTD consists of a fine
platinum wire wrapped around a mandrel and covered with a protective coating
(glass or ceramic).
measurement range supported by RTDs?
The RTD work on temperature range between-250 to 850 deg C.
Working of Thermocouple
Thermocouple consists of two strips or wires made up of different
metals and joined at one end. The temperature at that juncture induces and
electromotive force (emf) between the other ends. As the temperature goes up
the emf also increases. Through standard charts and tables the corresponding
temperature can be fond out.
The relationship between the thermocouple output and the
temperature is quite non linear. Different metallurgies produce different
outputs. The different metallurgies and different linearaities result in different
thermocouple designations as “J” “K” “N” “L”, etc.
is Cold junction compensation?
The industry accepted standard for the temperature at open end is
0 deg C. Therefore tables and chart make the assumption that the temp open end
is 0 deg C. In industry the open ends are always at actual room temperature and
not0 deg C. The emf adjustment because of difference between the temp and 0 deg
C is referred as Cold Correction (CJ Correction)
measurement range supported by thermocouple?
The thermocouple work on board temperature range ie- 270 to 2300.

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