99 PLC SCADA Interview Questions and Answers

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PLC SCADA Interview Questions and Answers

List of PLC SCADA Interview Questions and Answers

1. What is Automation?

Automation is delegation of human control functions to technical equipment for increasing productivity, better quality, reduce cost & increased in safety working conditions.

2. What 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.

3. What 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

4. Explain 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.

5. Difference 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 PLC & DCS?

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 transmitter?

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 level 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 signals.
Only current transmitters can be used as 2 wire transmitters.

13.      What is a “Smart” Transmitter?

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.

14.What 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.

15.What 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.

16. Explain 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).

17.Temperature measurement range supported by RTDs?

The RTD work on temperature range between-250 to 850 deg C.

18.Explain 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.

19.What 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)

20.Temperature measurement range supported by thermocouple?

The thermocouple work on board temperature range ie- 270 to 2300.

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